Class 10 - Cambridge_IGCSE_Physics_Coursebook.pdf

Cambridge_IGCSE_Physics_Coursebook.pdf

https://youtu.be/RawYzuakf5U?si=QE90xZep37pSwED9

1. Cambridge
UNIVERSITYPRESS
Physics
forCambridgeIGCSE™
c*r~oxxix
vUUi\JlDvJUlx
DavidSang,MikeFollows&SheilaTarpey

2. Cambridge
UNIVERSITYPRESS
Physics
forCambridgeIGCSE™
COURSEBOOK
DavidSang,MikeFollows&SheilaTarpey

3. Cambridge
*
UNIVERSITYPRESS
UniversityPrintingHouse,CambridgeCB28BS,UnitedKingdom
OneLibertyPlaza,20thFloor,NewYork,NY10006,USA
477WilliamstownRoad,PortMelbourne,VIC3207,Australia
314-321,3rdFloor,Plot3,SplendorForum,JasolaDistrictCentre,NewDelhi-110025,India
79AnsonRoad,#06-04/06,Singapore079906
CambridgeUniversityPressispartoftheUniversityofCambridge.
ItfurtherstheUniversity’smissionbydisseminatingknowledgeinthepursuitof
education,learningandresearchatthehighestinternationallevelsofexcellence.
www.cambridge.org
Informationonthistitle:www.cambridge.org/978-l-108-88807-3
©CambridgeUniversityPress2021
Thispublicationisincopyright.Subjecttostatutoryexception
andtotheprovisionsofrelevantcollectivelicensingagreements,
noreproductionofanypartmaytakeplacewithoutthewritten
permissionofCambridgeUniversityPress.
Firstedition2009
Secondedition2014
Thirdedition2021
2019181716151413121110987654321
PrintedinDubaibyOrientalPress
AcataloguerecordforthispublicationisavailablefromtheBritishLibrary
ISBN978-1-108-88807-3Coursebookwithdigitalaccess(2years)
ISBN978-1-108-97042-6Digitalcoursebook(2years)
ISBN978-1-108-74449-2CoursebookeBook
Additionalresourcesforthispublicationatwww.cambridge.org/delange
CambridgeInternationalcopyrightmaterialinthispublicationisreproduced
underlicenseandremainstheintellectualpropertyofCambridgeAssessment
InternationalEducation
NOTICETOTEACHERS
Thephotocopymastersinthispublicationmaybephotocopied
ordistributed[electronically]freeofchargeforclassroomuse
withintheschoolorinstitutionthatpurchasedthepublication.
Worksheetsandcopiesofthemremaininthecopyrightof
CambridgeUniversityPress,andsuchcopiesmaynotbe
distributedorusedinanywayoutsidethepurchasinginstitution.
Exam-stylequestions[andsampleanswers]havebeenwrittenby
theauthors.Inexaminations,thewaymarksareawardedmaybe
different.Referencestoassessmentand/orassessmentpreparation
arethepublisher’sinterpretationofthesyllabusrequirements
andmaynotfullyreflecttheapproachofCambridgeAssessment
InternationalEducation.
NOTICETOTEACHERSINTHEUK
Itisillegaltoreproduceanypartofthisworkinmaterialform
(includingphotocopyingandelectronicstorage)exceptunderthe
followingcircumstances:
(i)whereyouareabidingbyalicencegrantedtoyourschoolor
institutionbytheCopyrightLicensingAgency;
(ii)wherenosuchlicenceexists,orwhereyouwishtoexceedthe
termsofalicence,andyouhavegainedthewrittenpermission
ofCambridgeUniversityPress;
(iii)whereyouareallowedtoreproducewithoutpermissionunder
theprovisionsofChapter3oftheCopyright,Designsand
PatentsAct1988,whichcovers,forexample,thereproduction
ofshortpassageswithincertaintypesofeducationalanthology
andreproductionforthepurposesofsettingexamination
questions.

4. DEDICATED
L>TEACHER
AWARDS
Teachersplayanimportantpartinshapingfutures.
OurDedicatedTeacherAwardsrecognisethehard
workthatteachersputineveryday.
Thankyoutoeveryonewhonominatedthisyear;wehavebeeninspiredandmoved
byallofyourstories.Welldonetoallofournomineesforyourdedicationtolearning
andforinspiringthenextgenerationofthinkers,leadersandinnovators.
Congratulationstoourincrediblewinnerandfinalists!
Formoreinformationaboutourdedicatedteachersandtheirstories,goto
dedicatedteacher.cambridge.org
Hi
Cambridge
UNIVERSITYPRESS
BrighterThinking
BetterLearning
BuildingBrighterFuturesTogether

5. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Contents
*
Howtousethisseries
Howtousethisbook
Introduction
1Makingmeasurements
1.1 Measuringlengthandvolume
1.2 Density
1.3 Measuringtime
2Describingmotion
2.1 Understandingspeed
2.2 Distance-timegraphs
2.3 Understandingacceleration
2.4 Calculatingspeedandacceleration
3Forcesandmotion
3.1 Wehavelift-off
3.2 Mass,weightandgravity
3.3 Fallingandturning
3.4 Force,massandacceleration
3.5 Momentum
3.6 Moreaboutscalarsandvectors
vi
viii
X
3
5
9
20
24
25
31
44
47
49
52
54
59
6
7
8
9
Energystoresandtransfers
6.1 Energystores 104
6.2 Energytransfers 107
6.3 Conservationofenergy 110
6.4 Energycalculations 114
Energyresources
7.1 Theenergyweuse 125
7.2 EnergyfromtheSun 133
Workandpower
8.1 Doingwork 141
8.2 Calculatingworkdone 142
8.3 Power 145
8.4 Calculatingpower 146
Thekineticparticlemodelofmatter
9.1 Statesofmatter 155
9.2 Thekineticparticlemodelofmatter156
9.3 Gasesandthekineticmodel 160
9.4 TemperatureandtheCelsiusscale162
9.5 Thegaslaws 164
4
5
Turningeffects
4.1 Themomentofaforce
4.2 Calculatingmoments
4.3 Stabilityandcentreofgravity
Forcesandmatter
5.1 Forcesactingonsolids
5.2 Stretchingsprings
5.3 Thelimitofproportionalityandthe
springconstant
5.4 Pressure
5.5 Calculatingpressure
68
69
74
86
86
88
93
94
10
11
Thermalpropertiesofmatter
10.1Thermalexpansion
10.2Specificheatcapacity
10.3Changingstate
Thermalenergytransfers
11.1Conduction
11.2Convection
11.3Radiation
11.4Consequencesofthermal
energytransfer
172
175
179
191
196
200
204

6. Contents
12Sound
216
217
219
221
19Electricalcircuits
349
353
365
12.1Makingsounds 19.1Circuitcomponents
12.2Howdoessoundtravel? 19.2Combinationsofresistors
12.3Thespeedofsound 19.3Electricalsafety
12.4Seeingandhearingsounds
20Electromagneticforces
13Light 20.1Themagneticeffectofacurrent 378
13.1Reflectionoflight 231 20.2Forceonacurrent-carrying
13.2Refractionoflight 236
conductor 382
13.3Totalinternalreflection 241
20.3Electricmotors 384
13.4Lenses
13.5Dispersionoflight
246
253
20.4Beamsofchargedparticlesand
magneticfields
21Electromagneticinduction
386
14Propertiesofwaves
21.1Generatingelectricity 395
14.1Describingwaves 260
21.2Powerlinesandtransformers 401
14.2Speed,frequencyandwavelength 266
21.3Howtransformerswork 405
14.3Explainingwavephenomena 267
22Thenuclearatom
15Theelectromagneticspectrum 22.1Atomicstructure 415
15.1Electromagneticwaves
15.2Electromagnetichazards
15.3Communicatingusing
279
284
22.2Protons,neutronsandelectrons
23Radioactivity
418
electromagneticwaves 285 23.1Radioactivityallaroundus
23.2Radioactivedecay
429
431
16Magnetism 23.3Activityandhalf-life 436
16.1Permanentmagnets
16.2Magneticfields
295
297
23.4Usingradioisotopes
24EarthandtheSolarSystem
442
17Staticelectricity 24.1Earth,SunandMoon 453
17.1Charginganddischarging 310 24.2TheSolarSystem 456
17.2Explainingstaticelectricity
17.3Electricfields
312
314
25StarsandtheUniverse
25.1TheSun 468
18Electricalquantities
25.2Starsandgalaxies 469
18.1Currentinelectriccircuits 323
25.3TheUniverse 474
18.2Voltageinelectriccircuits
18.3Electricalresistance
327
330
Appendix 485
18.4Moreaboutelectricalresistance 335
Glossary 488
18.5Electricalenergy,workandpower 337Keyequations
Index
Acknowledgements
496
498
511

7. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
>Howtousethisseries
Theteacher’sresourcealsocontainsscaffolded
worksheetsandunittestsforeachchapter.
Answersforallcomponentsareaccessibleto
teachersforfreeontheCambridgeGOplatform.
Weofferacomprehensive,flexiblearrayofresourcesfortheCambridgeIGCSE™
Physicssyllabus.Weprovidetargetedsupportandpracticeforthespecificchallenges
we'veheardthatstudentsface:learningsciencewithEnglishasasecondlanguage;
learnerswhofindthemathematicalcontentwithinsciencedifficult;anddeveloping
practicalskills.
Thedigitalteachersresourcecontainsdetailedguidanceforalltopicsofthe
syllabus,includingcommonmisconceptionsidentifyingareaswherelearners
mightneedextrasupport,aswellasanengagingbankoflessonideasforeach
syllabustopic.Differentiationisemphasisedwithadvicefor
identificationofdifferentlearnerneedsand
suggestionsofappropriateinterventionsto
supportandstretchlearners.Theteacher’s
resourcealsocontainssupportforpreparing
andcarryingoutalltheinvestigationsinthe
practicalworkbook,includingasetofsample
resultsforwhenpracticalsaren’tpossible.
Cambridge
UNIVERSITYPRESS
Physics
forCambridgeIGCSE’“
ThecoursebookprovidescoverageofthefullCambridge
IGCSEPhysicssyllabus.Eachchapterexplainsfacts
andconcepts,andusesrelevantreal-worldexamplesof
scientificprinciplestobringthesubjecttolife.Together
withafocusonpracticalworkandplentyofactivelearning
opportunities,thecoursebookprepareslearnersforall
" aspectsoftheirscientificstudy.Attheendofeachchapter,
examination-stylequestionsofferpracticeopportunitiesfor
learnerstoapplytheirlearning.

8. Howtousethisseries
Theskills-focusedworkbookhasbeencarefullyconstructedtohelp
learnersdeveloptheskillsthattheyneedastheyprogressthrough
theirCambridgeIGCSEPhysicscourse,providingfurtherpractice
ofallthetopicsinthecoursebook.Athree-tier,scaffoldedapproach
toskillsdevelopmentenableslearnerstograduallyprogressthrough
‘focus’,‘practice’and‘challenge’exercises,ensuringthateverylearner
issupported.Theworkbookenablesindependentlearningandis
idealforuseinclassorashomework.
Cambridge
Physics
forCambridgeIGCSE'
TheCambridgeIGCSEpracticalworkbookprovideslearners
withadditionalopportunitiesforhands-onpracticalwork,
givingthemfullguidanceandsupportthatwillhelpthemto
developtheirinvestigativeskills.Theseskillsincludeplanning
investigations,selectingandhandlingapparatus,creating
hypotheses,recordinganddisplayingresults,andanalysing
andevaluatingdata.
Physics
forCambridgeIGCSE'
COMING
IN2022
Mathematicsisanintegralpartofscientificstudy,andonethat
learnersoftenfindabarriertoprogressioninscience.TheMaths
SkillsforCambridgeIGCSEPhysicswrite-inworkbookhasbeen
writtenincollaborationwiththeAssociationofScienceEducation
witheachchapterfocusingonseveralmathsskillsthatlearners
needtosucceedintheirPhysicscourse.
OurresearchshowsthatEnglishlanguageskillsarethesingle
biggestbarriertolearnersaccessinginternationalscience.
Thiswrite-inworkbookcontainsexercisessetwithinthe
contextofCambridgeIGCSEPhysicstopicstoconsolidate
understandingandembedpracticeinaspectsoflanguage
centraltothesubject.Activitiesrangefrompractisingusing
comparativeadjectivesinthecontextofmeasuringdensity,
towritingasetofinstructionsusingtheimperativeforan
experimentinvestigatingfrequencyandpitch.
COMING
IN2022
Physics
forCambridgeIGCSE'

9. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
>Howtousethisbook
Throughoutthisbook,youwillnoticelotsofdifferentfeaturesthatwillhelpyourlearning.Theseareexplainedbelow.
LEARNINGINTENTIONS
Thesesetthesceneforeachchapter,helpwithnavigationthroughthecoursebookandindicatethe
importantconceptsineachtopic.Thesebeginwith'Inthischapteryouwill:'.
Inthelearningintentionstable,Supplementcontentisindicatedwithalargearrowandadarker
background,asintheexamplehere.
GETTINGSTARTED
Thiscontainsquestionsandactivitiesonsubjectknowledgeyouwillneedbeforestartingthechapter.
SCIENCEINCONTEXT
Thisfeaturepresentsreal-worldexamples
andapplicationsofthecontentinachapter,
encouragingyoutolookfurtherintotopicsthat
maygobeyondthesyllabus.Therearediscussion
questionsattheendwhichlookatsomeofthe
benefitsandproblemsoftheseapplications.
EXPERIMENTAL SKILLS
Thisfeaturefocusesondevelopingyourpractical
skills.Theyincludelistsofequipmentrequiredand
anysafetyissues,step-by-stepinstructionssoyou
cancarryouttheexperiment,andquestionsto
helpyouthinkaboutwhatyouhavelearned.
KEYWORDS
Keyvocabularyishighlightedinthetextwhenit
isfirstintroduced,anddefinitionsaregivenin
boxesnearthevocabulary.Youwillalsofind
definitionsofthesewordsintheGlossaryatthe
backofthisbook.
Supplementcontent:Wherecontentisintendedfor
learnerswhoarestudyingtheSupplementcontentofthe
syllabusaswellastheCore,thisisindicatedinthemain
textusingthearrowandthebar,asontherighthere,and
thetextisinblue.Youmayalsoseethebluetextwith
justanarrow(andnobar),inboxedfeaturessuchasthe
KeyWordsortheGettingStarted.Symbolsinblueare
alsosupplementarycontent.
Questions
Appearingthroughoutthetext,questionsgiveyoua
chancetocheckthatyouhaveunderstoodthetopicyou
havejustreadabout.Theanswerstothesequestionsare
accessibletoteachersforfreeontheCambridgeGOsite.
ACTIVITY
Activitiesgiveyouanopportunitytocheck
yourunderstandingthroughoutthetextinamore
activeway,forexamplebycreatingpresentations,
postersortakingpartinroleplays.Whenactivities
haveanswers,teacherscanfindtheseforfreeon
theCambridgeGOsite.
KEYEQUATIONS
Importantequationswhichyouwillneedtolearn
andrememberaregivenintheseboxes.
vlll

10. Howtousethisbook
COMMAND WORDS
Commandwordsthatappearinthesyllabusand
mightbeusedinexamsarehighlightedinthe
exam-stylequestions.Inthemargin,youwillfind
theCambridgeInternationaldefinition.Youwill
alsofindthesedefinitionsintheGlossary.
SELF/PEERASSESSMENT
Attheendofsomeactivitiesandexperimental
skillsboxes,youwillfindopportunitiestohelp
youassessyourownwork,orthatofyour
classmates,andconsiderhowyoucanimprove
thewayyoulearn.
WORKEDEXAMPLE
Whereveryouneedtoknowhowtouseanequation
tocarryoutacalculation,thereareworkedexample
boxestoshowyouhowtodothis.
REFLECTION
Theseactivitiesaskyoutothinkaboutthe
approachthatyoutaketoyourwork,andhow
youmightimprovethisinthefuture.
PROJECT
Projectsallowyoutoapplyyourlearningfromthewholechaptertogroupactivitiessuchasmakingpostersor
presentations,orperformingindebates.Theymaygiveyoutheopportunitytoextendyourlearningbeyond
thesyllabusifyouwantto.
SUMMARY
Thereisasummaryofkeypointsattheendofeachchapter.
Supplementcontentisindicatedwithalargearrowinthemarginandadarkerbackground,ashere.
Supplementcontentisindicatedwithalargearrowinthemarginandadarkerbackground,ashere.
SELF-EVALUATION CHECKLIST
Thesummarychecklistsarefollowedby‘Ican’statementswhichmatchtheLearningintentionsatthebeginning
ofthechapter.Youmightfindithelpfultoratehowconfidentyouareforeachofthesestatementswhenyouare
revising.Youshouldrevisitanytopicsthatyourated‘Needsmorework’or‘Almostthere’.
See Needs Almost Confident
Topic...moreworkthere tomoveon
Core
Supplement
EXAM-STYLEQUESTIONS
Questionsattheendofeachchapterprovidemoredemandingexam-stylequestions,someofwhichmayrequire
useofknowledgefrompreviouschapters.Theanswerstothesequestionsareaccessibletoteachersforfreeonthe
CambridgeGOsite.
ix>

11. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
>Introduction
Studyingphysics
Whystudyphysics?Somepeoplestudyphysicsforthe
simplereasonthattheyfinditinteresting.Physicists
studymatter,energyandtheirinteractions.Theymight
beinterestedinobservingthetiniestsub-atomicparticles,
orunderstandingthevastnessoftheUniverseitself.
Onamorehumanscale,physicistsstudymaterialsto
trytopredictandcontroltheirproperties.Theystudy
theinteractionsofradiationwithmatter,includingthe
biologicalmaterialswearemadeof.
Otherpeoplearemoreinterestedintheapplicationsof
physics.Theywanttoknowhowitcanbeused,perhaps
inanengineeringproject,orformedicalpurposes.
Dependingonhowourknowledgeisapplied,itcan
maketheworldabetterplace.
Somepeoplestudyphysicsaspartoftheircourse
becausetheywanttobecomesomeothertypeofscientist
-perhapsachemist,biologistorgeologist.These
branchesofsciencedrawagreatdealonideasfrom
physics,andphysicsmaydrawonthem.
Thinkingphysics
Howdophysiciststhink?Oneofthecharacteristicsof
physicistsisthattheytrytosimplifyproblems-reduce
themtotheirbasics-andthensolvethembyapplying
someveryfundamentalideas.Forexample,youwill
befamiliarwiththeideathatmatterismadeoftiny
particlesthatattractandrepeleachotherandmove
about.Thisisaveryusefulmodel,whichhashelped
ustounderstandthebehaviourofmatter,howsound
travels,howelectricityflows,andmuchmore.
Onceafundamentalideaisestablished,physicistslook
aroundforotherareaswhereitmighthelptosolve
problems.Oneofthesurprisesof20thcenturyphysics
wasthat,oncephysicistshadbeguntounderstandthe
fundamentalparticlesofwhichatomsaremade,they
realisedthatthishelpedtoexplaintheearliestmoments
inthehistoryoftheUniverse,atthetimeofthe
BigBang.
Medicineisoftenseenasabiologicalcareerbutthisdoctorwillusemanyapplicationsofphysics,fromX-raystorobotic
limbs,inherwork.
X

12. Introduction
Physicsreliesonmathematics.Physicistsmeasure
quantitiesandanalysedata.Theyinventmathematical
models-equationsandsoon-toexplaintheir
findings.Infact,agreatdealofmathematicshasbeen
developedbyphysiciststohelpthemtounderstandtheir
experimentalresults.Anexampleofthisistheworkof
EdwardWitten,whodesignednewmathematicaltoolsto
unifydifferentversionsofsuperstringtheory-atheory
whichtriestounitealltheforcesandparticlesyouare
learningabout.
Computershavemadeabigdifferenceinphysics,
allowingphysiciststoprocessvastamountsofdata
rapidly.Computerscanprocessdatafromtelescopes,
controldistantspacecraftandpredictthebehaviourof
billionsofatomsinasolidmaterial.
InApril2019thefirstpictureswerereleasedofablackhole.
Thecentralareaissodensethatlightcannotescapeit.
Thisimagewastheresultofhundredsofscientistsusinga
networkofradiotelescopesaroundtheworld,processing
manypetabytesofdata-1petabyteisequalto1million
gigabytesor1x1015bytes.
Themoreyoustudyphysics,themoreyouwillcometo
realisehowtheideasjoinup.Indeed,theultimategoal
formanyphysicistsistolinkallideasintooneunifying
‘theoryofeverything’.
StephenHawkingwasabrilliantyoungstudentwhenhewas
diagnosedwithmotorneuronedisease.Hewasexpectedto
liveonlyafewyears,butatthetimeofhisdeathat76hewas
stillworkingasaprofessoratCambridgeUniversity.Oneof
hismainaimswastouniterelativity(whichexplainsthevery
large)andquantumphysics(whichexplainstheverysmall).
Hawkingcametobelievethiswouldnothappen,but
wasgladaboutthis:‘I’mnowgladthatoursearchfor
understandingwillnevercometoanend,andthatwe
willalwayshavethechallengeofnewdiscovery.Without
it,wewouldstagnate.’
Usingphysics
Thepracticalapplicationsofphysicsarefarreaching.
Manyphysicistsworkineconomicsandfinance,using
ideasfromphysicstopredicthowmarketswillchange.
Othersusetheirunderstandingofparticlesinmotionto
predicthowtrafficwillflow,orhowpeoplewillmovein
crowdedspaces.Thistypeofmodellingcanbeusedto
helpusunderstandthespreadofpathogens,suchasthe
viruswhichcausedthe2020Covid-19pandemic.
Physicsisbeingusedtofindsolutionsfortheworld’s
majorproblems.Newmethodsofgeneratingelectricity
withoutaddingtogreenhousegasemissionsarehelping
toreduceourdependenceonfossilfuels.Developments
inbatterytechnologyallowustostoreelectricalenergy,
makingelectricvehiclesareality.
xi

13. )CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
Ifthischilddrivesitwillprobablybeinanelectricvehiclelikethisone.Manycountriesaimtophaseoutpolluting,fossilfuel
poweredvehiclesbythemiddleofthe21stcentury.Physicistsareimprovingcardesignandbatterylifetomakethisfeasible.
Joiningin
So,whenyoustudyphysics,youaredoingtwothings.
iYouarejoininginwithabighumanproject-
learningmoreabouttheworldaroundusand
applyingthatknowledge.
iiAtthesametime,youarelearningtothinklike
aphysicist-howtoapplysomebasicideas,how
tolookcriticallyatdata,andhowtorecognise
underlyingpatterns.Whateverpathyoutake,these
skillswillremainwithyouandhelpyoumakesense
oftherapidlychangingworldinwhichwelive.

14. >Chapter1
Makino
measurements

15. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
GETTINGSTARTED
Inpairs,eithertakethemeasurementsorwritedownhowyouwoulddothefollowing:
measurethelength,widthandthicknessofthisbookandworkoutitsvolume
measurethethicknessofasheetofpaperthatmakesupthisbook
measurethelengthofajourney(forexample,onamap)thatisnotstraight.
Nowdiscusshowyouwouldworkoutthedensityof:
aregular-shapedsolid
anirregular-shapedsolid
aliquid.
AREWECLEVERERTHANOURANCESTORS WERE?
Figure1.1:Cubitrod.
TheAncientEgyptianswereexpertsatusingvery
simpletoolslikethecubitrod.Thisenabledthem
tobuildtheirpyramidsaccurately.Eratosthenes,
abrilliantscientistwholivedinEgyptinabout
300BCE,showedthesamecareandattentionto
detail.ThisallowedhimtoworkoutthattheEarth
hasacircumferenceof40000km(Figure1.2).
Incontrast,therearemanyrecentexampleswhere
incorrectmeasurementshaveledtoproblems.
AlthoughtheHubbleSpaceTelescopehadthemost
preciselyshapedmirrorevermade,theoriginal
imagesitproducedwerenotasclearasexpected.
Tinymistakesinmeasuringmeantthatithadthe
wrongshapeandittookalotofefforttoaccount
fortheseerrors.
Discussionquestions
1Youcannotalwaysdependonyoureyesto
judgelengths.LookatFigure1.3anddecide
whichlineislonger?Checkbyusingaruler.
Figure1.3:Whichlineislonger?
2Eratosthenesmayhavehiredamantopace
thedistancebetweenAlexandriaandSyene
(present-dayAswan)tocalculatetheEarth's
circumference.Peoplehavedifferentstride
lengthssosomepeopletakelongerstepsthan
others.Discussthepossiblewaysthatanyone
withanystridelengthcouldhavemeasured
thedistancebetweenthesetownsaccurately.
Peopletendtodismisspeoplewholivedinthe
pastaslessintelligentthanweare.Afterall,they
usedpartsoftheirbodiesformeasuringdistances.
Acubitwasthelengthoftheforearmfromthetip
ofthemiddlefingertotheelbow.However,the
ancientEgyptiansknewthisvariedbetweenpeople.
Therefore,inaround3000BCE,theyinventedthe
royalcubit(Figure1.1),markedoutonapieceof
graniteandusedthisasastandardtoproduce
cubitrodsofequallength.
Figure1.2:Eratosthenesusedshadowsandgeometryto
workoutthecircumferenceoftheEarth.
2

16. 1Makingmeasurements
1.1Measuringlength
andvolume
Inphysics,wemakemeasurementsofmanydifferent
lengths,forexample,thelengthofapieceofwire,the
heightofliquidinatube,thedistancemovedbyan
object,thediameterofaplanetortheradiusofitsorbit.
Inthelaboratory,lengthsareoftenmeasuredusinga
ruler(suchasametreruler).
Thepointhereistorecognisethatitisalwaysimportant
tothinkcriticallyaboutthemeasurementsyoumake,
howeverstraightforwardtheymayseem.Youhaveto
considerthemethodyouuse,aswellastheinstrument
(inthiscase,theruler).
Measuringlengthswitharulerisafamiliartask.Butwhen
youusearuler,itisworththinkingaboutthetaskand
justhowreliableyourmeasurementsmaybe.Consider
measuringthelengthofapieceofwire(Figure1.4).
Thewiremustbestraight,andlaidcloselyalongside
theruler.(Thismaybetrickywithabentpiece
ofwire.)
Lookattheendsofthewire.Aretheycutneatly,
oraretheyragged?Isitdifficulttojudgewherethe
wirebeginsandends?
Lookatthemarkingsontheruler.Theyare
probably1mmapart,buttheymaybequitewide.
Lineoneendofthewireupagainstthezeroonthe
scale.Becauseofthewidthofthemark,thismaybe
awkwardtojudge.
Lookattheotherendofthewireandreadthescale.
Again,thismaybetrickytojudge.
Nowyouhaveameasurement,withanideaofhowprecise
itis.Youcanprobablydeterminethelengthofthewireto
withinamillimetre.Butthereissomethingelsetothink
about-theruleritself.Howsurecanyoubethatitis
correctlycalibrated?Arethemarksattheendsofametre
rulerseparatedbyexactlyonemetre?Anyerrorinthiswill
leadtoaninaccuracy(probablysmall)inyourresult.
Figure1.4:Simplemeasurementsstillrequirecareful
technique,forexample,findingthelengthofawire.
KEYWORDS
standard:isanabsoluteorprimaryreferenceor
measurement
precise:whenseveralreadingsareclosetogether
whenmeasuringthesamevalue

calibrated:shouldagreecloselywithastandard
oragreeswhenacorrectionhasbeenapplied
Moremeasurementtechniques
Ifyouhavetomeasureasmalllength,suchasthe
thicknessofawire,itmaybebettertomeasureseveral
thicknessesandthencalculatetheaverage.Youcanuse
thesameapproachwhenmeasuringsomethingverythin,
suchasasheetofpaper.Takeastackof500sheetsand
measureitsthicknesswitharuler(Figure1.5).Then
divideby500tofindthethicknessofonesheet.
Figure1.5:Makingmultiplemeasurements.

17. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Forsomemeasurementsoflength,suchascurvedlines,
itcanhelptolayathreadalongtheline.Markthethread
ateitherendofthelineandthenlayitalongaruler
tofindthelength.Thistechniquecanalsobeusedfor
measuringthecircumferenceofacylindricalobjectsuch
asawoodenrodorameasuringcylinder.
Measuringvolumes
Therearetwoapproachestomeasuringvolumes,
dependingonwhetherornottheshapeisregular.
Foracubeorcuboid,suchasarectangularblock,
measurethelength,widthandheightoftheobjectand
multiplythemeasurementstogether.Forobjectsofother
regularshapes,suchasspheresorcylinders,youmay
havetomakeoneortwomeasurementsandthenlookup
theequationforthevolume.
Forliquids,measuringcylinderscanbeusedasshown
inFigure1.6.(Recallthatthesearedesignedsothat
youlookatthescalehorizontally,notatanoblique
angle,andreadthelevelofthebottomofthemeniscus.)
Themeniscusisthecurveduppersurfaceofaliquid,
causedbysurfacetension.Itcancurveupordownbut
thesurfaceofwaterinameasuringcylindercurves
downwards.Thinkcarefullyaboutthechoiceofcylinder.
A1litre(ora1dm3)cylinderisunlikelytobesuitable
formeasuringasmallvolumesuchas5cm3.Youwillget
amoreaccurateanswerusinga10cm3cylinder.
Figure1.6:Astudentmeasuringthevolumeofaliquid.
Hereyesarelevelwiththescalesothatshecanaccurately
measurewherethemeniscusmeetsthescale.
Measuringvolumeby
displacement
Mostobjectsdonothavearegularshape,sowecannot
findtheirvolumessimplybymeasuringthelengths
oftheirsides.Hereishowtofindthevolumeofan
irregularlyshapedobject.Thistechniqueisknownas
measuringvolumebydisplacement.
Selectameasuringcylinderthatisaboutthreeor
fourtimeslargerthantheobject.Partiallyfillitwith
water(Figure1.7),enoughtocovertheobject.Note
thevolumeofthewater.
Immersetheobjectinthewater.Thelevelofwater
inthecylinderwillincrease,becausetheobject
pushesthewateroutofthewayandtheonlywayit
canmoveisupwards.Theincreaseinitsvolumeis
equaltothevolumeoftheobject.
Unitsoflengthandvolume
Inphysics,wegenerallyuseSIunits(thisisshortfor
LeSystemeInternationald’UnitesorTheInternational
SystemofUnits).TheSIunitoflengthisthemetre(m).
Table1.1showssomealternativeunitsoflength,
togetherwithsomeunitsofvolume.Notethatthelitre
andmillilitrearenotofficialSIunitsofvolume,andso
arenotusedinthisbook.Onelitre(11)isthesameas
1dm3,andonemillilitre(1ml)isthesameas1cm3.
KEYWORDS
volume:thespaceoccupiedbyanobject
meniscus:curveduppersurfaceofaliquid
displace:movingsomethingtoanotherplaceso
waterismovedoutoftheway(upwards)whenan
objectisloweredintoit
immerse:tocoversomethinginafluid(usually
water)sothattheobjectissubmerged

18. 1Makingmeasurements
Figure1.7:Measuringvolumebydisplacement.
QuantityUnits
length metre(m)
1decimetre(dm)=0.1m
1centimetre(cm)= 0.01m
1millimetre(mm)= 0.001m
1micrometre(pm)=0.000001m
1kilometre(km)=1000m
volume cubicmetre(m3)
1cubiccentimetre(cm3)=0.000001m3
1cubicdecimetre(dm3)=0.001m3
Table1.1:SomeunitsoflengthandvolumeintheSIsystem.
Questions
1Thevolumeofapieceofwoodwhichfloatsinwater
canbemeasuredasshowninFigure1.8.
aWriteaparagraphtodescribetheprocedure.
bStatethevolumeofthewood.
Figure1.8:Measuringthevolumeofanobjectthatfloats.
2Astackofpapercontains500sheetsofpaper.
Thestackhasdimensionsof0.297mx21.0cmx
50.0mm.
aWhatisthethicknessofonesheetofpaper?
bWhatisthevolumeofthestackofpaperincm3?
1.2Density
Oureyescandeceiveus.Whenwelookatanobject,
wecanjudgeitsvolume.However,wecanonlyguessits
mass.Wemayguessincorrectly,becausewemisjudge
thedensity.Youmayoffertocarrysomeone’sbag,only
todiscoverthatitcontainsheavybooks.Alargeboxof
chocolatesmayhaveamassofonly200g.
Themassofanobjectisthequantity(amount)ofmatter
itismadeof.Massismeasuredinkilograms.Butdensity
isapropertyofamaterial.Ittellsushowconcentrated
itsmassis.Youwilllearnmoreaboutthemeaningof
massandhowitdiffersfromweightinChapter3.
Ineverydayspeech,wemightsaythatleadisheavierthan
wood.Wemeanthat,givenequalvolumesofleadand
wood,theleadisheavier.Inscientificterms,thedensity
ofleadisgreaterthanthedensityofwood.Sowedefine
densityasshown,inwordsandasanequation.
Densityisthemassperunitvolumeforasubstance.
KEYWORDS
mass:thequantityofmatterabodyiscomposed
of;masscausestheobjecttoresistchangesin
itsmotionandcausesittohaveagravitational
attractionforotherobjects
density:theratioofmasstovolumefora
substance
weight:thedownwardforceofgravitythatacts
onanobjectbecauseofitsmass
Thesymbolfordensityisp,theGreekletterrho.TheSI
unitofdensityiskg/m3(kilogramspercubicmetre).
Youmaycomeacrossotherunits,asshowninTable1.2.
5

19. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Unitofmass Unitofvolume Unitofdensity Densityofwater
kilogram,kg cubicmetre,m3 kilogramspercubicmetre 1000kg/m3
kilogram,kg cubicdecimetre,dm3 kilogramspercubicdecimetre1.0kg/dm3
gram,g cubiccentimetre,cm3 gramspercubiccentimetre 1.0g/cm3
Table1.2:Unitsofdensity.
Valuesofdensity
SomevaluesofdensityareshowninTable1.3.Gases
havemuchlowerdensitiesthansolidsorliquids.
Anobjectthatislessdensethanwaterwillfloat.Iceis
lessdensethanwaterwhichexplainswhyicebergsfloat
inthesea,ratherthansinkingtothebottom.Onlyabout
onetenthofanicebergisabovethewatersurface.Ifany
partofanobjectisabovethewatersurface,thenitisless
densethanwater.
Material Density/kg/m3
Gases air 1.29
hydrogen 0.09
helium 0.18
carbondioxide 1.98
Liquids water 1000
alcohol(ethanol)790
mercury 13600
Solids ice 920
wood 400-1200
polyethene 910-970
glass 2500-4200
steel 7500-8100
lead 11340
silver 10500
gold 19300
Table1.3:Densitiesofsomesubstances.Forgases,theseare
givenatatemperatureof0°Candapressureof1.0x105Pa.
Manymaterialshavearangeofdensities.Sometypes
ofwood,forexample,arelessdensethanwaterandwill
float.Othertypesofwood(suchasmahogany)aremore
denseandwillsink.Thedensitydependsonthenature
ofthewood(itscomposition).
Goldisdenserthansilver.Puregoldisasoftmetal,so
jewellersaddsilvertomakeitharder.Theamountof
silveraddedcanbejudgedbymeasuringthedensity.
Itisusefultorememberthatthedensityofwateris
1000kg/m3,1.0kg/dm3or1.0g/cm3.
Calculatingdensity
Tocalculatethedensityofamaterial,weneedtoknow
themassandvolumeofasampleofthematerial.
WORKEDEXAMPLE1.1
Asampleofethanolhasavolumeof240cm3.
Itsmassisfoundtobe190.0g.Whatisthedensity
ofethanol?
Step1:Writedownwhatyouknowandwhatyou
wanttoknow.
massm=190.0g
volumeV=240cm3
densityp=?
Step2:Writedowntheequationfordensity,
substitutevaluesandcalculatep.
_
m
P~V
190g
240cm3
=0.79g/cm3
Answer
Densityofethanol=0.79g/cm3

20. 1Makingmeasurements
Measuringdensity
Theeasiestwaytodeterminethedensityofasubstance
istofindthemassandvolumeofasampleofthe
substance.
Forasolidwitharegularshape,finditsvolumeby
measurement(seeSection1.1).Finditsmassusinga
balance.Thencalculatethedensity.
Questions
3AbrickisshowninFigure1.9.Ithasamassof
2.8kg.
Figure1.9:Abricklabelledwithitsdimensions.
aGivethedimensionsofthebrickinmetres.
bCalculatethevolumeofthebrick.
cCalculatethedensityofthebrick.
4Aboxfullof35matcheshasamassof6.77g.
Theboxitselfhasamassof3.37g.
aWhatisthemassofonematchingrams?
bWhatisthevolume(incm3)ofeachmatch.
Amatchhasdimensionsof42mmx2.3mmx
2.3mm?
cWhatisthedensityofthematches?
dHowdoyouknowifthesematcheswillfloat?
5TheEarthhasamassof6x1024kgandaradius
ofabout6400km.WhatisthedensityoftheEarth
(inkg/m3)?Thevolumeofasphereisgivenbythe
equationV=|w3,whereristheradius.
640drawingpins(thumbtacks)likethoseshown
inFigure1.10haveamassof17.55g.Whatisthe
volume(inmm3)ofonepinwhentheyaremadeof
metalwithadensityof8.7g/cm3?
Figure1.10:Apairofdrawingpins(thumbtacks).
7AyounggirlfromtheKayanpeopleinnorthern
Thailandwearsaneckringmadeofbrass(Figure
1.11).Itlooksasifthereare21individualringsbut
theringisactuallyonecontinuouslengthofbrass
fashioned(bent)intoacoil.Theheightofthebrass
coilis12cmanditsaveragecircumferenceis40cm.
Neckringsareusuallyonlyremovedtobereplaced
withabiggeroneasthegirlgrows.However,we
canestimatethemassofthisneckringwithout
removingit.
Figure1.11:AKayangirlwearinganeckring.

21. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
aWhatlookslike21individualringsaround
thegirl’sneckisactually21turnsofacoilof
brass.Eachturnhasacircumferenceof40cm.
Calculate(incm)thetotallengthofbrassused
tomakethegirl’sneckring.
bThecoilhasaheightof12cmandthecoilhas
21turns.Calculatetheradiusofthebrassincm.
cIfthebrasscoilisunwoundfromthegirl’sneck
andstraightenedout,itwouldbealong,thin,
cylinder.Calculatethevolumeofthiscylinder
incm3.Thevolumeofacylinderisgivenbythe
equationV=nr2h,where
r=radiusandh=height.
dCalculatethemassofbrassusedtomake
theneckringandexpressyouranswerinkg.
Thedensityofbrass=8.73g/cm3.
Whenliquidsmix,itisusuallybecausedheliquiddissolves
intheother.Forexample,orangesquashisaconcentrated
syrupthatisdilutedbydissolvingitinwater.
Findingthedensityofaliquid
Figure1.12showsonewaytofindthedensityofa
liquid.Placeameasuringcylinderonabalance.Setthe
balancetozero.Nowpourliquidintothecylinder.Read
thevolumefromthescaleonthecylinder.Thebalance
showsthemass.
Figure1.12:Measuringthemassofaliquid.
Whenliquidswithdifferentdensitiesarepouredinto
thesamecontainer,theywillarrangethemselvessothat
theliquidwiththelowestdensitywillbeatthetopand
theoneswiththehighestdensitywillbeatthebottom.
Thisisbecausethedenserliquidsdisplacethelessdense
liquids.Thisiseasiertoseewheneachliquidisgivena
differentcolour.InFigure1.13,thegreenliquidisless
densethantheredliquidandsoon.
Whenadistinctlayerformsinamixedsolution,the
liquidsaresaidtobeimmiscible,whichmeanstheydo
notmix.Thisiswhyoilfloatsonwater.However,notall
liquidsstayseparatedsoyouwouldbedisappointedif
youtriedthisathomewithsquashandwater,forexample.
Figure1.13:Liquiddensitytowers.
Apartfrommakingcolourfulliquiddensitytowers,
dovariationsinthedensityofliquidshavepractical
consequence?InChapter11,youwilllearnabout
convectioncurrentsinfluids(liquidsandgases),which
aredrivenbydifferencesindensity.Theseconvection
currentsincludethethermohalinecirculationinthe
oceans.Colderandsaltierwatersinks,displacing
(pushingup)warmerandlesssaltywater.
ACTIVITY1.1
Findingthedensityofaregularlyshapedsolid
Inpairs,createaworksheetonthecomputerfor
findingthedensityofaregularlyshapedsolidobject
(forexample,arectangularblock)usingaruleranda
massbalance.Yourworksheetshouldinclude:
amethodformeasuringthemassandworking
outthevolume
theequationforcalculatingdensity
atabletorecordthedata.
Youcouldincludeanoptionaltasktoworkoutthe
densityofaliquid.
Afteryourallottedtime,anotherpairisgoingto
testacopyofyourworksheet(perhapsbydoing
theexperiment).Theyaregoingtoaddanysteps
thataremissingormakesuggestionstomakeyour
worksheetclearer.Whenyougetyourworksheet
returned,editandsaveanewversionofit.

22. 1Makingmeasurements
CONTINUED
Findingthedensityofanirregularlyshapedsolid
Beforeyoustart,makeacopyofyourprevious
worksheetandsaveitunderanewname.Someof
whatyouincludedinthepreviousworksheetcan
bekeptandsomewillneedtobeedited.
Inpairs,createaworksheetforfindingthedensity
ofanirregularlyshapedsolidobjectusingamass
balance,ameasuringcylinder,somethread,apair
ofscissorsandaeurekacan(ifyouhaveaccessto
one).Yourmethodexplaininghowtomeasurethe
massandhowtocalculatethedensityshouldbe
thesame.However,youshould:
explainhowtomeasurevolumebydisplacement
saysomethingaboutchoosingasuitablysized
measuringcylinder
changeyourprevioustable
Youcouldincludeanoptionaltasktoworkout
thedensityofanirregularlyshapedsolidobject
thatislessdensethanwater.Findingitsmass
andcalculatingthedensityisstraightforward.The
challengingpartisexplaininghowtoworkoutthe
volumeofanobjectthatfloats.
Designaflowchartordecision-tree(optional)
Designaflowchartordecision-treeforuseby
anyonewhowantstoworkoutthedensityof
anyliquidoranysolidobject.Ensurethatyour
flowchartincludesenoughinformationsothat
someonecouldtakethemeasurements.Askyour
partnerorsomeoneelsewhohascompletedthe
firsttwopartstocheckandcorrectyourflowchart.
REFLECTION
Writedownonethingthatyoudidreallywellin
thisactivity.
Writedownonethingthatyouwilltrytodobetter
nexttime.Howwillyoudothis?
1.3Measuringtime
TheathleticscoachinFigure1.14isusinghisstopwatch
totimeasprinter.Forasprinter,afractionofa
second(perhapsjust0.01s)canmakeallthedifference
betweenwinningandcomingsecondorthird.Itis
differentinamarathon,wheretheracelastsformorethan
twohoursandtherunnersaretimedtothenearestsecond.
Figure1.14:Anathleticscoachusesastopwatchtotimea
hurdler,whocanthenlearnwhethershehasimproved.
ACTIVITY1.2
Howdensecanyoube?
Ingroupsofthree,writeamethodshowinghowyoucouldworkoutyourowndensity,orthatofafriendorof
ayoungersibling.Alternatively,planoutyourstrategyandbepreparedtoshareitwiththeclass.Thereareat
leasttwomethods:adrymethodandawetmethod.Discussoneorbothofthem.
Youwillneedtoinclude:
amethodthatisdetailedenoughforsomeonetofollow(thisshouldincludeadviceabouthowa
measurementshouldbetaken)
anycalculations
possiblesourcesofuncertaintyinthemeasurements
whatyouexpectyouranswertobe.
Ifyouactuallycarriedouttheexperiment,commentonhowcloseyourmeasurementwastowhatyouexpected.

23. >
CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
Inthelaboratory,youmightneedtorecordthe
temperatureofacontainerofwatereveryminute,or
findouthowlonganelectriccurrentisflowing.For
measurementslikethese,stopclocksandstopwatchescan
beused.Youmaycomeacrosstwotypesoftimingdevice.
Ananalogueclock(Figure1.15)islikeatraditional
clockwhosehandsmoveroundtheclock’sface.Youfind
thetimebylookingatwherethehandsarepointingon
thescale.Itcanbeusedtomeasuretimeintervalstono
betterthanthenearestsecond.
Figure1.15:Ananalogueclock.
Adigitalclock(Figure1.16)orstopwatchisonethat
givesadirectreadingofthetimeinnumerals.For
example,adigitalclockmightshowatimeof9.58s.A
digitalclockrecordstimetoaprecisionofatleastone
hundredthofasecond.Youwouldneverseeananalogue
watchrecordingtimesintheOlympicGames.
Figure1.16:Adigitalclockstartedwhenthegunfiredand
stopped9.58slaterwhenUsainBoltcrossedthefinishing
linetowinthe100matthe2009WorldChampionshipsin
worldrecordtime.
KEYWORDS
analogue:displayhashands(oraneedle)andis
oftennotveryprecise
digital:displayshowsnumbersandisoftenprecise
Whenstudyingmotion,youmayneedtomeasurethe
timetakenforarapidlymovingobjecttomovebetween
twopoints.Inthiscase,youmightuseadevicecalleda
lightgateconnectedtoanelectronictimer.Thisissimilar
tothewayinwhichrunnersaretimedinmajorathletics
events.Anelectronictimerstartswhenthemarshal’sgun
isfired,andstopsastherunnercrossesthefinishingline.
Youwilllearnmoreabouthowtouseelectronictiming
instrumentsinChapter2.
Measuringshortintervals
oftime
Figure1.17showsatypicallabpendulum.Amass,called
aplumbbob,hangsontheendofastring.Thestring
isclampedtightlyatthetopbetweentwowoodenjaws.
Ifyoupullthebobgentlytoonesideandreleaseit,the
pendulumwillswingfromsidetoside.
Thetimeforoneoscillationofapendulum(whenit
swingsfromlefttorightandbackagain)iscalledits
period.Asingleperiodisusuallytooshortatimeto
measureaccurately.However,becauseapendulum
swingsatasteadyrate,youcanuseastopwatchto
measurethetimeforalargenumberofoscillations
(perhaps20or50),andcalculatetheaveragetimeper
oscillation.Anyinaccuracyinthetimeatwhichthe
stopwatchisstartedandstoppedwillbemuchless
significantifyoumeasurethetotaltimeforalarge
numberofoscillations.
KEYWORDS
plumbbob:amass(usuallylead)hangingfroma
stringtodefineaverticalline
oscillation:arepetitivemotionorvibration
period:thetimeforonecompleteoscillationor
wave;thetimeittakesanobjecttoreturntoits
originalposition

24. 1Makingmeasurements
Figure1.17:Asimplependulum.
Questions
8High-speedvideocanrecordsportingeventsata
framerateof60framespersecond(frame/s).
aWhatisthetimeintervalbetweenoneframe
andthenext?
bIfwecansee24frame/sascontinuousmotion,
bywhatfactorcantheactionrecordedat
60frame/sbesloweddownandstilllook
continuous?
9Astudentwasinvestigatinghowtheperiodofa
pendulumvariedwiththelengthofthestringand
obtainedtheresultsinTable1.4.
Table1.4
Lengthof
string/m
Timefor20
oscillations/s
Timefor1
oscillation/s
0.00 0.0
0.20 18.1
0.40 25.1
0.60 28.3
0.80 39.4
1.00 40.5
1.20 44.4
1.40 47.9
aWhydidthestudentrecordthetimefor20
swings?
bMakeacopyofTable1.4and,foreachlength
ofthependulum,calculatethetimeforone
oscillationandrecordthevalueinthethird
columnofthetable.
cPlotagraphoftheperiodofthependulum
againstitslength(thatis,plotthelengthofthe
pendulumonthex-axis).
dUsethegraphtoworkoutthelengthofthe
pendulumwhentheperiodis2seconds.Thisis
thelengthofpendulumusedinagrandfather
clock.
ACTIVITY1.3
Figure1.18:Oneoscillationiswhenthe
plumbbobswingsonewayandthenthe
otherandreturnsbacktoitsoriginalposition.
Usingapendulumasaclock
In1656theDutchscientistChristiaanHuygensinventedaclock
basedonaswingingpendulum.Clockslikethesewerethe
mostpreciseintheworlduntilthe1930s.Oneoscillationofa
pendulumisdefinedasthetimeittakesforaplumbbobatthe
bottomofthestringtoreturntoitsoriginalposition(Figure1.18).
Youneedtodevelopaworksheetsothatstudentscanplota
graphofhowtheperiodofoscillationofapendulumvarieswith
thelengthofthestring.Theythenneedtousethegraphtofind
thelengththependulumneedstobetogiveaperiodofone
second(usefulforaclock).Yourworksheetneedsto:
11

25. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
definewhatanoscillationmeans(sothatastudentknowswhentostartandstopthestopwatch)
explainwhywetakethetimefor10or20oscillationswhenweonlyneedthetimeforoneoscillation
providealabelleddiagramoftheassembledapparatus(notjustalistofequipment)sothatstudents
knowhowtoputtheequipmenttogether
amethod(step-by-stepinstructions).
Swapcopiesofyourworksheetwithaclassmate.Writedownsuggestionsforanyimprovementsontheworksheet
youreceivebeforereturningittoitsowner.Notedownanyimprovementsifyouhaveaclassdiscussion.
PROJECT
Ingroupsofthreeorfour,produceapodcast(nomore
thanfiveminuteslong)ononeofthefollowingoptions.
Option1:Canwebuildonwhatwehavelearned
aboutdensity?
Thisisopportunitytorevisewhatyouhavelearned
aboutdensityandthenconsolidatethatknowledge
andunderstandingbyapplyingittooneofthetwo
examplesbelow.
Youmustexplainhowdensityiscalculated,
includingtheequation.
Youshoulddescribehowtomeasurethemass
andvolumeofbothregularandirregular
shapedobjects.
Youcoulddescribehowtoworkoutthedensity
ofanobjectthatcanfloat.
1RSSTitanic
ItwasclaimedthattheRSSTitanicwasunsinkable.
Howevertheshipsankin1912onitsfirstvoyage.
Youmustexplainwhyashipcanfloatdespite
beingmadeofmaterialthatisdenserthanwater.
Youshouldexplainwhyashipcansink,interms
ofchangesindensity.
Dosomeresearchtofindoutaboutbulkheads
inships:whataretheyandwhataretheyfor?
WhydidtheRSSTitanicsinkdespitebeing
fittedwithbulkheads?
2Submarinesandscubadivers
Youcoulddescribeonephenomenonthatdepends
onchangesordifferencesindensity.Youcouldthink
ofyourownorselectoneofthese:
Explainhowasubmarineorscubadivermovesup
anddowninthewatercolumn(orperhapsexplain
howaCartesiandiverdemonstrationworks).
Explainhowdifferencesinfluiddensitycan
leadtoconvection(somethingyouwillmeetin
Chapter11).Youmightwanttogoontodiscuss
howthisrelatestooceancurrentsorwind.
Option2:Whatwasthesolutiontothe
longitudeproblem?
Aclockbasedonapendulumisimpracticalonthe
movingdeckofa(sailing)shipbutknowingthetime
isimportantfornavigationasthisprovidesyour
longitudeonaspinningEarth.Linesoflongitude
aretheverticallinesonamap.Whenyoumoveeast
orwestyouarechangingyourlongitude;movefar
enoughandyouchangetimezone.
Youmuststartwithashortdescriptionofthe
longitudeproblem.
Youcoulddescribethevarioussuggested
solutionstothelongitudeproblem.
Youcoulddescribethefinalsolutiontothe
longitudeproblem.Forthis,youwouldneedto
lookupJohnHarrisonandhismarinechronometer.
Option3:HowdidtheAncientEgyptiansbuild
theirpyramidssoaccurately?
Thepyramidsareanincrediblefeatofengineering,
evenbytoday'sstandards.Usingverybasictools,the
Egyptians'pyramidsareperfectlysymmetrical.
Youcouldstartbyintroducingthedimensions
oftheGizapyramidandthenumberofblocks
requiredtobuildit.

26. 1Makingmeasurements
CONTINUED
YoucouldexplainhowtheEgyptiansmanaged
togetthesidesoftheirpyramidslinedupwith
truenorth(withoutacompass)andhowtheygot
thebaseofthemabsolutelylevel(flat)withouta
(spirit)level.
Option4:HowdidEratosthenesworkoutthe
circumferenceoftheEarth?
Eratostheneswasabrilliantscientist.Hewastold
that,atthesametimeeveryyear(12noonon
21June),verticalcolumnsinSyene(presentday
Aswan)castnoshadowswhilecolumnswherehe
livedinAlexandriacastshadows.Heusedthis
toworkoutthattheEarthisround.Eratosthenes
mayhavehiredamantomeasureoutthedistance
betweenAlexandriaandSyene.
Youcouldstartwithashortbiographyof
Eratosthenes.
Youshouldexplainwhytheobservationwiththe
shadowsshowsthattheEarthisasphere.You
mightwanttoincludeadiagramlikeFigure1.2.
Youshouldtryandshowhowthemanhired
byEratosthenescouldhaveworkedouthis
stride-length(thedistanceofeachstep)and
keptcountofhisstrides(steps).Thinkabouthis
possiblejourney:didhefollowastraightline;
werethereanyhillsintheway?Couldthishave
introducederrorsinmeasuringthedistance
betweenAlexandriaandSyene?
Finally,youcouldshowhowEratosthenesdid
thecalculation.
Option5:HowdidArchimedesreallyworkout
thatthegoldsmithhadreplacedsomeofthe
goldinHiero'scrownwithsilver?
Archimedeswasprobablythemostbrilliantscientist
ofhisera.Heissupposedtohavesolvedthe
problemofhowtoworkoutthedensityofthecrown
whilehavingabath.Legendhasitthathethenran
intothestreetsshouting'eureka'(I'vesolvedit).
Youcouldstartwithashortbiographyof
Archimedes.
Youcouldthendescribetheusualexplanation
ofhowheworkedoutthatsomegoldhad
beenstolen.Silverislessdensethangoldso
thesamemassofsilverhasabiggervolume
andwilldisplaceabiggervolumeofwater.
However,itwouldbedifficulttomeasurethe
differenceinvolume,especiallysincebubbles
ofaircouldclingtothesubmergedcrownand
therecouldbeothersourcesoferror.'
Youcoulddescribeabettermethod,which
usesamassbalance.Youwouldneedto
explainwhy,whenthemassesareequal,the
balancetipstowardsthedensermasswhen
loweredintowater.
Goldneedssomesilverimpurityoritwould
betoosoftandwouldbeeasytobendout
ofshape.Perhapsthegoldsmithwasfalsely
accused?Perhapsthisideacouldformpartofa
pieceofcreativewriting(someproseoraplay)
butbesuretoincludethephysics.
REFLECTION
Foryourproject,writedownsomethoughts
aboutwhatyoufeelwentwellandareaswhere
youcouldimprove.
Giveyourselfascoreoutoftenforhowmuch
youknowandunderstandthephysicsyou
included.Ifyouscoredten,writedownhow
youcouldhaveproducedamoreambitious
project.Ifyouscoredless,doyouneedto
thoroughlyreviewthematerialorareyou
makingcarelesserrors?Writedownwhat
concretestepsyouneedtotaketoimprovefor
nexttime.
Giveyourselfascoreoutoftenforthe
qualityofyourpresentation.Writedown
whatyouthoughtwasgoodabouttheother
presentationsoranyeffectivepresentation
ideasthatyoumightusenexttimeyoupresent.
13

27. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
SUMMARY
Lengthcanbemeasuredusingaruler.
Theperiodofoneoscillationcanbemeasuredbymeasuringthetimefor20oscillationsandthendividingthe
timeby20.
Thevolumeofacubeorcuboidcanbefoundbymeasuringthelengthofthethreesidesandmultiplyingthe
measurementstogether.
Thevolumeofaliquidcanbemeasuredusingameasuringcylinderwherethebottomofthemeniscusappears
onthescalewhenlookedathorizontally.
Allobjectsthatsinkinwaterdisplacetheirownvolumeofwater.
Thevolumeofanirregularlyshapedobjectcanbefoundfromthechangeintheheightofliquidinameasuring
cylinderwhenitisimmersedintheliquid.
Densityistheratioofmasstovolumeforasubstance:p=
Thedensityofwateris1000kg/m3or1.0g/cm3.
Anythinglessdensethanwaterwillfloatinwaterandanythingdenserthanwaterwillsinkinwater.
Icefloatsbecauseitislessdensethanwater.
Oneliquidwillfloatontopofanotherliquidifitislessdense.
Timecanbemeasuredusingaclockorwatch.
Ananalogueclockhashandsandcanonlymeasuretimetothenearestsecond.
Adigitalclockdisplaysnumbersandrecordstimetoaprecisionofatleastonehundredthofasecond.
EXAM-STYLEQUESTIONS
Usethistabletoanswerquestions1and2.Metal Density/g/cm3
gold 19.30
silver 10.49
lead 11.34
1Threemetalcubeshavethesamevolumebutaremadeofdifferentmetals.
Eachoneisloweredintoabeakerofwater.Usethedatainthetableto
decidewhichonewillcausethebiggestriseinwaterlevel.
Agold
Bsilver
Clead
Dallwillcausethesameriseinwaterlevel
[1]
14>

28. 1Makingmeasurements
CONTINUED
2Threemetalcubeshavethesamemassbutaremadeofdifferentmetals.
Eachoneisloweredintoabeakerofwater.Usethedatainthetableto
decidewhichonewillcausethebiggestriseinwaterlevel. [1]
Agold
Bsilver
Clead
Dallwillcausethesameriseinwaterlevel
3Astronautslandonanotherplanetandmeasurethedensityoftheatmosphere
ontheplanetsurface.Theymeasurethemassofa500cm3conicalflask
plusstopperas457.23g.Afterremovingtheair,themassis456.43g(1m3=
1000litres).Whatisthebestestimateofthedensityoftheair? [1]
A0.0000016kg/m3 C0.16kg/m3
B0.0016kg/m3 D1.6kg/m3
4Thegraphshowsthemassandvolumeofseveraldifferentobjects.
Volume
Whichtwoobjectshavethesamedensity? [1]
A2and3 Bland4 C2and4 D3and4
5Astudentmeasuresthecircumferenceofacircularcopperpipe.
Hewrapsalengthofstringfourtimesaroundthepipeandmarksitwith
ink,asshowninthephotograph.
15

29. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
aThestudentunwrapsthestringandholdsitagainstarulerwithacentimetre
scale.
Thephotographshowsthefirsttwoinkmarksonthestring.
iUsethephotographtoestimatethecircumferenceofthepipe. [1]
iiThestudentfindsthatthetotallengthofstringfor4turnsis354mm.
Calculatetheaverage(mean)circumferenceofthepipeusingthisvalue.[1]
[Total:2]
6Suggesthowyouwouldworkoutthethicknessofasinglesheetofpaper
iftheonlymeasuringdeviceavailablewasaruleranditssmallestdivision
was1mm. [1]
7Whatisthemassofamicroscopeslidethathasdimensionsof
75mmx26mmx1mmandhasadensityof2.24g/cm3? [2]
8Fourdifferentliquidsarepouredintoa100cm3measuringcylinderthatis
10cmtall.Eachliquidhasadifferentdensityandeachhasadifferentcolour.
aCalculatethemissingvaluesinthetable. [4]
Liquid Mass/g
Volume/
cm3
Density/
g/cm3
clear ethanol i 20.00 0.79
red glycerin 20.00 ii 1.26
green oliveoil 25.90 28.80 iii
blue turpentine30.00 35.30 iv
bCopythediagrambelow.Usingthedatafromthetableabove,writedown
thecolouroftheliquidyouwouldexpecttofindineachlayerandhow
thickthelayerwouldbe. [2]
Colouroflayer Thicknessoflayer/cm
-
9Metalsaredenserthanwater.Explainwhyametalshipcanfloat. [1]
10Suggesthowyoucouldworkoutthedensityofadrawingpin. [3]
%
COMMAND WORDS
explain:setout
purposesor
reasons;make
therelationships
betweenthings
evident;provide
whyand/orhowand
supportwithrelevant
evidence
calculate;workout
fromgivenfacts,
figuresorinformation
suggest:apply
knowledgeand
understanding
tosituations
wherethereare
arangeofvalid
responsesinorder
tomakeproposals/
putforward
considerations
16

30. 1Makingmeasurements
SELF-EVALUATIONCHECKLIST
terstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutosee
inygapsinyourknowledgeandhelpyoutolearnmoreeffectively.
Ican
See
Topic...
Needs
morework
Almost
there
Confident
tomoveon
Measurelength,volumeandtime. 1.1,1.3
Calculatethevolumeofacubeorcuboidfrom
measurementsusingaruler.
1.1
Determinethevolumeofanirregularlyshapedobject. 1.1
Measurethesizeoftinyobjects(forexample,thethickness
ofasheetofpaper,thevolumeofadrawingpin).
1.1
Calculatedensity. 1.2
Predictwhetheranobjectwillfloatorsinkinwater
basedonitsdensity.
1.2
Describeanexperimenttofindthedensityofaliquid. 1.2
Predictwhetheraliquidwillfloatontopofanotherliquid
iftheirdensitiesareknownandtheycannotmix.
1.2
Describeanexperimenttofindthedensityofacube
orcuboid.
1.2
Describeanexperimenttofindthedensityofan
irregularlyshapedobject.
1.2
Describethedifferencesbetweenanalogueanddigital
watchesorclocks.
1.3
17

31. definespeedandcalculateaveragespeed
plotandinterpretdistance-timeandspeed-timegraphs
workoutthedistancetravelledfromtheareaunderaspeed-timegraph
understandthataccelerationisachangeinspeedandthegradientofaspeed-timegraph
>Chapter2
Describing
motion
INTHISCHAPTERYOUWILL:
distinguishbetweenspeedandvelocity
defineandcalculateacceleration;understanddecelerationasanegativeacceleration
usethegradientofadistance-timegraphtocalculatespeedandthegradientofaspeed-timegraphto
calculateacceleration.

32. 2Describingmotion
.1ITINGSTARTED
Workinpairs.
1)nyourown,quicklysketchadistance-timegraph,perhapsbasedonyourjourneytoschool.Thenaskyour
I-irtnertowriteadescriptionofitonaseparatesheetofpaper.Discusseachother'sanswers.
ik<Itchaspeed-timegraphforasprinterrunningthe100minatimeof9.58s.Labelitwithasmuch
informationasyouknow.Showhowyourgraphcouldbeusedtoworkoutthesprinter'saccelerationat
tinstartoftheraceandthedistancehetravelled.Compareyoursketchwithyourpartner'sandaddtoor
orrectyourownwork.Bepreparedtoshareyourthoughtswiththeclass.
AROUNDTHEWORLDIN80DAYS
II"firstknowncircumnavigation(triparound
tinworld)wascompletedbyaSpanishshipon8
1
>iptember1522.Ittookmorethanthreeyears.
ILFrenchwriterJulesVernewrotethebook
Itourdumondeenquatre-vingtsj'ours(which
ntansAroundtheWorldinEightyDays)in1873.
Inhonourofthewriter,theJulesVerneTrophy
rprizeforthefastestcircumnavigationbya
.hht,nowheldbytheyachtIDECSport,which
liditinjustunder41daysin2017.In2002,the
-AmericanSteveFossettwasthefirsttomakeasolo
ir:umnavigationinaballoon,withoutstopping,
iifingjustover13days.In2006,heflewthe
ir<iinAtlanticGlobalFlyer(Figure2.1),thefirst
fi-
1»d-wingaircrafttogoaroundtheworldwithout
>ppingorrefuelling.Ittookhimjustunder
limedays.Hypersonicjetsarebeingdeveloped
th-itcouldflyat1.7kmpersecondsotheycould
iriumnavigatetheglobeinanincrediblesixanda
halfhours.
IIgure2.1:TheVirginAtlanticGlobalFlyerpassesover
thAtlasMountains.
Sometimestheseepicadventuresinspirethosewho
dothemtocampaignforabetterworld.TheBritish
sailorEllenMacArthur(Figure2.2)isjustsucha
person.Sheheldtheworldrecordforthefastest
solocircumnavigation,achievedon7February
2005.However,sheretiredfromcompetitive
sailingtosetuptheEllenMacArthurFoundation,
acharitythatworkswithbusinessandeducation
toacceleratethetransitiontoacirculareconomy.
Acirculareconomywouldcreatelesswasteas
thingsshouldbedesignedtolastalongtimeand
beeasytomaintain,repair,reuseorrecycle.
Figure2.2:EllenMacArthurcelebratesaftercompleting
herrecordsoloroundtheworldjourneyon7February
2005inFalmouth,England.
Discussionquestions
1Whatwerethespeedsofthesixjourneys
mentionedinthefirstparagraph?Assumethat
theEarth'scircumferenceis40000km.
2Howcouldthefastestboatnotwinaround-
the-worldyachtrace?
19

33. }CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
2.1Understandingspeed
Measuringspeed
Ifyoutravelonamajorhighwayorthroughalargecity,
thechancesarethatsomeoneiswatchingyou.Cameras
bythesideoftheroadandonoverheadroadsignskeep
aneyeontrafficasitmovesalong.Somecamerasare
theretomonitortheflow,sothattrafficmanagerscan
takeactionwhenblockagesdevelop,orwhenaccidents
occur.Othercamerasareequippedwithsensorstospot
speedingmotorists,orthosewhobreakthelawattraffic
lights.Insomebusyplaces,trafficpolicemayobservethe
roadsfromhelicopters.
Inthischapter,wewilllookatideasofmotionand
speed.InChapter3,wewilllookathowphysicistscame
tounderstandtheforcesinvolvedinmotion,andhowto
controlthemtomakeoureverydaytravelpossible.
Distance,timeandspeed
Thereismorethanonewaytodeterminethespeedofa
movingobject.Severalmethodstodeterminespeedrely
onmakingtwomeasurements:
thetotaldistancetravelledbetweentwopoints
thetotaltimetakentotravelbetweenthesetwopoints.
Wecanthenworkouttheaveragespeedbetweenthe
twopoints.
KEYEQUATION
„„ _
totaldistancetravelled
averagespeed=
totaltimetaken
KEYWORDS
speed:thedistancetravelledbyanobjectper
unitoftime
averagespeed:thespeedcalculatedfromtotal
distancetravelleddividedbytotaltimetaken
Wecanusetheequationforspeedinthedefinition
whenanobjectistravellingataconstantspeed.Ifit
travels10metresin1second,itwilltravel20metresin
2seconds.ItsspeedislOm/s.
Wecannotsaywhetheritwastravellingatasteadyspeed,
orifitsspeedwaschanging.Forexample,youcoulduse
astopwatchtotimeafriendcyclingoverafixeddistance,
forexample,100metres(seeFigure2.3).Dividing
distancebytimewouldtellyoutheiraveragespeed,but
theymighthavebeenspeedinguporslowingdownalong
theway.
Table2.1showsthedifferentunitsthatmaybeusedin
calculationsofspeed.SIunitsarethestandardunits
usedinphysics.Theunitsm/s(metrespersecond)should
remindyouthatyoudivideadistance(inmetres,m)bya
time(inseconds,s)tofindspeed.Inpractice,manyother
unitsareused.InUSspaceprogrammes,heightsabovethe
Earthareoftengiveninfeet,whilethespacecraft’sspeed
isgiveninknots(nauticalmilesperhour).Theseawkward
unitsdidnotpreventthemfromreachingtheMoon!
Figure2.3:Timingacyclistoverafixeddistance.Using
astopwatchinvolvesmakingjudgementsastowhenthe
cyclistpassesthestartingandfinishinglines.Thiscan
introduceanerrorintothemeasurements.Anautomatic
timingsystemmightbebetter.
Table2.1:Quantities,symbolsandunitsinmeasurements
ofspeed.
QuantitySIunit Otherunits
distancemetre,m kilometre,km
time second,s hour,h
speed metrespersecond,
m/s
kilometresper
hour,km/h
20>

34. 2Describingmotion
)RKEDEXAMPLE2.1
A\ilistcompleteda1500metrestageofaracein
i >sWhatwasheraveragespeed?
upI:Startbywritingdownwhatyouknow,and
whatyouwanttoknow.
distance=1500m
time=37.5s
speed=?
'<tcp2:Nowwritedowntheequation.
speed=
distance
time
'itcp3:Substitutethevaluesofthequantitiesonthe
right-handside.
._
1500m
Step4:Calculatetheanswer.
speed=40m/s
kiMwer
Ihecyclist’saveragespeedwas40m/s.
Questions
1aWhatwasUsainBolt’saveragespeedwhenhe
achievedhis100mworldrecordof9.58sin
2009?
bHowdoyouknowthathistopspeedmusthave
beenhigherthanthis?
JAcheetahruns100min3.11s.Whatisitsspeed?
IInformationaboutthreetrainstravellingbetween
stationsisshowninTable2.2.
Table2.2
Vehicle
Distance
travelled/km
Timetaken/
minutes
trainA 250 120
trainB 72 50
trainC 400 150
aWhichtrainhasthehighestaveragespeed?
bWhichtrainhasthelowestaveragespeed?
Determiningspeedin
thelaboratory
Therearemanyexperimentsyoucandointhelaboratory
ifyoucanmeasurethespeedofamovingtrolleyor
toycar.Figure2.4showshowtodothisusingoneor
twolightgatesconnectedtoanelectronictimer(orto
acomputer).Thelightgatehasabeamof(invisible)
infraredradiation.
infrared
beam
Figure2.4:Usinglightgatestomeasurethespeedofa
movingtrolleyinthelaboratory.
InthefirstpartofFigure2.4,thepegattachedtothe
trolleybreaksthebeamofonelightgatetostartthe
timer.Itbreaksthesecondbeamtostopthetimer.The
timerthenshowsthetimetakentotravelthedistance
betweenthetwolightgates.
21

35. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
InthesecondpartofFigure2.4,apieceofcard,called
aninterruptcard,ismountedonthetrolley.Asthe
trolleypassesthroughthegate,theleadingedgeof
theinterruptcardbreaksthebeamtostartthetimer.
Whenthetrailingedgepassesthegate,thebeamisno
longerbrokenandthetimerstops.Thefasterthetrolley
ismoving,theshorterthetimeforwhichthebeam
isbroken.Giventhelengthoftheinterruptcard,the
trolley’sspeedcanbecalculated.
KEYWORDS
lightgates:allowthespeedofanobjectpassing
betweenthemtobecalculatedelectronically
interruptcard:allowsthespeedofanobject
passingthroughalightgatetobecalculated;a
timerstartswhenthecardbreaksthebeamand
stopswhenthebeamisnolongerbroken
Rearrangingtheequation
Itisbettertorememberoneversionofanequation
andhowtorearrangeitthantotrytorememberthree
differentversions.Theequation
speed=
dis-tance
time
allowsustocalculatespeedfrommeasurementsofdistance
andtime.Thisequationcanalsobewritteninsymbols:
Thisissometimesknownastheinstantaneousspeed,
whichisthespeedataparticularinstantormomentin
time,whereasaveragespeedisworkedoutoveralonger
timeinterval.Beware,sinthisequationmeansdistance
(ordisplacement)andnotspeed.Wecanrearrangethe
equationtoallowustocalculatedistanceortime.
Forexample,arailwaysignallermightknowhowfasta
trainismoving,andneedstobeabletopredictwhereit
willhavereachedafteracertainlengthoftime:
distance=speedxtimeors=vt
Similarly,thecrewofanaircraft
mightwanttoknow
howlongitwilltakefortheiraircrafttotravelbetween
twopointsonitsflightpath:
time=
distance^or;=
£
speed v
WORKEDEXAMPLE2.2
AspacecraftisorbitingtheEarthatasteadyspeed
of8.0km/s(seeFigure2.5).Howlongwillittaketo
completeasingleorbit,adistanceof44000km?
Figure2.5
Step1:Startbywritingdownwhatyouknow,and
whatyouwanttoknow.
speed(v)=8.0km/s
distance(5)=40000km
time(t)=?
Step2:Choosetheappropriateequation,withthe
unknownquantity,time,asthesubject(on
theleft-handside).
Step3:Substitutevalues-itcanhelptoincludeunits.
t
_
40000km
8.0km/s
Step4:Performthecalculation.
t=5000s
Answer
Thetimetocompleteasingleorbit(44000km)is5500s.
Thisisabout92minutes(5500-r60=91.667).So,the
spacecrafttakes92minutestoorbittheEarthonce.
22

36. 2Describingmotion
"
i1edExample2.2illustratestheimportanceof
Ilingattheunits.Becausespeedisinkm/sand
Il
।.niceisinkm,wedonotneedtoconverttom/sand
milrWewouldgetthesameanswerifwedidthe
iumersion:
_
40000000m
(ime
8000m/s
=5000s
Questions
4Anaircrafttravels900metresin3.0seconds.
Whatisitsspeed?
hAcartravels400kmin3.5hours.Whatisthespeed
ofthecarinkm/handm/s?
6TheVoyagerspacecraftismovingat17000m/s.
Ilowfarwillittravelinoneyear?Giveyouranswer
inkm.
7Calculatehowmanyminutesittakessunlighttoreach
usfromtheSun.Lighttravelsat3x108m/sandthe
Sunisabout144millionkmaway.
8Acheetahcanmaintainitstopspeedof31m/s
overadistanceof100metreswhilesomebreedsof
gazelle,suchasThomson’sgazelle,haveatopspeed
of25m/s.Thisquestionconsidershowclosethe
cheetahneedstobetocatchthegazelleiftheyhave
bothjustreachedtopspeed.
aHowlongdoesittakeacheetahtocover100m?
bWhatistheclosingspeedofthecheetah,that
is,whatisthedifferenceinspeedbetweenthe
cheetahandthegazelle?
cHowfaraheadofthecheetahwouldthegazelle
needtobetoescape?(Hint:youneedthetime
youcalculatedinaandtheclosingspeedyou
calculatedinb.)
dHowlongwouldittakethecheetahtocatchthe
gazellewiththeclosingspeedyoucalculatedin
bandthedistanceapartyoucalculatedinc?
ACTIVITY2.1
Runningwiththewindbehindyou
In2011,JustinGatlinran100metresin9.45seconds
(fasterthanUsainBolt'sworldrecordby0.13seconds).
However,hewaspushedalongbya20m/stailwind
<pmeratedbygiantfansaspartofaJapanesegame
how.A100mor200msprintrecordcanstandonlyif
.itailwinddoesnotexceed2m/s.Whydoesthisrule
notapplytolongerevents?
Iirst,thinkabouthowyoumightapproach
thisproblem.
ThedayRogerBannisterranamileinfourminutes
(6May1954)healmostdecidednottoracebecause
itwastoowindy.Imaginethereisatailwindalong
thefinalstraightsectionofa400mtrackwhich
ipeedsyouup,andaheadwindontheopposite
sectionwhichslowsyoudown.Whydothe
effectsofthetailwindandheadwindnotcancelout?
(Hint:youneedtothinkaboutthetimeitwouldtake
youtorunthestraightsections.)
1Imaginethatyouarea400mrunnerwhocanrun
thedistancein40s(anewworldrecord)atthe
sameaveragespeedof10m/s.Assumethatthe
400mtrackisequallydividedsothatthestraight
sectionsandbendsareeach100mlong.
Plotyourtimeforthe400m(y-axis)againstwind
speed(x-axis).Whenyouarerunningagainstthe
windonthestraightsectionoppositethefinish
line,subtractthewindspeedfromyournormal
runningspeed.Whenyouarerunningwiththe
windonthefinalstraightsectionbeforethe
finishline,addthewindspeedtoyournormal
runningspeed.
Forexample,ifthereisawindspeedof1m/s,
yourspeedalongthestraightoppositethefinish
linewillbe9m/swhileitwillbe11m/salongthe
straightsectionbeforethefinishline.Thenyou
needtoaddthetimesforeachstraightsection
tothe20sforthebends.Repeatthis,increasing
thewindspeedby1m/seachtime,untilyou
reach10m/s.
2Couldyouhavereachedtheanswerwithout
plottingagraph?
3Discusswhetheritisrealistictoaddorsubtract
thewindspeedtoyournormalrunningspeed.
4Designanexperimenttotesthowwindspeed
affectsrunningspeed.Youmightneedto
includeequipmentthatyoudonothaveaccess
to(suchasthegiantfansusedontheJapanese
gameshow).
23

37. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
REFLECTION
Discussyouranswerstotheactivitywiththeperson
sittingnexttoyou.Havetheythoughtofanything
youhaven'tincludedinyouranswer?Wouldyou
addanythingtoyouranswersafteryourdiscussion?
2.2Distance-timegraphs
Youcandescribehowsomethingmovesinwords,
‘Thecoachdroveawayfromthebusstop.Ittravelledat
asteadyspeedalongthemainroad,leavingtown.After
fiveminutes,itreachedthehighway,whereitwasableto
speedup.Aftertenminutes,itwasforcedtostopbecause
oftraffic.’
Wecanshowthesameinformationintheformofa
distance-timegraph,asshowninFigure2.6a.Thisgraph
isinthreesections,correspondingtothethreesectionsof
thecoach’sjourney.
Figure2.6aandb:Agraphtorepresentthemotionofa
coach,asdescribedinthetext.Theslopeofthegraphtells
usaboutthecoach'sspeed.
InsectionA,thegraphslopesupgently,showingthatthe
coachwastravellingataslowspeed.
InsectionB,thegraphbecomessteeper.Thedistance
ofthecoachfromitsstartingpointisincreasingmore
rapidly.Itismovingfaster.
InsectionC,thegraphisflat(horizontal).Thedistance
ofthecoachfromitsstartingpointisnotchanging.Itis
stationary.
Theslopeofthedistance-timegraphtellsushowfastthe
coachismoving.Thesteeperthegraph,thefasteritis
moving(thegreateritsspeed).Whenthegraphbecomes
horizontal,itsslopeiszero.Thistellsusthatthecoach’s
speediszeroinsectionC.Itisnotmoving.
Figure2.6ashowsabrupt(instant)changesinspeed
betweenA,BandC.Itwouldnotbeaverycomfortable
rideforthepassengers!Insteadofabruptchangesin
speed,thespeedwouldchangemoreslowlyinthereal
worldandtherewouldbesmoothcurvesjoiningthe
sections(Figure2.6b).Theincreasinggradientofthe
upward-slopingcurvebetweenAandBwouldshow
thatthecoachwasspeedingup(accelerating)andthe
decreasinggradientofthecurvebetweenBandCwould
showthatthecoachwasslowingdown(decelerating).
However,wewillonlylookatgraphswithanglededges
asinFigure2.6a.
Questions
9Acarpulledawayfromthelightsandtravelledata
steadyspeedalonganemptyroad.After8minutes
itjoinedamainroad,whereittravelledatabout
twicetheoriginalspeedfor12minutes.Thecarthen
metatrafficjamandhadtoquicklyslowdownand
stop.Thetrafficclearedafter5minutesbutthenthe
cartravelledslowly,atabouthalftheoriginalspeed.
Sketchadistance-timegraphtoshowthecar’s
journey.
10Figure2.7showsthedistance-timegraphfora
womanrunningamountainmarathon.
Timeofday
Figure2.7:Distance-timegraph
24>

38. 2Describingmotion
aHowfardidshetravel?
bWhatwasheraveragespeedinkm/h?
cHowmanystopsdidshemake?
dTherulessaidshehadtostopforhalfanhour
forfood.Whendidshetakeherbreak?
eLatershestoppedtohelpaninjuredrunner.
Whendidthishappen?
fWhatwouldheraveragespeedhavebeenifshe
hadnotstoppedatall?
gWhatwasherhighestspeedandoverwhat
sectiondidthishappen?
Expresstrains,slowbuses
Anexpresstrainiscapableofreachinghighspeeds,
perhapsmorethan300km/h.However,whenitsetsoff
onitsjourney,itmaytakeseveralminutestoreachthis
topspeed.Thenittakesalongtimetoslowdownwhen
itapproachesitsdestination.TheFrenchTGVtrains
(Figure2.8)runonlinesthatarereservedsolelyfortheir
operation,sothattheirhigh-speedjourneysarenot
disruptedbyslower,localtrains.
Abusjourneyisfullofaccelerationsanddecelerations.
I‘hebusacceleratesawayfromthestop.Ideally,thedriver
hopestotravelatasteadyspeeduntilthenextstop.A
speedmeansthatyoucansitcomfortablyinyour
Thenthereisarapiddecelerationasthebusslowsto
halt.Alotofacceleratinganddeceleratingmeansthat
youarelikelytobethrownaboutasthebuschangesspeed.
1'hegentleaccelerationofanexpresstrainwillbarely
disturbthedrinkinyourcup.Thebus’srapidaccelerations
anddecelerationswouldmakeitimpossibletoavoid
tpillingthedrink(Figure2.9).
Figure2.8:France'shigh-speedtrains,theTGVs(Trainsa
C<randeVitesse),runondedicatedtracks.Theirspeedhas
madeitpossibletotravel600kmfromMarseilleinthesouth
l<>Parisinthenorth,attendameeting,andreturnhome
againwithinasingleday.
Figure2.9:Itcanbeuncomfortableonapackedbusasit
acceleratesanddeceleratesalongitsjourney.
2.3Understanding
acceleration
Somecars,particularlyhigh-performanceones,are
advertisedaccordingtohowrapidlytheycanaccelerate.
Anadvertmayclaimthatacargoes‘from0to100km/hin
5s’.Thismeansthat,ifthecaracceleratesatasteadyrate,
itreaches20km/hafter1s,40km/hafter2s,andsoon.
Wecouldsaythatitspeedsupby20km/heverysecond.
Inotherwords,itsaccelerationis20km/hpersecond.
So,wesaythatanobjectacceleratesifitsspeedincreases.
Itsaccelerationtellsustherateatwhichitsspeedis
changing,thatis,thechangeinspeedperunittime.
Whenanobjectslowsdown,itsspeedisalsochanging.
Wesaythatitisdecelerating.Insteadofanacceleration,
ithasadeceleration.
Speedandvelocity,vectors
andscalars
Inphysics,thewords‘speed’and‘velocity’havedifferent
meanings,althoughtheyarecloselyrelated:velocityis
anobject’sspeedinaparticularstateddirection.So,we
couldsaythatanaircrafthasaspeedof200m/sbuta
velocityof200m/sduenorth.Wemustgivethedirection
ofthevelocityortheinformationisincomplete.
Velocityisanexampleofavectorquantity.Vectorshave
bothmagnitude(size)anddirection.Anotherexample
ofavectorisweight-yourweightisaforcethatacts
downwards,towardsthecentreoftheEarth.
Speedisanexampleofascalarquantity.Scalarsonly
havemagnitude.Temperatureisanexampleofanother
scalarquantity.
YouwilllearnmoreaboutvectorsandscalarsinChapter3.
25

39. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
KEYWORDS
acceleration:therateofchangeofanobject's
velocity
velocity:thespeedofanobjectinastateddirection
vectorquantity:hasbothmagnitude(size)and
direction
scalarquantity:issomethingthathasmagnitude
butnodirection
Speed-timegraphs
Justaswecanrepresentthemotionofamovingobject
byadistance-timegraph,wecanalsorepresentitbya
speed-timegraph.Aspeed-timegraphshowshowthe
object’sspeedchangesasitmoves.Alwayscheckany
graphbylookingattheaxestoseethelabels.
Aspeed-timegraphhasspeedontheverticalaxisand
timeonthehorizontalaxis.
Figure2.10showsaspeed-timegraphforabus.The
graphfrequentlydropstozerobecausethebusstopsto
letpeopleonandoff.Thenthelineslopesup,asthebus
acceleratesawayfromthestop.Towardstheendofits
journey,thebusismovingatasteadyspeed(horizontal
graph),asitdoesnothavetostop.Finally,thegraph
slopesdownwardstozeroagainasthebuspullsintothe
terminusandstops.
Theslopeofthespeed-timegraphtellsusaboutthe
bus’sacceleration:
thesteepertheslope,thegreatertheacceleration
anegativeslopemeansadeceleration(slowing
down)
ahorizontalgraph(slope=0)meansaconstant
speed.
Figure2.10:Aspeed-timegraphforabusonabusyroute.
Atfirst,ithastohaltfrequentlyatbusstops.Towardsthe
endofitsjourney,itmaintainsasteadyspeed.
Graphsofdifferentshapes
Speed-timegraphscanshowusalotaboutanobject’s
movement.Wasitmovingatasteadyspeed,orspeeding
up,orslowingdown?Wasitmovingatall?
Figure2.11representsatrainjourney.Thegraphisin
foursections.Eachsectionillustratesadifferentpoint:
A:slopingupwards,sothespeedincreasesandthe
trainisaccelerating
B:horizontal,sothespeedisconstantandthetrain
istravellingatasteadyspeed
C:slopingdownwards,sothespeeddecreasesand
thetrainisdecelerating
D:horizontal,sothespeedhasdecreasedtozero
andthetrainisstationary.
Figure2.11:Anexampleofaspeed-timegraphforatrain
duringpartofitsjourney.
Questions
ThefactthatthegraphlinesarecurvedinsectionsA
andCtellsusthatthetrain’sacceleratiohwaschanging.
Ifitsspeedhadchangedatasteadyrate,theselines
wouldhavebeenstraight.
11Twostudentsliveinthesameapartmentblock
inHometownandattendthesameschoolin
Schooltown,asshowninFigure2.12.Forthis
question,workinkmandhours.
Figure2.12
26

40. 2Describingmotion
iArungetsalifttoschoolinhismother’scar.
Thetrafficisheavysotheaveragespeedforthe
journeyis40km/h.Howmanyminutesdoesit
takeAruntogettoschool?
bSofialeaveshomeatthesametimeasArun
butshewalksthe0.3kmtoHometownstation,
waits3minutes(0.05hour)forthetrain,travels
onthetraintoSchooltownstation(journey
distance22km)andwalksthe0.7kmfrom
Schooltownstationtotheschool.Thetrain
averages88km/handSofiawalksat5km/h.
HowmanyminutesdoesittakeSofiatogetto
school?
cHowmanyminutesshorterisSofia’sjourney
timethanArun’s?
dDrawaspeed-timegraphfortheirjourneys
onthesameaxesbutassumethatany
changeinspeedisinstant(donotshowthe
acceleration).
1'Lookatthespeed-timegraphinFigure2.13.
60
50
'
|40
®30
<D
CL
on20
10
0
Figure2.13
Namethesectionsthatrepresent:
asteadyspeed
bspeedingup(accelerating)
cbeingstationary
dslowingdown(decelerating).
13Acaristravellingat20m/s.Thedriverseesa
hazard.Afterareactiontimeof0.7s,sheperforms
anemergencystopbyapplyingthebrakes.Thecar
takesafurther3.3stocometoastop.Sketcha
speed-timegraphforherjourneyfromthemoment
sheseesthehazardtothemomentshebringshercar
toastop.Labelthegraphwithasmanydetailsas
youcan.
14aCopyTable2.3andsketchthemotiongraphs
foreachmotiondescribed.
Table2.3
Motionof
body
Distance-time
graph
Speed-time
graph
atrest
movingat
constantspeed
constant
acceleration
(speedingup)
constant
deceleration
(slowingdown)
bCopyTable2.4andsketchthespeed-time
graphsforeachaccelerationdescribed.
Motionof
body
constant
acceleration
increasing
acceleration
decreasing
acderation
accelerating
decelerating
Table2.4
Findingdistancetravelled
Aspeed-timegraphrepresentsanobject’smovement.
Ittellsusabouthowitsspeedchanges.Wecanalsouse
thegraphtodeduce(workout)howfartheobjecttravels.
Todothis,wehavetomakeuseoftheequation:
distance=areaunderspeed-timegraph
Theareaunderanystraight-linegraphcanbebroken
downintorectanglesandtriangles.Thenyoucan
calculatetheareausing:
areaofrectangle=widthxheight
areaofatriangle=xbasexheight
Tounderstandthisequation,considerWorkedExamples
2.3,2.4and2.5.
27

41. >
CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
WORKEDEXAMPLE2.3
Calculatethedistanceyoutravelwhenyoucyclefor
20sataconstantspeedoflOm/s(seeFigure2.14).
Figure2.14:Speed-timegraphforWorkedExample2.3.
*
Step1:Distancetravelledisthesameastheshaded
areaunderthegraph.Thisrectangleis20s
wideandlOm/shigh,soitsareaislOm/sx
20s=200m.
Step2:Checkusingtheequation:
distancetravelled=speedxtime
=lOm/sx20s-200m
Answer
Youwouldtravel200metres.
WORKEDEXAMPLE2.4
Yousetoffdownasteepskislope.Yourinitialspeedis
Om/s.After10syouaretravellingat30m/s(seeFigure
2.15).Calculatethedistanceyoutravelinthistime.
Step2:
Figure2.15:Speed-timegraphforWorkedExample2.4..
Answer
areaofatriangle=
1xbasexheight
—x10sx30m/s
2
=150m
Checkusingtheequation:
,initialvelocity+finalvelocity
averagespeed= - —
_
0m/s+30m/s
2
=15m/s
distancetravelled=averagespeedxtime
=15m/sx10s
=150m
Step1:Distancetravelledisthesameastheshaded
areaunderthegraph.Theshapeisatriangle
withaheightof30m/sandbaseof10s.
Youtravel150metres.
28>

42. 2Describingmotion
>RKEDEXAMPLE2.5
X11.mi’smotionisrepresentedbythegraphinFigureStep3:
I
(>('alculatethedistancethetraintravelsin60s.
i<iur«2.16:Speed-timegraphforWorkedExample2.5.
Findtheareaoftheorangetriangle.Ithasa
baseof40sandheightof14.0m/s-6.0m/s
=8.0m/s
areaofatriangle=±xbasexheight
=4x40sx8.0m/s
2
=160m
(Note:thistellsustheextradistancetravelled
bythetrainbecauseitwasaccelerating.)
Addthesetwoareastofindthetotalarea
and,therefore,thetotaldistancetravelled:
totaldistancetravelled=360m+160m
=520m
।pI:Distancetravelledisthesameastheshaded
areasunderthegraph.Thisgraphhastwo
shadedareas:thepinkrectangleandthe
orangetriangle.
>p2:Findtheareaofthepinkrectangle.
Itis60swideand6.0m/shigh,soitsarea=
60sx6.0m/s=360m
(Note:thistellsushowfarthetrainwould
havetravelledifithadmaintainedaconstant
speedof6.0m/s.)
Step5:Checkusingtheequation
distancetravelled=averagespeedxtime
Thetraintravelledfor20satasteadyspeed
of6.0m/s,andthenfor40satanaverage
speedof10.0m/s.So:
distancetravelled=(6.0m/sx20s)+
(10.0m/sx40s)
=120mx400m
=520m
Answer
In60s,thetraintravelled520metres.
Question
1 iDrawaspeed-timegraphtoshowacarthatacceleratesuniformlyfrom6m/sfor5sthentravelsatasteady
speedof12m/sfor5s.
I>Onyourgraph,shadetheareathatshowsthedistancetravelledbythecarin10s.
iCalculatethedistancetravelledinthistime.
ACTIVITY2.2
Ihe4x100metrerelay
II"purposeofthisactivityistoapplywhatyouhavelearnedaboutmotion(andparticularlysketching
।
hhid-timegraphs)toarealproblem.Ifyougetthechancetotakethisactivityoutontoarunningtrack,you
illneedtotaketimeanddistancemeasurements(somethingyoulearnedaboutinChapter1).
iressina4x100mrelayracedependsbothonthespeedoftherunnersandeffectivebatonexchange
I
"
tweentherunners.Thebatonmustpassbetweenrunnerswithina30mchangeover(orpassing)zone,
v/hichincludesa10macceleration(orfly)zone.Figure2.17showsthefirstofthesethreepassingzones.
29y

43. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
idealexchange
pointforbaton
Figure2.17:Thefirstbendofa400mathleticstrack.
Eachathleteactuallysprintsformorethan100m,
asshowninTable2.5.Byplanningforthebatonto
exchangebetweenrunnersatthebeginningorend
ofthechangeoverbox,youcanadjustthedistance
eachrunnerruns.Youmighthaveaslightlyshorter
distancefora60msprinterandalengthened
distancefora100mrunnerwhoalsoruns200m,
whichalsomakesthemusedtorunningbends.
Usually,eachrunnerkeepsthebatoninthesame
handandpassesittotheoppositehandofthe
nextrunnertoexchangethebaton.Usually,thefirst
runnercarriesthebatonintheirrighthand.
1Inwhathandwillrunnersreceiveandcarrythe
batononsubsequentlegs?
2Whataretheadvantagesofpassingthebaton
totheoppositehand?
Ideally,duringthebatonexchangethespeeds
oftherunnersshouldbethesame.Toachieve
thistheoutgoingrunnerstartshisrunwhenthe
incomingrunnerreachesacheckmark.
3Howwouldyouworkoutwheretoplacethe
checkmark?(Hint:itmighthelpifyousketch
speed-timegraphsonthesameaxesforboth
runners,startingwhentherunnerreceivingthe
batonstartsrunning.)Whatotherinformation
wouldyouneedtomakethisaccurate?
4EvenatOlympicfinalsteamscanbedisqualified
(stoppedfromtakingpart)iftheydropthebaton
orpassitoutsideofthechangeoverzone.Why
doesthishappensooften?
5Imagineyouaretheschool'sathleticscoach.
Table2.5liststhetimesforrunnerswhooften
competeinthesenior4
*
100mrelay.
Usethisinformationtoselectyourteamand
decidewhichlegeachrunnershouldrunand
entertheirnamesontheteamsheet.Doyou
haveastrategyfordecidingwhichathlete
runswhichleg?Whatotherinformationmight
youwanttogatherbeforemakingadecision?
Forexample,Sajjansuggeststhatheisthebest
starter.Someathletesarebetteratrunning
bends.Somearebetteratpassingorreceiving
thebaton.
Table2.5
Athletename
100m
personal
best/s
200m
personal
best/s
Right-handed,left-handed,
orambidextrous(happy
usingeitherhand)
Goodbendrunner
SajjanSidhu 12.1 25.8 right prefersbends
GarPsiHo 11.8 24.3 right,ambidextrous prefersbends
AndrewKerr-Chin 11.1 24.4 ambidextrous
prefersstraightsbut
goodatboth
TomSchofield 11.7 25.1 right,ambidextrous betteratstraights
OliverHudson 12.6 26.3 ambidextrous happytorunbends
30)

44. 2Describingmotion
CONTINUED
6Collectdatafromyourowngroup.Usethistoselecta4x100mteamanddecidewhoshouldruneachleg.
Copyandcompletethisteamsheet.
Table2.6
Teamsheet
LegTypicaldistanceactuallyrun/mAthletename 100mpersonalbest
1 105
2 125
3 125
4 120
SELF-ASSESSMENT
Inicienceitisoftenhelpfultovisualisetasks.
Iorquestion3,didyouhaveaclearideaofhowto
workoutwheretoplacethecheckmarks?Didthe
ide.iofsketchingthespeed-timegraphsforthe
runnershelp?(Thedifferenceintheareaunderthe
Iwographsuptothemomentofbatonexchange
shouldtellyouhowfarinfrontoftheacceleration
netoplacethecheckmark).
Wh.itotherinformationdidyouneedbefore
I,idingwhichrunnershouldruneachleg?
2.4Calculatingspeed
andacceleration
Iimiadistance-timegraph,wecanfindhowfast
micihingismoving.Figure2.18showsinformation
al'litacarjourneybetweentwocities.Thecartravelled
111<iiislowlyatsometimesthanatothers.Itiseasierto
thisifwepresenttheinformationasagraph.
Iimithegraph,youcanseethatthecartravelledslowly
Ithestartofitsjourney,andalsoattheend,whenit
Willravellingthroughthecity.Thegraphissteeperin
ilmiddlesection,whenitwastravellingontheopen
iiIbetweenthecities.
11';iiphalsoshowshowtousethegradientto
ihulutethecar’sspeedontheopenroad:
=gradientofdistance-timegraph
*
li'detailisgiveninWorkedExample2.6.
WORKEDEXAMPLE2.6
UsethegradientofthegraphinFigure2.18to
calculatethecar’sspeedontheopenroad.'
Table2.7:Dataforacarjourney.
Distancetravelled/km Timetaken/h
0 0.0
10 0.4
20 0.8
100 1.8
110 2.3
Figure2.18:Distance-timegraphforacarjourney.
31

45. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
speed=gradientofdistance-timegraph
Step1:Identifytherelevantstraightsectionofthe
graph.Here,wearelookingatthestraight
sectioninthemiddleofthegraph,wherethe
car’sspeedwasconstant.
Step2:Drawhorizontalandverticallinesto
completearight-angledtriangle.
Step3:Calculatethelengthsofthesidesofthe
triangle.
Step4:Dividetheverticalheightbythehorizontal
widthofthetriangle(‘updividedbyalong’).
verticalheight=80km
horizontalwidth=1.0h
.80km
srad,mt=Toit
=80km/h
Answer
Thecar’sspeedwas80km/hforthissectionofits
journey.
Note:Ithelpstoincludeunitsinthecalculation
becausethentheanswerwillautomaticallyhavethe
correctunits,inthiscase,km/h.
Question
16Table2.8showsinformationaboutatrainjourney.
Table2.8
Station
Distance
travelled/km
Timetaken/
minutes
Hornby 0 0
KirbyLonsdale 10 30
Ingleton 20 45
Dolphinholme 46 60
Galgate 56 80
UsethedatainTable2.8toplotadistance-time
graphforthetrain.Findthetrain’saveragespeed
betweenKirbyLonsdaleandDolphinholme.Give
youranswerinkm/h.
Calculatingacceleration
Pictureanexpresstrainsettingofffromastationona
long,straighttrack.Itmaytake300storeachavelocity
of300km/halongthetrack.Itsvelocityhasincreasedby
1km/heachsecond,andsowesaythatitsaccelerationis
1km/hpersecond.
Thesearenotveryconvenientunits,althoughtheymay
helptomakeitclearwhatishappeningwhenwetalk
aboutacceleration.Tocalculateanobject’sacceleration,
weneedtoknowtwothings:
itschangeinvelocity(howmuchitspeedsup)
thetimetaken(howlongittakestospeedup).
Theaccelerationoftheobjectisdefinedasthechangeof
anobject’svelocityperunittime.
, changeinvelocity
acceleration=
timetaken
Wecanwritetheequationforaccelerationinsymbols
withAvforchangeinvelocityandAtfortimetaken.
Sowecanwritetheequationforaccelerationlikethis:
Alternatively,becausetherearetwovelocities,we
couldusetwosymbols:u=initialvelocityandv=final
velocity.Nowwecanwritetheequationforacceleration
likethis:
Theadvantageofthisequationisthatifthefinalvelocity
islessthantheinitialvelocity,theanswerisnegative.
Thistellsyouthattheaccelerationisnegative(i.e.that
theobjectisdecelerating).
Intheexampleoftheexpresstrain,wehaveinitial
velocityu=0km/h,finalvelocityv=300km/handtime
takent=300s.
c ... 300km/h-0km/h,.„
So,accelerationa= =1km/hper
300s
F
second.WorkedExample2.7usesthemorestandard
velocityunitsofm/s.
32

46. 2Describingmotion
Unitsofacceleration
bi>rkedExample2.7,theunitsofaccelerationare
i'i>cnasm/s2(metrespersecondsquared).Theseare
lluil.mdardunitsofacceleration.Thecalculation
du/.thattheaircraft’svelocityincreasedby2m/s
"
ivsecond,orby2metrespersecondpersecond.It
iimplesttowritethisas2m/s2,butyoumaypreferto
dunkofitas2m/spersecond,asthisemphasisesthe
uhtilingofacceleration.
>RKEDEXAMPLE2.7
aircraftacceleratesfrom100m/sto300m/sin
I<H)Whatisitsacceleration?
SupI:Startbywritingdownwhatyouknow,and
whatyouwanttoknow.
initialvelocityu=100m/s
finalvelocityv=300m/s
timet=100s
accelerationa=?
?lep2:Nowcalculatethechangeinvelocity.
changeinvelocity=300m/s-100m/s
=200m/s
sup3:Substituteintotheequation.
, changeinvelocity
acceleration=
timetaken
_
200m/s
100s
=2.0m/s2
Alternatively,youcouldsubstitutethevalues
ofu,vandtdirectlyintotheequation.
_
300-100
100
=2m/s2
swer
IInaircraft’saccelerationis2.0m/s2
IIonareworkingouttheaccelerationofanobjectthat
।"lowingdown,thenthisalternativemethodshownin
w
'hkedExample2.7willgiveanegativeanswer.Ifthe
Hi'iaftwasslowingdownfrom300m/sto100m/sthen
ii.i<celerationwouldbe:
_
v-m_
lOOm/s-300m/s_
-
,,
a:—= — =~2m/s2
/ 100s
Thisisbecauseaccelerationisavectorquantity:
ithasadirection.Itcanbeforwards(positive)or
backwards(negative).Soitisimportantalwaystothink
aboutvelocityratherthanspeedwhenworkingout
accelerations,becausevelocityisalsoavectorquantity.
Questions
17Whichofthefollowingcouldnotbeaunitof
acceleration?
km/s2 mph/s km/s m/s2
18Acarsetsofffromtrafficlights.Itreachesaspeed
of21m/sin10s.Whatisitsacceleration?
19Atrain,initiallymovingat15m/s,speedsupto
39m/sin120s.Whatisitsacceleration?
20Thespeedofacarincreasesfrom12m/sto20m/sin
4seconds.
aSketchthespeed-timegraph.
bCalculatetheacceleration.
cUsethegraphtoworkoutthedistancecovered
inthose4seconds.
dCalculatethedistancetravelled.
eIfyouranswerstopartscanddarenotthe
same,thenworkoutwhereyouhavemadea
mistake.
Accelerationfrom
speed-timegraphs
Aspeed-timegraphwithasteepslopeshowsthatthe
speedischangingrapidly-theaccelerationisgreater.
Itfollowsthatwecanfindtheaccelerationofanobject
bycalculatingthegradientofitsspeed-timegraph:
acceleration=gradientofspeed-timegraph
Threepointsshouldbenoted:
Theobjectmustbetravellinginastraightline;its
velocityischangingbutitsdirectionisnot.
Ifthespeed-timegraphiscurved(ratherthana
straightline),theaccelerationischanging.
Ifthegraphisslopingdownwards,theobjectis
decelerating.Thegradientofthegraphisnegative.
Soadecelerationisanegativeacceleration.

47. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
*WORKEDEXAMPLE2.8
Atraintravelsslowlyasitclimbsupalonghill.Then
itspeedsupasittravelsdowntheotherside.Table2.9
showshowitsspeedchanges.Drawaspeed-timegraph
toshowthisdata.Usethegraphtocalculatethetrain’s
accelerationduringthesecondhalfofitsjourney.
Time/s Speed/m/s
0 6.0
10 6.0
20 6.0
30 8.0
40 10.0
50 12.0
60 14.0
Theslopingsectionshowsthatthetrainwas
thenaccelerating.
Figure2.19:Speed-timegraphforWorkedExample2.8.
Table2.9:Speedofatrain.
Beforestartingtodrawthegraph,itisworthlooking
atthedatainthetable.Thevaluesofspeedaregiven
atequalintervalsoftime(every10s).Thespeedis
constantatfirst(6.0m/s).Thenitincreasesinequal
steps(8.0,10.0,andsoon).Infact,wecanseethatthe
speedincreasesby2.0m/severy10s.Thisisenoughto
tellusthatthetrain’saccelerationis0.2m/s2.However,
wewillfollowthroughthedetailedcalculationto
illustratehowtoworkoutaccelerationfromagraph.
Step1:Drawthespeed-timegraphusingthedatain
Table2.9;thisisshowninFigure2.19.
Step2:Drawinatriangletocalculatetheslopeof
thegraph,asshownonFigure2.19.This
givesustheacceleration.
_
14.0m/s-6.0m/s
60s-20s
_
8.0m/s
40s
=0.20m/s2
Answer
Theinitialhorizontalsectionshowsthatthe
train’sspeedwasconstant(zeroacceleration).
Thetrain’saccelerationdownthehillis0.20m/s2.
AlthoughWorkedExample2.8usestheequationfor
acceleration,youarefindingthegradientoftheslopein
Figure2.19.
Figure2.20showsthespeed-timegraphforaskydiver
fromthemomentsheleavesanaircraft.Shejumps
from5000mandopensherparachutewhenshereaches
1500m,60saftershejumps.Youhavealreadylearned
thatyoucanfindtheaccelerationfromthegradientofa
speed-timegraph.However,thereareplaceswherethe
gradientofthegraphischanging(whenthegraphisnot
astraightline).Tofindtheaccelerationatanymomentin
time,atangenttothegraphisdrawn.Thisworksforany
graph:straightorcurved.
Time/s
Figure2.20:Thespeed-timegraphforaskydiver,showing
thefirst105softhejump.
34

48. I'rihapsyoucanalreadyexplainwhyheracceleration
ih.mgesasshefallsbutitwillbeexplainedin
iipter3.Canyouseewhensheopensherparachute
iligure2.20?Recallinghowtoworkoutdistanceon
peedtimegraph,canyouworkouthowfarshehas
I.illenwhensheopensherparachute?Canyouworkout
ih.itshelands160saftershestartsherjump?
Question
JIAcardriverhastodoanemergencystop.Thisis
whenthedriverneedstostopthecarintheshortest
possiblestoppingdistance.Thereisadelaybetween
teeingahazardandapplyingthebrakes.Thisisdue
tothereactiontimeofthedriver,sometimescalled
thethinkingtime.Thedistancethecarmovesinthis
time(whenthecarhasnotchangedspeed)isthe
thinkingdistance.Thedistancethecarmovesonce
thebrakesareappliedanduntilthecarcomestoa
stopisthebrakingdistance.Thestoppingdistance
=thinkingdistance+brakingdistance.
Acaristravellingat20m/swhenthedriverseesa
hazard.Shehasareactiontimeof0.7sandbrings
hercartoastop4.0safterseeingthedanger.
aDrawaspeed-timegraphtorepresentthecar’s
motionduringthe4.0sdescribed.Assume
thatthedeceleration(negativeacceleration)is
constant.
bUsethegraphtodeduce(workout)thecar’s
decelerationasitslowsdown.
cUsethegraphtodeducehowfarthecartravels
duringthe4.0sdescribed.
2Describingmotion
IookatFigure2.20.Whatistheskydiver’s
.iccelerationat:
m0s
b5.5s?
I'Hrta
Partb
Step1:Drawatangenttothegraphatt=5.5s
(shownbelowbytheblueline).
Step2:Drawinatriangletocalculatetheslopeof
thegraph(shownbelowbythedashedlines).
0 5 10 15 20
Time/s
Step3:Calculatetheslopeofthegraph.Thisgives
ustheacceleration.
Step3:Calculatetheslopeofthegraph.Thisgivesus
theacceleration.
Answer
Theparachutisthastheaccelerationoffree-fall
(9.8m/s2)themomentshejumpsoutoftheaircraft
(att=0s)andheraccelerationdecreaseswithtime
untilshereachesaconstantspeed.After5.5sher
accelerationis3.8m/s2.
Step1:Drawatangenttothegraphatt=0s(shown
belowbytheblueline).
Step2:Drawinatriangle(shownbelowbythe
dashedlines).
“O
o
o
CL
LO
70
60
50
40
30
20
10
0
10
me/s
WORKEDEXAMPLE2.9

49. >
CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
3
4
1
1
2
3
4
5
2
Table2.10
Figure2.21:Tickertape.
ACTIVITY2.3
Usingtickertapetofindtheaccelerationofa
trolleydownaramp
Youaregoingtoinvestigatethemotionofatrolley
downaramp.Sometickertapeisattachedtothe
trolley-andatickertimermarksthepaper50times
asecond(Figure2.21).Asthetrolleyaccelerates,the
distancebetweenthedotsincreases.
Cutacopyofthethetickertapeintolengths
correspondingtoeveryfifthdot(0.1stime
interval).
Stickthelengthssidebyside(likeahistogram)
ontographpaper,withthebottomofeachstrip
onthehorizontalaxis.
Plotaspeed-timegraph.
Usethegradientfromthegraphtocalculatethe
acceleration.
Alternativeapproach
Everyfifthdothasbeennumbered.Thiscorresponds
tothedistancetravelledevery0.1s.
Drawalinethroughthedotatthetopofeach
strip(orthemiddleofthetopofeachstrip,ifthe
dotismissing).
Workoutthescaleforeachaxis.Thewidthof
eachstripisequaltoatimeintervalof0.1s.
Workoutthegradientofthespeed-timegraph
youhaveconstructed.
Thetickertimermarksthepaper50timesa
second.Whatintervaloftimedoeseachgap
represent?
Tofindthespeedataparticulardot,youneed
tomeasurethedistancecoveredoverashort
intervaloftimecentredonthedot.Measurethe
distancebetweenthepreceding(previous)dot
andsucceedingdot(theonethatfollows).For
example,tofindthespeedatdot15,weneed
tofindthedistancecoveredbetweendot14
anddot16(13mm)andthendividebythetime
takentocoverthisdistance(2x0.02s=0.04s),
usingtheequation:
speed=^ce=13mm
=0325m/s
time 0.04s
CopyandcompleteTable2.10,usingtheticker
tapetohelpyou.
Dotnumber Timesincetapestarted/s Distancecovered/mm Speed/m/s
0 0.0
5 0.1
10 0.2
15 13 0.325
20
25
30

50. 2Describingmotion
PROJECT
i>urteacherwilldecidewhetheryouwillworkon
y>urown,inpairsoraspartofasmallgroup.Your
।rikistoplanathree-partrevisionlessononthe
materialinthischapterfortherestofyourclass,
।
.itticularlythelinkbetweenmotiongraphsandthe
ju.itionsofmotion.Writedownaplantoshow
/hityouwoulddoandwhatresourcesyouwould
uIfyouhavetime,youcanproduceandteach
ilrlessontosmallgroupsofyourclassmatesorthe
hHeofyourclass.Thefollowingpointswillhelp
iasyouplantherevisionlesson.
Youneedtobeabletoanswerquestionson
motiongraphsandequationsofmotionsothat
youcanthenusethemasabasistowriteyour
own ownquestions.
Youneedtoproducemodelanswersforyour
questionsorcomeupwithabetterwayof
gettingtheideasacross.
Insistthatyourclassmatesshowtheirworking.
Youneedtolabelwhatpartsofyourquestions
iresupplementary.
।kiaresomesuggestedquestionswhichyoucan
UwInyourplanforthelesson:
Part1:Howtointerpretmotiongraphs
u.tionforyourclassmatestoanswer:'Copyand
npletethetablebystatingwhatfeatureofthe
nrliongraphcanbeusedtoobtainthevariable
IIintheleft-handcolumn.Thefirstcellhas
I'ndoneforyou.'
Distance-time
graph
Speed-time
graph
di.tance
readoffthe
verticalaxis
t»|>ead
deration
mightwanttosuggestthatyourclassmates
Inircodethetableinsomeway.
।
iyouthinkofabetterwayofgettinginformation
lmmotiongraphs?
Part2:Linkingmotiongraphstoequations
ofmotion
Questionforyourclassmatestoanswer:'Abody
movingat2m/sacceleratesfor2secondsuntil
itreachesaspeedof4m/s.Showthatthebody
travelsadistanceof6mandacceleratesat1m/s2.'
Youneedyourclassmatestogetthesameanswer
forthequestionyouproduceusingtwodifferent
methods.
Method1:Usetherelevantequations(for
accelerationanddistance)
Someofyourclassmateswillgetthedistancewrong
becausetheydonotusetheaveragespeedinthe
equationfordistance.
Method2:Sketchthemotiongraph
Yourclassmatesshouldusethegradientofthe
graphtofindtheaccelerationandtheareaunder
thecurvetofindthedistance.However,someof
yourclassmateswillsketchthemotionfromthe
origin(insteadoffrom2m/s)andwillworkout
theareaofatriangle(insteadofatriangleplusa
square)sowillgetadistancetravelledof2metres.
Otherswillmeasurethehorizontalandvertical
distanceswitharulertoworkoutthegradient
insteadofusingthescaleontheaxestoworkout
thechangesinthespeedandtimetoworkout
thegradient.
Youneedtocomeupwithsimilarquestions
(differentnumbers)andtheirmodelanswers.
Perhapstryyourquestiononafewofyourfriends
tocheckthatitisclearandtopickupcommon
mistakes.Youcouldprovideyourquestionanda
wrongsolutionandaskothermembersofyourclass
tospotandcorrectthemistakes.
Part3:Puttinglearningintopractice
Questionsforyourclassmatestoanswer:
BloodhoundLSRisbeingdevelopedtoachievea
newlandspeedrecordof1000mph.Thevehicle
willbetimedovera'measuredmile'half-waydown
a12milelongsaltpaninSouthAfrica.
IfBloodhoundachievesWOOmph,howlong
wouldittaketocompletethe'measuredmile'?
37

51. )CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
CONTINUED
Figure2.22:BloodhoundLSRduringapracticerunon
theHekskeenpaninSouthAfrica.
Sketchaspeed-timegraphforitsjourney.
Labelitwithsignificantspeedsandtimes.
AssumethatBloodhoundacceleratesuniformly
untilitreachesthe'measuredmile'andthen
deceleratesuniformlysothatitcomestorest
12milesfromthestart(andbeforetheendof
thesaltpan).
Whatisthetotaltimeforthe12milejourney?
Whatistheaccelerationordecelerationof
thevehicleandhowdoesthiscomparetothe
accelerationoffreefall(9.81m/s2)?'
First,youneedtoanswerthequestionyourself.
Whenyousetyourquestion,decidewhether
toconvertthedatainthequestiontoSIunits
orgetyourclassmatestodoitthemselves
(1mph=0.447m/s;1mile=1610m).
Youcouldintroducethequestionwithashortvideo
clipaboutthevehicle.Adaptthequestionsabove
andproduceamodelanswersothatyourpeers
cancheckandcorrecttheirsolutions.Forexample,
youcouldflipthequestionbytellingyourpeers
themaximumaccelerationanddecelerationofthe
vehicleandgetthemtoworkouttheminimum
distancethe'track'needstobe,oryoucould
changethedatawhilekeepingitrealistic^
SUMMARY
Speedisdistancedividedbytime.
Averagespeedistotaldistancedividedbytotaltime.
Lightgatesandinterruptcardscanbeusedtomeasurespeedinthelaboratory.
Theequationthatrelatesspeed,distanceandtimecanbere-arrangedtofindanyoneofthevariables,giventhe
valuesoftheothertwo.
Thegradient(slope)ofadistance-timegraphrepresentsspeed.
Accelerationisachangeinspeed.
Thegreaterthegradient(slope)ofaspeed-timegraph,thebiggertheacceleration.
Distancetravelledcanbecalculated(workedout)fromtheareaunderaspeed-timegraph.
Speedcanbecalculatedfromthegradientofadistance-timegraphandaccelerationcanbecalculatedfromthe
gradientofaspeed-timegraph.
Speedisascalarandvelocityisavector.
Accelerationcanbecalculatedfromthechangeofspeeddividedbytimeandanegativeaccelerationisthesame
asadeceleration.
38
>

52. AM-STYLEQUESTIONS
1thisgraphforquestions1and2.
।Ilowisaconstantvelocityshownonthegraph? [1]
Atheslopinglineatthestart
Ithehorizontalpartoftheline
Ctheareaundertheline
()theslopinglineattheend
Ilowisthedistancetravelledshownonthegraph? [1]
Atheslopinglineatthestart
I?thehorizontalpartoftheline
Ctheareaundertheline
Dtheslopinglineattheend
<\snailtakespartinasnailrace.Thesnailcompletesthe180cmcoursein
0minutes.Whatistheapproximateaveragespeedofthesnail? [1]
A0.43m/s B26m/s C0.26m/s D0.0043m/s
1 Ihevelocity-timegraphshowstheperformanceofaFormula1®racingcaras
itacceleratesfromrestfor7.33secondsandthenbrakes,comingtoastopin
.69seconds.Itcoversadistanceof520metres.
Whatistheapproximatemaximumvelocityofthecar? [1]
A50m/s B75m/s C105m/s D175m/s
2Describingmotion
39>

53. >
CAMBRIDGE 1GCSE™PHYSICS:COURSEBOOK
CONTINUED
5ThetableshowsUsainBolt’ssplittimesfromhisworldrecord100mrunin
Berlinin2009.Eachsplittimeisfora10msectionofthe100mdistance.
Thetimeforthefirst10mincludeshisreactiontimeof0.146sbeforeheleft
hisblocks.
Section/m 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80
£
o
co 90-100
Time/s 1.890.990.900.860.830.820.810.820.830.83
aCalculatethetimethatUsainBolttakestorunthefirst10metresfrom
themomenthestartsmoving. [1]
bCalculateUsainBolt’saveragespeedoverthefirst10metresfromthe
momenthestartsmoving. [2]
cCalculateUsainBolt’smaximumspeedoverthefirst10metres.
Ignorehisreactiontimeandassumehisaccelerationisconstant. [2]
dCalculateUsainBolt’saccelerationoverthefirst10metres.
Ignorehisreactiontimeandassumehisaccelerationisconstant. [2]
eCalculateUsainBolt’stopspeedintherace.Showyourworking. [2]
[Total:8]
6Anaircrafthappenedtobeflyingnearavolcanowheniterupted.Theco-pilot
tooksomevideofootage.Hehandedthefootageovertoscientistsforanalysis.
Thescientistsspottedahugeboulderthatwasmovingataconstantspeed
horizontally(sideways)inthefirstframeandfallinginsubsequentframesofthe
video.Theywantedtoworkouthowfartheashandrockwouldspread.
aPlotagraphofthepositionoftheboulderatintervalsof5seconds.
Plottheverticalheightoftheboulder(verticalaxis)againstthe
horizontaldistancetravelled(horizontalaxis). [3]
Time/s
Horizontaldistance
travelled/m
Verticalheight/m
0 0 4420
5 525 4292
10 1050 3924
15 1580 3311
20 2100 2453
25 2630 1349
30 3150 0
bExplaintheshapeofthegraph. [11
COMMAND WORDS
calculate:workout
fromgivenfacts,
figuresorinformation
explain:setout
purposesor
reasons;make
therelationships
betweenthings
evident;provide
whyand/orhowand
supportwithrelevant
evidence
40

54. 2Describingmotion
CONTINUED
cThescientiststhoughttheaircrafthadbeenatanaltitude(height)of
4420metreswhenthevideowastakenbutitwasat3600metres.Useyour
graphtoestimatethe(horizontal)distancetheboulderwillhavetravelled
fromthepointthatitwasrecordedonvideotowhereithittheground.[2]
dUseyourgraphtoestimatehowlongittookthebouldertohitthe
groundfromwhereitwasfilmed. [1]
eCalculatethehorizontalspeedoftheboulder. [2]
fSuggestwhythereisashandrocksoverawideareaandnotjustacircle
ofdebristhatyouranswertocmightsuggest. [1]
[Total:10]
COMMAND WORD
suggest:apply
knowledgeand
understanding
tosituations
wherethereare
arangeofvalid
responsesinorder
tomakeproposals/
putforward
considerations
SELF-EVALUATIONCHECKLIST
fterstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutosee
>ygapsinyourknowledgeandhelpyoutolearnmoreeffectively.
Ican
See
Topic...
Needs
morework
Almost
there
Confident
tomoveon
Workoutspeedfromdistancetravelledandtimetaken. 2.1
Workoutaveragespeedfromtotaldistancetravelled
andtotaltimetaken.
2.1
Describeexperimentstomeasurespeedinthe
laboratory.
2.1
Workoutdistancetravelledfromaspeed-timegraph. 2.3
Findspeedonaspeed-timegraph. 2.3
Calculateaccelerationfromthechangeinvelocityand
limetaken.
2.4
Workoutaccelerationfromaspeed-timegraph. 2.4
Explainthedifferencebetweenspeedandvelocity. 2.4

55. *
>Chapter3
Forces
andmotion
discoverthedifferencesbetweenmassandweight
describethewaysinwhicharesultantforcemaychangethemotionofabody
findtheresultantoftwoormoreforcesactingalongthesameline
findoutabouttheeffectoffriction(orairresistanceordrag)onamovingobject
INTHISCHAPTERYOUWILL:
learnaboutcircularmotion
learnhowforce,massandaccelerationarerelated
definewhataforceis,understandtheconceptsofmomentumandimpulseandapplytheprincipleof
theconservationofmomentum
understandthedifferencebetweenscalarsandvectorsandlearnhowtodeterminetheresultantoftwo
vectorsactingatrightanglestoeachother.

56. 3Forcesandmotion
2
3
4
.fr
.1ITINGSTARTED
HYPERLOOPONE:SUPERSONICTRAVELINSIDEVACUUMTUBES
Figure3.1:TheEarthorbitingtheSun.Whichofthepaths
wouldEarthfollowiftheSunsuddenlystopped'existing?
attract)pullsthetrainfromthefrontwhilemagnetic
repulsionpushesitfrombehind.Thetrainswilltravel
throughtubeswithmostoftheairremovedusing
pumps.ThiswillallowthemtotravelatMach7(that
is,seventimesthespeedofsoundatsealevel).Thisis
about2000m/s,muchfasterthansupersonicaircraft.
In2016,HyperloopOnelauncheditsHyperloopOne
GlobalChallengeandselectedfivecountriesforthe
developmentofthehyperloopnetworks:US,UK,
Canada,Mexico,andIndia.
Ir.insportsystemswouldbemuchmoreefficientif
h
1
energywaswastedworkingagainstfrictionor
.»irresistance.HyperloopOnepromisestogetrid
Iboth.ElonMuskproposediton12August2013
i।fasteralternativetoairtravel.Itcombinestwo
-istingtechnologies-maglev(magneticlevitation)
»ndvactrain(vacuumtubetrain).Maglevtrainsuse
magneticrepulsion(likepolesrepel)tomakethetrain
ll<».it,whicheliminatesfriction.Alinearmotorthen
.!■<eleratesthetrain:magneticattraction(unlikepoles
IIflure3.2a:Theideaofpassengerstravellingthroughatubeisnotnew.Passengerstakingarideinthefirstpneumatic
I
।isengerrailwayintheUS,erectedattheExhibitionoftheAmericanInstituteattheAmory,NewYorkCity,in1867.
blAHyperlooptubeondisplayduringthefirsttestofthepropulsionsystemattheHyperloopOneTestandSafetysite
nilMay2016inLasVegas,Nevada.
I"okatthefollowingquestions.Yourteacherwillgive
yousometimetothinkaboutthemonyourown.You
m.jyalsotakesometimetodiscussthemwiththe
।
'>irsonsittingnexttoyou.Bepreparedtoshareyour
irriwerswiththeclass.
Whilesittingonyourseat,describeanyforces
actingonyou.
Imagineaballthrownintheair.Sketchtheball.
Drawanarroworarrowstoshowanyforcesacting
ontheballand,ifpossible,labelthearrows.
Describedailylifewithoutfriction.Whatdoyou
thinkwouldchangethemost?
LookatFigure3.1anddecidewhichpaththeEarth
wouldfollowifgravitystoppedacting.

57. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
Discussionquestions
1DescribethewaysinwhichfrictionwillbereducedinHyperloopOne.
2DescribeanypotentialdangersoftravellinginHyperloopOne.
3.1Wehavelift-off
Ittakesanenormousforcetoliftagiantspaceshuttleoff
itslaunchpad,andtopropelitintospace(Figure3.3).
Theboosterrocketsthatsupplytheinitialthrustprovide
aforceofseveralmillionnewtons.Asthespacecraft
acceleratesupwards,thecrewexperiencethesensation
ofbeingpressedfirmlybackintotheirseats.Thatishow
theyknowthattheircraftisaccelerating.
Figure3.3:Aspaceshuttleacceleratingawayfromits
launchpad.Theforceneededisprovidedbyseveralrockets.
Onceeachrockethasusedallitsfuel,itwillbejettisoned
(dropped),toreducethemassthatisbeingcarriedup
intospace.
Unbalancedforces
changemotion
Onemoment,theshuttleissittingontheground,
stationary.Thenextmoment,itisacceleratingupwards,
pushedbytheforceprovidedbytherockets.
Inthischapter,wewilllookathowforces-pushesand
pulls-affectobjectsastheymove.Youwillbefamiliar
withtheideathattheunitusedformeasuringforcesisthe
newton(N).Togiveanideaofthesizesofvariousforces,
herearesomeexamples:
Youliftanapple.Theforceneededtoliftanappleis
roughlyonenewton(1N).
Youjumpupintheair.Yourlegmusclesprovidethe
forceneededtodothis,about1000N.
Youreachthemotorwayinyourhigh-performance
car,andpresstheacceleratorpedal.Thecar
acceleratesforwards.Theengineprovidesaforceof
about5000N.
YouarecrossingtheAtlanticinaBoeing747jumbo
jet.Thefourenginestogetherprovideathrust
ofabout500000N.Intotal,thatisabouthalf
thethrustprovidedbyeachofthespaceshuttle’s
boosterrockets.
Someimportantforces
Forcesappearwhentwoobjectsinteractwitheachother.
Figure3.4showssomeimportantforces.Eachforceis
representedbyanarrowtoshowitsdirection.Usually,
thelongerthearrow,thebiggertheforceis.Noticethe
conventionthatthearrowusuallypointsawayfromthe
objectofinterest.
44

58. 3Forcesandmotion
lbweightofanobjectisthe
Iillofgravityonit.Weight
i1 i.actsvertically
Inwnwards.Whentwoobjects
nu<h,thereisacontactforce.It
Ilbcontactforcethatstops
Ifillingthroughthefloor.
Frictionopposesmotion.Think
aboutthedirectioninwhichan
objectismoving(ortryingto
move).Frictionactsinthe
oppositedirection.
iiijinu3.4:Somecommonforces.
Airresistanceordragisthe
forceoffrictionwhenanobject
movesthroughairorwater.
Upthrustistheupwardpushofa
liquidorgasonanobject.The
upthrustofwatermakesyoufloat
intheswimmingpool.
IIn
’urshowninFigure3.5ismovingrapidly.The

inincisprovidingaforcetoaccelerateitforwards,but
I'iisanotherforceacting,whichtendstoslowdown
Uli<n.Thisisairresistance,aformoffrictioncaused
Ininobjectmovesthroughtheair.(Frictionisalso
litddrag,especiallyformotionthroughliquids.)The
hiIi.igsontheobject,producingaforcethatactsinthe
।tiledirectiontotheobject’smotion.
airresistance
Il#ut«3.5:Acarmovesthroughtheair.Airresistanceacts
11oppositedirectiontoitsmotion.
\IiIverwhoneedstostopquicklywillapplythebrakes
I
।i.ikiadvantageofsolidfriction,whentwosurfaces
'
111contact(thebrakepadsandbrakediscs,inthis
।.in)I'hekineticenergyofthecartransfersintothermal
iiiy,raisingthetemperatureofthebrakes.Youcan
Imonstratethisforyourselfbyrubbingyourhands
I Iher.Energytransformationslikethesearediscussed
inmoredetailinChapter6.Solidfrictionexistseven
*
Innthesurfacesarenotmovingagainsteachother,
mik.oneorbothsurfacesarelikeiceandofferalmost
ii1
1listancetomotion(thentheyaresaidtohavea
oivlowcoefficientoffriction).Comparestandingon

।mniewithstandingonice.Thereismuchmoresolid
111liononconcreteandyoudonothavetothinkabout
i
11pingyourbalance.Thesolidfrictionbetweenthe
olyourshoesandconcreteimpedesmotion(so
I-
diuithepossibilityofslipping)and,becausethere
isnomotionofonesurfaceagainstanother,thereisno
increaseinthermalenergy.Ifyouarerunningquicklyon
concreteandfalloveryoumaynoticethatthegrazeon
yourkneefeelshot.Thisisbecauseyourkineticenergy
transfersintothermalenergyduetothesolidfriction
betweenyourskinandconcrete.A‘shootingstar’isa
meteor(lumpofrockthatbumsupinouratmosphere).
Itshowsthatairresistanceanddragleadtothetransfer
ofkineticenergytothermalenergy.
KEYWORDS
force:theactionofonebodyonasecondbody;
unbalancedforcescausechangesinspeed,shape
ordirection
newton(N):theforcerequiredtogiveamassof
1kganaccelerationof1m/sI
2
airresistance:frictionactingonanobjectmoving
throughair
friction:theforcethatactswhentwosurfacesrub
overoneanother
drag:frictionthatactsonanobjectasitmoves
throughafluid(aliquidoragas)
solidfriction:theresistancetomotioncaused
whentwosurfacesareincontact
Unbalancedforces
produceacceleration
ThecardriverinFigure3.6aiswaitingforthetraffic
lightstochange.Whentheygogreen,hemovesforwards.
Theforceprovidedbytheenginecausesthecarto
accelerate.Inafewseconds,thecarismovingquickly
45
>

59. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
alongtheroad.Thearrowinthediagramshowstheforce
pushingthecarforwards.Ifthedriverwantstogetaway
fromthelightsmorequickly,hecanpressharderonthe
accelerator.Theforwardforceisthenbigger,andthe
car’saccelerationwillbegreater.
Figure3.6:Aforcecanberepresentedbyanarrow.
a:Theforwardforceprovidedbytheenginecausesthecar
toaccelerateforwards,b:Thebackwardforceprovidedby
thebrakescausesthecartodecelerate,c:Asidewaysforce
causesthecartochangedirection.
Thedriverreachesanotherjunction,wherehemuststop.
Heappliesthebrakes.Thisprovidesanotherforceto
slowdownthecar(seeFigure3.6b).Thecarismoving
forwards,buttheforceneededtomakeitdecelerate
isdirectedbackwards.Ifthedriverwantstostopina
hurry,abiggerforceisneeded.Hemustpresshardon
thebrakepedal,andthecar’sdecelerationwillbegreater.
Finally,thedriverwantstoturnacorner.Heturnsthe
steeringwheel.Thisproducesasidewaysforceonthecar
(Figure3.6c),sothatthecarchangesdirection.
Tosummarise,wehaveseenseveralthingsaboutforces:
Theycanberepresentedbyarrows.Aforcehasa
direction,shownbythedirectionofthearrow.
Aforcecanmakeanobjectchangespeed.A
forwardforcemakesitspeedup(accelerate),whilea
backwardforcemakesitslowdown(decelerate).
Aforcecanchangethedirectioninwhichanobject
ismoving.
Thiscanbesummarisedbysayingthatabodywill
remainatrestorwillmoveataconstantspeedina
straightlineunlessacteduponbyaresultantforce.
Therearealternativewaysofsayingthesamething.
Forexample,aresultantforcewillchangethespeed
ordirectionofabody.However,theproblemwiththis
statementisthatsomepeopleforgettoincludestarting
andstoppingaschangesinspeed.Anotheralternative
istosaythataresultantforcewillchangethevelocity
ofabody,butasvelocityisavector,aresultantforce
canchangethedirectionaswellasthespeedofabody.
Aresultantforcecanchangebothspeedanddirectionat
thesametime.
Question
1Figure3.7showsthreeobjectsthataremoving.
Aforceactsoneachobject.Foreach,sayhowits
movementwillchange.
Figure3.7:Threemovingobjects.
Twoormoreforcesalongthe
samestraightline
ThetwoforcesactingonthecarinFigure3.8aare:
pushofengine=600Ntotheright
dragofairresistance=400Ntotheleft.
46

60. 3Forcesandmotion
Question
2Theforcesactingonthreeobjectsareshownin
Figure3.9.
Figure3.9:Forcesactingonthreeobjects.
Foreachofa,bandc:
Iluure3.8:Acarmovesthroughtheair.Airresistanceacts
hilloppositedirectiontoitsmotion.
innworkoutthecombinedeffectofthesetwoforces
Imbtractingonefromtheothertogivetheresultant
Imactingonthecar.Theresultantforceisthesingle
Iinthathasthesameeffectastwoormoreforces.So,
hiIigure3.8a:
inltantforce=600N-400N
=200Ntotheright
IInresultantforcewillmakethecaracceleratetothe
iii'lilbutnotasmuchasiftherewasnoairresistance.
InIigure3.8b,thecarismovingevenfaster,andair
nritanceisgreater.Nowthetwoforcescanceleach
lliciout.So,inFigure3.8b:
Iult.intforce=600N-600N=ON
Winythattheforcesonthecararebalanced.Thereis
H i.ultantforceandsothecarnolongeraccelerates.
Il।mtinuesataconstantspeedinastraightline.
11noresultantforceactsonanobject,itwillnot
iccelerate;itwillremainatrestoritwillcontinueto
moveataconstantspeedinastraightline.
11anobjectisatrestorismovingataconstant
ipcedinastraightline,wecansaythatthereisno
1
1“sultantforceactingonit.
।WORDS
'’Itantforcethesingleforcethathasthesame
If
Htonabodyastwoormoreforces
istatewhethertheforcesarebalancedor
unbalanced
iiiftheforcesareunbalanced,calculatethe
resultantforceontheobjectandgiveitsdirection
iiistatehowtheobject’smotionwillchange.
3.2Mass,weight
andgravity
Ifyoudropanobject,itfallstotheground.Itisdifficult
toseehowafallingobjectmoves.However,amulti-flash
photographcanshowthepatternofmovementwhenan
objectfalls.
Figure3.10showsaballfalling.Therearesevenimages
oftheball,takenatequalintervalsoftime.Theballfalls
furtherineachsuccessivetimeinterval.Thisshowsthat
itsspeedisincreasing-itisaccelerating.
Figure3.10:Theincreasingspeedofafallingballis
capturedinthismulti-flashimage.
47

61. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Whenanobjectaccelerates,theremustbeaforcethatis
causingittodoso.Inthiscase,theforceofgravityispulling
theballdownwards.Thenamegiventothegravitational
forceactingonanobjectthathasmassisitsweight.Because
weightisaforce,itismeasuredinnewtons(N).
EveryobjectonorneartheEarth’ssurfacehasweight.
ThisiscausedbytheattractionoftheEarth’sgravity.
TheEarthpullswithaforceofION(approximately)on
eachkilogramofmatter,soanobjectofmass1kghas
aweightofaboutION.Actually,theEarthpullswith
aforceof9.8Noneachkilogramsoanobjectofmass
1kghasaweightof9.8N.
BecausetheEarthpullswiththesameforceonevery
kilogramofmatter,everyobjectfallswiththesame
accelerationclosetotheEarth’ssurface.Ifyoudropa
5kgballanda1kgballatthesametime,theywillreach
thegroundatthesametime.
TheaccelerationcausedbythepulloftheEarth’sgravity
iscalledtheaccelerationoffreefallortheacceleration
duetogravity.Thisquantityisgiventhesymbolgandits
valueisapproximatelyconstantclosetothesurfaceof
theEarth.Itisapproximately9.8m/s2.
Calculatingweightand
gravitationalfieldstrength
Wehaveseenthatanobjectofmass1kghasaweightof
9.8N;anobjectofmass2kghasaweightof19.6N;andso
on.Tocalculateanobject’sweightWfromitsmassm,we
multiplyby9.8,thevalueoftheaccelerationoffreefallg.
Wecanwritethisasanequationinwordsandinsymbols:
weight=massxaccelerationoffreefall
W=mg
Thegravitationalfieldstrengthatapointisthegravitational
forceexertedperunitmassplacedatthatpoint.Fromthe
equation,W=mg,thegravitationalfieldstrength,g,is:
OntheEarth’ssurface,thegravitationalfieldstrengthis
9.8N/kg.Ithasthesamevalueastheaccelerationoffree
falloraccelerationduetogravitythatwemetearlierand
isoftenroundedupto10N/kg.
Distinguishingmass
andweight
Itisimportanttounderstandthedifferencebetweenthe
twoquantities,massandweight.
Themassofanobject,measuredinkilograms,tells
youhowmuchmatteranobjectiscomposedof.
Theweightofanobject,measuredinnewtons,isthe
gravitationalforcethatactsontheobject.
KEYWORDS
gravity:theforcethatexistsbetweenanytwo
objectswithmass
accelerationoffreefall:theaccelerationofan
objectfallingfreelyundergravity
accelerationduetogravity:theaccelerationof
anobjectfallingfreelyundergravity
gravitationalfieldstrength:thegravitational
forceexertedperunitmassplacedatthatpoint
IfyoutakeanobjecttotheMoon,itwillweighlessthan
itdoesonEarth,becausetheMoon’sgravityisweaker
thantheEarth’s.Flowever,itsmasswillbeunchanged,
becausetheobjectismadeofjustasmuchmatteras
whenitwasonEarth.
Whenweweighanobjectusingabalance,weare
comparingitsweightwiththatofstandardweights
ontheothersideofthebalance(Figure3.11).Weare
makinguseofthefactthat,iftwoobjectsweighthe
same,theirmasseswillbethesame.Wealwaystalkabout
weighinganobject.However,ifthebalanceweusehas
ascaleinkilogramsorgrams,wewillfinditsmass,not
itsweight.
Figure3.11:Whenthebalanceisbalanced,weknowthat
theweightsonoppositesidesareequal,andsothemasses
mustalsobeequal.
48>

62. 3Forcesandmotion
Questions
> I1stthedifferencesbetweenmassandweight.
4\bagofsugarhasamassof1kgsoitsweightonEarthis10N.Whatcanyousayaboutthebag’smassandits
weightifyoutakeit:
atotheMoon,wheregravityisweakerthanonEarth?
btoJupiter,wheregravityisstronger?
I <iLookatTable3.1.Calculatethemissingvaluesi-v.Showyourmethod.
Table3.1
Planetorasteroid
Objectonplanetor
asteroid
Massof
object/kg
Accelerationdueto
gravity/m/s2
Weightof
object/N
Earth astronaut 70 9.8 i
Moon astronaut ii 1.60 112.0
Jupiter tinofbeans 0.44 23.0 iii
Geographus(asteroid1620) bus iv 0.00153 7.650
Toro(asteroid1685) astronaut 70 V 0.538
l>Whatdoyounoticeaboutthemassoftheastronaut?
<ExplainwhyatinofbeansweighsmoreonJupiterthanabusweighsontheasteroidGeographus.
3.3Fallingandturning

Il|iIsfalltothegroundbecausetheyhaveweight,
llnweightiscausedbythegravitationalfieldofthe
Iuihpullingdownwardsontheirmass.TheMoon’s
nationalfieldismuchweaker,whichiswhyobjects
ii'lllesswhentheyareontheMoon.
Inthissection,wewilllookattwosituationsinwhich
'havetotakecarefulaccountofthedirectionsofthe
Inncmactingonanobject.
Fallingthroughtheair
111Earth'sgravityisequallystrongatallpointsclose
i"lhiEarth’ssurface.Ifyouclimbtothetopofatall
hiiilding,yourweightwillstaythesame.Wesaythat
IIn11<isauniformgravitationalfieldclosetotheEarth’s
Ian,Thismeansthatallobjectsfallwiththesame
hi*riorationastheballshowninFigure3.10,provided
IIn11isnootherforceactingtoreducetheiracceleration.
Ihimanyobjects,theforceofairresistancecanaffect
llnliacceleration.
I'machutistsmakeuseofairresistance.Afree-fall
putuhutist(Figure3.12a)jumpsoutofanaircraftand
hi*
।(cratesdownwards.Figure3.12bshowstheforceson
hi'nachutistatdifferentpointsinhisfall.Noticethat
hisweightdoesnotchange(sothelengthofthe
downward-pointingarrowdoesnotchange).Atfirst,air
resistancehaslittleeffect.However,airresistanceincreases
withthespeedofmotion.Astheparachutistfallsfaster,
eventuallyairresistancebalanceshisweight.Thenthe
parachutiststopsaccelerating:hefallsatasteadyrate
knownastheterminalvelocity.Theresultantforceonthe
free-fallparachutististheresultoftwoforcesactingalong
thesamelineandactinginoppositedirections.
KEYWORDS
terminalvelocity:thegreatestspeedreachedby
anobjectwhenmovingthroughafluid
Openingtheparachutegreatlyincreasesitsareaand
hencetheairresistance.Nowthereisamuchbigger
forceupwards.Theforcesontheparachutistareagain
unbalanced,andheslowsdown.Theideaistoreacha
new,slower,terminalvelocityofabout10m/s,'atwhich
speedhecansafelyland.Atthispoint,weight=drag,
andsotheforcesontheparachutistarebalanced.
Figure3.12cshowshowtheparachutist’sspeedchanges
duringafall.
Whenthegraphishorizontal,speedisconstantand
forcesarebalanced.Whenthegraphissloping,speedis
49>

63. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
changing.Theparachutistisacceleratingordecelerating,
andforcesareunbalanced.
Figure3.12a:Free-fallparachutists,beforetheyopentheir
parachutes.Theycanreachaterminalvelocityofmorethan
50m/s.b:Theforcesonafallingparachutist.Noticethat
hisweightisconstant.Whenairresistanceequalsweight,
theforcesarebalancedandtheparachutistreachesasteady
speed.Theparachutistisalwaysfalling(velocitydownwards),
althoughhisaccelerationisupwardswhenheopenshis
parachute,c:Aspeed-timegraphforafallingparachutist.
Question
6Lookatthespeed-timegraphinFigure3.12c.Find
apointatwhichthegraphisslopingupwards.
aIstheparachutistacceleratingordecelerating?
bWhichofthetwoforcesactingontheparachutist
isgreaterwhenthegraphisslopingupwards?
cExplaintheshapeofthegraphafterthe
parachuteisopened.
Goingroundincircles
Whenacarturnsacomer,itchangesdirection.Any
objectmovingalongacircularpathischangingdirection
asitgoes.Aforceisneededtodothis.Figure3.13shows
threeobjectsfollowingcurvedpaths,togetherwiththe
forcesthatacttokeepthemontrack.
InFigure3.13a,theboyisspinninganapplearoundon
theendofapieceofstring.Thetensioninthestring
pullsontheapple,keepingitmovinginacircle.
InFigure3.13b,anaircraft‘banks’(tilts)tochange
direction.Theliftforceonitswingsprovidesthe
necessaryforce.
InFigure3.13c,theMoonisheldinitsorbitaroundthe
EarthbythepulloftheEarth’sgravity.
Figure3.13:Examplesofmotionalongacurvedpath.
Ineachcase,thereisasidewaysforceholdingtheobjectin
itscircularpath.
Foranobjectfollowingacircularpath,theobjectis
actedonbyaforceperpendicular(atrightangles)toits
motion.Theforcethatkeepsanobjectmovinginacircle
alwaysactstowardsthecentreofthecircle.Iftheforce
soy

64. 3Forcesandmotion
happears,theobjectwillmoveoffatatangenttothe
inIe;itwillnotflyoutwards,awayfromthecentre.
11\movinginacircle,anobjectwillbechangingdirection
imitinuously(allthetime).Therefore,eveniftheobject
imovingataconstantspeed,itsvelocityischanging.
I'<memberthatvelocityisavectorandsohasdirection
Iliwellasmagnitude(size).Ifthevelocityofanobject
l
ihanging,itmustbeaccelerating.Thismeansthatan
unbalancedforceisactingontheobjectandthisforce
i'titowardsthecentreofthecircle.Theresultantforce
thatactstowardsthecentreofacircleistheresultofa
liteactingperpendiculartothemotionoftheobject.
Ihesizeoftheresultantforceneededtomakeanobject
11nvelinacircledependsontheobject’smass,itsspeed
miltheradiusofthecircleinwhichitismoving.A
Inrperforceisneededif:
theobject’smassisbigger(andspeedandradius
staythesame)
theobject’sspeedisbigger(andmassandradius
slaythesame)

theradiusofthecircleissmaller(andmassand
speedstaythesame).
\iiobjectmovinginacircleischangingdirection.It

1
1
iiIreslessforcetochangethedirectionofanobject
thiihaslessmass.Aninsectiseasiertodeflectthana
ilimoceros,eveniftherhinoismovingslowly.Whenyou
uhhavellinginacar,youwillfeelabiggerforcewhen
uIravelfasterroundabendintheroadorwhenthe
Ihndissharper.
Hiwallofdeath’isastuntwheremotorcyclesorcars
ippcuttodefygravityastheyaredrivenaroundthe
inulurwallinsideagiantcylinder(Figure3.14a).The
hklesdonotslidedownthewallbecausetheirweight
।Imlnticedbyfriction.Astheymovefaster,theforceof
111<Iionincreasesbecausetheforcepushingthevehicle
Inwardsthecentreofthecylinderincreases.Iftheywere
in<Iop,thevehicleswouldslidedownthewall.
5ipmdryerworksbyspinningaboutitsaxisof
»immiitry.Thewallspushtheclothestowardsthecentre
il11k'drum.Waterdropletspassoutthroughtheholes
winictheydonotexperiencethisforce.
ofyouwillhaveriddenacarouselormerry-
W
<i'>uild(Figure3.14b).Passengerssitonacircular
I«1Himinthatismadetospin.Itfeelslikeyouarebeing
|uiIiIoutwards,awayfromthecentre,butthisisan
Illusion.Ifyouwereluckyyourmerry-go-roundhad
Wal.witharailingalongthecircumferencewhereyou
।mill'illfacingthecentre;itwouldfeelliketherailing
Figure3.14a:The'wallofdeath'inRajkot,India,
b:Amotherandhersonsplayingonamerry-go-round.
waspushingyoutowardsthecentre.Imaginetherailings
disappearedandtherewasnothingtoholdonto.Where
wouldyougo?Youwouldmoveatatangenttothe
circularmotion(alongpathAinFigure3.1).
Ifyourollaballalongaflatsurfaceitwillmoveina
straightlineuntilfrictionbringsittoastop.Ifsomeone
pushesitatrightanglestoitsmotion,itwillchange
direction.Youmayhaveseenafootballergentlytapa
balltodivertitintothegoal.
Youmightwanttotrythefollowingchallengewitha
smallerball.Gathersomeclassmatesaroundasquare
table.Oneofyourollsamarbleorothersmallballclose
toandparalleltoanedgeofthetable.Astheballpasses
eachperson,theypushitoncewithaflatedgelikea
rulerorbookbutonlytowardsthecentreofthetable.
Seeifyoucangettheballtotravelinacirclebeforeit
issloweddownbyfriction.Astheballslowsdown,you
willnoticethatyouneedtoapplyasmallerforcetokeep
itmovinginacircle.Ifsomeonemissestheirturn,notice
whichwaytheballmoves.Itshouldmoveinastraight
51

65. I
>
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
lineatatangenttothecircle,solongasthetableis
perfectlyhorizontalandthereisnospinontheball.If
theSunsuddenlystoppedexistingtheEarthwouldtravel
inastraightlineinthedirectionthatitwasmovingthe
instanttheSundisappeared.
Questions
7Drawadiagram(seenfromabove)toshowthe
forcesactingonacarasitturnsacomer.
8Whatprovidestheforcekeepingtheplanetsinorbit
roundtheEarth?
9ThrowingthehammerisanOlympicsport(Figure
3.15),wherethethrowerspinsaroundinsideacircle
whileswinginga‘hammer’.Throwersspinasfast
astheycanbeforereleasingthehammer.Looking
downfromabove,sketchthepathofthehammer
movinginacirclefollowedbyitspathafteritis
released.
Figure3.15:ZhengWangofChinacompetesinthe
Women'shammerthrowinthe2019WorldChampionships.
Describehowtheforceneededbytheathletewould
changeif:
athespeedofthehammerincreased
bthelengthofthehammerwasreduced
cthemassofthehammerincreased.
3.4Force,massand
acceleration
Acardriverusestheacceleratorpedaltocontrolthe
car’sacceleration.Thisalterstheforceprovidedbythe
engine.Thebiggertheforceactingonthecar,thebigger
theaccelerationitgivestothecar.Doublingtheforce
producestwicetheacceleration;threetimestheforce
producesthreetimestheacceleration,andsoon.
Thereisanotherfactorthataffectsthecar’sacceleration.
Supposethedriverfillsthebootwithalotofheavyboxes
andthencollectsseveralchildrenfromcollege.Hewill
noticethedifferencewhenhemovesawayfromthetraffic
lights.Thecarwillnotacceleratesoreadily,becauseits
masshasincreased.Similarly,whenheappliesthebrakes,
itwillnotdecelerateasreadilyasbefore.Themassof
thecaraffectshoweasilyitcanaccelerateordecelerate.
Driverslearntotakeaccountofthis.
Thegreaterthemassofanobject,thesmallerthe
accelerationitisgivenbyaparticularforce.
So,big(moremassive)objectsarehardertoaccelerate
thansmall(lessmassive)ones.Ifwedoublethemass
oftheobject,itsaccelerationforagivenforcewillbe
halved.Weneedtodoubletheforcetogiveitthesame
acceleration.
Thistellsuswhatwemeanbymass.Itisthepropertyof
anobjectthatresistschangesinitsmotion.
Forcecalculations
Theserelationshipsbetweenforce,massandacceleration
canbecombinedintoasingle,veryuseful,equation:
KEYEQUATION
force=massxacceleration
F—ma
Theforcevectorandtheaccelerationvectorareinthe
samedirection.Thisseemsobviouswhenwearetalking
aboutincreasingtheengineforcetomakeacaraccelerate
alongastraightroad,butrecallfromtheprevious
sectionthattheEarthmustbeacceleratingtowardsthe
SunbecauseofthepulloftheSun’sgravity.
Thequantitiesinvolvedinthisequation,andtheirunits,
aresummarisedinTable3.2.Theunitofforceisthe
newton.WorkedExamples3.1and3.2showhowtouse
theequation.
Quantity Symbol SIUnit
force F netwon,N
mass m kilogram,kg
acceleration a
metrespersecond
squaredm/s2
Table3.2:Thethreequantitiesrelatedbytheequation
force=massxacceleration.
52

66. 3Forcesandmotion
Ii<|ure3.16:AnAirbusA380.
Questions
11a
b
c
12
I4>lc3.3
)RKEDEXAMPLE3.1
FORKEDEXAMPLE3.2
AnAirbusA380aircraft(Figure3.16)hasfourjet
<ngines,eachcapableofproviding320000Nofthrust.
Ihemassoftheaircraftis560000kgwhenloaded.
Whatisthegreatestaccelerationthattheaircraftcan
achieve?
Anaccelerometerisadevicethatcandetectand
calculateacceleration.Calculatetheaccelerationof
a0.15kgmassthatexperiencesaforceof10N.
10I,ookatTable3.3.Calculatethemissingvaluesa-d.
Showyourworking.
Calculatetheweightofabrickthathasamass
of2.4kg.
Thesamebrickfallswithanaccelerationof
9.8m/s2.Calculatetheforceonthebrick.
Whatcanyousayaboutyouranswerstopartsa
andb?
Answer
Youneedtogivetheballaforceof50N.
Notethatmassmustbeinkg(thebaseSIunit),notg,
iftheforceistoworkoutinN.
Step1:Writedownwhatyouknowandwhatyou
wanttofindout:
force=4x320000N=1280000N
'
(thegreatestforceprovidedbyallfour
enginesworkingtogether)
mass=560000kg
acceleration=?
Whenyoustrikeatennisballthatanotherplayerhas
lilttowardsyou,youprovidealargeforcetoreverse
Ilsdirectionoftravelandsenditbacktowardsyour
opponent.Yougivetheballalargeacceleration.
Whatforceisneededtogiveaballofmass0.10kgan
inderationof500m/s2?
Nlep1:Writedownwhatyouknowandwhatyouwant
tofindout:
mass=0.10kg
acceleration=500m/s2
force=?
Step2:Substituteintheequationtofindtheforce:
force=massxacceleration
=0.10kgx500m/s2
=50N
Answer
2.29N/kgisthegreatestaccelerationtheaircraftcan
achieve.
Step2:Substituteintheequationtofindthe
acceleration:
acceleration=
mass
_
1280000N
560000kg
=2.29N/kg

67. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
ACTIVITY3.1
Canmosquitoesflyintherain?
Workinpairs.Usingyourphysicsknowledgeand
theinformationanddatabelow,trytoanswerthe
question:canmosquitoesflyintherain?
Onceyouhavearrivedatananswer,discussit
withanotherpairandbepreparedtoshareyour
reasoningwiththeclass.
Ifyouarenotsurewheretostart,workthroughthe
followingquestions:
1Whatistheaccelerationofthemosquitowhen
hitbyaraindropinmid-air?
2Calculatehowmanytimesbiggerthisisthan
theaccelerationoffreefall.
3Doyouthinkahumancouldsurvivethis
acceleration?
4Statetheequationthatrelatesforce,massand
acceleration.
5Calculatetheforcethatthemosquito
experiences.
6Willamosquitosurviveamid-aircollisionwith
araindrop?
7Willamosquitosurviveifitissittingona
hardsurfacewhenstruckwitharaindrop?
Calculatetheforcearaindropwouldexert
onamosquitoiftheinsectwassittingona
hardsurface(suchasatreebranch)andthe
raindropcametoastopin2x10-3s.
Inheavyrain,amosquitomightcollidewith
araindroptwiceaminute.
Ifaraindrophitsamosquitoinmid-air,
themosquitofallswiththeraindropfora
fewcentimetresandthemosquito'sspeed
increasesfromzeroto2.1m/sin1.5x10~3s.
Ifaraindrophitsamosquitowhenitison
asolidsurface,suchasatreebranch,the
raindropstopsmovingin2x10-3s.
speedofraindrops=10m/s
massofraindrops=upto100mg
massofmosquito=2mg
forcethatmosquitoexoskeletoncansurvive
=0.03N
REFLECTION
Didyouneedtousethequestionstohelpyou
duringtheactivity?Usingthesequestionsgives
yousomeinsightintohowscientistsmightgo
aboutansweringaquestion:theybreakaquestion
orproblemdownintosmallerstepsorquestions.
Werethequestionshelpful?Ifnot,canyouthinkof
questionsthatwouldbemorehelpful?Couldyou
suggestotherquestionstoyourteacherorclassmates?
3.5Momentum
Aforcewillchangeanobject’smotion.Itwillmakethe
objectaccelerate;itmaymakeitchangedirection.The
effectofaforceFdependsontwothings:
howbigtheforceis
thetimeintervalAtitactsfor.
Thebiggertheforceandthelongeritactsfor,themore
theobject’smotionwillchange.Theimpulseequation
sumsthisup:
FNt-mv-mu
Thequantityontheleft,F^t,iscalledtheimpulseofthe
force.Ontherightwehavemv(massxfinalvelocity)and
mu(massxinitialvelocity).
Thequantitymassxvelocityisknownasthemomentum
(p)oftheobject(p=mv),sotheright-handsideofthe
equationmv-muisthechangeintheobject’smomentum,
whichcanbewrittenas:
\p=mv—
mu=A(mv)
Wecanwritetheimpulseequationlikethis:
impulseofforce=changeofmomentum
Impulseandmomentumarebothdefinedbyequations:
impulse=forcextimeforwhichitacts=FAr
changeinmomentum=Ap=A(mv)so
impulse=Fkt=A(wv)
KEYWORDS
impulse:thechangeinanobject'smomentum,Ap,
ortheforceactingonanobjectmultipliedbythe
timeforwhichtheforceacts(FxAt)
momentum:thequantitymassxvelocity,p=mv
54

68. 3Forcesandmotion
IQUATIONS
tinnuilium=massxvelocity
p=mv
Ihi|mlforcextimeforwhichtheforceacts
iiu|'iil<FNt=N(mv)
RKEDEXAMPLE3.3
A<illofmass600kgismovingat15m/s.
i» <'alculateitsmomentum.
Ili''driveracceleratesgentlysothataforceof30N
uhonthecarfor10seconds.
h <alculatetheimpulseoftheforce.
r
('alculatethemomentumofthecarafterthe
acceleratingforcehasactedonit.
AlMwer
mmomentum=massxvelocity
=600kgx15m/s=9000kgm/s
Itimpulse=forcextime=30Nx10s=300Ns
rI'heimpulseoftheforcetellsushowmuchthe
car’smomentumchanges.Thecarisspeeding
up,soitsmomentumincreasesby300Ns.
finalmomentum=initialmomentum+impulse
offorce
=9000+300
=9300kgm/s
lobthattheunitofmomentumiskgm/s;thisisthe
lineasNs,theunitofimpulse.
Questions
11('alculatethemomentumofabulletofmass10.5g
movingat553m/s.
14Aforceof500Nactsonarocketfor600s,causing
therocket’svelocitytoincrease.

Calculatetheimpulseoftheforce.
bBylowmuchdoestherocket’smomentum
increase?
IRememberthatF=ma.Weknowthata
=^-=-
u
NtNt
Inwecansubstituteforatogive:
Expandingoutthebracketsgives:
„mv-mu
f=~nT~
KnowingthatNp=mv-mu,wecanwritetheequationas:
Nt
So,theresultantforceisthechangeinmomentumper
unittime.
Thisdefinestheforceastherateofchangeof
momentumbutitisreallyjustadifferentwayofwriting
themorefamiliarequationF=mainawaythatmakes
iteasiertoexplainsomephysics.Forexample,‘itexplains
whycarsarefittedwithseatbelts,airbagsandcrumple
zones.Italsoexplainswhyweautomaticallybendour
kneeswhenwejumpdownfromatallobject.
1 Nt
oftheequation smaller.Thisreducestheforce
Whenacarcrashesatagivenspeedsay,(10m/s),the
passengersexperiencethesameimpulseorchangein
momentumwhetherornottheyarewearingseatbelts.
However,thetimetakentocometoastopincreasesfor
thosewearingseatbelts.Thismakestheright-handside
experiencedbythepassengers,whichreducestherisk
ofinjury.Thesamereasoningcanbeappliedtoother
safetymeasures,includingtheuseofairbagsinWorked
Example3.4.
WORKEDEXAMPLE3.4
Crashtestsallowscientiststoinvestigatetheforces
onpassengersduringcollisions.Inthefirsttest,acar
travellingat15m/scrashesintoawall(Figure3.17).
Thecrashtestdummy,whichhasamassof70kg,
comestoastopin0.03s.Inthesecondtest,thecar
isfittedwithanairbagsothedummytakesfivetimes
longertocometoastop.Whatforcesareactingon
thedummyinbothtests?
55

69. 1
yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Figure3.17:Acrashtest.
Step1:Startbywritingdownwhatyouknowand
whatyouwanttoknow.
Firsttest Secondtest
m=70kg m=70kg
v=15m/s v=15m/s
AZ=
0.03s AZ=0.15s
F=? F=?
Step2:Nowwritedowntheequation.
F=—
M
Step3:Substitutethevaluesofthequantitiesonthe
right-handsideandcalculatetheanswer.
Firsttest Secondtest
_70kgx15m/s
0.03s
=3.5x104N
70kgx15m/s
0.15s
=7.0x103N
Answer
Inthefirsttest,theforceonthecrashtestdummywas
3.5xKFN.Inthesecondtest,theforcewas7.0x103N.
Theairbagincreasedthetimethatittookforthedummy
tocometoastopandreducedtheforcebyafactorof
five.Anairbagreducestheriskof,injury.
CONTINUED
Question
15Inacarcrashthedriverandhispassengerboth
experiencethesameimpulse.However,thedriver
iswearingaseatbeltsoittakeshimlongertostop
moving.Explainwhyhisinjuriesarelessserious
thanthoseofhispassenger.
16Supermanisafictionalaharacterwhowasmadefor
lifeontheplanetKryptonbutarrivedonEarthas
achild.Whilehecouldhavegrownuptobecome
averageonKrypton,thisquestionexploreswhyhe
appearstohavesuperpowersonEarth.
aWritedownanequationforforceintermsof
momentumandtime.
bAssumethatSupermanhasamassof100kg
andlauncheshimselfupwardswithaspeedof
60m/s.Whatforcewouldheapplyifhespent
0.25spushingofffromtheground?
cWhatisSuperman’sweight?
dHowmanytimesbiggeristheforceheexerted
(youranswertopartb)comparedtohisweight
(youranswertopartc)?
eCalculatethegravitationalfieldstrengthof
Krypton.(Hint:Youneedtoknowthevalueof
thegravitationalfieldstrengthfortheEarth’s
surfaceandthat,whenmostpeoplejump,they
applyaforceapproximatelyequaltotheirweight.)
ACTIVITY3.2
Publicawarenesscampaign
Workinpairstodevelopyourideasandthenjoin
withanotherpairtocompletetheactivity.Develop
apublicawarenesscampaign(posters,videoclips)
tohighlightroadtrafficaccidents(RTAs).Theaim
ofyourcampaignistoreduceseriousinjury.
Inyourcampaignmakesureyouincludewhythe
followingareimportantinreducingseriousinjury
inRTAs:
keepingtospeedlimits
wearingseatbelts
installingairbags
crumplezonesoncars
notdrivingaftertakingdrugs
notdrivingwhentired.
Rememberthattheaudiencearenotscientists,
soyouwillneedtoexplain,byusingphysics,why
theseprecautionsreduceseriousinjuryinRTAs.
Momentuminacollision
Figure3.18showsagameofswingball,inwhichaball
hangsfromalengthofstring.Theplayerhitstheball
horizontallywitharacket.
56

70.
|l1l*!Hittingaballwithatennisracket,a:Beforethe
limth*hit.
I*
ihicusetheideaofmomentumtodescribewhat
’
Weneedtothinkaboutmomentumbeforethe
pb।<ollidcswiththeball,andthenafterthecollision.
I H8a,beforethecollision,theracketismoving
iiphl;ithasmomentum.Theballisstationary,so
IM*momentum.
inidi118b,afterthecollision,theracketismoving
I"11phI.butmoreslowlythanbefore.Ithaslost
mumumTheballismovingrapidlytotheright.It
ligiilnidmomentum.
K
u
'inseethat,whentheracketexertsaforceon
illmomentumistransferredfromtherackettothe
ill**
heneveraforceactsonanobject,itsmomentum
thesametime,themomentumoftheobject
inmitheforcealsochanges.Ifoneobjectgains
nixntiim,thentheotherlosesanequalamount
ii'nuiitum.Thisisknownastheprincipleofthe
ionofmomentum.
'
)RDS
Withinthemeetingofparticlesorofbodiesin
hachexertsaforceupontheother
Inclpl*oftheconservationofmomentum:the
ilinjmentumisconstantanddoesnotchange
।-nr
"
ofaninteractionbetweenbodies(such
11illi'llons)
Wvintatetheprincipleinadifferentway.Whenever
l»n"b|ritsinteract,thetotalamountofmomentum
Il
1>hIhoyinteractisthesameasthetotalamountof
Hlimi'iliumafterwards:
Ini.iImomentumbefore=totalmomentumafter
"
1
<1Example3.5showshowwecanusethistowork

ihi'ilasttheballinFigure3.18willbemovingafter
IIImImenhitbytheracket.
3Forcesandmotion
WORKEDEXAMPLE3.5
Duringagameofswingball,aplayerhitstheball
horizontallywitharacket.
massoftennisracket=3.0kg
velocityoftennisracketbeforeitstrikesthe
ball=20m/s
velocityoftennisracketafteritstrikesthe
ball=18m/s
massoftennisball=0.25kg
velocityoftennisballbeforetheracketstrikes
it=Om/s
Find:
athemomentumoftheracket
ibeforethecollision
iiafterthecollision
bthemomentumoftheballafterthecollision
cthevelocityoftheball.
Answer
amomentum=massxvelocity
iBeforethecollision:
momentum=3.0kgx20m/s=60kgm/s
iiAfterthecollision:
momentum=3.0kgx18m/s=54kgm/s
Note:Afterthecollision,theracketismoving
moreslowlyandsoitsmomentumisless.
bThemomentumgainedbytheballisequalto
themomentumlostbytheracket.
So,momentumofball=60kgm/s-54kgm/s
=6.0kgm/s
cWecancalculatethevelocityoftheballby
rearrangingtheequationformomentum:
velocity
J
mass
_6.0kgm/s
0.25kg
=24m/s
Theballwillmoveoffwithavelocityof24m/s
totheright.
57>

71. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
ACTIVITY3.3
Findingthevelocityofatennisballusingconservationofmomentum
Youwanttocarryoutaninvestigationtofindthespeedofatennisball,butmostoftheinstructionsare
missing.Youneedtofinishtheplan.
Thisistheonlyguidanceyouhave.Thisistheonlyguidanceyouhave.
E^uipmenf.:tennisball(orsimilar),carJboarXbox,
newspaperOrbubblewrap,stopwatch,measuring
tapeormetrerulers,massbalances.
Method:FillaCardboardboxwithlooselyCrumpled
newspaperOrbubblewrap.PuttheboxOna
smoothflatfloorandthrowtheballhorizontally
intothebox(Figure3.1^5.Theboxwillslide
beforecomingtoastop. Figure3.19
Onyourown,spendthreeminutesthinkingcarefullyaboutthefollowingactivityandanswerthequestions.
Thenspendthreeminutessharingideasabouttheactivitywiththepersonsittingnexttoyou.Beprepared
toshareyouranswerswiththeclass.
Followthesestepstofinishtheplan:
1Explainhowthespeedoftheballcanbecalculatedusingtheprincipleofconservationofmomentum.
(Hint:thisisacollisionbetweentheballandthebox.)
2Explainhowthespeedoftheballcanbecalculatedusingonlytheequipmentavailable.Itmighthelpto
sketchaspeed-timegraphforthebox,asthiswillsuggestwhatmeasurementsyoucantake.
3Writedownthesteps(method)requiredtocompletetheinvestigation,includingcalculationsneededand
anysafetyprecautions.
Optional
4Compareyourexplanationandmethodwithanothergroupandimproveyourown.
5Ifyouhaveaccesstotheequipment,carryouttheexperiment.
6Usinganalternativetechnique(suchasvideoanalysis)checkwhetheryoumeasuredthecorrectspeed
andaccountforanydifference.
PEERASSESSMENT
Didtheothergroupcorrectlyandclearly:
describeandexplainwhatmeasurementstotake?
describeandexplainwhatcalculationstodo?
Wouldyouhavebeenabletofollowtheirmethodtoproducereliableresults?
Didthegroupincludesensiblesafetyprecautions?
Provideconstructivewrittenandverbalfeedback.Aswellaspointingoutimprovements,praisegood
aspectsoftheirwork.
Whenyougetyourworkreturnedtoyou,makeimprovementsbasedonthefeedback.
58>

72. 3Forcesandmotion
3.6Moreaboutscalars
andvectors
"inwillrecallthatscalarshavemagnitude(size)
illmdnodirection.Wecanrepresentforcesusing
itowsbecauseaforcehasadirectionaswellasa
inigmtude.Thismeansthatforceisavectorquantity
*
(hapter2).Table3.4listssomescalarandvector
juunities.Everyvectorquantityhasadirection.
11iivovcr,itisnotalwaysnecessarytostatethedirection
ilIInsisobvious-forexample,wemightsay,‘The
ith!oftheblockisION,’withoutsayingthatthis
iiiactsdownwards.
1quantities Vectorquantities
llutonce velocity
limn force
|>rod weight
Innis acceleration
unurgy momentum
bmperature electricfieldstrength
gravitationalfield
strength
Iill'3.4:Somescalarandvectorquantities.
Addingforces
Ahuthappensifanobjectisactedonbytwoormore
iiniFigure3.20ashowssomeonepushingacar.
Iiictionopposestheirpushingforce.Becausetheforces
<ractinginastraightline,itissimpletocalculate
ilaresultantforce,providedwetakeintoaccountthe
Inetionsoftheforces:
hultantforce=500N-350N
=150Ntotheright
’hilithatwemustgivethedirectionoftheresultant
Innc,aswellasitsmagnitude.Thecarwillaccelerate
I-
.irdstheright.
Irlire3.20bshowsamoredifficultsituation.Afirework
i'hketisactedonbytwoforces.
I'hethrustofitsburningfuelpushesittowards
theright.
1tsweightactsverticallydownwards.
Figure3.20a:Addingtwoforcesinastraightline.
b:Addingtwoforcesatrightangles.
WorkedExample3.6showshowtofindtheresultant
forcebydrawingavectortriangle(agraphical
representationofvectors)orusingPythagoras’theorem.
KEYWORDS
vectortriangle:agraphicalrepresentationof
vectorsintwodimensionssothattheresultant
vectorcanbecalculated
Rulesforvectoraddition
Youcanaddtwoormoreforcesusingthefollowing
method.Simplykeepaddingarrowsend-to-end.
Drawarrowsend-to-end.sothattheendofoneis
thestartofthenext.
Chooseascalethatgivesalargetriangle.
Jointhestartofthefirstarrowtotheendofthelast
arrowtofindtheresultant.
Othervectorquantities(forexample,twovelocities)
canbeaddedinthisway.Imaginethatyousetoutto
swimacrossafast-flowingriver.Youswimtowards
theoppositebank,buttheriver’svelocitycarriesyou
downstream.Yourresultantvelocitywillbeatanangle
tothebank.
Airlinepilotsmustunderstandvectoraddition.Aircraft
flyathighspeed,buttheairtheyaremovingthrough
isalsomovingfast.Iftheyaretoflyinastraightline
towardstheirdestination,thepilotmusttakeaccountof
thewindvelocity(bothitsspeedanddirection).
Onceyouhavemastereddrawingavectortriangle,you
couldusePythagoras’theoremtofindthelengthofthe
resultantvectorandtrigonometrytofindtheangle.
59

73. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
WORKEDEXAMPLE3.6
Findtheresultantforceactingontherocketshownin
Figure3.20b.Whateffectwilltheresultantforcehave
ontherocket?
Method1:Drawascalediagram
Step1:LookatFigure3.20b.Thetwoforcesare4.0N
horizontallyand3.0Nvertically.
Younowneedtodrawascalediagram
(avectortriangle)torepresenttheseforces.
Useascaleof1.0cmtorepresent1.0N.
Step2:Drawahorizontalarrow,4.0cmlong,to
representthe4.0Nforce.Markitwithan
arrowtoshowitsdirection.
Step3:Usingtheendofthisarrowasthestartofthe
nextarrow,drawaverticalarrow,3.0cmlong,
torepresentthe3.0Nforce.
Step4:Completethetrianglebydrawinganarrow
fromthestartofthefirstarrowtotheendof
thesecondarrow.Thisarrowrepresentsthe
resultantforce.
Step5:Measurethisarrow,andusethescaleto
determinethesizeoftheforceitrepresents.
lengthofline=5.0cm
resultantforce=5.0N
Step6:Useaprotractortomeasuretheangleofthe
force.(Youcouldalsocalculatethisangleusing
trigonometry.)
angleofforce=37°belowhorizontal
Figure3.21showswhatyourvectortriangleshould
looklike.
*
Figure3.21:Vectortriangle.
Method2:UsingPythagoras’Theorem
Step1:LookatFigure3.20b.Thetwoforcesareat
rightanglessoPythagoras’theoremcanbe
used.Thesquareofthehypotenuse(theside
oppositetherightangle)isequaltothesum
ofthesquaresoftheothertwosides.
Step2:FindtheresultantforceusingPythagoras’
theorem.
c2=b2+a2
soc=/u2+b2
soc=A2+32=^16+9=/25=5N
Step3:Findtheanglebelowthehorizontalusing
trigonometry.
adjacent4N
6=tan'1=37°
Answer
Theresultantforceactingontherocketis5.0N
actingat37°belowthehorizontal.Therocketwillbe
givenanaccelerationinthisdirection.
Noticethatbothmethodsgivethesameanswer.
Question
17YouarerowingacrosstheLembehStraitat1.5m/s.
aCalculateyourvelocityifyourowagainsta
currentof0.2m/s?
bUseavectortriangleorcalculateyourresultant
velocityifyoutravelat4m/sinaspeedboat
duesouthandacurrentpushesyoutowards
theeastat3m/s.Givebothyourspeedand
direction.
60>

74. 3Forcesandmotion
HQJECT
HyrilopOnewasdescribedatthebeginningof
Bhipter.IfHyperloopOneissuccessful,trains
*
Inivolathighspeedalongtubes.Itwilluse
Mai-ticrepulsionbetweentwounlikemagnetic
I’"1'>lifttrainsoffthefloorofthetubesto
Blinmatesolidfriction.Byremovingairfromthe
llllu,ietrainswillpushagainstlessairresistance.
Wlweoftenwanttoreducefrictionthereare
«i"ituationswherefrictionishelpfuloreven
ti.il.Lateryouwilldiscoverotherexamples
ifph.icswheresomethingcanbebothhelpful
1h।irdous(forexample,ionisingradiation
I"
1
h.ipter23).
In"nnethatthereisacharactercalledFrictionwho
LIonchargedwiththecrimeofimpedingthe
Bimdhrunningofthenaturalworld.Evidenceis
i»11«<todforandagainsthimbeforehiscasegoes
iIoln.ll.
evidencefortheprosecution
(gainstfiction)
N*mhatleastoneexamplethatyouhavenot
'
ikiymetinthischapterwhereitishelpfulto
I-Isolidfrictionordrag.
evidenceforthedefence(ofFriction)
Minhatleastoneexampleofsolidfrictionand
In'ampleofairresistancethatisnecessary
»I
”
l|ful.Forexample,weneedsolidfriction

’
<1cartyresandtheroad.Withoutthisfriction
iwouldnotbeabletochangespeed(orstart
itlip)orchangedirectioninordertofollowthe
liiindturnsintheroad.However,thisfriction
|i’li'esheatandthisleadstowastedenergy.
Thejudgement
Writethetranscriptfromthetrial(whatwassaidand
whosaidit).Thismightincludestatementsbythe
prosecutor,defencelawyer,expertwitnesses,and
thetrialjudge.
Optionaltask:promotinganefficientmass
transportsystem
Byreducingfriction,HyperloopOnepromises
moreefficientuseofenergy.However,itrequires
thebuildingoftubesforthetrainstoruninside,
whichrequiresbuildingmaterialsandenergy
forconstruction.
UsetheInternettofindoutaboutefficientmass
transportsystems(suchastramorsubwaysystems
forcities)thathavealowcarbonfootprint.Itcould
beanexistingsystemoronethatisproposedfor
thefuture(suchasHyperloopOne).Tolearnabout
themaglevtechnologyinHyperloopOne,you
couldreadChapter16todiscoverwhymagnets
attractandrepel.
Onceyouhavefoundthemostpromisingsystem,
imaginethatyouareanenvironmentaljournalist
withaphysicsbackground.Writeanarticle
(maximum500words)insupportofit,whichurges
readerstopressurethegovernmenttoadopt
thetransportsystemyouareproposing.Aswell
aswritingsomethingthatgrabstheattentionof
readers,youneedto:
justifywhyyousupportthesystemyouhave
chosen,basedonphysics
explaintherelevantphysicssothatitcanbe
understoodbythepublic
includerelevantimages.
61
>

75. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
SUMMARY
Theunitofforceisthenewton(N).
Forcesappearwhentwoobjectsinteractwitheachother.
Aforcecanberepresentedbyanarrowtoshowitsdirection,whilelengthisproportionaltothesizeof
theforce.
Solidfriction,airresistance,anddragactintheoppositedirectiontotheobject’smotionandcan
produceheating.
Theresultantforceisthesingleforcethathasthesameeffectastwoormoreforces.
Aresultantforcecanchangethespeedand/ordirectionofanobject.
Theforceofgravitypullsobjectsdownwardsandisnormallycalledtheweightoftheobject.
I
terationcausedbythepulloftheEarth’sgravityiscalledtheaccelerationoffreefallorthe
ationduetogravity.Itisgiventhesymbolganditsvalueis9.8m/s2closetothesurfaceoftheEarth.
issofanobject,measuredinkilograms,tellsyouhowmuchmatterthatobjectiscomposedof.
ightofanobject,measuredinnewtons,isthegravitationalforcethatactsonthatobject.
alvelocityisthenameforthemaximumconstantspeedreachedwhentheresultantforceactingonan
becomeszero.Itisoftenappliedtoparachutistswhentheupwardsforceofairresistancebecomesequal
positetoweight.
bjectmovesinacircle,aforcemustbeactingtowardsthecentreofthepath,perpendicular(atright
tothespeedoftheobject.
itioninacircularpath,abiggerforceisrequiredifthebodyismoremassive,movingfasterormovingin
:rcircle.
-massxacceleration,F=ma.
itumisthequantitymassxvelocity,p=mv.
incipleoftheconservationofmomentummeansthatthetotalmomentumafteraninteractionbetween
(forexample,acollision)isthesameasitwasbeforetheinteraction.
pulse(ofaforce)canbedefinedasthechangeinanobject’smomentum(mv-mu)ortheforceactingon
:ctmultipliedbythetimeforwhichtheforceacts(Ft),soimpulse=Fht.
:anbedefinedastherateofchangeofmomentum,F-—
.
°
;ultantoftwovectorsthatdonotactalongthesamelinecanbefoundbydrawingavectortriangleor
ulation.
—
62

76. 3Forcesandmotion
X
acceleration=0.7m/s2
70N
mass=35kg
320N
[1]
1000N 700N
800kg 700kg
[1]
[1]
3.1N
2.6N
1.4N
1.8N 1.8N
2.6N 2.6N
'MSTYLEQUESTIONS
C1.25m/s2totheleft
D0.38m/s2totheleft
A\=70N;Y=119.0N
H\=70N;T=94.5N
CX=320N;y=94.5N
D2f=320N;Y=119.0N
Aitincreased
11itstayedthesame
Citreducedbutnottozero
Ditreducedtozero
Ihrdiagramshowstheforcesduringatugofwarcompetition.TeamAhas
totalmassof800kgandpullswithaforceof1000Ntotheleft.TeamB
hi,itotalmassof700kgandpullswithaforceof700Ntotheright.
A.trongropejoinsthem.
WhatistheaccelerationofteamA?
A0.20m/s2totheleft
B 1.25m/s2totheright
Ihesefourdiagramsshowtheforcesonraindropsthatarefallingtowards
ih<ground.
Whichraindropisslowingdown?
Iherearefourforcesactingontheobject.Whatarethevaluesoftheforces
LibelledTandT?
\nobjectwithamassof35kgacceleratesat0.7m/s2totheright,asshownin
tindiagram.
in’014,AlanEustacesettheworldrecordforthehighestfreefallparachute
Iiimp(fromaheightofmorethan41km).AshefelltowardstheEarth,he
।.uhedaterminalvelocityofalmost370m/s.Theairwasverythinwherehe
I.iitedhisjumpandbecamethickerthecloserhegottotheground.What
li।ppenedtohisterminalvelocity,beforeheopenedhisparachute? [1]
teamA— teamB

77. 1
yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
^O^TINUED
5Anaircraftofmass4000kgproducesathrustof10kN.Theaircraftneeds
totravelat35m/stotakeoff.
aStatetheequationthatlinksforce,massandacceleration. [1]
bCalculatetheaccelerationoftheaircraft. [1]
cTheaircraftstartsfromrest.Calculatethetimeittakestobecome
airborne. [2]
dSketchaspeed-timegraphfortheaircraftfromthemomentitstarts
untilittakesoff.Assumethatitsaccelerationisconstant. [2]
eCalculatetheminimumlengthoftherunwayrequiredforthisaircraft
totakeoff. [11
fWhenitlandsattheendoftheflight,ittravelsontheground(taxis)at
aconstantspeed.Explainhowthisaircraftcanaccelerateevenwhenit
istravellingataconstantspeed. [1]
[Total:8]
6Scientiststestthesafetyfeaturesofacarbycrashingitintoalargeblock
ofconcrete.
Acrashtestdummysitsinthedriver'sseat.Avideocamerarecordsthe
crash.Inonetest,thecaristravellingat13m/sandthedummyhasamass
of83kg.
aStatetheequationthatlinksmomentum,massandvelocity. [1]
bCalculatethemomentumofthedummy. [1]
cInanothertest,themomentumofthedummychangesby1250kgm/s
inatimeof0.17s.Calculatetheaverageforceactingonthedummy
duringthistime. [2]
dThesetestshelptomakeourroadssafer.Useideasaboutmomentum
toexplainhowseatbeltsandthecrumplezonesofacarhelptoreduce
injuriesduringacrash. [3]
[Total:7]
COMMAND WORDS
state:expressin
clearterms
calculate:workout
fromgivenfacts,
figuresorinformation
sketch:makea
simplefreehand
drawingshowingthe
keyfeatures,taking
careoverproportions
explain:setout
purposeso.r
reasons/make
therelationships
betweenthings
evident/provide
whyand/orhowand
supportwithrelevant
evidence
64

78. 3Forcesandmotion
'.ELF-EVALUATIONCHECKLIST
Afterstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutosee
ungapsinyourknowledgeandhelpyoutolearnmoreeffectively.
1can
See
Topic...
Needs
morework
Almost
there
Confident
tomoveon
Recallthetwowaysthataforcecanchangethemotion
ofabody.
3.1
Recallthesignificanceofboththelengthanddirection
ofarrowsusedtorepresentaforce.
3.1
Recallthatfrictionactsbetweentwosolidsurfaces;
frictionactsintheoppositedirectiontomotionandcan
producethermalenergy.
3.1
Recallthatairresistanceanddragarelikefriction. 3.1
Calculatetheresultantforcewhentwoormoreforces
actalongthesameline.
3.1
Definemassandweightandrecallthedifferences
betweenthem,includingtheunitsused.
3.2
Recallthatweightisthenamegiventotheforceof
gravityonabodyandthatitalwaysactsdownwards.
3.2
RecallandusetheequationW=mgandrecallthat
g=9.8m/s2.
3.2
Defineterminalvelocity. 3.3
Recallthedirectionoftheforcethatactsonabodyto
makeitmoveinacircle.
3.3
Recallhowthesizeoftheforcethatactsonabodyto
makeitmoveinacircledependsonthemassandspeed
ofthebodyandtheradiusofthecircle.
3.3
Recallandusetheequationsforforceandmomentum. 3.4,3.5
Defineimpulseandperformcalculationsbyrecalling
andusingtheassociatedequations.
3.4
Applytheprincipleoftheconservationofmomentum. 3.5
Definewhataforceisandrecallandusetheassociated
equation.
3.4
Calculate,ordrawavectortriangletoworkout,the
resultantforcewhentheforcesdonotactalongthe
sameline.
3.6

79. INTHISCHAPTERYOUWILL:
applytheprincipleofmomentswhenthereismorethanonemomentoneachsideofapivot.
describeandcalculatetheturningforce
investigateandapplytheprincipleofmoments
describetheconditionsneededforanobjecttobeinequilibrium
describehowthecentreofgravityofanobjectaffectsitsstability
>Chapter4
Turningeffects

80. 4Turningeffectsofforces
l|"lid60secondsthinkingonyourown,have30secondsofdiscussionwithapartnerandthenbeprepared
||limyouranswerstothefollowingquestionswiththeclass.Youmightfindithelpfultodrawsketches.
1
।ilninhowthestreetperformer
n|iInndinFigure4.1aappears
ithit.ite(floatinspaceabove
pound)andtheperformer
tinwninFigure4.1bappearsto
HIvlb'forceofgravity.
I
I
iimhowlifeboats(seeFigure
II)
'anbeself-righting(turn
IniKjhtwayupaftercapsizing).
Figure4.1a:Astreetperformerappearstolevitate,b:Astreetperformer
appearstodefytheforceofgravity,c:Anengravingofaself-rightinglifeboat
fromthe19thcentury.
Iliiladfactthatwarcreatesopportunities
Itnadvancesintechnology.Chinainventedthe
'•■I>imhetinthe5thcenturyanditwasaneffective
|l»ii'weaponuntiltheinventionofgunpowder.
Anauchetisacatapultthathasaswingingarmto
i
'
projectile(athrownobject).
Ilu>tnbuchetismadefromalongbeamthatpivots
ininaxle.Theattackingsoldiersattachasling

inhliningtheprojectiletotheendofthelonger
Mlion.Theyattachacounterweighttotheshorter
IIofirethetrebuchet,thesoldiersallowthe
।mnterweighttofallwhichappliesaturningforce
iimomentonthebeam.Becausetheprojectile
I*luriherfromtheaxle(orpivot),theprojectile
Huiv<muchfasterthanthecounterweightand
tl"dingattheendextendstheeffectivelength
ilthebeam,makingtheprojectilemoveeven
Iilor.Thetrebuchetusesenergytransfersaswell
i.lurningforces.Thelargesttrebuchetshada
Imetrebeam,a9000kgcounterweightandcould
Inuia140kgstoneblocktoarangeofalmost
I00metres.LoupdeGuerre(orWolfofWar)was
'hibiggesttrebucheteverbuilt,byEdwardI,who
ikingofEnglandinthelate13thcentury.He
iTriedthesurrenderofScottishdefenderssothat
Ii"ioulduseitagainstStirlingCastle.
I'l’■cussionquestions
1Tryexplaininghowatrebuchetworks.
Describehowtheperformanceofthe
trebuchetwouldchangeiftheprojectileand
Figure4.2:Trebuchetsarenotapartofmodernwarfare.
Here,anenthusiastuseshistrebuchettothrowapumpkin
duringarecentNorthAmericanPumpkinLaunch.
counterweightswappedpositions.Hint:Think
abouttherelativespeedsofthetwoendsof
thebeam.Drawasketchifithelps.
2Atrebuchetusesenergytransfersaswellas
turningforces.Canyouthinkofotherideas
inphysicsthatneedmorethanonetopicto
explainit?
3Whatdoyouthinkwouldhavedonemore
damagetothewallsofStirlingCastle-a
projectilethatistwiceasmassiveortwiceas
fast?(Ifyouhavealreadymettheequationfor
kineticenergy,seeifyoucanworkitout.)
beam
counter-
HUGSTARTED
ITTHEWOLFOFWAR

81. r
yCAMBRIDGE 1GCSE™PHYSICS:COURSEBOOK
4.1Themomentofaforce
Figure4.3showsaboywhoistryingtoopenaheavy
doorbypushingonit.Hemustmaketheturningeffect
ofhisforceasbigaspossible.Howshouldhepush?
Figure4.3:Openingadoor-howcantheboyhaveabig
turningeffect?
Firstofall,lookforthepivot
-thefixedpointabout
whichthedoorwillturn.Thisisthehingeofthedoor.
Toopenthedoor,thepersonmustpushwithasbiga
forceaspossible,andasfaraspossiblefromthepivot
-attheotheredgeofthedoor.(Thatiswhythedoor
handleisfittedthere.)Tohaveabigturningeffect,
thepersonmustpushhardatrightanglestothedoor.
Pushingatadifferentanglegivesasmallerturningeffect.
Thequantitythattellsustheturningeffectofaforce
aboutapivotisitsmoment.
Themomentofaforceisbiggeriftheforceisbigger.
Themomentofaforceisbiggerifitactsfurther
fromthepivot.
Themomentofaforceisgreatestifitactsat90°to
theobjectitactson.
KEYWORDS
turningeffect:whenaforcecausesanobjectto
rotateorwouldmaketheobjectrotateifthere
werenoresistiveforces
pivot:thefixedpointaboutwhichaleverturns;
alsoknownasthefulcrum
moment:theturningeffectofaforceabouta
pivot,givenbyforcexperpendiculardistance
fromthepivot
Makinguseofturningeffects
Figure4.4showshowunderstandingmomentscan
beuseful.
Whenusingacrowbartoliftaheavyrock,pullnearthe
endofthebar,andat90°,tohavethebiggestpossible
turningeffect.
Whenliftingaloadinawheelbarrow,thelonghandles
helptoincreasethemomentoftheliftingforce.
Figure4.4:Understandingmomentscanhelpinsome
difficulttasks.
Balancingabeam
Figure4.5showsasmallchildsittingontheleft-handend
ofasee-saw.Herweightcausesthesee-sawtotipdownon
theleft.Herfatherpressesdownontheotherend.Ifhe
canpresswithaforcegreaterthanherweight,thesee-saw
willtiptotherightandshewillcomeupintheair.
Now,supposethefatherpressesdownclosertothepivot.
Hewillhavetopresswithagreaterforceiftheturning
effectofhisforceistoovercometheturningeffectofhis
daughter’sweight.Ifhepressesathalfthedistancefrom
thepivot,hewillneedtopresswithtwicetheforceto
balanceherweight.
68>

82. 4Turningeffectsofforces
whh)htofgirl father'spush
।pii>4.5:Twoforcesarecausingthissee-sawtotip.
iiir1'-weightcausesittotiptotheleft,whileherfather
1'.aforcetotipittotheright.Hecanincreasethe
in;ffectofhisforcebyincreasingtheforce,orby
।Illi")downatagreaterdistancefromthepivot.
\m
'.awisanexampleofabeam,along,rigidobject
'i'llpivotedatapoint.Thegirl’sweightismakingthe
Imiliponeway.Thefather’spushismakingittipthe
><iliiWay.Ifthebeamistobebalanced,themomentsof
lli.Iwoforcesmustcanceleachotherout.
Equilibrium
Inlenceandinothersubjects,youwilloftenhear
♦«imlIhingsthatareinequilibrium.Thisalwaysmeans
1bitiwoormorethingsarebalanced.Whenabeamis
Imeed,wesaythatitisinequilibrium.Whenanobject
1'inequilibrium:
Iheforcesonitmustbebalanced(noresultantforce)
Iheturningeffectsoftheforcesonitmustalsobe
balanced(noresultantturningeffect).
"
Innaresultantforceactsonanobject,itwillstartto
inm<’offinthedirectionoftheresultantforce.Ifthereis
I
'■
'
illantturningeffect,itwillstarttorotate.
IYWORD
Iequilibrium:whennonetforceandnonet
momentactonabody
Questions
I(hoosethecorrectoptioninthefollowing
statements.
aThemomentofaforceisbiggeriftheforceis
{smaller/bigger}.
1
'
Themomentofaforceisbiggerifitacts
{closer/further}fromthepivot.
cThemomentofaforceisgreatestifitactsat
{0°/45°/90°}totheobjectitactson.
2Threedifferentforcesarepullingonaheavy
trapdoor(Figure4.6).Whichforcewillhavethe
biggestturningeffect?Explainyouranswer.
hingetrapdoor
Figure4.6
3Explainwhysomebodywoulduseaspannerwitha
longerhandleiftheyneededtoundoatightbolt.
4aExplainwhyatreeismorelikelytobeblown
overinastrongerwind.
bExplainwhyatallertreeismorelikelytobe
blownoverthanashortertree.
4.2Calculatingmoments
Wehaveseenthat,thegreateraforceandthefurtheritacts
fromthepivot,thegreaterisitsmoment.Wecanwritean
equationforcalculatingthemomentofaforcelikethis:
KEYEQUATION
momentofaforce=forcexperpendiculardistance
fromthepivot
Nowletusconsidertheunitofmoment.Sincemomentis
aforce(N)multipliedbyadistance(m),itsunitissimply
thenewtonmetre(Nm).Thereisnospecialnameforthis
unitintheSIsystem.Takecare:ifdistancesaregivenin
cm,theunitofmomentwillbeNcm.Takecarenottomix
thesedifferentunits(NmandNcm)inasinglecalculation.
Figure4.7showsanexample.The40Nforceis
2.0metresfromthepivot,so:
momentofforce=40Nx2.0m-80Nm
m 2.0m J
Ta I
beampivot |4QN
Figure4.7:Calculatingthemomentofaforce.
69>

83. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Balancingmoments
LookbackatFigure4.5,whereafatherandhisdaughter
areplayingonasee-saw.Onherown,shewouldmake
thesee-sawturnanticlockwise;herweighthasan
anticlockwisemoment.Tomakethesee-sawbalance,her
fatherneedstopushdownontheright-handendofthe
see-saw,applyingaclockwisemoment.
Theideathatanobjectisbalancedwhenclockwise
andanticlockwisemomentsareequalisknownasthe
principleofmoments.Wecanusethisprincipletofind
thevalueofanunknownforceordistance,asshownin
WorkedExample4.1.
WORKEDEXAMPLE4.1
ThedaughterinFigure4.5hasaweightof500Nand
issitting2.0metrestotheleftofthepivot.Herfather
hasaweightof800N.Howfartotherightofthe
pivotshouldhesitsothatthesee-sawisbalanced?
Step1:Writedownwhatyouknowandwhatyou
wanttofindout.
anti-clockwiseforce=500N
distance-2.0m
clockwiseforce=800N;distance=?
Step2:Sincethesee-sawhastobeinequilibrium,
wecanwrite:
totalanticlockwise=totalclockwise
moment moment
Step3:SubstituteinthevaluesfromStep1,andsolve.
500Nx2.0m=800Nxperpendicular
distance
1000Nm=800Nxperpendiculardistance
... _
l000Nm_1oc
perpendiculardistance=
gpg
~1-25m
Answer
Thefatherneedstosit1.25mtotherightofthepivot
sothatthesee-sawisbalanced.
Questions
5Writedown,inwords,theequationforfindingthe
momentofaforceaboutapoint,statingcarefully
theunitsforeachquantity.
6AboltistightenedbyApplyingaturningforceof
30Ntotheendofthespanner(Figure4.8).
aWhichofthethreedistancemeasurements
shouldyouuse?
bUsethisdistancetocalculatethemoment.
7Auniformmetrerulerisbalancedatitsmidpoint
(Figure4.9).
Figure4.9
CalculatetheunknownforceF.
8Aboyofmass60kg,andhissisterofmass49kg,
playonasee-sawpivotedatitscentre.Iftheboysits
2.7metresfromthepivotofthesee-saw,calculate
thedistancefromtheothersideofthepivotthegirl
mustsittomakethesee-sawbalanced.
KEYWORDS
anticlockwise:turningintheoppositedirection
fromthehandsonaclock
clockwise:turninginthesamedirectionasthe
handsonaclock
principleofmoments:whenanobjectisin
equilibrium,thesumofanticlockwisemoments
aboutanypointequalsthesumofclockwise
momentsaboutthesamepoint
70>

84. 4Turningeffectsofforces
fIVITY4.1
f'■vlngfingersalongametreruler
>dwithapartner.Takeitinturnstobalance
imetreruler(orapairofroundpencils)across
,"iiindexfingerswithyourhandswideapartbut
Hl।liIferentdistancesfromtheendsoftheruler.
Iiliwlybringyourhandstogether.Discusswhat
."iobservewithyourpartner.Aspartofyour
liiitissions,youbothneedtodescribeandexplain
Lilyouobserve.
IMwworkindependentlytowriteadescription
iiiiI.»nexplanation.Yourdescriptionshould
H
pludeadiagramofwhatwashappening.You
|anLibelthediagramaspartofyourexplanation,
whichneedstoincludethefollowingwords:
liniments,centreofgravity,andfriction.
/lienyouhavebothfinishedwriting,swapyour
w4anddecidewhohaswrittenthebetter
n
।
'Linationandwhyitisbetter.Forexample,isit
।Ie.norormorescientificallyaccurate?Updateyour
wnworkwithanyimprovements.
1lowtrythesamethingwitha100gmasstaped
totheruler(oruseapoolcueif
illlble).Whatdoyouobservethistime?Can
ymioxplainwhatishappening?
hineedtodescribeandexplainasyoudidbefore.
Morebalancingmoments
tInihicechildreninFigure4.10havebalancedtheir
mwitisinequilibrium.Theweightofthechildon
FUhli'ltistendingtoturnthesee-sawanticlockwise.So
Hnweightofthechildonthelefthasananticlockwise
I Theweightsofthetwochildrenontheright
Ilinvidockwisemoments.FromthedatainFigure4.10,
-
imicalculatethesemoments:
b|llll<lockwisemoment=500Nx2.0m=1000Nm
ilockwisemoments=(300Nx2.0m)+(400Nx1.0m)
=600Nm+400Nm
=1000Nm
IInhiticketsareincludedasaremindertoperformthe
mil(plicationsbeforetheaddition.
"
'<miseethat,inthissituation:
totalclockwisemoment=totalanticlockwisemoment
|ntinsee-sawinFigure4.10isbalanced.
2.0m 1.0m 1.0m
Figure4.10:Abalancedsee-saw.Onherown,thechild
ontheleftwouldmakethesee-sawturnanticlockwise;her
weighthasananticlockwisemoment.Theweightofeach
childontherighthasaclockwisemoment.Sincethesee¬
sawisbalanced,thesumoftheclockwisemomentsmust
equaltheanticlockwisemoment.
WORKEDEXAMPLE4.2
ThebeamshowninFigure4.11is2.0metreslong
andhasaweightof20N.Itispivotedasshown.A
forceof10Nactsdownwardsatoneend.Whatforce,
Fmustbeapplieddownwardsattheotherendto
balancethebeam?
Figure4.11:Balancingabeam.
Step1:Identifytheclockwiseandanticlockwise
forces.Twoforcesactclockwise:20Nata
distanceof0.5m,and10Nat1.5m.One
forceactsanticlockwise:theforceFat0.5m.
Step2:Sincethebeamisinequilibrium,wecan
writetotalclockwisemoment=total
anticlockwisemoment
Step3:SubstituteinthevaluesfromStep1,andsolve.
(20Nx0.5m)+(10Nx1.5m)=Fx0.5m
lONm+15Nm=Fx0.5m
25Nm=Fx0.5m
F=
25Nm
0.5m
=50N
71

85. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
Answer
Aforceof50Nmustbeapplieddownwardsatthe
otherendtobalancethebeam.
Youmighthavebeenabletoworkthisoutinyour
head,bylookingatthediagram.The20Nweight
requires20Ntobalanceit,andtheIONat1.5m
needs30Nat0.5mtobalanceit.Sothetotalforce
neededis50N.
Inequilibrium
InFigure4.10,threeforcesareshownactingdownwards.
Thereisalsotheweightofthesee-sawitself,200N,
toconsider,whichalsoactsdownwards,throughits
midpoint.Iftheseweretheonlyforcesacting,they
wouldmakethesee-sawacceleratedownwards.Another
forceactstopreventthisfromhappening.Thereisan
upwardcontactforcewherethesee-sawsitsonthepivot.
Figure4.12showsallfiveforces.
Figure4.12:Aforcediagramforthesee-sawshownin
Figure4.10.Theupwardcontactforceofthepivotonthe
see-sawbalancesthedownwardforcesofthechildren's
weightsandtheweightofthesee-sawitself.Thecontact
forcehasnomomentaboutthepivotbecauseitacts
throughthepivot.Theweightofthesee-sawisanother
forcethatactsthroughthepivot,soitalsohasnomoment
aboutthepivot.
Becausethesee-sawisinequilibrium,wecancalculate
thiscontactforce.Itmustbalancethefourdownward
forces,soitsvalueis(500+200+400+300)N=1400N,
upwards.Thisforcehasnoturningeffectbecauseitacts
throughthepivot.Itsdistancefromthepivotiszero,so
itsmomentiszero.
Nowwehavesatisfiedthetwoconditionsthatmustbe
metifanobjectistobeinequilibrium:
theremustbenoresultantforceactingonit
totalclockwisemoment=totalanticlockwise
moment.
Youcanusethesetworulestosolveproblemsconcerning
theforcesactingonobjectsinequilibrium.
Sometimesweknowthattheforcesandmomentsacting
onanobjectarebalanced.Thenwecansaythatitisin
equilibrium.Sometimesweknowthereverse,namely,that
anobjectisinequilibrium.Thenwecansaythatthereis
noresultantforceonit,andnoresultantmoment.
Questions
9Auniformmetrerulerisbalancedatitscentre
(Figure4.13).
76N
0.44m
Figure4.13:Balanceduniformmetreruler.
aCalculatethedistancetotherightofthepivot
thatthe125Nloadneedstobeplacedforthe
rulertobebalanced.
bCalculatethecontactforceactingatthepivot.
10Abeamisbalancedonapivot0.33metresfromits
left-handside(Figure4.14).
Thebeambalanceswhenaweightof0.47Nis
suspended0.13metresfromthesameend.
Figure4.14:Balancedbeam.
aCalculatetheanticlockwisemomentofthe
0.47Nforce.
bWhatisthemomentduetotheweightoftherod?
cTheweightoftherodis0.79N.Calculatethe
positionofitscentreofgravitytotherightof
thepivot.

86. 4Turningeffectsofforces
IA>vitionofbalance

।
11usneedtodesignstructuresthatdonotfall
IAwwnwhentheforcesonthemchangelocationor
IhmiHltude(size).Forexample,abuildingneeds
Bittyt.indingwhenwindblowsagainstitwith
Idlllt".'(itstrengthsorfromdifferentdirections.
HKldunishouldnotbendorcollapsewhenvehicles
11""llm.Youwilltestwhetherthereisaresultant
Winnuntonanobjectthatisinequilibrium.You
WilliIthemomentofaforceandtheprincipleof
it',inthisexperiment.
willneed:
imetreruler
two10Nspringbalances
"1of100gmasses
twoclamp-standswithclampsand
Ih>sses
threecottonloops
minA
it"4.15:Lorryonabridge.
UtotyTakecarenottodropthemasseson
BuriMt.
1
"
iilnqstarted

A/litismeantbyequilibrium?

Mi.itisthemomentofaforce?

Wl
।itistheprincipleofmoments?

Whorewouldyouexpectthecentreofgravity
tI
"forametreruler?

IIgure4.15showsalorryabouttobedriven
"
rossabridgesupportedbycolumnsAandB.
I।tribetheforceatcolumnAasthelorrytravels
I
"
tweenAandBandthenbeyondcolumnB.
Figure4.16:Apparatusforinvestigation.
2
3
4
5
6
moveable
loadof9.8N
loopA
atthe5cm
mark
Onthesameaxes,plotlinegraphsofFA,FB
andFa+Fbontheverticalaxisagainstthe
distancefromAonthehorizontalaxis.
Notedownthereadingsonbothofthespring
balances,FAandFB,andcalculate(FA+FB).
Recordallthevaluesinacopyofthetablebelow:
loopB
atthe95cm
mark
Hangthe9.8Nfromtherulersothatdistance
A=10cmanddistanceB=80cm.
Movetheloadabout10cmalongtheruler
towardsB.Makesuretherulerishorizontaland
thenrecordallthemeasurementsrequiredto
completethenextrowofthetable.
Repeatthisprocessuntiltheloadisata
distanceof0.8mfromA.
Method
1SetuptheapparatusasshowninFigure4.16
withtheloopsforthespringbalancesabout
5cmfromeachendoftheruler.Youaregoing
tomovetheloadtodifferentpositionsalong
theruler.Makesurethattherulerishorizontal
beforeyoutakeeachmeasurementbymoving
theclampsholdingoneofthespringbalances
upordownitsclamp-stand.
spring
balanceB,.Q
supportedfromE
clampstand J
distanceB n
spring
balanceA
Ssupportedfrom
LIclampstand
HdistanceA
Distance
fromA/m
fa/nfb/nfa+fb/n
0.1
l.'IMENTALSKILLS4.1

87. r
>
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
Questions
1Whydidyouhavetomakesurethattheruler
washorizontalbeforeeachmeasurementwas
made?
2Calculatethemeanofyour(FA+FB)values.
Thisshouldbemorethantheloadof9.81N
hangingfromtheruler.Canyouexplainwhy?
3Measurethemassoftherulerandworkoutits
weight.Doesthisaccountforthedifferencein
thelastquestion?
4Insteadofdoingtheexperiment,itcanbe
solvedmathematicallybytakingmomentsabout
eitherspringbalance.Forexample,theforceat
springbalanceA,FA,canbesolvedbytaking
momentsaboutB(andtakingintoaccountof
therulertogetamoreaccurateresult).
ACTIVITY4.2
Understandingtheshadoof
TheAncientEgyptiansusedtheshadooftolift
waterandirrigatetheland.Itisstillinusetoday
(seeFigure4.17).
Figure4.17:ASudanesemanirrigateshislandusinga
shadoof.
Theshadoofhasacounterweightattheshortend
andabucketatthelongendofabeam.Ittakesas
muchefforttomovethebucketdownasitdoesto
pullitup.
Unlessyourteachergivesyouatime,spend
oneminutethinkingonyourown,oneminuteof
discussionwithyourneighbourandthenbeprepared
toshareyouranswerstoquestion1withtheclass.
1aWhatistheadvantageofmakingittakeas
muchefforttopushthebucketdownasit
doestopullitup?
bExplainhowtheshadoofdoesthis.
2Drawadiagramoftheshadoofandincludethe
pivot,thecounterweightandtheforcesacting
onit.Youcouldestimatethelengthsofthe
beamandassumethatthecounterweighthasa
weightof150N.
Ifyouaregiventimeandtheequipment,makea
workingmodel.
REFLECTION
DidyouunderstandthephysicsinActivities4.1
and4.2?Agoodtestofyourunderstandingis
whetheryoucanexplainitclearly.Areyouableto
supportyourexplanationswithclearandcorrect
forcediagrams?Ifyouarestillunsure,aska
classmatetoexplainit.
4.3Stabilityandcentre
ofgravity
Peoplearetallandthin,likeapencilstandingonend.
Unlikeapencil,wedonottoppleoverwhentouchedby
theslightestpush.Weareabletoremainupright,and
towalk,becausewemakecontinualadjustmentstothe
positionsofourlimbsandbody.Weneedconsiderable
brainpowertocontrolourmusclesforthis.Theadvantage
isthat,withoureyesaboutametrehigherthanifwewere
onall-fours,wecanseemuchmoreoftheworld.
74

88. 4Turningeffectsofforces
«»«.ii.lltislessuchastightropewalkersandhigh-wire
MliU'lln'ure4.18)havedevelopedtheskillofremaining
upii><hlIoahighdegree.Theyuseitemssuchaspolesor
|mi..ibitohelpthemmaintaintheirbalance.Theideaof
Hwiiihnt<anhelpustounderstandwhysomeobjectsare
(MW*hiIeothersaremorelikelytotoppleover.
9l«W»418:Thishigh-wireartisteisusingalongpoleto
M*'1anhorstabilityonthewire.Ifshesensesthather
|l<r
1Iillghtlytoofartotheleft,shecanredressthe
Ml*
"
bymovingthepoletotheright.Frequent,small
iitsallowhertowalksmoothlyalongthewire.
’
Im11hisscanbeknockedovereasily-itisunstable.
111"hI19showswhathappensiftheglassistilted.
fiiiui"4.19:Atallglassiseasilytoppled.Oncethelineof
0’itsweightisbeyondtheedgeofthebasetheglass
Mmn|hlover.
IInine4.19a,theglassisupright.Itsweightacts
4Nii"irdsandthecontactforceofthetableacts
mi'“udsThetwoforcesareinline,andtheglassisin
miiiilihnum.
||iIlimit4.19b,theglassistiltedslightlytotheright,

n11heforcesarenolongerinline.Thereisapivotatthe
i"nlwherethebaseoftheglassisincontactwiththe
table.Thelineoftheglass’sweightistotheleftofthis
pivot,soithasananticlockwisemoment,whichtendsto
tiptheglassbacktoitsuprightposition.
InFigure4.19c,theglassistippedfurther.Itsweightacts
totherightofthepivot,andhasaclockwisemoment,
whichmakestheglasstiprightover.
Centreofgravity
InFigure4.19,theweightoftheglassisrepresentedby
anarrowstartingatapointinsidetheliquid.Whyisthis?
Thereasonisthattheglassbehavesasifallofitsmass
wereconcentratedatthispoint,knownasthecentreof
gravity.Theforceofgravityactsonthemassoftheglass
-eachbitoftheglassispulledbytheEarth’sgravity.
However,ratherthandrawinglotsofweightarrows,one
foreachbitoftheglass,itissimplertodrawasingle
arrowactingthroughthecentreofgravity.(Becausewe
canthinkoftheweightoftheglassactingatthispoint,
itissometimesknownasthecentreofgravity.)
KEYWORDS
stableanobjectthatisunlikelytotoppleover,
oftenbecauseithasalowcentreofgravityanda
widebase
unstable:anobjectthatislikelytotoppleover,
oftenbecauseithasahighcentreofgravityanda
narrowbase
centreofgravity:allthemassofanobjectcould
belocatedhereandtheobjectwouldbehavethe
same(whenignoringanyspin)
Figure4.20showsthepositionofthecentreofgravity
forseveralobjects.Apersonisfairlysymmetrical,so
theircentreofgravitymustliesomewhereontheaxis
ofsymmetry.(Thisisbecausehalfoftheirmassison
onesideoftheaxis,andhalfontheother.)Thecentre
ofgravityisinthemiddleofthebody,roughlylevel
withthenavel.Aballismuchmoresymmetrical,andits
centreofgravityisatitscentre.
Foranobjecttobestable,itshouldhavealowcentreof
gravityandawidebase.ThepyramidinFigure4.20isan
exampleofthis.Thehigh-wireartisteshowninFigure
4.18hastoadjustherpositionsothathercentreof
gravityremainsaboveherbase,whichisthepointwhere
herfeetmakecontactwiththewire.
75>

89. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Figure4.20:Theweightofanobjectactsthroughitscentreofgravity.Symmetrycanhelptojudgewherethecentreof
gravitylies.Anobject'sweightcanbeconsideredtoactthroughthispoint.Notethat,forthetable,itscentreofgravityisin
theairbelowthetabletop.
Findingthecentreofgravity
Balancingisthecluetofindinganobject’scentreof
gravity.Ametrerulerbalancesatitsmidpoint,sothatis
whereitscentreofgravitymustlie.
Theprocedureforfindingthecentreofgravityofamore
irregularlyshapedobjectisshowninFigure4.21.Inthis
case,theobjectisapieceofcard,describedasaplane
lamina.Thecardissuspendedfromapin.Ifitisfreeto
move,ithangswithitscentreofgravitybelowthepoint
ofsuspension.Thisisbecauseitsweightpullsitround
untiltheweightandthecontactforceatthepinarelined
up.Thenthereisnomomentaboutthepin.Aplumb-line
isusedtomarkaverticallinebelowthepin.Thecentre
ofgravitymustlieonthisline.
Theprocessisrepeatedfortwomorepinholes.Now
therearethreelinesonthecard,andthecentreof
gravitymustlieonallofthem,thatis,atthepointwhere
theyintersect.Twolinesmighthavebeenenough,but
itisadvisabletouseatleastthreepointstorevealany
inaccuracies.
Figure4.21:Findingthecentreofgravityofanirregularly
shapedpieceofcard.Thecardhangsfreelyfromthepin.
Thecentreofgravitymustlieonthelineindicatedbythe
plumb-linehangingfromthepin.Threelinesareenoughto
findthecentreofgravity.
KEYWORD
lamina:flattwo-dimensionalshape
EXPERIMENTAL SKILLS4.2
Centreofgravityofaplanelamina
Youwillinvestigateatechniqueforfindingthe
centreofgravityofapieceofrectangularcard.
Onceyougainconfidencethatthetechniqueworks,
youwillapplyittoanirregularshape,perhapsthe
mapofyourcountry.Locatingthecentreofgravity
ofabodyisimportantwhenconsideringitsstability.
Youwillneed:
atleasttwopiecesofcard
aholepunch
apencil
apairofscissors
aruler
aclampstandandboss
apin(orshortthinmetalrod)
alengthofstringwithaweight(plumb
bob)attached.
Safety:Thepinandthescissorsshouldbe
handledwithcaretoavoidcuttingsomeone.Wear
eyeprotectionifusingapin(asopposedtoathin
metalrod).Ensurethatthepinisnotateyeheight
andispointingawayfromtheedgeofthebench.
76

90. 4Turningeffectsofforces
PEERASSESSMENT
SwapyourworkwithapartnerforExperimental
skills4.2.
Givethemasmileyfaceforthefollowing:
threeneatlydrawnandcloselyintersecting
linesontheirrectangularlamina(asthis
showscarefulexperimentaltechnique)
threecloselyintersectinglinesontheir
irregularshape
correctanswertotheGettingStarted
questions
aclearandcorrectexplanationofwhythe
centreofgravityisverticallybelowthepivot.
Finally,discussanythingthatyoucanlearnfrom
eachother.
Questions
11Onacopyoftheshapesbelow,markthecentreof
gravityforeachwithanX.Wherepossible,showlines
thathelpedyoulocatethecentreofgravity.
Figure4.22:Laminarshapes.
12Busesandothervehicleshavetobetilt-testedtoan
angleofatleast28degreesfromtheverticalbefore
theycancarrypassengers.
busA busB
Figure4.23:Bothofthesedouble-deckerbuseswould
toppleoveriftiltedanyfurther.
lUHIngstarted
»I
n.dietwherethecentreofgravityislocated
ontherectangularlamina.

I।dietwherethecentreofgravityislocated
intheshapeyoupropose.

(
.inyousuggestshapeswherethecentreof
pivitywouldnotbeonthecard?
Method:Part1
IH1.1th<centreofgravityofarectangularsheetof
i<iIIhisisyourlamina.
IIf.,theholepunchtomakethreeholesfar
>|>nrtaroundtheedgeofthelamina.
/Ilthepinhorizontallyintheclamp.
1Usingonehole,hangthelaminafromthepin.
M<ikesurethatitcanswingfreely.
4ILingthestringfromthepinsothattheweight
m.lkesithangvertically.Marktwopointsonthe
liminaalongthelengthofthestring.
K<ipeatsteps3and4usingtheothertwoholes.
AIuythelaminaonthebenchand,usinga
ruler,drawlinesjoiningeachpairofpoints.
Wherethelinescrossisthecentreofgravity
4thelamina.Itshouldbewherethelinesof
symmetrycoincidebut,ifthethreelinescross
<actlyatapoint,youhavedonewell!
Method:Part2
i
pt.itpart1afteryouhavecutashapeoutofthe
Lunin,i.Seeifyoucangetamapofyourcountry
I
HIntodonthecardsothatyoucanfinditscentre
tilgravity.
<nmitions
II)idthethreelinesyoudrewintersectatthe
point?
.'Ifthethreelinesdidnotintersectatthesame
point,howdidyoudecidewherethecentreof
pavityislocated?
tIfthethreelinesdidnotintersectatthesame
point,howmuchconfidencedoyouhavein
thelocationyouhavechosen?
'4'.uggestawayofcheckingthatyouhave
located(found)thecentreofgravity?
'.Ixplainwhythecentreofgravityofthelamina
Ilesonaverticallinebelowthepin(pivot).
rr>

91. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
aUsetheideasofstabilityandcentreofgravity
toexplainwhyeitherbusinFigure4.23would
toppleoveriftiltedanyfurther.Youcandraw
oncopiesofthediagramstohelpwithyour
explanation.
bExplainhowthestabilityofthebuswould
beaffectedbyhavingmorepassengersonthe
upperdeck.
cExplainwhybagsofsandareonlyputonthe
topdeckofbusBandnotthelowerdeck.
13Figure4.24showstheforcesactingonacyclist.
aExplainhowyoucantellthatthecyclistshown
inFigure4.24aisinequilibrium.
bAretheforcesonthecyclistinFigure4.24b
balancednow?Howcanyoutell?
cWouldyoudescribethecyclistasstableor
unstable?Explainyouranswer.
Figure4.24:Forcesactingonacyclist.
PROJECT
TheItalianJob
TheItalianJobwasafilmmadein1969.Inthis
filmagangstealsgoldworth$4millioninTurin,
northernItaly.Asthegangescapesthrough
Switzerlandonabus,thedriverlosescontrol.The
busendsupwithitsrearhalfhangingoverthe
edgeofaverticaldrop.Anyattempttoreachthe
goldatthebackofthecoachriskssendingthe
bus,themenandthegoldcrashingintothevalley
below.ThefilmfinisheswithCrokersaying:'Hang
onaminutelads,I'vegotagreatidea'.Thefilmis
availableonlinebutonlythelastfiveminutesare
importantfromaphysicsviewpoint.
YouneedtosuggestwhatCroker's'greatidea'
mighthavebeentosavethegoldandgetoffthe
coachsafely.Youshouldworkingroupsofthree.
Startbydescribingtheproblemusing
correctscientifictermslike'pivot',
'centreofgravity','equilibrium',
'moment'and'theprincipleofmoments'.
Makeastoryboardofyoursolutionand
includeitaspartofatwo-minutepitch
forasequeltothefilm.
Selectthebestpitch(withcorrector
correctedphysics)fromthreeorfour
groupsofthreetopresenttotherestof
theclass.

92. 4Turningeffectsofforces
IARY
momentofaforceisameasureofitsturningeffect.
forceordistancefromthepivotincreasesthemomentofaforce.
mmillofaforce=forcexperpendiculardistancefromthepivot.
In I|<itisinequilibriumwhentheforcesonitarebalanced(noresultantforce)andtheturningeffectsofthe
Uh**onitarebalanced(noresultantturningeffect).
Innliofgravityisthepointatwhichtheweightofanobjectappearstobeconcentrated.
In*lotitionofthecentreofgravityaffectsstability.
79>

93. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
80>

94. 4Turningeffectsofforces
twowaysthattheturningforcecanbeincreased. [2]
Statetwopiecesofevidencethatwouldtellyouthatabodyisnotin
equilibrium. [2]
Ihediagramshowsanangle-poiselamp.Thelightisattheendofanarm.
Ihearmcanbemovedaboutthepivot.Thecordpassesverticallydowntoa
stretchedspring.
Iheweightofthelampandarmis3.75Nandactsatadistanceof25cm
11omthepivot.
iWritedowntheequationusedtocalculatethemomentofaforce. [1]
Ialculatethemomentofthe3.75Nforceaboutthepivotwhenthe
armishorizontal. [2]
<Thearmisraisedasshowninthediagram.
iExplainwhathashappenedtothemomentofthe3.75Nforce
aboutthepivot. [1]
iiExplainwhathashappenedtotheclockwisemomentproducedby
thespring. [1]
[Total:5]
pivot
25cm
3.75N
3.75N
pivot
pivot
«—pivot
spring
state:command
termnotsupplied
calculate:workout
fromgivenfacts,
figuresorinformation
explain:setout
purposesor
reasons;make
therelationships
betweenthings
evident;provide
whyand/orhowand
supportwithrelevant
evidence
ONTINUED
COMMAND WORDS

95. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
8Thediagramsshowawindsurferpullingupthesailofasailboard.Thesail
pivotswhereitjoinswiththeboardatpointP.
aAsthesailispulledupfromAtoC,howdoestheforceneeded
Thewindsurferandthesailboardshowninthediagrambelowarein
equilibrium.
Thewindsurferweighs900N.Thewindisblowingwithaforceof300N.
Thewindsurfermaintainsequilibrium.
cCalculatehowfartotherightofthepivotthewindsurferhastomove
hiscentreofgravity. [2]
dWhatwouldthewindsurferneedtodoiftheforceofthe
winddecreased?Explainyouranswer. [2]
[Total:7]
82>

96. 4Turningeffectsofforces
IF-EVALUATIONCHECKLIST
itudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutosee
ipsinyourknowledgeandhelpyoutolearnmoreeffectively.
1can
See
Topic...
Needs
morework
Almost
there
Confident
tomoveon
(iiveeverydayexamplesofaturningforce. 4.1
1hiderstandthatincreasingforceordistancefromthe
pivotincreasesthemomentofaforce.
4.1
<.ilculatethemomentusingtheproductforcex
pitpendiculardistancefromthepivot.
4.2
Applytheprincipleofmomentstobalancingabeam. 4.2
Applytheprincipleofmomentstodifferent
filiations,includingwhenthereismorethanone
momentoneithersideofthepivot.
4.2
1''1formanexperimenttofindthecentreofgravity
o|apieceofcard.
4.3

1
''
scribehowthelocationofthecentregravity
ilkictsstability.
4.3

97. matter
*
INTHISCHAPTERYOUWILL:
>Chapter5
Forcesand
>
p
recallandusetheexpressionk=—,wherekisthespringconstantandFisforceperunitextension
recognisethesignificanceofthe'limitofproportionality'foranload-extensiongraph
\recallandusetheequation^p=pgkh.
recognisethataforcemaychangethesizeandshapeofabody
plotandinterpretload-extensiongraphsanddescribetheassociatedexperimentalprocedure
p
relatepressuretoforceandareaandrecalltheassociatedequationp=—
71
relatethepressurebeneathaliquidsurfacetodepthandtodensity

98. 5Forcesandmatter
IFINGSTARTED
insideout,apoppercanjumpseveralmetresinto
theair.Usingyourknowledgeofphysics,explain
howthiscanhappen.
Iujure5.1a:Rubberpop-uppopperinitsnormalstate,b:Whenithasbeenturnedinsideout,justbeforespringingback

»Itnormalshape.
OWTHEMANTISSHRIMPPUNCHESABOVEITSWEIGHT
Iiijure5.2showsamantisshrimp.Thismarine
niturecanpunchitsprey(usuallycrabs)witha
Iinofmorethan700N,overathousandtimesits
'
iwnbodyweight.Ithashingedclubsatthefront
'Iitsbodycalleddactyls.Theycanreachspeeds
il’3m/sinlessthanthreemilliseconds,morethan
Zhi)timestheaccelerationduetogravity.Itachieves
ilil'bystoringelasticpotentialenergyinapartofits
xoukeletonthatisshapedlikeasaddle.Thispiece
'1oxoskeletonbehaveslikeacompressedspringor
flurubberpoppershowninFigure5.1.Turningthe
।"
i|>perinsideoutchangesitsshape.Theshrimp
IM",itsmusclestochangetheshapeofpartof
Hoxoskeletonbytransferringchemicalpotential
norgyintoelasticpotentialenergy.Humansachieve
lh«samethingwithacrossbow.Onceenough
i">rgyhasbeenstored,alatchcanreleaseit.
I<l4cussionquestions
1Whatweaponshavehumansdevelopedthatactsinthesamewayastheshrimp'sdactyl?
Canyouthinkofotherapplications?
iCoulda50gmantisshrimpinjureyou?Apunchfromamantisshrimpexertsthesameforceasthe
weightofatypicaladult.
85>
Injure5.1showsapop-uppoppertoy.Figure5.1a
howsthepopperinitsnaturalstateandFigure5.1b
howsitwhenitisturnedinsideout.Onceturned

99. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
5.1Forcesacting
onsolids
Forcescanchangethesizeandshapeofanobject.They
canstretch,squash,bendortwistit.Figure5.3shows
theforcesneededforthesedifferentwaysofdeforming
anobject.Youcouldimagineholdingacylinderoffoam
rubber,whichiseasytodeform,andchangingitsshape
ineachoftheseways.
undeformed
(tensileforces)(compressiveforces)
(bendingforces)(torsionalforces)
Figure5.3:Forcescanchangethesizeandshapeofa
solidobject.Thesediagramsshowfourdifferentwaysof
deformingasolidobject.
Foamrubberisgoodforinvestigatinghowthings
deform,because,whentheforcesareremoved,itsprings
backtoitsoriginalshape.Herearetwomoreexamples
ofmaterialsthatdeforminthisway:
Firstly,whenafootballiskicked,itiscompressedfora
shortwhile(seeFigure5.4a).Thenitspringsbacktoits
originalshapeasitpushesitselfoffthefootoftheplayer
whohaskickedit.Thesameistrueforatennisballwhen
struckbyaracket.
Secondly,bungeejumpersrelyonthespringinessofthe
rubberrope,whichbreakstheirfallwhentheyjump
fromaheight(seeFigure5.4b).Iftheropebecame
permanentlystretched,theywouldstopsuddenlyatthe
bottomoftheirfall,ratherthanbouncingupanddown
andgraduallycomingtoahalt.
Figure5.4a:ThisX-rayimageshowshowafootballis
compressedwhenitiskicked.Itreturnstoitsoriginalshape
asitleavestheplayer'sboot.(Thisisanexampleofanelastic
deformation.)Thebootisalsocompressedslightlybut,because
itisstifferthantheball,theeffectislessnoticeable,b:When
thebungeecordreachesitsmaximumextension,itwillreturntc
itsoriginallength,pullingthebungeejumperupwards.
Somematerialsarelessspringy.Theybecome
permanentlydeformedwhenforcesactonthem.
Whentwocarscollide,themetalpanelsoftheir
bodyworkarebent.
Goldandsilveraremetalsthatcanbedeformed
byhammeringthem(seeFigure5.5).Peoplehave
knownforthousandsofyearshowtoshaperings
andotherornamentsfromthesepreciousmetals.
Figure5.5:ATibetansilversmithmakingawristband.Silver
isarelativelysoftmetalatroomtemperature,soitcanbe
hammeredintoshapewithouttheneedforheating.
5.2Stretchingsprings
Toinvestigatehowobjectsdeform,itissimplesttostart
withaspring.Springsaredesignedtostretchalongway
whenasmallforceisapplied,soitiseasytomeasure
howtheirlengthchanges.
86y

100. 5Forcesandmatter
ii5.6:Investigatingthestretchingofaspring.
KEYWORD
WORD
n5.6showshowtocarryoutaninvestigationon
lungaspring.Thespringishungfromarigidclamp,
ihHitstopendisfixed.Weightsarehungontheendof
pimg-thesearereferredtoastheload.Astheloadis
ni;d,thespringstretchesanditslengthincreases.
extension:theincreasedlengthofanobject(for
example,aspring)whenaload(forexample,
weight)isattachedtoit
Table5.1:Resultsfromanexperimenttofindouthowa
springstretchesastheloadonitisincreased.
M^iiim5.7:Stretchingaspring.Atfirst,thespringdeforms
h?b.,|||y.Itwillreturntoitsoriginallengthwhentheloadis
Bn,,n.JEventually,however,theloadissogreatthatthe
ip.damaged.
Table5.1showshowtouseatablewiththreecolumnsto
recordtheresultsofanexperimenttostretchaspring.
Thethirdcolumnisusedtorecordthevalueofthe
extension,whichiscalculatedbysubtractingtheoriginal
lengthfromthevalueinthesecondcolumn.
Toseehowtheextensiondependsontheload,wedraw
aload-extensiongraph(Figure5.8).Youcanseethatthe
graphisintwoparts.
Atfirst,thegraphslopesupsteadily.Thisshowsthatthe
extensionincreasesinequalstepsastheloadincreases.
Thenthegraphcurves.Thishappenswhentheload
issogreatthatthespringhasbecomepermanently
damaged.Itwillnotreturntoitsoriginallength.
YoucanseethesamefeaturesinTable5.1.Lookatthe
thirdcolumn.Atfirst,thenumbersgoupinequalsteps.
Thelasttwostepsarebigger.
Extensionofaspring
Astheforcestretchingthespringincreases,thespringgets
longer.Itisimportanttoconsidertheincreaseinlengthof
thespring.Thisquantityisknownastheextension.
lengthofstretchedspring=originallength+extension
Thismeansthat,ifyoudoubletheloadthatisstretching
aspring,thespringwillnotbecometwiceaslong.Itis
theextensionthatisdoubled.
Itad;theforce(usuallyweight)stretchesanobject
i.ispring)
Iuuh''.7showsthepatternobservedastheloadis
Ci
।linedinregularsteps.Thelengthofthespring
iies(alsoinregularsteps).Atthisstagethespring
Iiliumtoitsoriginallengthiftheloadisremoved.
McIver,iftheloadisincreasedtoofar,thespring
imupermanentlystretchedandwillnotreturntoits
length.Ithasbeeninelasticallydeformed.
Load/N Length/cm Extension/cm
0.0 24.0 0.0
1.0 24.6 0.6
2.0 25.2 1.2
3.0 25.8 1.8
4.0 26.4 2.4
5.0 27.0 3.0
6.0 27.6 3.6
7.0 28.6 4.6
8.0 29.5 5.6

101. CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
Figure5.8:Aload-extensiongraphforaspring,basedon
thedatainTable5.1.
Questions
1Apieceofelasticcordis75cmlong.Whenitis
stretched,itslengthincreasesto97cm.Whatis
itsextension?
2Table5.2showstheresultsofanexperimentto
stretchanelasticcord.Copyandcompletethetable,
anddrawagraphtorepresentthisdata(withload
ontheverticalaxis).
Table5.2
Load/N Length/cm Extension/cm
0 75 0
2 81
4 87
6 93
8 99
10 105
12 118
14 135
16 156
5.3Thelimitof
proportionalityandthe
springconstant
Themathematicalpatternofthestretchingspringwas
firstdescribedbytheBritishscientistRobertHooke.He
realisedthat,whentheloadonthespringwasdoubled,
theextensionalsodoubled.Threetimestheloadgave
threetimestheextension,andsoon.Thisisshownintl
graphinFigure5.9.Thegraphshowshowtheextensior
dependsontheload.Atfirst,thegraphisastraightline
leadingupfromtheorigin.Thisshowsthattheextensio
isproportionaltotheload.
Atacertainpoint,thegraphcurvesandtheline
slopesuplesssteeply.Thispointiscalledthelimitof
proportionality.Ifthespringisstretchedbeyondthis
point,itwillbepermanentlydamaged.Iftheloadis
removed,thespringwillnotreturnallthewaytoits
original,undeformedlength.
KEYWORDS
limitofproportionality:uptothislimit,the
extensiononaspringisproportionaltoload
Figure5.9a:Aload-extensiongraphforaspring.Beyond
thelimitofproportionality,thegraphisnolongerastraight
line,andthespringispermanentlydeformed,b:This
graphshowswhathappenswhentheloadisremoved.The
extensiondoesnotreturntozero,showingthatthespring
nowlongerthanatthestartoftheexperiment.
88y

102. 5Forcesandmatter
PERIMENTALSKILLS5.1
InMitigatingsprings
Iitninghowdifferentmaterialsbehavewhena
ipiIsappliedtothemisveryimportantifthey
inmedtomakesomething.ThehulloftheRMS
IIIinkwascomposedofmildsteelplatesheld
।
"rtherby3millionrivets.Whentheshipstruckan
4
"
irgin1912theforcebrokesomeoftheserivets
1
'Ithiscontributedtotheshipsinking.
"Iwillinvestigatehowtheextensionofthespring
1
1mgesastheloadisincreasedandwhetherthe
h'Dsiononthespringisproportionaltotheload.
Y<'Uwillneed:
eyeprotection
Iclampstand,bossandclamp
I
spring
fhangerwithslottedmasses
G-clamp
ruler
Iplumbline(apieceofthreadwithalumpof
metalontheend).
Youneedtoweareyeprotectionbecause
Ihrnisadangerthatthespringwillflyinto
K
",i.tone'seyeifitbreaksundertension.Placea
onthefloorbeneaththemassessothat,ifthe
piIngsnaps,themasseswillnotdamagethefloor.
Avijldstandingonthematbecausethemassesmay
fondonyourfeetifthespringsnaps.
ilottingstarted
1Ixplainthepurposeof:
atheG-clamp
btheplumbline.
'Iachslottedweightis100g.Calculatewhat
loadthisrepresents.
1Identifytheindependent(input)anddependent
(output)variables.
Method
Figure5.10:Apparatusforinvestigation.
2Arrangetherulerwithzeroatthetop.This
meansthat,asthesteelspringstretches,the
readingsontherulerincrease.
3Usetheheadingsbelowtodrawaresultstable.
Remembertoaddmorerows.
Loadon
hanger/
N
Ruler
reading/
cm
Spring
extension/
cm
Doesthespring
returntooriginal
lengthwhen
unloaded?Y/N
4Attachyourhangertothebottomofthespring
sothatthespringhangsvertically.
5Recordtheloadaszeroandrecordthereading
ontherulerwhereitlinesupwiththebottomof
thehanger.
89

103. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
6Addaslotted100gmass(equaltoaloadof
1.0N)andrecordthenewrulerreadingwhereit
linesupwiththebottomofthehanger.
7Removethemassandrecordwhetherthesteel
springreturnstoitsoriginallength.
8Repeatsteps6and7,addinganother100g
masseachtimeuntilyouhavefilledthetableor
thespringbreaks.
9Remember,theextensionisthedifference
betweenthelengthofthespringwiththeload
attachedandtheoriginallengthwhenjustthe
hangerwasattached.Tocalculatethespring
extension,subtractyourrulerreadingfora
loadof0Nfromallofyourrulerreadings.This
meansthatthespringextensionshouldbezero
(0cm)whentheloadiszero(0N).
10Plotagraphofloadagainstextension(thatis,
extensiononthehorizontalaxis).Includeatitle,
axislabelsandalineofbestfit.
Questions
1Didyourgraphpassthroughtheorigin?Ifnot,
didyouremembertocorrectfortheoriginal
lengthofthespring?
2Howcanyouidentifywhereonthegraphthe
forceonthesteelspringisproportionaltothe
load?Whatisthenameofthepointwherethis
nolongerhappenstothespringandcanyou
locateitonyourgraph?
3Whatarethevaluesfortheloadandextension
correspondingtothelimitofproportionalityfor
yourspring?
4Didthespringcontinuereturningtoitsoriginal
lengthbeyondthelimitofproportionality?
frontcover
Figure5.11:Atextbookbentoutofshape.
ACTIVITY5.1
book
spine
Youwillseethebookspineontheleft-handsideisat
rightanglestoboththefrontandbackcovers.The
pagesslidepasteachothersothatthelengthofall
thepagesandthebookcovershavenotchanged.
Asolidblockofpaper(suchasastackofpaperinits
wrapper)isdifficulttobendbut,ifyoucouldbendit,
thetopsurfacewouldstretchandthebottomsurface
wouldcompressandtheright-handedgewould
beatright-anglestothetopandbottomsurfaceas
shownbythedottedredlinetotherightofthebook.
Thelittletriangleofpapertotherightofthedotted
linewouldnotbepresent.Inthisexample,itwould
backcover
Thiswouldbethe
differenceinlength
betweenthefront
andbackcoversif
thebookwasasolid
blockofpaper.
Elasticglass
Glassisbrittle.Itshattersonceitreachesitslimitof
proportionality.
So,whycanglassfibres(usedinopticfibresand
loftinsulation)bendsoeasilywithoutshattering?In
thisactivityyouwillexplainwhyyoucanbendglass
fibresintoacirclebutaglassblockwillshatterifyou
trytobendit.
First,thinkabouthowyouwouldapproachthis
problemwithouttheguidancethatfollows;make
somenotesofyourideas.
Whenyoubendanobject(suchasapencileraser),
onesurfacestretchesandtheoppositesurface
compresses(getsshorter).Takeyourtextbookand
measurethewidthofthebottomedge.Itshould
beabout21cm.Nowbenditintotheshapeofan
archwiththespineontheleft-handsideasshownin
Figure5.11.
90y

104. 5Forcesandmatter
3
4
5
Otherthingstothinkabout
1
2
i
»NTINUED
KEYWORDS
WORKEDEXAMPLE5.1
Thebreakingstrainofordinaryglassismuchlower
thanthis,probablybecauseithasmicroscopic(tiny)
imperfectionswherestresstendstoconcentrate.
loadisplottedagainstextension(i.e.withextensionon
thehorizontalaxis).
।'lit।equation,Fistheload(force)stretchingthe
Mg,kisthespringconstantofthespring,andxisthe
l«inionofthespring.Thespringconstantisdefined
iihrforceperunitofextension,whichisobviouswhen
lawisexpressedintermsofk:
springconstant:theconstantofproportionality,
themeasureofthestiffnessofaspring
Whatistherelationshipbetweenthelengthof
thedouble-headedarrowandthethicknessof
thebook?
Whatistherelationshipbetweenthelengthof
thedouble-headedarrowandhowmuchthe
bookisbent?
lithcliaviourofthespringisrepresentedbythegraph
}1Igurc5.10aandcanbedescribedas:
Inintensionofaspringisproportionaltotheload
irli'dtoit,providedthelimitofproportionalityis
vneeded.
Wouldglassasthickasyourbookshatterifbent
throughtheangleshowninFigure5.11?
Calculatetheextension(orcompression)foran
individualpage.Youlearnedhowtoworkout
thethicknessofasinglepageinChapter1.How
doesextensionvarywiththethicknessofthe
objectthatisbeingbentoutofshape?
Wouldglassasthickasapageinyourbookshatter
ifbentthroughtheangleshowninFigure5.11?
I’"springconstantisameasureofthe‘stiffness’ofthe
Bumr
(hestifferthespring,thebiggertheloadrequired
BlIniDgcitslengthandthesteeperthegradientwhen
liiiinnlsoknownasHooke'slaw.Wecanwritethe
MWiourofaspringasanequation:
!A।
Aspringhasaspringconstantk=20N/cm.What
loadisneededtoproduceanextensionof2.5cm?
Step1:Writedownwhatyouknowandwhatyou
wanttofindout.
springconstantk=20N/cm,
extension=2.5cm
loadF=?
Step2:Writedowntheequationlinkingthese
quantities,substitutevaluesandcalculate
theresult.
F=kx
F=20N/cmx2.5cm=50N
Answer
Aloadof50Nwillstretchthespringby2.5cm.
blitinanapproximatedifferenceofabout2cmin
Iwiqthbetweenthetopandbottomsurfaces.Half
11hisdifferencewouldbebecausethetopsurface
iii'ichedandtheotherhalfwouldbebecausethe
j^ltomsurfacecompressed.
IEithertakemeasurementsfromFigure5.11
ortrytheexperimentyourself(inpairs).One
personcanholdandbendthebookwhilethe
Otherpersonusesasetsquaretofindwherethe
dashedredlineshouldbe(noneedtomarkthe
bookthough).Usearulertomeasurethelength
indicatedbythereddouble-headedarrow.
Theextension(orcompression)ishalfthelength
ofthedouble-headedarrow.
11|ineerscalculatestrain,whichistheextension
liidedbytheoriginallength.Forthebookinthe
I"lurethiswillberoughly1cmdividedby21cm
|(105or5%).Engineershavealsoworkedoutthat
ll«rentmaterialshavedifferentbreakingstrains.
Ail.issfibrehasabreakingstrainofabout2%.

105. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Howrubberbehaves
Arubberbandcanbestretchedinasimilarwayto
aspring.Aswithaspring,thebiggertheload,the
biggertheextension.However,iftheweightsareadded
withgreatcare,andthenremovedonebyonewithout
releasingthetensionintherubber,thefollowingcan
beobserved.
Thegraphobtainedisnotastraightline.Rather,it
hasaslightlyS-shapedcurve.Thisshowsthatthe
extensionisnotexactlyproportionaltotheload.
Eventually,increasingtheloadnolongerproduces
anyextension.Therubberfeelsverystiff.When
theloadisremoved,thegraphdoesnotcomeback
exactlytozero.
Hookeandsprings
WhywasRobertHookesointerestedinsprings?Hooke
wasascientist,buthewasalsoagreatinventor.Hewas
interestedinspringsfortworeasons.
Springsareusefulformakingweighingmachines,
andHookewantedtomakeaweighingmachine
thatwasbothverysensitive(toweighverylight
objects)andveryaccurate(tomeasureveryprecise
quantities).
Healsorealisedthataspiralspringcouldbeusedto
controlaclockorevenawristwatch.
Figure5.12showsasetofdiagramsdrawnbyHooke,
includingalongspringandaspiralspring,complete
withpansforcarryingweights.Youcanalsoseesomeof
hisgraphs.
Figure5.12:RobertHooke'sdiagramsofsprings.
Forscientists,itisimportanttopublishresultssothat
otherscientistscanmakeuseofthem.Hookewasvery
secretiveaboutsomeofhisfindings,becausehedidnot
wantotherpeopletousethemintheirowninventions.
Forthisreason,hepublishedsomeofhisfindingsin
code.Forexample,insteadofwritinghislawofsprings
asgivenabove,hewrote:ceiiinosssttuv.Later,whenhe
feltthatitwassafetopublishhisideas,herevealedthat
thiswasananagramofasentenceinLatin.Decoded,
itsaid:Uttensio,sicvis.InEnglish,thisis:Asthe
extensionincreases,sodoestheforce.’Inotherwords,
theextensionisproportionaltotheforceproducingit.
YoucanseeHooke’sstraight-linegraphinFigure5.12.
Questions
3Aspringrequiresaloadof7.5Ntoincreaseits
lengthby5.0cm.Whatloadwillgiveitanextension
of12cm?
4Aspringhasanunstretchedlengthof13.0cm.Its
springconstant,kis7.0N/cm.Whatloadisneeded
tostretchthespringtoalengthof18.0cm?
5Theresultsofanexperimenttostretchaspringare
showninTable5.3.Usetheresultstoplota
loadextensiongraph.Onyourgraph,markthe
limitofproportionalityandstatethevalueofthe
loadatthatpoint.
Load/N Length/m
0.0 1.396
2.6 1.422
5.3 1.448
7.9 1.475
10.6 1.501
13.2 1.536
15.9 1.579
Table5.3
REFLECTION
ThinkbacktoActivity5.1.Thiswasanexampleof
athoughtexperiment.GreatscientistslikeAlbert
Einsteinhaveusedthisapproachtomakehuge
progressinscience.
92

106. 5Forcesandmatter
ONTINUED
I
'
Iyouneedtheguidance?Ifso,wasithelpful?
Whenbendinganobject,youcompressitonthe
IIIIethatgetsshorterandstretch(orextend)iton
flu'otherside.
1
ouldyouimaginewhatwashappening?When
ytujrollonesheetofpaperintoacylinder,the
'i'decircumferenceandoutsidecircumference
Hi'nearlythesamelength:therehasbeenvery
Illistretching(extension)orcompression.This
I*thesameastheglassfibre.Canyouseethat,
1.makingthepaperverymuchthicker,the
iillorenceinlengthbetweentheinnerandouter
।i»iumferenceswouldgetbiggerandsowouldthe
!<msionandcompression?Glassisquitebrittle
"'.iretchingittoomuchwillmakeitshatter.
1
inyouthinkofwaysofdevelopingthisthinking

ill.indpractisingitinthenexttwoactivitiesin
flu(hapterandbeyond?
5.4Pressure
N>midiveintoaswimmingpool,youwillexperiencethe
I*
meofthewateronyou.Itprovidestheupthrust,
Inlipushesyoubacktothesurface.Thedeeperyougo,
Ik
।it.iterthepressureactingonyou.Submarinesand
iniiirexploringvehicles(Figure5.13)mustbedesignedto
'I'Lindverygreatpressures.Theyhavecurvedsurfaces,
tinliMremuchstrongerunderpressure,andtheyaremade
I
।Imkinetal.
hi-5,13:Thisunderwaterexploringvehicleisusedto
♦•iiiitPitstodepthsof600metres,wherethepressureis61
11it.itthesurface.Thedesignmakesuseofthefactthat
m"i>ilndcylindricalsurfacesstandupwelltopressure.The
'Iwindowismadeofacrylicplasticandis9.5cmthick.
Thispressurecomesaboutbecauseanyobjectunder
waterisbeingpresseddownonbytheweightofwater
aboveit.Thedeeperyougo,thegreatertheamount
ofwaterpressingdownonyou(seeFigure5.14a).
Inasimilarway,theatmosphereexertspressure
onus,althoughwearenotnormallyconsciousof
this.TheEarth’sgravitypullsitdownwards,sothat
theatmospherepressesdownwardsonourheads.
MountaineersclimbingtothetopofMountEverestrise
throughtwo-thirdsoftheatmosphere,sothepressureis
onlyaboutone-thirdofthepressuredownatsealevel.
Thereismuchlessairabovethem,pressingdown.
Thepressurecausedbywaterismuchgreaterthanthat
causedbyairbecausewaterismuchdenserthanair.
Figure5.14bshowshowadamisdesignedtowithstand
thepressureofthewaterbehindit.Becausethepressure
isgreatestatthegreatestdepth,thedammustbemade
thickestatitsbase.
Inafluidsuchaswaterorair,pressuredoesnotsimplyact
downwards-itactsequallyinalldirections.Thisisbecause
themoleculesofthefluidmovearoundinalldirections,
causingpressureoneverysurfacetheycollidewith.
Figure5.14a:Pressureiscausedbytheweightofwater(or
otherfluid)aboveanobject,b:Thisdamisthickestnearits
base,becausethatiswherethepressureisgreatest.
93

107. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
ACTIVITY5.2
Drinkingthroughstrawsandbreathing
throughsnorkels
1Whenyoudrinkthroughastraw,are
youpullingtheliquidupthestraworis
atmosphericpressurepushingtheliquidup
thestraw?Trytoexplainwhatisgoingon.
2Trythiswithfamilyorfriends.Drinkthrough
twostraws,withtheendofonestrawbelow
thesurfaceoftheliquidandtheendofthe
otherstrawabovetheliquidsurface.Explain
whyyouoryourfriendsfailtodrawupany
liquidthrougheitherstraw.
3Wecanmodify(bend)astrawanduseitto
breatheunderwater.Figure5.15ashows
someoneusingasnorkel.Normally,snorkelers
holdtheirbreathanddivetoexploredeeper
underwater.Butcouldsomeonejustbreathe
fromalongersnorkel?Isthereapractical
limittothelengththatasnorkelcanbeorthe
depthyoucanbreathefromone?Ifso,tryto
explainwhy.
4Anelephantcanswimunderwateranduse
itstrunkasasnorkel(Figure5.15b).Usethe
Internettoresearchhowanelephantcan
breathewhendeeperinthewaterthanwecan
andpresentyourworkonanA4orA3poster.
Figure5.15a:Asnorkeler.b:Elephantscanusetheir
trunksassnorkelstohelpthembreatheandcanswim
deeperthanhumans.
5.5Calculatingpressure
Alargeforcepressingonasmallareagivesahigh
pressure.Wecanthinkofpressureastheforceperunit
areaactingonasurface,andwecanwriteanequation
forpressure,asshown:
force F
= P-~.
Nowletusconsidertheunitofpressure.Ifforce,F,is
measuredinnewtons(N)andarea,A,isinsquaremetre;
(m2),thenpressure,p,isinnewtonspersquaremetre
(N/m2).IntheSIsystemofunits,thisisgiventhename
pascal(Pa).Itisequivalenttoonenewtonpersquare
metre(1N/m2).
KEYWORDS
pressure:theforceactingperunitareaatright
anglestoasurface
pascal:theSIunitofpressure,equivalenttoone
newtonpersquaremetre;1Pa=1N/m2=1Pa
WORKEDEXAMPLE5.2
Stilettoheelshaveaverysmallsurfacearea.(‘Stiletto’
isanItalianwordmeaningasmallandmurderous
dagger.)Suchnarrowheelscandamagefloors,and
dancehallsoftenhavenoticesrequiringshoeswith
suchheelstoberemoved.
Calculatethepressureexertedbyadancerweighing
600Nstandingonasingleheelofarea1cm2.The
surfaceofthedancefloorisbrokenbypressuresover
5millionpascals(5.0MPa).Willitbedamagedby
thedancer?
Step1:Tocalculatethepressure,weneedtoknowthe
force,andtheareaonwhichtheforceacts,inm2.
forceF=600N
areaA=1cm2=0.0001m2=10-4m2
Step2:Nowwecancalculatethepressurep.
_
600N
0.0001m2
=6000000Pa
=6.0MPa
Answer
Thepressureis6.0x106Pa,or6.0MPa.Thisismore
thantheminimumpressureneededtobreakthe
surfaceofthefloor,soitwillbedamaged.
94>

108. 5Forcesandmatter
Questions
ID
11
b
c
d
ACTIVITY5.3
REFLECTION
WORKEDEXAMPLE5.3
IYEQUATION
Theactivitiesinthischapteraskedyoutoimagine
whatishappening.Itcanbehelpfulforscientists
tovisualise(imaginesomethingintheirmind).
Canyouthinkofwaysthatyoucandevelopyour
scientificimagination?
Explainwhythishappens.Thinkaboutwhatforces
areactingonthecard.Whatisthetallestcolumn
ofwateryoucouldusebeforethe'trick'nolonger
works?
Statetheequationlinkingweight,massandg.
Usetheequationtocalculatetheweightsofthe
elephantandthewoman.
Statetheequationthatlinks(solid)pressure,
forceandarea.
Calculatethepressuresexertedbytheelephant
andwoman.(Hint:Rememberthatweightisa
forceandthatyouneedtoconverttheareaof
thestilettoheelintom2.)
Usethepressurevaluesyoucalculatedto
suggestwhythewomenmightcausemore
damagetoafloorthananelephant.
Step2:Writedowntheequationforpressure,
substitutevaluesandcalculatetheanswer.
Ap=pg^h=1000kg/m3xlON/kgx2.5m
=2.5x104Pa
Calculatethepressureonthebottomofaswimming
poolthatis2.5metresdeep.Howdoesthepressure
comparewithatmosphericpressure,105Pa
(100000Pa)?Thedensityofwater=1000kg/m3.
Step1:Writedownwhatyouknow,andwhatyou
wanttoknow.
hh=2.5m
p=1000kg/m3
g=10N/kg
^p=^
Writedownanequationthatdefinespressure.
WhatistheSIunitofpressure?
Whichexertsagreaterpressure,aforceof200N
actingon1.0m2,orthesameforceactingon2.0m2?
Whatpressureisexertedbyaforceof50000N
actingon2.5m2?
Aswimmingpoolhasalevel,horizontal,bottom.
Itsdimensionsare25.0mby5.0m.Thepressure
ofthewateronthebottomis15000Pa.Whattotal
forcedoesthewaterexertonthebottomofthe
pool?Whatistheweightofwater?
Anelephanthasamassof5000kg.Theareaofan
elephant’sfootis0.13m3.
Awomanhasamassof60kg.Theareaofher
siilettoheelis25mm2.
Atmosphericstopper
Fillacontainer,suchasabottleorbeaker,with
watertothebrim(top).
Placeapieceofstiffcardontopofthebottleor
beakersothatitmorethancoverstheopening
(inotherwords,itiswiderthantheopening).You
coulduseatabletennisballinsteadofcard,ifthe
bottlehasaverynarrowneck.
Ensurethatyoudothenextstepoverasink.While
holdingthecardfirmlytothetopofthecontainer,
turnthecontainerupsidedownandslowlyremove
yourhandfromthecard.Thewatershouldstayin
thecontainer.
Pressure,depthanddensity
Wrhaveseenthatthedeeperonedivesintowater,the
Fuller
thepressure.Pressurepisproportionaltodepth
Iweusetheletterh,forheight).Twicethedepthmeans
beethepressure.Pressurealsodependsonthedensity
i'nlthematerial(wherepistheGreekletterrho).Ifyou
Ikeintomercury,whichismorethantentimesasdense
»water,thepressurewillbemorethantentimesasgreat.
Hiiinwriteanequationforthechangeinpressureata
drplhAinafluidofdensityp:
Iaigeinpressure=densityxaccelerationdueto
gravityxdepth
Np=pg^h
hangeinpressure=densityxaccelerationdueto
gravityxdepth
Ap=pg^h

109. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
Answer
Thisisone-quarterofatmosphericpressure.Welive
atthebottomoftheatmosphere.Thereisabout
10kmofairaboveus,pressingdownwardsonus-
thatistheoriginofatmosphericpressure.
Questions
12Thedensityofwateris1000kg/m3.Calculatethe
pressureduetothewateronadiverwhenheis
25metresunderthesurface.
13Figure5.16showsatankthatisfilledwithoil.
Thedensityoftheoilis920kg/m3.
Figure5.16:Atankfilledwithoil.
aCalculatethevolumeofthetank.
bCalculatetheweightoftheoilinthetank.
cThepressureonthebottomofthetankis
causedbytheweightoftheoil.Calculatethe
pressureusing:
dNowcalculatethepressureusing:
hp=pg^h
Doyoufindthesameanswerasinpartc?
14Atmosphericpressureis100000Pa.Theroofof
abuildinghasanareaof50m2.
aWhatforcedoestheatmosphereexertdown
ontheroof?
bExplainwhytheroofdoesnotcollapse.
cWhatwouldhappentotheroofifitwere
possibletoremovealloftheairfrominside
thebuilding?Explainyouranswer.
15Calculatethedepthofthepoolinquestion10.
Assumethedensityofwater=1000kg/m3and
g=lON/kg.
PROJECT
Underpressure
Developaresourceaboutpressureforafuture
IGCSEclass.
Howtopresentyourwork
Choosethemediumyouthinkismostsuitable
foryourwork(suchasaPowerPointpresentation
orpodcast).
Asfaraspossible,produceyourowninformative
drawings,photographsorvideoclips(whichshould
lastforlessthanoneminute).
Listallthesourcesofyourinformation(websites,
books,televisiondocumentaries,andsoon)with
enoughdetailsforotherpeopletofindthesources.
Whatyouneedtoinclude
1Introduceeachofthetwoequationsyouhave
metinthischapterattheappropriatepointin
yourresourceand,foreachequation,includean
examplequestionwithitssolution.
2Describeasituationinwhichpressureis
deliberatelyincreased(byincreasingtheforceor
reducingthearea).
3Describeasituationinwhichpressureis
deliberatelyreduced(byincreasingthearea).
4Chooseoneofthefollowing:
Explainwhythecollapsingcanexperiment
worksusingthefirstpressureequationyou
metinthischapter.Unlessthishasbeen
demonstratedinclass,youmightneedto
searchforitontheInternet.
Explainwhypressureincreaseswith
increasingdepthinafluidandtherefore
whyadammustbemadethicker(orwider)
incross-sectionwithincreasingdepth.
Explainhowahydraulicbrakingsystemin
acarworks.Itisimportanttoemphasise
thatpressureinafluiddoesnotsimply
pushdownfromabove.Itpushesfromall
directions.
96>

110. 5Forcesandmatter
Iuuicanchangethesizeandshapeofanobject.
I\nobject(forexample,aspring)willstretchwhenaloadisattachedtoit.
HluIculatetheextensionofaspringforagivenload,itsoriginallengthmustbesubtractedfromitsnewlength.
I-Iktihingaspringbeyonditslimitofproportionalitypermanentlydeformsitanditwillnotreturntoits
I'ibinalsizeandshape.
I11«extensiononaspringisproportionaltotheloadappliedtothespringuntilthespringreachesthelimitof
1''itionality.
>il<id-extensiongraph,theextensiononaspringisproportionaltotheloadappliedtothespringwherethe
iiiiraight.
i> iiphstartstocurveatthelimitofproportionality;beyondthispoint,theextensiononaspringisnot
l
ilionaltotheloadappliedtothespring.
F
I
imgconstantisdefinedastheforceperunitextension,k=—
.
11.Ii;erthespringconstant,themoredifficultitistostretchthespring.Wesaythattheobjectisstiffer.
1».1.ireturnstoitsoriginalsizeandshapeunlesstheloadisincreasedafterithasstoppedstretching.
I'lV1Hi'istheforce(forexample,weight)perunitarea(perpendiculartothesurface).
I'lrhi!■inafluid(liquidorgas)iscausedbytheweightoffluidaboveit.
iiiiucthepressureisgreatestatthegreatestdepth,adamisthickestatitsbase.
i idireinafluid,itismoreusualtousetheequationchangeinpressure=densityxgravitationalfield
depth,hp=pghh.
MSTYLEQUESTIONS
Itthegraph.Whichmaterialisthestiffest? [1]
Extension/cm
97>

111. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
COMMAND WORDS
state:expressin
clearterms
calculate:workout
fromgivenfacts,
figuresorinformation
suggest:apply
knowledgeand
understanding
tosituations
wherethereare
arangeofvalid
responsesinorder
tomakeproposals/
putforward
considerations
98

112. 5Forcesandmatter
11INUED
densityofwateris1000kg/m3.
COMMAND WORD
[1]
[1]
[1]
[1]
[1]
[1]
[1]
[1]
[1]
[1]
[1]
explain:setout
purposesor
reasonsImake
therelationships
betweenthings
evidentIprovide
whyandIorhowand
supportwithrelevant
evidence
unwaterthatlandsontheroofofabuildingcanbecollected(harvested)
Ineclingtherainfromtheguttersintowaterbutts(sometimescalledrain
11elsorrainwatertanks).
Statetheequationlinkingdensity,massandvolume.
। <'alculatethemassofwaterinwaterbuttA.
diagramshowstwocontainersthatstorerainwater.Thecontainershave
Apipejoinsthetaps.Thetapsareclosed.Bothcontainershaveabaseof
it).O7m2.
Statetheequationlinking(hydrostatic)pressuredifference,height,
densityandg.
Isingthisequation,calculatethepressureatthebaseofwater
buttAandcompareittotheansweryoufoundineii.
Ihetapsbetweenthewaterbuttsarenowopened.
<>nacopyofthediagram,showthefinaldepthof
waterinwaterbuttAandwaterbuttB.
I\plainintermsofpressureandforceswhythewaterstarts
toHowandthenstops.
isplainwhythepressureatthebottomofeachwaterbuttisdifferent.
ilculatethevolumeofthewaterinwaterbuttA.
Statetheequationlinkingweight,massandg.
a(alculatetheweightofwaterinwaterbuttA.
Statetheequationthatlinks(solid)pressure,forceandarea.
a Isingthisequation,calculatethepressureatthebaseofwater
buttA.
[3]
[Total:14]

113. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
7Carbrakingsystemsmultiplyforces.Inthediagrambelow,therearethree
pistons:oneeffortpistonandtwoloadpistons.
aTheeffortpistonhasanareaof2.0cm2.Thedriverexertsaforceof
50Nontheeffortpiston.Whatisthepressureofthebrakefluid? [2]
bThispressureisthesameeverywhereinthebrakefluid,includingatthe
loadpistons.Theloadpistonshaveatotalareaof40cm2.
Calculatetheforceatthebrakedisc. [2]
[Total:4]
SELF-EVALUATION CHECKLIST
Afterstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutosee
anygapsinyourknowledgeandhelpyoutolearnmoreeffectively.
Ican
See
Topic...
Needs
morework
Almost
there
Confident
tomoveon
Recallthatforcescanchangethesizeandshapeofan
object.
5.1
Plotandinterpretload-extensiongraphsanddescribe
theassociatedexperimentalprocedure.
5.2
Recallhowtocalculatetheextensionofaspringfora
givenload.
5.2
Recallwhathappenstoaspringifitisstretchedbeyond
itslimitofproportionality.
5.2
Recalltheequationfordeterminingthespringconstant.5.3
Identifythelimitofproportionalityonaload-extension
graphandwhereonthegraphtheextensionisproportional
totheload.
5.3
Understandthesignificanceofthespringconstantand
recognisehowanobjectwithadifferentspringconstant
wouldappearonaload-extensiongraph.
5.3
100>

114. 5Forcesandmatter
r
()NTINUED
Iran
See
Topic...
Needs
morework
Almost
there
Confident
tomoveon
I*'calltheequationthatrelatespressure,forceandarea. 5.5
1'allwhatcausesthepressuretoincreasewith
tinleasingdepthintowater.
5.5
1'illandusetheequationforpressureinafluid. 5.5
101>

115. >Chapter6
Energystores
andtransfers
INTHISCHAPTERYOUWILL:
calculatepotentialenergyandkineticenergy.
identifychangesindifferentenergystores
recognisedifferentenergytransfersandinterpretenergyflowdiagrams
understandthemeaningofenergyefficiency
applytheprincipleofconservationofenergy

116. 6Energystoresandtransfers
Discussionquestions
2
ITTINGSTARTED
OMETS-FRIENDORFOE?
Cometscancrossourpath(andcollidewithEarth)
becausetheirorbitsarehighlyelliptical(shapedlike
squashedcircles).ThespeedofEarthinitsorbit
aroundtheSunisnearlyconstantbecauseitsorbit
isnearlycircular.Thespeedofacometchanges.
AsitmovesclosertotheSun,moreofthecomet's
gravitationalpotentialenergyistransferredinto
kineticenergy,soitspeedsup.Asitgetsfurther
awayfromtheSun,someofitskineticenergy
transfersintogravitationalpotentialenergy,andit
slowsdown.
Withaclassmate,drawanenergymindmapto
includeeverythingyouknowaboutthistopic,
includingtheprincipleofconservationofenergy.
IH|ure6.1a:Flashlightswitchedon.b:Wounduptoy.c:Movingradio-controlledcar.d:Bunsenburner,e:Loudspeaker
hiU".f:Ringingbicyclebell,g:Solar-poweredbattery,h:Hairdryer.

hjim-6.2:TheengravingthatshowsHalley'sComet
iiI()06.TheMonkofMalmesburyseesitasawarning
Iil
"
NormanConquest,whentheNormansinvaded
Iinil.ind.
Cioplehavedifferentopinionsaboutcomets,both
iIandbad.Somepeoplethinkcometsare'dirty
uiwballs'thatbroughtwatertoourplanetwhich
illowsustolive.However,alongwithasteroids,
inn0tsalsothreatenlifeonEarth.Forexample,
II"impactofanasteroidorcometwipedoutthe
lim"iaurs66millionyearsago.Itisnotsurprising
Iiilcometsusedtobeseenaswarningsofdisaster
(Imure6.2).However,thankstophysics,comets
annowlessmysteriousandwecanpredicttheir
1
1illAstronomersworkingforSpaceguardsearch
II"’’(kies,lookingforandtrackingobjects(including
ilimits)thatmightcollidewiththeEarth.
Workwithaclassmatetodescribetheenergy
ti.msfersthataretakingplaceineachdiagram.
Whatdoyoualreadyknowaboutenergy?
WhenwastheEarth'slastmajorcollisionwith
anasteroidorcometandwhathappened?
DescribeasystemonEarthwhereenergy
transfersfromgravitationalpotentialenergyto
kineticenergyandbackagain.
Describetheenergytransferswhenyouthrow
aballintotheairandrelatethistotheenergy
transfersforacometorbitingtheSun.

117. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
6.1Energystores
Energycanbedividedintoenergystoresandenergy
transfers.Energy,andenergytransfers,areinvolvedinall
sortsofactivities.Wewilllookattwoexamplesandsee
howwecandescribethemintermsofenergy.Weneedto
havetheideaofstoresofenergy.
KEYWORD
energy:quantitythatmustbechangedor
transferredtomakesomethinghappen
Example1:running
Atthestartofarace,youarestationary,waitingfor
thestarter’spistol.Energyisstoredinyourtoned-up
muscles,readytobereleased.Asyousetoff,theenergy
fromyourmusclesgetsyoumoving.Ifyouarerunninga
marathon,youwillneedtomakeuseoftheenergyinthe
longer-termstoresofthefattytissuesofyourbody.
TheenergytransfersinvolvedareshowninFigure6.3.
Yourmusclesstorechemicalenergy.Theenergyisstored
bychemicalsinyourmuscles,readytobereleasedat
amoment’snotice.Yourmusclesstartyoumoving,
andyouthenhavekineticenergy.Runningmakesyou
hot.Thistellsusthatsomeoftheenergyreleasedin
yourmusclesiswastedasthermalenergy,ratherthan
becomingusefulkineticenergy.Fitnesstraininghelps
peopletoreducethiswaste.
Figure6.3a:Atthestartofarace,therunner'smusclesarestores
ofchemicalenergy,b:Astherunnerstartstomove,chemical
energyistransferredtokineticenergyandthermalenergy.
Example2:switchingonalight
Itisevening,andthedaylightisfading.Youswitchon
thelight.Yourelectricitymeterstartstoturnalittle
faster,recordingthefactthatyouaredrawingmore
energyfromthedistantpowerstation.
TheenergychangesinvolvedareshowninFigure6.4.
Electricityisusefulbecauseitbringsenergy,availablea
theflickofaswitch.Wecanthinkoftheenergybeing
transferredelectrically.Inthelightbulb,thisenergy
istransferredbylight.Everylightbulbalsoproduces
thermalenergy.
Figure6.4:Switchingonthelightrequiresasupplyof
electricity.Inthelightbulb,electricalenergyistransferred
lightandheating.
Namingenergy
Example1andExample2highlightsomeofthevariot
energystoresandtransfers.Wewillnowtakeabrieflo
atexamplesofthese.
Amovingobjecthaskineticenergy.Thefasteranobject
moves,thegreateritskineticenergy.Weknowthisbecau
weneedtotransferenergytoanobjecttogetitmoving.
Ifyouliftanobjectupwards,yougiveitgravitational
potentialenergy(g.p.e.).Thehigheranobjectisabove
theground,thegreateritsg.p.e.Ifyoulettheobjectfal
youcangettheenergybackagain.Thisisexploitedin
manysituations.Thewaterstoredbehindahydroelectr
damhasg.p.e.Asthewaterfalls,itcanbeusedtodrive
aturbinetogenerateelectricity.Agrandfatherclockha
weightsthatmustbepulledupwardsonceaweek.The:
astheygraduallyfall,theydrivethependulumtoopen
theclock’smechanism.
KEYWORDS
kineticenergy:theenergystoreofamovingobject
gravitationalpotentialenergy(g.p.e):the
energystoreofanobjectraisedupagainstthe
forceofgravity;moregenerally,itisthedistance
betweenparticlesorbodies
104

118. 6Energystoresandtransfers

Iujchascoalorpetrol(gasoline)arestoresof
ilenergy.Weknowthatafuelisastoreof
(ui\because,whenthefuelburns,thestoredenergyis
Mil*icd,usuallyasheatandlight.Therearemanyother
,ofchemicalenergy(seeFigure6.5).AsFigure6.3
ho".energyisstoredbychemicalsinourbodies.
Nullrilesarealsostoresofenergy.Whenabatteryispart
"(
.iiompletecircuit,thechemicalsstarttoreactwith
Humotherandanelectriccurrentflows.Thecurrent
.Hi.energytotheothercomponentsinthecircuit.

jure6.5:Somestoresofchemicalenergy-breadand
!<■mutbutter,petrol,batteries.Ourbodieshavelong-term
itnr.ofenergyintheformoffattytissues.
inillectriccurrentisagoodwayoftransferringenergy
111mioneplacetoanother.Whenthecurrentflowsthrough
iomponentsuchasaheater,itgivesupsomeofitsenergy.
'
iiniumisanexampleofanuclearfuel,whichisa
liiiiofnuclearenergy.Allradioactivematerialsarealso
ilnn.ofnuclearenergy.Inthesesubstances,theenergy
luredinthenucleusoftheatoms-thetinypositively
hugedcoreoftheatom.Anuclearpowerstationis
li><ignedtoreleasethenuclearenergystoredinuranium.
IIyoustretcharubberband,itbecomesastoreofstrain
"
Thebandcangiveitsenergytoapaperpelletand
wilditflyingacrosstheroom.Strainenergyistheenergy
byanobjectthathasbeenstretchedorsquashed
hiinelasticway(sothatitwillspringbacktoitsoriginal
dimensionswhenthestretchingorsquashingforcesare
nmoved).Forthisreason,itisalsoknownaselasticenergy.
Ihemetalspringsofacarareconstantlystoringand
releasingelasticenergyasthecartravelsalong,sothat
theoccupantshaveasmootherride.Awind-upclock
storesenergyinaspring,whichistheenergysource
neededtokeepitsmechanismoperating.
Ifyouheatanobjectsothatitgetshotter,youaregiving
energytoitsatoms.Theenergystoredinahotobjectis
calledinternalenergy.Wecanpicturetheatomsofahot
objectjigglingrapidlyabout-theyhavealotofenergy.
ThispictureisdevelopedfurtherinChapter9.
Ifyougetclosetoahotobject,youmayfeelthermal
energycomingfromit.Thisisenergytravellingfroma
hotterobjecttoacolderone.Thedifferentwaysinwhich
thiscanhappenaredescribedinChapter11.
Itisimportantnottoconfuseinternalenergyandthermal
energy.Theinternalenergyofanobjectisthetotal
kineticandpotentialenergiesoftheparticlesitismade
of.Theinternalenergyofanobjectwillbehigherifthese
particlesaremovingfaster(higherkineticenergy)orthey
arefurtherapart(biggerpotentialenergy).Heatingan
object(givingitmorethermalenergy)raisesitsinternal
energyandthiscanraiseitstemperatureorchangeits
state(fromwatertosteam,forexample).Steamhasmore
internalenergythanboilingwatereventhoughtheyare
atthesametemperature.Theparticles(watermolecules)
insteamhavemorepotentialenergythanwatermolecules
inboilingwaterbecausetheyarefurtherapart.Thermal
energyspreadsoutfromahotobject.
Veryhotobjectsglowbrightly.Theyaretransferringenergy
bylight.Lightradiatesoutwardsallaroundthehotobject.
Anotherwayinwhichenergycanbetransferredtoan
object’ssurroundingsisbysound.Anelectriccurrent
transfersenergyelectricallytoaloudspeaker.Energyis
transferredtothesurroundingsassoundandthermal
energy(seeFigure6.6).
KEYWORDS
chemicalenergy:energystoredinbonds
betweenatomsthatcanbereleasedwhen
chemicalreactionstakeplace
nuclearenergy:energystoredinthenucleusof
anatom
strainenergy/elasticenergy:energystoredin
thechangedshapeofanobject
internalenergy:theenergyofanobject;the
totalkineticandpotentialenergiesofitsparticles
thermalenergy:energytransferredfroma
hotterplacetoacolderplacebecauseofthe
temperaturedifferencebetweenthem
105

119. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Energystores,energytransfers
Imaginethatenergyislikemoney.Theamountofmoney
youhavedetermineswhatyoucanbuy.Theamountof
energyyouhavedetermineswhatyoucando.
LetusimaginethattheamountofmoneyIhaveisfixed
(Icannotearnanyorspendit).Someofmymoneyisstored
inmybankaccount,someinmywalletandsomedown
thebackofmysofa.Ican*transfer(move)moneybetwet
thesestoresbutthetotalmoneyIhavedoesnotchange.
Energystoresarepotentialenergy.Energycanalsotransff
betweenstores,butthetotalamountofenergynever
changes.So,energycanbestoredoritcanbetransferred.
Table6.1listsenergyundertwoheadings:energystores
andenergytransfers.
Table6.1:Energycanbeclassifiedasstoresortransfers.
Energystores Energytransfers
kineticenergy(k.e.) electrical
gravitationalpotentialenergy(g.p.e.) thermal(heat)
chemicalenergy radiation(suchaslight)
elastic(strain)energy
mechanical(suchassound,whichisawayof
transferringvibrationalkineticenergy)
nuclearenergy
internalenergy
electrostaticenergy
Figure6.6:Atamajorrockconcert,giantloudspeakers
transfersoundtotheaudience.Extrageneratorsmayhave
tobebroughtontothesitetoactasasourceofenergyto
powerthespeakersystems.Muchoftheenergysupplied
iswastedasthermalenergy,becauseonlyafractionofthe
energyistransferredbysound.
Figure6.7:Whenacatapultfiresaball,energyis
transferredfromtheelasticstoreofthecatapulttothe
kineticstoreoftheball.Iftheballisfiredverticallyupward:
energyfromthekineticenergystoreistransferredtothe
g.p.e.store,untilthereisnothingleftinthekineticenergy
store.Theballstopsmovingupwardsandstartsfalling,wit
energytransferringfromthegravitationalstorebacktoth<
kineticstore.
106>

120. 6Energystoresandtransfers
IJiimycanbetransferredfromonestoretoanother,
fwnwithinthesameobject.
>ample,whenyouclimbahill,youaretransferring
iryfromyourchemicalstoretoyourgravity(or
1
1'
>)store.Herearefourdifferentwaysinwhichenergy
Unbetransferred:
Byaforce(mechanicalworking).Ifyoulift
something,yougiveitgravitationalpotentialenergy
youprovidetheforcethatliftsit.Alternatively,
youcanprovidetheforceneededtostartsomething
moving-yougiveitkineticenergy.Firingacatapult
(Figure6.7)isanotherexampleofamechanical
Iriinsfer.Whenenergyistransferredfromoneobject
Ioanotherbymeansofaforce,wesaythattheforce
isdoingwork.ThisisdiscussedindetailinChapter8.
Uyheating(thermalworking).Wehavealready
teenhowthermalenergyspreadsoutfromhot
objects.Nomatterhowgoodtheinsulation,
energyistransferredfromahotobjecttoitscooler
mrroundings.ThisisdiscussedindetailinChapter11.
Uyradiation(light).LightreachesusfromtheSun.
ThatishowenergyistransferredfromtheSunto
IheEarth.Someoftheenergyisalsotransferredas
infraredandultravioletradiation.Theseareexamples
ofelectromagneticradiation(seeChapter15).
Uyelectricalcurrents(electricalworking).An
electriccurrentisaconvenientwayoftransferring
energyfromplacetoplace.Theelectricitymaybe
generatedinapowerstationmanykilometresaway
fromwheretheenergyisrequired.Alternatively,an
electricalcurrenttransfersenergyfromthechemical
energystoreofaflashlightbatterytotheinternal
energyofabulb.Thisincreasedinternalenergy
storeofthebulbistransferredtothesurroundings
vialightradiation.ThisiscoveredinChapter18.
IYWORDS
doingwork:transferringenergybymeansofa
lorce
electromagneticradiation:energythatis
Imnsferredusingelectromagneticwaves
Questions
1Whatnameisgiventotheenergyofamoving
object?
!Whatdothelettersg.p.e.standfor?Howcanan
objectbegiveng.p.e.?
IWhatenergyisstoredinastretchedspring?
4Explainwhysteamislikelytoleadtoamoreserious
skinburnthanboilingwater.
5LookatthelistofenergystoresshowninTable6.1.
Foreach,giveanexampleofanobjectormaterial
thatstoresthisenergy.
6Lookatthephysicalcluesintheleftcolumnof
Table6.2andwritedownwhichenergystoreis
changing.
Table6.2
Physicalclue
Whichenergystoreis
changing?
materialchanging
shape
objectchangesspeed
chemicalreaction
changeoftemperature
nuclearfissionorfusion
distancebetween
objectschanges

6.2Energytransfers
Wehavealreadymentionedseveralexamplesofenergy
transfers.Nowwewilllookatafewmoreandthinka
littleabouthowenergyistransferredbetweenstores
duringeventsandprocesses,andhowthesetransferscan
berepresentedbyenergyflowdiagrams.
Strikingamatchisanexampleofaneventwhileburning
isaprocess.Aneventissomethingthathappensor
takesplace,oftenataspecifictimeandplace.Aprocess
isaseriesofactionsorsteps,oftentakingplaceovera
longperiodoftime.Climbingamountainwouldbean
exampleofaprocess,whilefallingoverwouldbeanevent.
Sometimes,itisdifficulttotellthedifferencebetweenan
eventandaprocess.Theimportantthingtorememberis
thatenergyisonlytransferredorchangedduringevents
andprocesses;inotherwords,whensomethinghappens.
KEYWORDS
event:somethingthathappensortakesplace,
oftenataspecifictimeandplace
process:aseriesofactionsorsteps,oftentaking
placeoveralongperiodoftime
107>

121. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Figure6.8showsscubadiversusingflashlightsduringa
diveatnight.Thetransferofenergytothelightbulbsis
aprocess.Thechemicalenergystoredinthebatteryis
transferredelectricallythroughthewirestothelightbulbs,
whichincreasesitsstoreofinternalenergy.Thelamp
transfers(useful)energybylighttothesurroundingsas
wellasbyheating,whichiswasted.Thediverswouldbein
seriousdangeriftheirflashlightranoutofcharge.They
cannotreplaceorrechargetheirbatteriesunderwater.
Theyneedtheirflashlightstobeefficientsothatmostof
thechemicalenergyistransferredusefullybylightand
verylittleiswastedasthermalenergy.Amoreefficient
flashlightwillproducelightforalongertime.
Theenergystoresandtransfersinaflashlightcanbe
representedbytheflowdiagraminFigure6.9.Theblue
boxesshowtheenergystores(andwherethosestores
are)andthegreenboxeswitharrowsshowtheenergy
transfers.
Figure6.8:Diversusingflashlights.
Figure6.10:MarsCuriosityrover.
Adevicecalledaradioisotopethermoelectricgenerator
(RTG)isasourceofenergyusedinspaceprobes.The
ApollomissionsandtheMarsCuriosityroverusedthen
(Figure6.10).Theyareidealforremoteplaceswhere
batteries,generatorsorsolarcellsarenotpractical.Also
becausethedeviceshavenomovingparts,theyaremore
reliablethanalternativesandrequireverylittlemaintenant
TheenergytransfersinanRTGareanotherexample
ofaprocess.InanRTG,acontainersealsaradioactive
source(usuallyplutonium-238).Theradioactivesource
producesthermalenergy,raisingtheinternalenergyof
thefuel.Thermocouplespassthroughthewallsofthe
container,withtheinnerendofeachthermocouplekep
hotbythefuelwhiletheouterendisconnectedtoahet
sinksothatitstayscold.
battery lightbulb surroundings
surroundings
Figure6.9:Theenergystoresandtransfersassociatedwithabattery-poweredlamp.Thestoresareindicatedbyblueboxes
andthetransfersbythegreenboxeswitharrows.
108

122. 6Energystoresandtransfers
surroundings
luui«6.11:TheenergyflowdiagramfortheRTG.Theblueboxesrepresentstoresandthegreenboxesarethetransfers.
109>
radioactive
source
radioactive
sourcein
theRTG
Figure6.12:Thisrocketusesrocketmotorstoliftitupinto
space.Eachrocketmotorburnsaboutonetonneoffueland
oxygeneveryminutetoprovidetheenergyneededtomove
therocketupwards.
along
thermocouple
|felloithermalenergymovesalongthethermocouple
humthehotendtothecoldend(downatemperature
|iidunt).Thetemperaturedifferenceatthejunctionof
Hitl
1ometalsproducesavoltage.
11"nuclearenergyinthefuelchangesstoreseveraltimes.
IlHimlersfromthenuclearstoretotheinternalenergy
H
'irntthefuel.Itthentransfersasthermalenergyasit
hi ilongthethermocouples.Atthejunctionofthe
'
inietals,ittransfersaselectricalenergy.
ihktearenergy(store)—
>internalenergy(store)—
Iheimalenergy(transfer)—
electricalenergy(transfer)
energychangescanberepresentedintheenergy
flv11mgraminFigure6.11.
|
'Mlmganarrowisanevent,butarocketlaunchisa
|mnr.,TherocketinFigure6.12isliftingofffromthe
|imildasitcarriesanewspacecraftupintospace.Its
miKcomesfromitsstoreofchemicalenergy(tanks
।In|iudhydrogen)andoxygen.Whenthehydrogenfuel
innmoxygen,itsstoreofchemicalenergyisreleased.
1Imiixketisaccelerating,sowecansaythatitskinetic
|*gVIsincreasing.Itisalsorisingupwards,soits
ginsHiilionalpotentialenergyisincreasing.InFigure6.12,
।inseelightcomingfromtheburningfuel.Youcan
I
।imaginethatlargeamountsofthermalenergyand
tumidenergyaretransferredintotheatmosphere.
I iienergychangescouldberepresentedasanenergy
'
।">diagramasbeforeorasanequation:
ihemicalenergy—
k.e.+g.p.e.+thermalenergy+
lightenergy+soundenergy

123. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Question
7Whatenergytransfersaregoingoninthefollowing?
Ineachcase,writeanequationtorepresentthe
energytransfer.
aCoalisburnedtoheataroomandtoprovidea
supplyofhotwater.
bAstudentusesanelectriclampwhilesheis
doingherhomework.
cAhairdryerisconnectedtothemains
electricitysupply.Itblowshotairattheuser’s
wethair.Itwhirrsasitdoesso.
ACTIVITY6.1
Energychanges
Examinesomedevicesthattransferenergy.Some
ideasareshowninFigure6.1inthe'Getting
Started'box.
Inpairs,examineeachofthedevicesyouare
providedwith.Foreachofthem,describewhat
energytransfersaregoingoninthedevice.
Withyourpartner,decidehowtorecord
andpresenttheenergytransfersyouhave
describedforeachdevice.
Compareyouranswerswiththeanswersof
othermembersoftheclassandcorrector
addtoyourownanswers.
6.3Conservation
ofenergy
Whenenergyistransferredfromonestoretoanother,itis
oftenthecasethatsomeoftheenergyendsupasunwanted
energy.Theenergytransfersinalightbulbwereshownin
Figure6.9.Thebulbtransferslight(whichwewant)and
heat(whichisnotwanted).
Thisisanexampleofaveryimportantidea,theprinciple
ofconservationofenergy:
Inanyenergytransfer,thetotalamountofenergy
beforeandafterthetransferisconstant.
Thistellsussomethingveryimportantaboutenergy:
itcannotbecreatedordestroyed.Thetotalamount
ofenergyisconstant.Ifwemeasureorcalculate
theamountofenergybeforeatransfer,andagain
afterwards,wewillalwaysgetthesameresult.Ifwefin
anydifference,wemustlookforplaceswhereenergym
beenteringorescapingunnoticed.
KEYWORDS
principleofconservationofenergy:energy
cannotbecreatedordestroyed;itcanonlybe
storedortransferred
WORKEDEXAMPLE6.1
Acarbums3x105Joffuel(chemicalenergy)per
second.Ithas1.3x105Jofkineticenergyandgains
0.7x105Jofgravitationalpotentialenergyasitgoes
upaslope.Howmuchenergytransfersawayfrom
thecarthroughthermalenergytransfer?Assumetha1
accelerationduetogravityg=10m/s2.
Step1:Writedownwhatyouknow,andwhatyou
wanttoknow.
inputenergy:
chemicalenergy=3xIO5J
outputenergy:
kineticenergy=1.3x105J
gravitationalpotentialenergy=0.7x105J
thermalenergytransferred=?
Step2:Writedownanyequationsoruseful
principles.
Accordingtotheprincipleofconservation
ofenergy,thetotalinputenergyshouldequal
thetotaloutputenergy.
Step3:Applytheprincipletothisproblemand
substituteknownvaluestosolvetheproblem
chemicalenergy=k.e.+g.p.e.+thermal
energy
3x105J=1.3x105J+0.7x105J+thermal
energy
=1.0x105Jtransfersawayfrom
thecarthroughthermalenergy
transfer
Answer
1.0x105Jtransfersawayfromthecarasthermal
energy.

124. 6Energystoresandtransfers
uestion
IYWORDS
kinetic
energy
increasedg.p.e.
storeoftheblocks
IthebeginningofChapter1youwereintroduced
IheAncientEgyptiansandtheirpyramids.The
yvptiansbuilttheirpyramidsbydragginglimestone
docksuprampsandFigure6.14showstheSankey
gravitational
potential
energy
4Jofenergyaretransferredfromthelamp
eachsecondaslight.Howmanyjoulesof
energyaretransferredeachsecondbyheating?
Inimical
energy
billedin
IijoIand
'kygen)
increasedinternalenergy
ofthesurroundings
S.tnkeydiagram:aflowdiagramthatrepresents
Iheprincipleofconservationofenergy;thewidth
ofthearrowsisproportionaltoenergy
diagramforthis.Bydoingmechanicalwork,they
transferredenergyfromthechemicalenergystorein
theirbodiestothe(useful)gravitationalpotentialenergy
gainedbytheblocks.Atthesametime,someoftheir
storeofchemicalenergyistransferred,byheating,tothe
(useless)internalenergyofthesurroundings.Thisheat
camefromtheirbodiesandbecauseoffrictionbetween
theblocksandtheramp.
Figure6.14:ASankeydiagramforblocksbeingdraggedup
aramp.
1'(jure6.13:Theenergychangesgoingonasarocketlike
InFigure6.12acceleratesupwards.Chemicalenergyin
fuelisreleasedwhenitburnsinoxygenandistransferred
Ithreeotherenergystores.
internalenergyofthe
surroundings
chemicalenergy
storeofthe
Egyptianworkers
Whenfuelsareburned(perhapstogenerateelectricity,
ortodriveacar),heatisproduced.Anykindofengine
needsadifferenceintemperaturetocreatemovement.
Thermalenergytransfersfromthehotparttothecold
partoftheengineandkineticenergyisproduced.Butno
matterhowwellinsulatedthehotpartis,itwilltransfer
thermalenergytothesurroundings.Or,thecoldpart
hastobecooledtomaintain(keep)thetemperature
difference.So,powerstationsproducewarmcooling
waterandcarsproducehotexhaustgases.
Frictionisoftenaproblemwhenthingsaremoving.
Lubricationcanhelptoreducefrictionandnodoubtthe
Egyptianslubricatedtherampstomakeiteasierforthe
blockstobedraggedupthem.Astreamlinedcardesign
canreduceairresistance.Butitisimpossibletoeliminate
(remove)frictionentirelyfrommachineswithmoving
parts.Frictiongeneratesheat.
Anothercommonwastedenergytransferissound.
Noisymachinery,loudcarenginesandsoon,all
transfersoundtotheatmosphere.However,evenloud
noisescontainverylittleenergy,sothereislittletobe
gained(intermsofenergy)byreducingnoise.
Energyefficiency
Mostwastedenergyistransferredawayasheat.There
aretwomainreasonsforthis.
ankeydiagrams
lirlfectivewaytorepresenttheprincipleof
i<irvationofenergyisbyusingaSankeydiagram,
rocketmotorwesawearlier(Figure6.12)does
lianicalworktotransferchemicalenergyintok.e.
notp.e.(energystoresthatwedowant),whileheat,
hplilandsoundtransferenergytotheinternalenergy
burofthesurroundings(anenergystorethatwedonot
*unttoincrease).ThisisshowninFigure6.13.
Alightbulbissuppliedwith60Jofenergyeach
wcond.
Howmanyjoulesofenergyaretransferred
fromthebulbeachsecond?

125. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Itisimportanttomakegooduseoftheenergyresources
availabletous.Thisisbecauseenergyisexpensive,
suppliesareoftenlimited,andouruseofenergycan
damagetheenvironment.Sowemustuseresources
efficiently.Hereiswhatwemeanbyefficiency:
Efficiencyisthefraction(orpercentage)ofenergy
suppliedthatisusefullytransferred.
Becareful,theword,‘efficiency’isoftenusedineveryday
life,butoftenitisusedtomeanquickly,whichisnotthe
sameasthescientificmeaning.
KEYWORDS
lubrication:usuallyaliquid,itallowstwosurfaces
toslidepasteachothermoreeasily
efficiency:thefraction(orpercentage)ofenergy
suppliedthatisusefullytransferred
Table6.3showsthetypicalefficienciesforsomeimportant
devices.Youcanseethateventhemostmoderngas-fired
powerstationisonly50%efficient.Halfoftheenergyitis
suppliedwithiswasted.
Device Typicalefficiency
electricheater 100%
largeelectricmotor 90%
washingmachinemotor 70%
gas-firedpowerstation 50%
dieselengine 40%
carpetrolengine 30%
steamlocomotive 10%
Table6.3:Energyefficiencies.Mostdevicesarelessthan
100%efficientbecausetheyproducewasteheat.Anelectric
heateris100%efficientbecausealloftheelectricalenergy
suppliedistransferredtothermalenergy.Thereisno
problemwithwastehere.
Questions
9aInwhatwayisenergyusuallywasted?
bNameanotherwayinwhichenergyisoftenwasted.
10Givethreereasonswhyitisimportantnottowaste
energy.
Makingbetteruseofenergy
Figure6.15showsaSankeydiagramthatrepresents
energyflowsinthewholeoftheUKinatypicalyear.
MostoftheenergyflowingintotheUKcomesfrom
fuels,particularlycoal,oilandgas.Energyiswastedii
twogeneralways:whenitischangedintoelectricity,a
whileitisbeingused(forexample,inlightbulbs).
Figure6.15:EnergyflowsintheUKintheyear2000.
Allnumbersarex10,8J.Alargepropdrtianoftheenergy
suppliedbyfuelsiswastedinenergytransferprocesses
andduringitsfinaluse.Someofthiswasteisinevitable,t
betterinsulationandmoreefficientmachinescouldreduc
thewasteandenvironmentaldamage,andsavemoney.
Figure6.16showsonewaytomakemoreefficientuse
ofelectricity.Weuselightbulbstoprovideuswithlig]
Thelowerlightbulbisafilamtntlamp;theotherone
isanenergy-efficientlamp.TheSankeydiagramsshow
theenergyeachlightbulbtransferseachsecond.The
diagramshowsthateachofthetwobulbsproducesth
sameamountoflight.However,becauseitwastesmu<
lessenergyasheat,theenergy-efficientlamprequirese
muchsmallerinputofenergyandismoreefficient.
electrical
energy
25J
wasteheat10J
electrical
energy
100J
Figure6.16:Eachofthesetwolightbulbsprovidesthe
sameamountoflight.Theenergy-efficientlampwastes
muchlessenergyasheat.

126. 6Energystoresandtransfers
efficiency=
percentageefficiency=
Questions
11
12
13
14
efficiency=
|)ercentageefficiency=
WORD
।YEQUATIONS
<.inseefromTable6.3thatefficiencyisoften
tiusapercentage.Wecancalculatetheefficiency
11percentageefficiencyofanenergychangeasfollows:
nottomixstoresandtransfersonthesameSankey
inn.Figures6.13and6.14showsenergystoreswhile
>>h6.16showstransfers.Figure6.16showstheenergy
Ibythelightbulbspersecond.Energytransferred
acondisknownaspowerandissomethingyou
meetinChapter8.Thishighlightsanimportant
itncebetweenstoresandtransfers.Transfersarea
ofenergy.
Describetheenergytransferstakingplacewhen
chargingamobilephone,includingtheenergythat
iswasted.
tlpated:energythatisspreadoutisnot
noful(wasted)
Efficiencyisexpressedasanumber(nounits)uptoa
valueof1.Thisnumbercanbemultipliedby100toget
percentageefficiency.Percentageefficiencygreaterthan
100%isimpossible.
WhenthefilamentlampfromFigure6.16issupplied
with100Jofenergy,itproduces15Jofusefullight.Its
efficiencyisthus:
Calculatetheefficiencyoftheenergy-efficientlamp
fromthedatashowninFigure6.16.
Atidal-powerstationisexpectedtoproduce32TJ
ofenergy(1TJ=1012J)whenthetidesprovideit
with100TJofgravitationalpotentialenergy.What
istheefficiencyofthepowerstation?
Atungsten-filamentlampis4%efficient.Howmuch
electricalenergymustbesuppliedtothelampeach
secondwhenitproduces6Joflightpersecond?
ergybecomingdissipated
luiveseenthatenergychangesareusuallylessthan
..efficient.Energyescapesandiswastedasheat.
i«meansthatobjectsandtheirsurroundingsare
imod(andgainsomeinternalenergy).Itisvery
ulttogetthatenergyback.Wesaythatenergytends
^0dissipated(spreadout)duringanenergytransfer.
IImikabout,forexample,abatteryinaflashlight.
N|»<iconvenient,compactstoreofenergy.Onceithas
IiiUsed,someofitsenergyhasbeenchangedtolight
histhenabsorbedbythesurfacesitfallson,causing
ntowarmslightly(raisingtheirinternalenergy).The
oftheenergyisdissipatedasthermalenergyinthe
ponentsoftheelectriccircuitintheflashlight.
. usefulenergyoutput,„„„,
iientageefficiency= ; x100%
totalenergyinput
usefulenergyoutput
।Uhicncy=
;
totalenergyinput
usefulenergyoutput
totalenergyinput
usefulenergyoutput
x
totalenergyinput
usefulpoweroutput
totalpowerinput
usefulpoweroutput
totalpowerinput
_
. usefulenergyoutput
efficiency=
totalenergyinput
=
JlL
=0.15
100J
„. usefulenergyoutput
percentageefficiency= x100%
totalenergyinput
=
x100%=15%
100J
Similarequationscanbeusedtocalculatetheefficiency
andpercentageefficiencyintermsofpowerasfollows:

127. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
ACTIVITY6.2
Energychangesduringthepolevault
Energyistransferredbetweendifferentenergystores
duringthepolevault.Snapshots(labelled1-5)of
anathleteatdifferentstagesofthepolevaultareas
showninFigure6.17.Betweeneachsnapshot,the
energyistransferredbetweenstores.
1Copyandcompletethistable:
Snapshot
Main
energy
store
Additional
energy
stores
Wasted
energy
1
2
3
4
5
2Howistheenergytransferredbetweeneachstore?
3UsingFigure6.9asaguide,drawanenergyflow
diagramthatshowsthemainenergystoresand
theenergytransfersbetweenthem.
4Decidewhetheryouthinkthepolevaultisan
eventoraprocessandjustifyyouranswer.
5Ifyourteachergivesyouthetimetodoso,
compareyouranswerswithyourneighbourand
trytoresolveanydifferences.Bepreparedto
discussyourthinkingwiththeclass.
SELF-ASSESSMENT
ThinkaboutActivity6.2.Didyoufindthisactivityeasy?Ifyoufounditdifficult,youcouldthinkaboutenergy
transfersthatyoucomeacrosseveryday(forexample,thetransportyouusetogettoandfromschool)and
askafriendtocheckyouranswer.
6.4Energycalculations
Energyisnotsimplyanidea,itisalsoaquantitythat
wecancalculate.
Gravitationalpotentialenergy
(g.p.e.)
MountaineeringontheMoonshouldbeeasy(see
Figure6.18).TheMoon’sgravityismuchweakerthan
theEarth’s,becausetheMoon’smassisonlyone¬
eightiethoftheEarth’s.Thismeansthattheweight
astronautontheMoonisafractionofhisorherwe
ontheEarth.Inprinciple,itispossibletojumphigt
ontheMoonthanontheEarth.
Earlier,wesawthatanobject’sg.p.e.dependsonits
heightabovetheground.Thehigheritis,thegreatei
g.p.e.Ifyouliftanobjectupwards,youprovidethe1
neededtoincreaseitsg.p.e.Theheaviertheobject,t
greatertheforceneededtoliftit,andhencethegrea
itsg.p.e.

128. 6Energystoresandtransfers
I
g.p.e.
g.p.e.=40Nx1
2.5m
1m
ION
Mlilggeststhatanobject’sgravitationalpotential
Hkipv(g.p.e.)dependsontwofactors:
40N
Q
6.18:AstronautsontheMoon.Thegravitationalfield
M"|thonthesurfaceoftheMoonisone-sixthofwhatitis
II.Iih.ExperimentsontheMoonhaveshownthatagolf
Mil
'
anbehitmuchfurtherthanonEarth.Thisisbecause
IIIbIhamuchgreaterdistancehorizontallybeforegravity
j**!1IIbacktotheground.
HuiitillustratedinFigure6.19.Fromthenumbersin
Hiiliigram,youcanseethatachangeing.p.e.issimply
|ah11lutedbymultiplyingweightbyheight.(Here,we
«i,iiwmingthatanobject’sg.p.e.iszerowhenitisat
Bbinmllevel.)Wecanwritethisasanequationforg.p.e.:
=40Nx2.5m
=100J
Iheobject’sweight,mg-thegreateritsweight,the
rteateritsg.p.e.
Iheobject’sheight,h,abovegroundlevel-the
greateritsheight,thegreateritsg.p.e.
Hgiu
*
6.19:Thegravitationalpotentialenergyofanobject
Hl'-i>i"asitisliftedhigher.Thegreateritsweight,the
Ii.Iitsg.p.e.
ining.p.e.=weightxchangeinheight
&Ep=mgEh
<1pe.
iri.
EQUATION
WORKEDEXAMPLE6.2
Anathleteofmass50kgrunsupahill.Thefootof
thehillis400metresabovesea-level.Thesummitis
1200metresabovesea-level.Byhowmuchdoesthe
athlete’sg.p.e.increase?Assumethataccelerationdue
togravityg=10m/s2.
Step1:Assumethatg.p.e.iszeroatthefootofthe
hill.Calculatetheincreaseinheight.
A/z=1200m-400m=800m
Step2:Writedowntheequationforg.p.e.,substitute
valuesandsolve.
A£p=weightxchangeinheight
=mgkh
=50kgx10m/s2x800m
=400000J
=400kJ
Answer
Theathlete’sg.p.e.increasesby400kJ.
Anoteonheight
Wehavetobecarefulwhenmeasuringorcalculatingthe
changeinanobject’sheight.
First,wehavetoconsidertheverticalheightthroughwhich
itmoves.Atrainmaytravel1kmupalongandgentle
slope,butitsverticalheightmayonlyincreaseby10metres.
AsatellitemaytravelaroundtheEarthinacircularorbit.
ItstaysataconstantdistancefromthecentreoftheEarth,
andsoitsheightdoesnotchange.Itsg.p.e.isconstant.
Second,itisthechangeinheightoftheobject’scentreof
gravitythatwemustconsider.Thisisillustratedbythe
pole-vaultershowninFigure6.20.Ashejumps,hemust
trytoincreasehisg.p.e.enoughtogetoverthebar.In
fact,bycurvinghisbody,hepassesoverthebarbuthis
centreofgravitymaypassunderit.
Figure6.20:Thispole¬
vaulteradoptsacurved
posturetogetoverthebar.
Hecannotincreasehisg.p.e.
enoughtogethiswhole
bodyabovethelevelofthe
bar.Hiscentreofgravitymay
evenpassunderthebar,so
thatatnotimeishisbody
entirelyabovethebar.

129. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
ACTIVITY6.3
Moonflighthighjump
Table6.4
Highjumprecord/mAthlete Heightofathlete/m Year
men 2.45 JavierSotomayor(Cuba) 1.95 1993
women 2.09 StefkaKostadinova(Bulgaria) 1.80 1987
Table6.4liststhecurrentworldrecordsforthe
highjump.
Predictwhatthehighjumprecordwouldbeonthe
lunarsurface.
Nowfollowthesestepstoseeifyourprediction
wascorrect.
1LetusassumethattheMoonhasthesame
atmosphereasEarth,andthattheathletescan
reachthesamerun-upspeed.Imaginethat
thegravitationalfieldstrengthontheMoonis
reducedtoonesixthofthevalueithasonthe
Earth'ssurface(10N/kg)onlyafterthejumpers
haveliftedofftheground.Predictwhatyouthink
thehighjumprecordswouldbeontheMoon.
Writedownyourworkingandyouranswers.
2Nowassumethatthecentreofgravityofaperson
islocatedhalf-wayuptheirbody.Throughwhat
heighthavetheseathletesmovedtheircentreof
gravityinordertoachievetheirworldrecords?
3Theathletesaredoingphysicalworktoraise
theircentresofmassoverthebar.Nowthat
youknowthejumpersareraisingtheircentres
ofgravity,workoutarevisedpredictionforthe
records,buttakecare,asthereisstillapotenti
trapfortheunwary.
4Mosthighjumpersnowuseatechniquethat
allowstheircentreofgravitytopassbelowthe
bar,byasmuchas20cm.Explainorsketchho'
thisispossible.
5Canyouexplainwhytheheightgainedbythe
athletewhentheyjumpisnotthedistance
betweenthebarandtheground?
6Usephysicstoexplainwhysuccessfulhigh
jumperstendtobetallandslim.
7Makeacaseformedalseingawardedto
athleteswhocanraisetheircentresofgravity
throughthebiggestheight.
Kineticenergy
Ittakesenergytomakethingsmove.Youtransferenergy
toaballwhenyouthrowitorhitit.Acarusesenergy
fromitsfueltogetitmoving.Elasticenergystoredina
stretchedpieceofrubberisneededtofireapelletfrom
acatapult.Soamovingobjectisastoreofenergy.This
energyisknownaskineticenergy(k.e.).
Weoftenmakeuseofanobject’skineticenergy.Todo
this,wemustslowitdown.Forexample,movingair
turnsawindturbine.Thisslowsdowntheair,reducing
itsk.e.Theenergyextractedcanbeusedtoturna
generatortoproduceelectricity.
Thissuggeststhatthekineticenergyofanobject
dependsontwofactors:
theobject’smassm-thegreaterthemass,the
greateritskineticenergy
theobject’sspeedv-thegreaterthespeed,the
greateritskineticenergy.
Thesearecombinedinanequationforkineticenergy
KEYEQUATION
kineticenergy=
xmassxspeed2
£k=~mv2
2
WorkedExample6.3showshowtousetheequation
calculatethekineticenergyofamovingobject.Note
alsothatkineticenergyisascalarquantity,despitetl
factthatitinvolvesv.Itisbesttothinkofvhereass
ratherthanvelocity.

130. 6Energystoresandtransfers
Questions
15
c
16
17
18
19
20
21
)RKEDEXAMPLE6.3
Inthefollowingexamples,istheobject’sg.p.e.
increasing,decreasingorremainingconstant?
Whenthevan’sspeeddoublesfromlOm/sto20m/s,its
kineticenergyincreasesfrom100kJto400kJ.Inother
words,whenitsspeedincreasesbyafactoroftwo,its
kineticenergyincreasesbyafactoroffour.Thisisbecause
kineticenergydependsonspeedsquared.Ifthespeed
trebled(increasedbyafactorofthree),thekineticenergy
wouldincreasebyafactorofnine(seeFigure6.21).
Figure6.21:Thefasterthevantravels,thegreateritskinetic
energy.Thegraphshowsthatkineticenergyincreasesmore
andmorerapidlyasthevan'sspeedincreases.
a
b
Aballoonrisesintheair.
Abirdfliesataconstantheightonitsmigration
route.
Araindropfallsfromthesky.
ItisclaimedthatSupermancanjump200metres
verticallyupwards.Ifhehasamassof100kg,by
howmuchdoeshisg.p.e.increase?
Araindropweighs1x10"3N.Itsg.p.e.decreases
by0.8Jwhenitfallsfromacloud.Howhighwas
thecloud?
tiHwer

Thevan’sk.e.whentravellingat10m/sis100kJ.
hThevan’sk.e.increasesby300kJwhenitspeeds
upfrom10m/sto20m/s.
Whenthevanstartsmovingfromrestandspeedsup
In10m/s,itskineticenergyincreasesfrom0to100kJ.
Whenitsspeedincreasesbythesameamountagain,
hnin10m/sto20m/s,itskineticenergyincreasesby
’HOkJ,threetimesasmuch.Ittakesalotmoreenergy
Inincreaseyourspeedwhenyouarealreadymoving
Iulckly.Thatiswhyacar’sfuelconsumptionstartsto
miitaserapidlywhenthedrivertriestoacceleratein
In‘fastlaneofamotorway.
Slop3:Calculatethechangeinthevan’skinetic
energy.
changeinkineticenergy=400kJ-100kJ
=300kJ
WhatdoesvrepresentintheequationEk=^mv
2?
Howmuchkineticenergyisstoredbyabulletwitha
massof10.5gtravellingat553m/s?
UsainBolthasamassof86kg.Whenherunsat
12m/s,whatishiskineticenergy?
Whichhasmorekineticenergy,a2.0gbeeflyingat
1.0m/s,ora1.0gwaspflyingat2.0m/s?
\vanofmass2000kgistravellingat10m/s.
Calculateitskineticenergy.
hItsspeedincreasesto20m/s.Byhowmuchdoesits
kineticenergyincrease?
<lop1:Calculatethevan’skineticenergyat10m/s.
£=lwv2
k
2
=|x
2000kgx(10m/s)2
=100000J
=100kJ
Iop2:Calculatethevan’skineticenergyat20m/s.
-I 2
Ev=-mv*
k
2
=|
x2000kgx(20m/s)2
=400000J
=400kJ
HiiWorthlookingatWorkedExample6.3indetail,
»im।itillustratesseveralimportantpoints.
Ii'iicalculatingkineticenergyusingE^-^mv2,take
pulOnlythespeedissquared.Usingacalculator,start
iquaringthespeed.Thenmultiplybythemass,and
Im.illydividebytwo.

131. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
REFLECTION
Howeasydidyoufindthistopic?
Howwillyoulearnthedifferentenergystoresandtransfersandrememberthedifferencebetweenthem?
Ifyoudonotknowthedifferencebetweenaneventandaprocess,howareyougoingtofindout?
PROJECT
Chooseoneoftheoptionsbelowandeither
produceashortreport(lessthan500words)
alongwithrelevantillustrationsorproduceashort
presentation(twoorthreeminutes),withsuitable
visualaids.
Option1:Inventionsforremoteplaces
Researchaninventionthatprovidesusefulenergy
inalocationwithoutanobviousorreliableenergy
supply.Ifyoucannottrackdownanotherinvention,
focusononeofthefollowingexamples.
Youshouldalreadyhavemettheradioisotope
thermoelectricgenerator(RTG)earlierin
thebook.
TrevorBaylisinventedthewind-upradio,
whichworkedwithoutbatteriesoraccesstoan
electricalpowersource.
Option2:Efficiency
Itisimportanttoincreaseefficiencytoreduce
waste,reduceenvironmentaldamage,andsave
money.Investigateeffortstoimprovetheefficiency
ofonedevice(forexample,alightbulb,oracar)or
createbetterinsulationforhomes.
SUMMARY
Transfersbetweendifferentstoresofenergycanoccurbecauseofaneventorprocess.
Acollisionisaneventthatwillchangethekineticenergyofabody.
Heatingabodywillincreaseitsinternalenergy.
Changingtheshapeofabodywillchangeitselastic(strain)energy.
Liftingabodywillincreaseitsg.p.e.

132. 6Energystoresandtransfers
Burningasubstancewillreduceitschemicalenergy.
I,nergycanbetransferredbetweenenergystores,whichcanbeillustratedusinganenergyflowdiagram.
Mechanicalworkcantransferg.p.e.toanobject,byliftingit.
IElectriccurrentstransferenergyelectrically.
Thermalenergycantransferinternalenergyfromahotobjecttoacoldobject.
Ilisimportanttoincreaseefficiencytoreducewaste,reduceenvironmentaldamage,andsavemoney.
Whenaprocessisnot100%efficient,thewastedenergyspreadoutsandisnotuseful(usuallythermalenergy).
Inergyisconserved.Itcannotbecreatedordestroyed;itcanonlytransferfromonestoretoanother.
\Sankeydiagramillustratestheprincipleofconservationofenergy.
Illiciencyisthefractionofthetotalenergythatisuseful.
hangeingravitationalpotentialenergy=weightxchangeinheightorchangeingravitationalpotentialenergy
massxgravitationalfieldstrengthxchangeinheightorAEp=mg^h.
ImeticenergyisEk=jmv2.
Whenworkingoutkineticenergy,onlythespeedissquared.
KAM-STYLEQUESTIONS
IThisdiagramshowsanamusementparkrollercoasterride(notdrawntoscale).
.1Onwhatpartoftherideisthecarmovingslowest?
bOnwhatpartoftherideisthecarmovingfastest?
[1]
[1]
<ThecarbecomesstuckatpointP,whichis50metresabovetheground.
Tothereliefofthepassengers,thecareventuallymovesagainandpasses
pointRat20m/s.ApproximatelyhowhighispointR?Thecarandits
passengershaveacombinedmassof700
kg(thoughthequestioncanbe
answeredwithoutthisinformation). [1]
A
B
C
D
35m/s
30m/s
25m/s
20m/s
[Total:3]
119>

133. >
CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
CONTINUED
2Copyandcompletethetable.Foreachdescription,writedownthename
oftheassociatedenergyandtewhetheritisastoreortransfer. [2]
Description NameofenergyStoreortransfer
energyofamovingobject
energyinahotobject
energyinafuel
energythatwecansee
energyinasquashedspring
energycarriedbyanelectric
current
energyinthenucleusofan
atom
energyescapingfromahot
object
3Whichofthefollowingstatementsisclosesttothemeaningoftheprinciple
ofconservationofenergy? [1]
AEnergycanonlybestoredortransferred
BEnergyiscreatedbyenergystores
CEnergycanbedestroyedbytransfers
DEnergycanonlybetransferred
4Thisdiagramrepresentsanenergytransfer.
usefulenergy
energyinput output
wasteenergy
Copyandcompletethefollowingtwowordequationsforthisenergychange:
awastedenergy= [1]
befficiency= [1]
[Total:2]
COMMAND WOR
state:expressin
clearterms
120>

134. 6Energystoresandtransfers
before after
V.
d
[1]
f
nottoscale
7J
93J
208J
ONTINUED
COMMAND WORDS
b
power
station
Hl
[2]
[1]
[2]
Scientistsuseaballisticpendulumtoworkoutthespeedofaprojectilethat
hitsit.Theblockhasamassof4.7kgandmoveswithaninitialspeedof
I24m/swhenitishit.
a
b
c
d
transmission
lines
Statetheequationlinkingkineticenergy,massandvelocity.
alculatethekineticenergyoftheblock.
Astheblockswings,itgainsg.p.e.Whatisthemaximumg.p.e.
thatcanbegainedbytheblock.
Calculatethemaximumheighttheblockgains.
Itmightnotbeeasytomeasuretheheightincrease.Suggestanother
variablearesearchercouldmeasuremoreeasilyandthenusetoget
theheight.
Researchersfindthekineticenergyoftheprojectileismuchhigherthan
thekineticenergyoftheblock.Whathappenedtothekineticenergy
thatwasnottransferredfromtheprojectiletotheblock?
Calculatetheinputenergytothepowerstation.
Calculatetheefficiencyofthepowerstation.
Calculatetheefficiencyofthelightbulb.
Statetheenergydissipated(wasted)inthelamp.
ThisistheSankeydiagramfora100Wlightbulb.
calculate:workout
fromgivenfacts,
figuresorinformation
suggest:apply
knowledgeand
understanding
tosituationswhere
therearearange
ofvalidresponses
inordertomake
proposals/putforward
considerations
[1]
Hl
[11
[1]
[Total:4]
[1]
[Total:8]
lamp
4J

135. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
SELF-EVALUATIONCHECKLIST
Afterstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutosee
anygapsinyourknowledgeandhelpyoutolearnmoreeffectively.
Ican
See
Topic...
Needs
morework
Almost
there
Confident
tomoveon
Recallthedifferentnamesofenergystores(k.e.,g.p.e.,
chemical,elastic,nuclear,internal,electrostatic,magnetic)
andtransfers(electrical,thermal,radiation,mechanical).
6.1,6.2
Recognisehowenergyistransferredduringeventsand
processes.
6.2
Interpretenergyflowdiagrams. 6.2
Understandthemeaningofefficiency. 6.3
Understandandapplytheprincipleofconservationof
energy.
6.3
Calculateusingtheequationsforpercentageefficiency. 6.3
Explainthat,inanyeventorprocess,theenergytends
tobecomemorespreadoutamongtheobjectsand
surroundings.
6.3
Recallandusetheequationforkineticenergy,Ek=^mv2.
6.4
Recallandusetheequationforachangeing.p.e.,
Ep=mgAA.
6.4
122>

136. Chapter7
nergy
esources
INTHISCHAPTERYOUWILL:
understandthatenergyisreleasedbynuclearfusionintheSun.
yunderstandthattheSunisthesourceofenergyforallourenergyresourcesexceptgeothermal,
nuclearandtidal

137. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
GETTINGSTARTED
Listalltheenergyresourcesthatyouknow.
Examplesofenergyresourcesincludewoodfor
heatingandcooking.
Whichoftheseenergyresourcesarerenewable?
Cananyoftheseresourcesbetracedbackto
sunlight?
ISTHORIUMTHEPERFECTFUEL?
KirkSorensenworkedforNASAtocomeupwith
areliablesourceofenergyforaMoonbase.None
oftheenergyresourcesthatareusedonEarth
weresuitable.Butthenhefoundabookabout
liquidfluoridethoriumreactors(or'lifters'),an
environmentallyfriendlyandsafeversionofnuclear
power.TheywerebeingdevelopedbytheUSA
sothataircraftcarryingnuclearbombswouldonly
havetolandtochangecrewsandtakeonsupplies.
Buttheexperimentwasabandonedin1956
becausemissilescouldmoreeasilysendnuclear
bombsovergreatdistances.
Nuclearpowerstationsneedwaterbutalifter
wouldnot,makingitsuitablefortheMoon.
ButSorensonthought,'whynothavethemhere
onEarth?'Therearehugereservesofthorium
fuelavailable,theyproducetinyamountsof
radioactivewaste,anaccidentalmeltdownwould
beimpossible,anditwouldbeextremelydifficult
tomakeanuclearbombusingalifter.
Discussionquestions
1ExplainwhyenergyresourcesusedonEarth
wouldnotbesuitablefortheMoon.
2Wouldyoubeinfavourofnbclearpower
basedonalifter?Explainyouranswer.
Figure7.1:Pelletsofthorium.
124)

138. 7Energyresources
oil(34%)
coal(27%)
KEYWORDS
natural
gas(24%)
Figure7.2:Worldenergyuse,byfuel.Thischartshows
people'senergyconsumptionofdifferentfuelsacrossthe
worldin2018.Around85%ofallenergycomesfromfossilfuels.
non-renewables:anenergyresourcethatisgone
foreveronceithasbeenused
renewables:anenergyresourcethatwillbe
replenished(replaced)naturallywhenused
Renewablesandnon-renewables
Figure7.2showsthatmostoftheenergysupplieswe
usearefossilfuels-coal,oilandgas.Oilandnatural
gasareexpectedtorunoutthiscenturybutreservesof
coalshouldlastanother200years.Theyaredescribedas
non-renewables.Onceused,theyaregoneforever.
Othersourcesofenergy,suchaswind,solarandbiofuel,
aredescribedasrenewables.Thisisbecause,whenwe
usethem,theywillsoonbereplaced.Thewindwillblow
again,theSunwillshineagain.Afterharvestingabiofuel
crop,wecangrowanothercrop.
Ideally,ourenergysupplyshouldbebasedon
renewables.Thenwewouldnothavetoworryabout
suppliesrunningout.Aswewillsee,non-renewable
resourcesalsocausesignificantenvironmentalproblems.
Burningfossilfuelscausesglobalwarmingwhilenuclear
powerproducesdangerousradioactivewaste.
7.1Theenergyweuse
IloreonEarth,werelyontheSunformostoftheenergy
?cuse.TheSunisafairlyaveragestar,150million
1ilometresaway.Theheatandlightwereceivefromit
likeabouteightminutestotravelthroughemptyspace
Iogethere.Plantsabsorbthisenergyintheprocessof
photosynthesis,andanimalsarekeptwarmbyit.
IheEarthisataconvenientdistancefromtheSunfor
livingorganisms.TheSun’sraysarestrongenough,but
nottoostrong.TheEarth’saveragetemperatureisabout
15°C,whichissuitableforlife.IftheEarthwerecloser
Io(heSun,itmightbeintolerablyhotlikeVenus,where
Iheaveragesurfacetemperatureisover400°C.Further
out,thingsarecolder.Saturnisroughlytentimesas
InfromtheSun,sotheSunintheskylooksone-tenth
ilthediameterthatweseeit,anditsradiationhas
onlyone-hundredthoftheintensity.Saturn’ssurface
temperatureisabout-180°C.
oftheenergyweusecomesfromtheSun,butonly
iimallamountisuseddirectlyfromtheSun.Onacold
hutsunnymorning,youmightsitinthesunshineto
"iirmyourbody.Yourhousemightbedesignedtocollect
"
urmthfromtheSun’srays,perhapsbyhavinglarger
'Indowsonthesunnyside.However,mostoftheenergy
usecomesonlyindirectlyfromtheSun.Itmustbe
Hunsferredinamoreusefulform,suchaselectricity.
IIgure7.2showsthedifferentfuelsthatcontributetothe
>0rld’senergysupplies.Thischartreflectspatternsof
<uergyconsumptionin2018.Manypeopletodaylivein
industrialisedcountriesandconsumelargeamountsof
riiergy,particularlyfromfossilfuels(coal,oilandgas).
Peoplelivinginless-developedcountriesconsumefar
Irisenergymostlytheyusebiomassfuels,particularly
Athousandyearsago,thechartwouldhave
lookedverydifferent.Fossilfuelconsumptionwasmuch
hisimportantthen.Mostpeoplereliedonburningwood
Iosupplytheirenergyrequirements.Wewillnowlookat
thesegroupsoffuelsinturn,inadditiontootherenergy
nsources.Energyresourcesarenotthesameasthestores
mdtransfersofenergyyoustudiedinChapter6.
Ilowever,wefirstneedtocategorise(grouptogether)
।uergyresourcestomakeiteasiertocomparethem.So,
Wewilllookathowmostofthemcanbeusedtogenerate
electricityandwhetherornottheyarerenewable.
other
renewables(4%)
nuc|ear(4%)
hydro(7%)

139. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
ACTIVITY7.1
Createapresentationonanenergyresource
Researchaparticularenergyresourceandpresent
yourworktotheclass.Workingroupsoftwo
orthree.
Yourtwotothreeminutepresentationshould
answerthesequestions:
Whatistheoriginoftheenergyresource?
Istheenergyresourcerenewableor
non-renewable?
Iftheresourceisusedtogenerateelectricity,
whatenergytransfershappenandhowit
isdone?
Whataretheadvantagesanddisadvantages
ofusingthisenergyresourcecompared
withothers?
Thepresentationshouldmakeuseofaudio-visual
technology(suchaspresentationsoftware),oryou
couldproduceadocumentary.
Thepresentationwillbegradedbytherestofthe
classonitsscientificcontentandthequalityofits
delivery(inotherwords,howwellitispresented).
Youshouldalsoprepareahandoutconsistingof
onesideofA4thatsummarisesthepointsmadein
yourpresentation.
EnergydirectfromtheSun
Inhot,sunnycountries,solarpanelsareusedtocollect
energytransferredbylightfromtheSun.TheSun’srays
fallonalargesolarpanel,ontheroofofahouse,for
example.Thisabsorbstheenergyoftherays,andwater
insidethepanelheatsup.Thisprovideshotwaterfor
washing.Itcanalsobepumpedroundthehouse,through
radiators,toprovideacheapformofcentralheating.
Wecanalsogenerateelectricitydirectlyfromsunlight
(Figure7.3).TheSun’sraysshineonalargearrayof
solarcells(alsoknownasaphotocellsorphotovoltaic
cells).Thesolarcellsabsorbtheenergyoftherays,and
electricityisproduced.
Whilesolarpower(fromsolarpanelsandphotocells)is
renewableanddoesnotcontributetoglobalwarming,it
isunreliablebecausetheintensityofsunlightvaries(and
dropstozeroatnight)andalargeareaofsolarpanelsis
requiredtocapturetheenergy.
However,asthistechnologybecomescheaper,itisfind
moreandmoreuses.Itisusefulinremotelocations.
Forexample,forrunningarefrigeratorthatstores
medicinesincentralAfrica,orforpoweringroadside
emergencyphonesindesertregionssuchastheAustral
outback.Solarcellshavealsobeenusedextensivelyfor
poweringspacecraft.Ideally,asolarcellisconnectedto
rechargeablebattery,whichstorestheenergycollected,
thatitcanbeavailableduringthehoursofdarkness.
KEYWORDS
solarpanel,usedtocollectenergythatis
transferredbylightfromtheSun
solarcell/photocell/photovoltaiccell:an
electricaldevicethattransferstheenergyof
sunlightdirectlytoelectricity,byproducinga
voltagewhenlightfallsonit
Figure7.3:AnarrayofsolarcellsinsidetheVansadNatic
Park,India.
Windpower
WindandwavesarealsocausedbytheeffectsoftheSu
TheSunheatssomepartsoftheatmospheremorethan
others.Heatedairexpandsandstartstomovearound-
thisisaconvectioncurrent(seeChapter11).Thisisthe
originofwinds.Therearemanytechnologiesforextraci
energyfromthewind.Windmillshavebeenusedforak
timeforgrindingandpumping,andmodemwindturbi
cangenerateelectricity(seeFigure7.4).
Windisrenewableanddoesnotcontributetoglobal
warming.However,itisunreliablebecausethespeed
ofthewindcanvaryandoncalmdaysnopoweris
produced.Windturbinesneedaminimumwindspeed
ofabout5m/sandareswitchedoffwhenwindspeeds
exceed25m/stopreventthembeingdamaged.

140. 7Energyresources
Questions
।
Figure7.5:ThegiantItaipudamontheParanaRiverin
SouthAmericagenerateselectricityforBrazilandParaguay.
Windisadiluteenergyresource.Itwouldtakea‘wind
liirm’ofseveralhundredwindturbines(spreadover
>«|veralsquarekilometres)toproducethesameenergyas
itypicalfossilfuelpowerstation.Windturbinesarealso
liqisyandmanypeoplethinktheyspoiltheappearance
'
>1placeswheretheyarelocated.
Iigure7.4:Thesegiantturbinesarepartofawindfarm
«1XinjianginChina.Theyproduceasmuchelectricityasa
modium-sizedcoal-firedpowerstation.
Explainwhywindpowercanbetracedback
tosunlight.
Whatisthedifferencebetweenasolarpaneland
asolarcell?
Describetheadvantagesanddisadvantagesof
solarpower.
Wavepower
Mostoftheenergyofwindsistransferredtothesea
1iwavesareformedbyfrictionbetweenwindand
"liter.Likewind,wavepowerisrenewableanddoesnot
।oiltributetoglobalwarming.However,itisunreliable
liecausetheheightofwavescanvaryand,whenthere
ntnowaves,nopowerisproduced.Itisalsodifficult
inconverttheup-and-downmotionofwavesintothe
pinningmotionrequiredforaturbineinthewave
nergyconverters,whichfloatonthewater.Thecostis
highbecausethesemachinescorrodeinthesaltwaterand
inbedamagedinstorms.
Hydroelectricpower
OneofthesmallestcontributionstothechartinFigure7.2
ishydroelectricpower.Forcenturies,peoplehaveusedthe
kineticenergyofmovingwatertoturnwaterwheels,which
thendrivemachinery.Forexample,theyareusedtogrind
comandothercrops,pumpwaterandweavetextiles.Today
wehavehydroelectricpowerstations(seeFigure7.5).
Waterstoredbehindadamisreleasedtoturnturbines,
whichmakegeneratorsspin.Thisisaverysafe,cleanand
reliablewayofproducingelectricity,butitisnotwithoutits
problems.Anewreservoirfloodslandthatmightotherwise
havebeenusedforhuntingorfarming.Peoplemaybe
madehomeless,andwildlifehabitatsdestroyed.
Hydroelectricpowerstationshaveaveryshortstartup
time(thetimebetweenswitchingonapowerstationand
energybeingdelivered).Thismakesthemveryusefulfor
storingenergyuntilthereisasuddensurge(increase)in
demand.Thedemandforelectricityvariesduringthe
day:itishighestduringthedaytime(whenmostpeople
areawake)andlowestduringthenight.Powerstations
thatusefossilfuelsandnuclearfuelstakealongtime
tostartupandstopso,oncestarted,theyareallowed
tocontinuerunning.Itwouldtaketoolongtostop
themwhendemandislowandthenstartthemagain
forthenextriseindemand.Thismeansthatsometimes
(usuallyatnight)toomuchelectricityissuppliedand
batterytechnologyisnotcurrentlygoodenoughtostore
largeamountsofenergy.Insomehydroelectricpower
stations(calledpumpedstoragesystems),theturbines
canbereversedsothatwatercanbepumpedbackupa
mountaintothereservoirsothatenergycanbestoredas
gravitationalpotentialenergy.Thiswatercanbeallowed
tofallbackdownthemountaintoproduceelectricity
whendemandrises.

141. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Biomassfuels
Formanypeopleintheworld,woodisthemostimportant
fuel.Itwarmstheirhomesandprovidestheheatnecessary
forcookingtheirfood.Woodismadebytreesandshrubs.
Itstoresenergythattheplanthascapturedfromsunlight
intheprocessofphotosynthesis.Whenwebumwood,we
arereleasingenergythatcamefromtheSunintherecent
past,perhapstenorahundredyearsago.
Woodisjustoneexampleofabiofuel.Othersinclude
animaldung(Figure7.6)andbiogas,generatedbyrotting
vegetablematter.
Figure7,6:AMaasaiblowingintoelephantdungtomake
fireinavillageinWestKilimanjaro,Tanzania.
Biomassfuelsaccountforroughly10%ofglobalenergy
consumptionbut,becauseno-onekeepstrackofall
thewoodconsumedasfuel,itisaroughestimate.This
meansthatitisrarelyincludedinglobalfiguresanddoes
notappearinFigure7.2.Abouttwo-thirdsofbiomass
fuelisusedindevelopingcountriesforcookingand
heating.The25%ofpeopleinthedeveloped(industrial)
nationsconsumeaboutsixtimesasmuchenergyasthe
other75%ofpeoplelivinginthedevelopingworld.
Biomasshastheadvantagethatitisrenewableanddoes
notcontributetoglobalwarming.Itisreliablebecauseit
canbeburnedwhenneeded.However,burningbiofuels,
particularlyindoors,canleadtorespiratoryandother
healthproblems.
Fossilfuels
Oil,coalandgasareallexamplesoffossilfuels.These
areusuallyhydrocarbons(compoundsofhydrogenand
carbon).Whentheyareburned,theycombinewith
oxygenfromtheair.Inthi^process,thecarbonbecomes
carbondioxide.Thehydrogenbecomesdihydrogen
monoxide,whichweusuallycallwater.Energyisreleasee
Wecanwritethisasanequation:
hydrocarbon+oxygen—carbondioxide+water
+energy
Hence,wecanthinkofafossilfuelasastoreofchemic:
energy.Wherehasthisenergycomefrom?
Fossilfuels(Figure7.7)aretheremainsoforganisms
(plantsandanimals)thatlivedinthepast.Manyofthe
Earth’scoalreserves,forexample,formedfromtrees
thatlivedintheCarboniferousera,between286and
360millionyearsago.(Carboniferousmeanscoal¬
producing.)ThesetreescapturedenergyfromtheSun
byphotosynthesis.Theygrewandeventuallytheydied.
Theirtrunksfellintoswampyground,buttheydidnot
rotcompletely,becausetherewasinsufficientoxygen.
KEYWORDS
biofuel:material,recentlyliving,qsedasafuel
fossilfuels:material,formedfromlong-dead
material,usedasafuel
Figure7.7:Coalisafossilfuel.Afossilisanylivingmaterial
thathasbeenpreservedforalongtime.Usually,itschemica
compositionchangesduringtheprocess.Coalsometimes
showsevidenceoftheplantmaterialfromwhichitformed.
Sometimesyoucanseefossilisedcreaturesthatlivedin
theswampsoftheCarboniferousera.Thesecreaturesdied
alongwiththetreesthateventuallybecamecoal.
128

142. nuclearfission:theprocessbywhichenergyis
releasedfromnuclearfuelsbythesplittingofa
largeheavynucleusintotwoormoresmallernuclei
geothermalenergy:energystoredinhotrocks
underground
7Energyresources
A«materialbuiltupontopoftheseancienttrees,the
iHireonthemincreased.Eventually,millionsofyears
IIioinpressionturnedthemintoundergroundreserves
I1ionl(Figure7.7).Today,whenweburncoal,thelight
hmweseeandthewarmththatwefeelhavetheirorigins
llieenergyfromtheSuntrappedbytreeshundredsof
billionsofyearsago.
IHliindgasareusuallyfoundtogether.Theyareformed
|limnilarwaytocoal,butfromtheremainsoftiny
'iilinp-likecreaturescalledmicroplanktonthatlived
IIlheoceans.TheoilfieldsoftheArabianGulf,North
IileaandtheGulfofMexico,whichcontainhalf
।
1Ihiworld’sknownoilreserves,allformedinthe
IlInccousera,75to120millionyearsago.
hainingfossilfuelsreleasescarbondioxideintothe
♦innsphere.Thisenhances(increases)thegreenhouse
'Ii(andisthecauseofrecentglobalwarming.Coal
Iinducesmorecarbondioxidethanoilandnatural
1
11Burningcoalandoilusuallyalsoproducessulfur
IIIitide,whichleadstoacidrainanddamageto
'
ystemsandbuildings.
(Juestions
IWhatformofenergyisstoredinfossilfuels
iindbiofuels?
Whatisthedifferencebetweenbiofuelsandfossilfuels?
|IJescribehowfossilfuelsareformed.
Nuclearfuels
NiuIcarpowerwasdevelopedinthesecondhalfofthe
1IIIicentury.Itisaverydemandingtechnology,which
|litresverystrictcontrols,becauseoftheserious

iuniigethatcanbecausedbyanaccident.
iInfuelforanuclearpowerstation(Figure7.8)is
uihillyuranium,sometimesplutonium.Theseare
idloactivematerials.Insideanuclearreactor,the
'
Iinactivedecayofthesematerialsisspeededupsothat
|li*'iiergytheystoreisreleasedmuchmorequickly.This
1«ihiprocessofnuclearfission.
1
111niumisanuclearfuel.Itisaveryconcentratedstore
Mitiergyintheformofnuclearenergy.Atypicalnuclear
[■iwurstationwillreceiveaboutonetruckloadofnew
nmliiachweek.Coalisalessconcentratedenergystore.
Aunilarcoal-firedpowerstationislikelytoneeda
link'trainloadofcoaleveryhour.Awindfarmcapable
mneratingelectricityatthesameratewouldcovera
Ihiitareaofground-perhaps20squarekilometres.
Figure7.8:BellevillenuclearpowerstationinFrance.
Insomecountriesthathavefewotherresourcesfor
generatingelectricity,nuclearpowerprovidesalotof
energy.InFrance,forexample,nuclearpowerstations
generatethree-quartersofthecountry’selectricity.
Excessproductionisexportedtoneighbouringcountries,
includingSpain,SwitzerlandandtheUK.
Nuclearfuelisarelativelycheap,concentratedenergy
resource.However,nuclearpowerhasprovedtobe
expensivebecauseoftheinitialcostofbuildingthe
powerstations,andthecostsofdisposingofthe
radioactivespentfuelanddecommissioningthestations
attheendoftheirworkinglives.Also,accidentslike
thoseatChernobylin1986andFukushimain2011can
causeradioactivematerialtobespreadoverawidearea.
Geothermalenergy
TheinterioroftheEarthishot.Thiswouldbeauseful
sourceofenergy,ifwecouldgetatit.Peopledomakeuse
ofthisgeothermalenergywherehotrocksarefoundata
shallowdepthbelowtheEarth’ssurface.Theserocksarehot
becauseofthepresenceofradioactivesubstancesinsidethe
Earth.Tomakeuseofthisenergy,waterispumpeddown
intotherocks,whereitboils.High-pressurestearhreturnsto
thesurface,whereitcanbeusedtogenerateelectricity.
KEYWORDS
KEYWORDS

143. >
CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
Suitablehotundergroundrocksareusuallyfoundinplaces
wherethereareactivevolcanoes.Iceland,forexample,has
severalgeothermalpowerstations.Thesealsosupplyhot
watertoheatnearbyhomesandbuildings.Whileenergy
fromgeothermalresourceshasnoobviousdisadvantages,
therearefewplacesonEarthwhereitisavailable.
Tidalenergy
Atidalpowerstationissimilartoahydroelectricpower
station:electricalpowerisgeneratedbymovingwater.
Abarrage(dam)isbuiltacrossariverestuary(wherea
rivermeetsthesea)creatingareservoir.Asthetidegoes
inandout,waterpassesthroughturbinesinthedam.
Tidalpowerhastheadvantageofbeingrenewable.Also,
tidesarepredictablemakingitafairlyreliableenergy
resource.However,byfloodingestuaries,atidalpower
stationcandestroywetlands,animportanthabitatfor
wildlife,particularlymigratingbirdsthatuseittofeed
andrestbeforethenextlegoftheirjourney.Thebarrage
canalsoblockshippingroutes.
Questions
7Listtheadvantagesanddisadvantagesof
nuclearpower.
8Whatenergyisavailablefrommovingwater?
9Inwhatwayisahydroelectricpowerstationlike
acell(orbattery)andhowcanitbeadaptedto
actlikearechargeablecell?
Usingenergyresourcesto
generateelectricity
Manyoftheenergyresourcesinthischapterproduce
electricitysothatitcanbetransferredtowhereitis
needed.Thethermalenergyproducedwhenfossilfuel
areburned(Figure7.9)orwhennuclearfissiontakes
placeisusedtoheatwaterinaboilertoformsteam.
Thesteamturnsthebladesofaturbine,transferring
thermalenergyintokineticenergy.Theturbineislinki
byanaxletoageneratorwhereavoltageisinducedin
conductingwireswhentheymoveinamagneticfield.
YouwilllearnmoreaboutgeneratorsinChapter21.
Thedetailsofhowthermalenergyisproducedinpowe:
stationsthatusefuelwillvary,buttheywillallhavea
boiler,turbineandgenerator.Thoseenergyresourcestl
donotuseafuelwillnotneedaboiler,buttheywillstil
useaturbinelinkedtoageneratortoproduceelectricity
Movingair(wind)andmovingwater(hydroelectricity,
waveandtidal)canturnaturbinedirectly.
KEYWORDS
boiler:devicewherethermalenergyistransferrei
towatertoturnitintosteam
turbine:adevicethatismadetoturnbymovingaii
steamorwater;oftenused(ogenerateelectricity
generator:adevicewhichgenerateselectricity
usingelectromagneticinduction
Figure7.9:Aschematicofcoal-firedpowerstation.Thedetailsofhowthermalenergyisproducedinpowerstationsthati.
fuelwillvarybuttheywillallhaveaboiler,turbineandgenerator.
130>

144. 7Energyresources
Part1
SangheraIsland(Figure7.10)isaremotefictitious
island.Ithasnofossilfuels.Itishotwithjungle
vegetation,thoughitcansometimesbecold
atnight.Highrainfallandmountainousterrain
leadstofast-flowingstreams.Youareoneof25
membersofascientificexpeditionplanningto
studytheislandforthreeyears.Atthesametime
Io*t
*■lumldseparateinitialcostsfromrunningcosts.A
toiniIIisexpensivetobuybuttherearenocostsfor
-unlightisfree!Whilenuclearfuelischeap,the
lioldecommissioningnuclearpowerplantsarehigh.
*
iilfuelpowerstationcanbecompactandstillsupply
Kilt"population.Itwouldtakeseveralsquaremetres
।Incellstosupplyasmallhousehold.Alternatively,
Hiililometimestalkabouthowconcentratedordilute
P
i।»ipyresourceis.Whentalkingaboutfuels,theyare
i"l'11mghowmuchenergyisstoredinacertainmassof
IteiitIParticularlywhencomparingwindturbineswith
|Hii
।nergyresources,peoplewilltalkabouttheland
M»ii<|uiredtogeneratethesameamountofenergy.
Avnil.ibility
ft
uhrusesnuclearpowertoproduceabout75%ofits
I'
iui(ybecauseithasfewalternativeenergyresources.
ihasplentyofrainfallandmountainssogenerates
*ml5%ofitselectricityfromhydroelectricpower.
h
1milusesgeothermalenergy,whichisquitelocalised.
Hnllnbility
f
'
'nergysupplyconstantlyavailable?Thewindis
*«iithlc,sowindpowerisunreliable.Warsandtrade
..Tmescaninterruptfuelsupplies.
(ornparingenergyresources
k.iihfossilfuelsalotbecausetheyrepresent
|UHrintratedsourcesofenergy.Amoderngas-fired
F
|11nationmightoccupythespaceofafootball
uniandsupplyatownof100000people.Toreplace
diawindfarmmightrequire50ormorewind
mluiripreadoveranareaofseveralsquarekilometres.
Ila
।indisamuchmoredilutesourceofenergy.
IImillustratessomeoftheideasthatweusewhencomparing
illenergyresources.Eachhasitsadvantagesand
Iullages.Weneedtothinkaboutthefollowingfactors.
M«n«wability
ihaveseen,therearelimitedreservesoffossilfuels.
IInmineappliestouraniumnuclearfuel.However,
CH
nrplentifulreservesofalternativenuclearfuels
Ihoiium.Allotherenergyresourcesarerenewable,
i>liulingallthosethatcanbetracedbacktoradiation
h 11heSun.Themostimportantadvantageof
ableresourcesisthat,onceinstalled,theydonot
miilbutetoglobalwarming.
SangheraIsland
N
1km
Environmentalimpact
Theuseoffossilfuelsleadstoclimatechange.
Ahydroelectricdammayfloodusefulfarmland.
Everyenergysourcehassomeeffectontheenvironment.
Questions
10Whichofthefollowingenergyresourcesisrenewable?
Aoil Cbiofuels
Bnuclear Dcoal
11Whichofthefollowingenergyresourcesisnot
renewable?
Ahydroelectricpower Ctidal
Bwind Dnuclear
Figure7.10:MapofSangheraIsland.
Key
forest
stream
Ohotsprings

spotheights
tracks
ACTIVITY7.2

145. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
asminimisingyourimpact(ecologicalfootprint)
ontheisland,youaretheexpertgiventhejobof
providingalltheenergytheteamwillneed.
Answerthefollowingquestionstodevelopaplan
thatyouwillpresenttotheteam.
1Describetwowaysofprovidinghotwaterand
heatforthebuildings(livingaccommodation
andlaboratory).
2Describetwopossiblewaysofsupplyingheat
forcookingfood.
3Howwouldyousupplytheelectricityneeded
forlighting,PCsandmachinery?
4Therewillbearefrigeratorforlife-saving
medicines,aswellaschemicals,thatneedtobe
storedataconstanttemperature.Howwould
youensureaconstantsupplyofelectricity?
5Whichnaturalresourceontheislandshould
beconserved?
6Onacopyofthemap,markwhereyouplan
tolocatebuildingsandtheenergyharvesters
(devicesthatcollectenergyfromthe
environment),showinghowyouwouldgetthe
energyfromtheharvesterstothebuildings,if
necessary.Explainyourreasonsforchoosing
theseparticularsites.
7Wouldyouranswersbedifferentiftheteam
hadalimitedbudget?Explainyouranswer.
8Describewhatotherinformationyou
mightneedbeforeyoucanmakea
recommendationtotheteam.
Part2
Inagroupoffour,designyourownislandand
abrieforsetofquestionsforanothergroupto
decidehowtheywillprovidetherequiredenergy.
Bepreparedtoanswerquestionsandhaveyour
ownsolutioninmind.
REFLECTION
Isthereaneffectivemethodformemorisingthe
differentenergyresources?Forexample,could
youputthemintocategories?Howwillyoulearn
theiradvantagesanddisadvantages?
ACTIVITY7.3
Planningmeeting
Workinagroupofthree.Eachgroupwillberandom
allocateda'powerstation'thatusesadifferentenerc
resource(suchasawindfarm).Youwillbegiventhe
roleofeitheradvocating(supporting)oropposing
planningpermissionforyourallocatedpowerstation
tobebuiltclosetoyourcommunity.
Youwillneedtoidentifythegeneraladvantages
ordisadvantagesofyourpowerstationbefore
identifyingthespecificadvantagesordisadvantage
oflocatingitclosetoyourcommunity.
Eachgroupwillpresenttheircaseasaspoken
presentation,withawhiteboardandpensthe
onlyresourcespermitted.
Youwillbegivenashorttimetoagree,asa
class,thecriteriaforjudgingeachcase.For
example,youmightdecidetodismissacast
ifthescienceisincorrect.
Youwillthenbeallocatedyourpowerstatioi
Unlessitisgivenasahomeworktask(when
youcouldspendtimedoingmoredetailed
research),spendaboutfiveminutesinyour
groupspreparingyourcase.
Thoseadvocatingandopposingeachpower
stationwillbecalledtothefrontoftheclass
andeachgivenamaximumofoneminuteto
puttheircasetotherestoftheclass.Theclass
willhaveamaximumof30secondstoquestio
eachcase.Whilelisteningtoeachcase,note
downtheadvantagesanddisadvantagesof
eachpowerstationinatablesothatyouhav
asummary.Questionsfromtheclassand
answersfromyourteachershouldidentify
anyerrors.
Whileyouareintheaudience,youwillscore
eachpresentationoutof10accordingtothe
criteriaagreed.Subtractyourvoteforthe
oppositionfromyourvotefortheadvocatesto
getanoverallscore.Twoexamplesaregivenir
Table7.1.Anegativescoresuggestsoppositio
but,ifallproposalsarenegative,youmightbe
forcedtochoosetheleast-worstoption.
Table7.1:Examplescoresheet.
Typeofpower
station
ForAgainstOverall
score
windfarm 2 4 -2
nuclear 7 3 4
132
>

146. 7Energyresources
REFLECTION
DidyoufindActivity7.3helpfulinsummarising
theadvantagesanddisadvantagesofthe
differentenergyresources?Isthereabetter
methodforlearningthismaterial?
.2EnergyfromtheSun
listoftheenergyweusecanbetracedbackto
li.itionfromtheSun.Tosummarise:
Fossilfuelsarestoresofenergythatcamefromthe
Sunmillionsofyearsago.
Radiation(lightandheat)fromtheSuncanbe
absorbedbysolarpanelstoprovidehotwater.
Sunlightcanalsobeabsorbedbyarraysofsolar
cells(photocells)togenerateelectricity.Insome
countries,youmayseetheseontheroofsofhouses.
IliewindiscausedwhenairisheatedbytheSun.
Warmairrises;coolairflowsintoreplaceit.This
movingaircanbeusedtogenerateelectricityusing
windturbines.
Mosthydroelectricpowercomesultimatelyfrom
theSun.TheSun’srayscausewatertoevaporate
fromtheoceansandlandsurface.Thiswater
vapourintheatmosphereeventuallyformsclouds
nthighaltitudes.Rainfallsonhighground,and
vanthenbetrappedbehindadam.Thisispartof
thewatercycle.WithoutenergyfromtheSun,there
wouldbenowatercycleandnohydroelectricpower.
jwuver,wemakeuseofasmallamountofenergy
ntdoesnotcomefromtheSunasradiation.Hereare
Ktexamples:
IheMoonandtheSunbothcontributetothe
oceans’tides.Theirgravitationalpullcausesthe
leveloftheocean’ssurfacetoriseandfallevery
iwclve-and-a-bithours.Athightide,watercanbe
11appedbehindadam.Later,atlowertides,itcanbe
ileasedtodriveturbinesandgenerators.Because
Ihisdependsongravity,andnottheSun’sheatand
light,wecanrelyontidalpowerevenatnightand
whentheSunishiddenbytheclouds.
Nitclearpowermakesuseofnuclearfuels-mostly
in.inium-minedfromunderground.Uraniumisa
radioactiveelement,whichhasbeeninthe
ItroundeversincetheEarthformed,togetherwith
IhrrestoftheSolarSystem,4.5billionyearsago.
KEYWORDS
watercycle:waterevaporatesfromthesurface
oftheEarth,risesintotheatmosphere,cools,
condenses,andfallsasrain
nuclearfusion:theprocessbywhichenergyis
releasedwhentwosmalllightnucleijointogether
toformanewheaviernucleus
Geothermalenergyalsodependsonthepresenceof
radioactivesubstancesinsidetheEarth.Thesehave
beentheresincetheEarthformed;theyhavebeen
continuouslyreleasingtheirstoreofenergyever
since.
ThesourceoftheSun'senergy
TheSunreleasesvastamountsofenergy,butitisnot
burningfuelinthesamewayaswehaveseenforfossil
fuels.Itisnotachemicalreaction.TheSunconsists
largelyofhydrogen,butthereisnooxygentoburnthis
gas.Instead,energyisreleasedintheSunbytheprocess
ofnuclearfusion.Innuclearfusion,fourenergetic
hydrogenatomscollideandfuse(jointogether)toform
anatomofhelium.
Nuclearfusionrequiresveryhightemperaturesand
pressures.ThetemperatureinsidetheSuniscloseto15
milliondegrees.Thepressureisalsoveryhigh,sothat
hydrogenatomsareforcedveryclosetogether,allowing
themtofuse.Atthistemperaturealltheatomsare
ionised.Alltheelectronshavebeenremovedfromallthe
atoms,creatingplasmaofpositivenucleiandnegative
electrons.Atomicnucleiallhaveapositivechargeand
likechargesrepelsoatemperatureofabout100million
degrees(andhighpressure)isrequiredtoovercomethis
electrostaticrepulsionandgetthenucleicloseenough
tofuse.Themassofthefinalnucleusisslightlylessthan
thecombinedmassoftheinitialnucleiandthedifference
inmassisturnedintoenergyaccordingtoEinstein’s
famousequation:E=me1.Theenergy,E,releasedisbig
becausethemass,m,ismultipliedbythespeedoflight,c,
squared(whichisabignumber).
Nuclearfusionreactors-
artificialSunsonEarth?
Scientistshavebeentryingtorecreatethesameprocess
artificiallyhereonEarthsincethe1950sasitoffers
acleansourceofalmostunlimitedenergy.Itwillnot
producegreenhousegasesornuclearwaste.
133

147. CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Itisessentialtoholdthehotplasmainplaceforlong
enoughforfusiontotakeplace.IntheSun,thestar’s
enormousgravitationalfieldpreventstheplasma
escaping.Inthe1950s,Sovietphysicistscameupwiththe
ideaofatokamak(Figure7.11)tocontaintheplasma.
Figure7.11:China'snewtokamakunderconstruction
inChengdu.
Thisisacontainershapedlikeatorus(ordoughnut)
withacomplicatedarrangementofmagnetstostopthe
plasmatouchingthewalls.Iftheplasmaweretotouch
thecontainerwalls,itwouldcool(andfusionwould
stop)andthecontainerwallswouldbedamaged.Thisis
whyfusionisaverychallengingengineeringproblem.
FusionreactorsonEarthwillfusedeuteriumand
tritium(twoisotopesofhydrogen)toproducehelium
andaneutron.Onlychargedparticles(thataremoving)
canexperienceamagneticforceandbeconfinedbya
magneticfield.Neutronsareneutral(havezerocharge)
socannotbeconfinedbythemagneticfieldandsothey
hitthewallsofthetokamak.Thesecollisionsproduce
thermalenergy.Heatexchangersinthewallsconductthe
thermalenergytoheatupwatertomakesteamtoturna
turbineandproduceelectricity.
Sofar,nofusionreactorWasproducedmoreenergythai
needstobeputintokeeptheplasmahot.In1997Joint
EuropeanTorusclaimedtheworldrecordforgetting
out67%oftheinputenergy.Itishopedthatreactors
willgetouttentimesmoreenergythanisputin.The
InternationalThermonuclearExperimentalReactor
(ITER)project(Figure7.12)isbeingbuiltatCadaracht
inFrance.Thisisaninternationalcollaborationthat
involvesscientistsfromcountriesthatrepresenthalfthe
world’spopulation.
Figure7.12:Employeesinlate2018buildingthetokamak
insidetheITER(InternationalThermonuclearExperimental
Reactor)constructionsiteinFrance.
\
Questions
12Whatisplasma?
13Comparehowplasmaisconfinedinastarwithhoi
wewillachievethesameeffecthereonEarth.
14Discusstheadvantagesanddisadvantagesof
nuclearfusion.
15Whatisthedifferencebetweennuclearfusionand
nuclearfission?
134>

148. 7Energyresources
Thefutureofenergyresources
Thereisahugevarietyofpotentialprojectsinthis
Importantareaofphysics.Weneedenergybut
gettingitbyburningfossilfuelscontributestoglobal
warming.Whatevertopicyouchoose,youneed
toposeaquestionliketheonedescribingthorium
reactorsatthestartofthechapter.Youranswer
ihouldbeclear,conciseandcoherent.Writeitin
yourownwordsandlimityourselfto1000words.
Useinformativediagramswhereyoucan.Whatever
mediumyouchoosetoconveyyouranswer,tryto
reachanaudiencebeyondyourownclassroom.
Or,youcouldpromote(forexample,toyourfriends
.Indfamilyonsocialmedia)goodworkthatyouhave
discoveredduringyourresearch.Youcould:
Investigatewaystoreducedemandforenergy.
Incoldcountriesthismightincludeefforts
toimprovebuildinginsulationwhile,inhot
countries,itmightbeworththinkingaboutthe
developmentofwindtowerstoreducedemand
forairconditioning.Oryoucouldfocusonthe
developmentofmoreefficienttransport(such
aselectricvehicles).
Investigatewaystoincreasethesupplyof
energy.Thiscouldfocusononeoftheenergy
resourcesyouhavealreadymetinthischapter,
oryoumightinvestigateonethatisunder
development(forexample,onebasedonalgae).
Investigatethechallengesfacingdevelopment
oftheliftersyoumetatthestartofthechapter.
Youshouldlookattheprocessandmakea
comparisonwithnuclearpowerbasedonthe
uraniumcycle.
Investigatefracking.Youshouldexplainthe
processitselfandofferabalancedassessment
oftheadvantagesanddisadvantages.
Investigatedevelopmentsinenergystorage,
includingimprovedbatterytechnology,which
mightincludetheenvironmentalimpactof
lithiummining.
PROJECT

149. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
SUMMARY
SolarpanelsareusedtocollectenergyfromtheSun.
Solarcells(alsoknownasphotocells)generateelectricityusingenergyfromtheSun.
Oil,coalandnaturalgasareallexamplesoffossilfuels.
Coalformsfromtrees,andoilandnaturalgasformfrom microplankton.
Non-renewableenergyresourceswillrunout.Thisincludesfossilfuelsandnuclearfuelbutnotbiofuel.
Renewableenergyresourcesarereplenished(replaced)aftertheyhavebeenused.
Biofuelsarerenewable,reliable,cheaptosetupanduse,butarediffuse.
Geothermalenergyisharvested(collected)wherehotrockisclosetotheEarth’ssurface.
Windpower,wavepowerandsolarpowerarerenewablebutunreliableanddiluteenergyresources.Running
costsarelowbuttheyareexpensivetosetup.
Hydroelectricpower,tidalpowerandgeothermalpowerarerenewable,reliableandconcentratedenergy
resourcesbutsuitablelocationsarelimited,andtheyareexpensivetosetup.
Nuclearpowerstationsusenuclearfuel,whichproducesthermalenergybynuclearfissionwhenheavynuclei
breakapart.
TheSunistheoriginofallourenergyresourcesexceptgeothermal,nuclearandtidal.
ThesourceoftheSun’senergyisnuclearfusion,whenhydrogenfuses(joins)togethertoformhelium.
Wearetryingtodevelopnuclearfusionreactors.
EXAM-STYLEQUESTIONS
1Whichofthefollowingenergyresourcesdoesnotdependonsunlight? [1]
Ahydroelectricpower Ccoal
Btidal Dwind
2Whichofthefollowingenergyresourcesproducesgreenhousegases? [1]
Ahydroelectricpower Cbiofuels
Bnuclear Dnaturalgas
3Whichofthesepowerstationsgenerateselectricityfromgravitational
potentialenergy? [1]
Atidalpowerstation Cwindpower
Bgeothermalpowerstation Dnuclearpowerstation
4Whichofthesepowerstationsgenerateselectricityfromthermalenergy?[1]
Ahydroelectricpowerstation Ccoal-firedpowerstation
Bwindfarm Dsolarfarm
136y

150. 7Energyresources
[4]
[1]
[2]
[4]
NTINUED
COMMAND WORDS
state:expressin
clearterms
11<11isalistofenergyresourcesavailabletotheworld.Someoftheseare
nnewableandsomearenon-renewable.
>ti
1nergycompanyisproposinganewpowerstationbuthastodecide
11 tenasolarpowerstationandageothermalpowerstation.
uplnnhowthelocationandtheclimatemightaffectthedecision.
explain:setout
purposesor
reasons;make
therelationships
betweenthings
evident;provide
whyand/orhowand
supportwithrelevant
evidence
।
Ithetable.Inthefirstblankcolumn,putatickbyanythreeresources
Hiiiirenon-renewable.
Inihisecondblankcolumn,putatickbyanythreeresourcesthatare
iiiwable.
Hu
।uestionisabouthydroelectricpowerstations.Waterisstoredbehind
।111in.Waterreleasedfromthedamflowsdownhillandspinsaturbine.
Ihspinningturbinecausesageneratortoturn,whichproduceselectricity.
4Writedownthetwoenergytransfersthatoccurinahydroelectricpower
i.ition.
>lainhowelectricityfromahydroelectricpowerstationrelieson
।nergyfromtheSun.
I Ihedemandforelectricityisnotconstant.Explainhowahydroelectric
I>werstationcanhelpmatchsupplytodemand. [2]
[Total:5]
11-firedpowerstationandawindturbinebothproduceelectrical
i.Thepowerstationproduces2500MWandthewindturbine
miuces2.0MW.
oneadvantageofusingwindturbinesinsteadofacoal-fired
1
1<>werstationtoproduceelectricity. [1]
>al-firedpowerstationsstillaccountforonethirdoftheworld’senergy
insumption.Explainwhywindturbineshavenotreplacedthem. [2]
[Total:3]
|Energyresource Non-renewable Renewable
wavepower
|hydroelectricity
quothermal
|ioal
nuclearenergy
I
oil
|'.olarenergy
naturalgas
lidalenergy
[windenergy

151. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
9Imaginethatyouarewritingasciencefictionstoryinwhichfossilfuelshave
runoutandtheSunstopsshining.Onlypowerstationsbasedonthreedifferent
energyresourceswillcontinueproducingpower.
aYouwanttomakeyourstorybelievable,sowhatpowerstationsshould
peopleuseiftheywanttosurvive? [1]
bAsthetemperaturefalls,explainwhyoneofthesepowerstations
mightstopworking. [1]
[Total:2]
SELF-EVALUATIONCHECKLIST
Afterstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutose<
anygapsinyourknowledgeandhelpyoutolearnmoreeffectively.
Ican
See
Topic...
Needs
morework
Almost
there
Confiden
tomove
Appreciatethatelectricityisaconvenientwayofmoving
energytowhereitisused.
7.1
Recallalltheenergyresources:fossilfuels,biofuels,
nuclear,geothermal,solar,hydroelectric,tidal,wind
andwave.
7.1
Describehoweachenergyresourcecanproduce
electricityorotherusefulformsofenergy.
7.1
Giveadvantagesanddisadvantagesofeachenergy
resourceintermsofrenewability,cost,reliability,
availability,scaleandenvironmentalimpact.
7.1
UnderstandthattheSunistheoriginofallourenergy
resourcesexceptgeothermal,nuclearandtidal.
7.2
Understandthatnuclearfusionisthesourceofthe
Sun’senergy.
7.2
Understandthattherehasbeenalotofresearchinto
developinganuclearfusionreactor.
7.2
138
>

152. Chapter8
A/ork
andpower
INTHISCHAPTERYOUWILL:

153. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
GETTINGSTARTED
Whatistheeverydaymeaningof'work'?Doyouknowwhattheword'work'meansinphysics?
Whatistheeverydaymeaningof'power'?Doyouknowwhattheword'power'meansinphysics?
THEMACHINEAGE
Scottishengineer,JamesWatt(Figure8.1),
developedthesteamenginejustovertwocenturies
ago.ItdeliveredthepowerforBritaintoenter
theindustrialage.Accordingtolegend,Watt's
firstcustomerwantedasteamengineonlyifit
couldpumpatleastasmuchwaterfromawell
tohisbreweryasoneofhishorses.Thebrewer
pickedhisstrongesthorseandworkedhimhard
foreighthours.Thehorsepumped33000lb
(1lb=0.453592kg)ofwatertoaheightofonefoot
(0.3048metres)perminute;thisistheoriginofthe
unitofhorsepower.Asyouwillsee,thisisequivalent
toabout750watts.Thewatt,W,istheSIunitfor
power,namedafterJamesWatt.Longbeforesteam
power,humansdevelopedmechanicalmachinesto
performtasksthatmightotherwisebeimpossible
(suchastheramptobuildthepyramids).Electric
motorshavebeendevelopedsince.
Discussionquestions
1Whataretheadvantagesanddisadvantages
ofusingsteampower?
2Canweproducepowerwithoutanyofthe
disadvantagesofsteampower?
3Thesteamengineisamachinethatchanges
thermalenergy(fromadifferencein
temperature)intomechanicalwork.What
othermachinesarementioned
inthissection,
andwhatenergytransferstakeplace?
Figure8.1:TheGoldenBoysstatueintheUK,whichshowsMatthewBoulton,JamesWatt(centre)andWilliamMurdoch
whoallplayedapartinimprovingthesteamengine.
140>

154. 8Workandpower
KEYWORDS
IIIUBiHI
IIgure8.2:Liftinganobjectrequiresanupwardforce,pulling
.1'iilnstgravity.Astheboxrisesupwards,theforcealsomoves
i|>Wards.Energyisbeingtransferredbytheforcetothebox.
workdone:theamountofenergytransferred
whenonebodyexertsaforceonanother;the
energytransferredbyaforcewhenitmoves;
workdone=energytransferred
Threefurtherexamplesofforcesdoingworkareshown
inFigure8.3.
Mechanicalorelectricalworkisequaltoenergytransferred.
Inthischapter,wearefocussingonmechanicalwork.
ElectricalworkisdiscussedinChapter18.
Howmuchwork?
Thinkaboutliftingaheavyobject,asshowninFigure8.2.
Aheavyobjectneedsabigforcetoliftit.Theheavier
theobjectis,andthehigheritislifted,themoreits
g.p.e.increases.Thissuggeststhattheamountofenergy
transferredbyaforcedependsontwothings:
thesizeoftheforce-thegreatertheforce,themore
workitdoes
thedistancemovedinthedirectionoftheforce-the
furtheritmoves,themoreworkitdoes.
Soabigforcemovingthroughabigdistancedoesmore
workthanasmallforcemovingthroughasmalldistance.
8.1Doingwork
1
Ipure8.2showsonewayofliftingaheavyobject.
Itillingontheroperaisestheheavybox.Asyoupull,the
।rcmovestheboxupwards.
I।liftanobject,youneedastoreofenergy(aschemical
ncrgy,inyourmuscles).Yougivetheobjectmore
»invitationalpotentialenergy(g.p.e.).Theforceisyourmeans
iiltransferringenergyfromyoutotheobject.Thenamegiven
li'thistypeofenergytransferbyaforceisdoingwork.
IInmoreworkthataforcedoes,themoreenergyit
minsfers.Theamountofworkdoneissimplytheamount
illenergytransferred:
workdone=energytransferred
Iflure8.3:Threeexamplesoffereesdoingwork.Ineachcase,theforcemovesasittransfersenergy,a:Pushingashopping
Holleytostartitmoving.Thepushingforcedoeswork.Ittransfersenergytothetrolley,andthetrolley'skineticenergy
(I
increases,b:Anapplefallingfromatree.Gravitypullstheappledownwards.Gravitydoeswork,andtheapple'sk.e.
increases,c:Brakingtostopabicycle.Thebrakesproduceaforceoffriction,whichslowsdownthebicycle.Thefrictiondoes
work,andthebicycle'sk.e.istransferredtotheinternalenergyofthebrakes,whichgethot.

155. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Wordsinphysics
Youwillbynowunderstandthat‘work’isawordthat
hasaspecialisedmeaninginphysics,differentfromits
meaningineverydaylife.Whenphysiciststhinkaboutthe
ideaofwork,theythinkaboutforcescausingmovement.
Ifyouaresittingthinkingaboutyourhomework,no
forcesarecausingmovementandyouaredoingnowork.
Itisonlywhenyoustarttowritethatyouaredoingwork
inthephysicssense.Tomaketheinkflowfromyourpen,
youmustpushagainsttheforceoffriction,andthenyou
reallyareworking.Similarly,youaredoingwork(inthe
senseofphysics)whenyouliftupthisheavybook.
Manywordshavespecialisedmeaningsinscience.In
earlierchapters,weusedthesewords:‘force’,‘mass’,
‘weight’,‘velocity’,‘moment’and‘energy’.Eachhasa
carefullydefinedmeaninginphysics.Thisisimportant
becausephysicistshavetoagreeonthetermsthey
areusing.However,ifyoulookthesewordsupin
adictionary,youwillfindthattheyhavearangeof
everydaymeanings,aswellastheirspecialisedscientific
meaning.Thisisnotaproblem,providedyouknow
whetheryouareusingaparticularwordinitsscientific
senseorinamoreeverydaysense.(Somephysicistsget
veryupsetiftheyhearshopkeeperstalkingaboutweights
inkilograms,forexample,butno-onewillunderstand
youifyouaskfor10newtonsoforanges!)
8.2Calculatingworkdone
Whenaforcedoeswork,ittransfersenergytotheobject
itisactingon.Theamountofenergytransferredisequal
totheamountofworkdone.Wecanwritethisasa
simpleequation:
W-^E
Inthisequation,weusethesymbolA(Greekcapital
letterdelta)tomean‘amountof’or‘changein’.So,
hE=changeinenergy
Howcanwecalculatetheworkdonebyaforce?Thework
donedependsontwothings:
thesizeoftheforce,F
thedistance,d,movedbytheforce.
Wecanthenwriteanequationforthis:
workdonebyaforce=forcexdistancemovedby
theforceinthedirectionoftheforce.
Insymbols:
W=Fd=^E
KEYEQUATION
workdonebyaforce=forcexdistancemovedby
theforceinthedirectionof
theforce
W=Fd=^E
Thephrase‘inthedirectionoftheforce’willbeexplainec
shortly.Asamountofworkdoneisthesameasthe
amountofenergytransferred,itismeasuredinjoules(J)
theunitofenergy.
Joulesandnewtons
Theequationfortheworkdonebyaforce(W=Fxd)
showsustherelationshipbetweenjoulesandnewtons.
IfwereplaceeachquantityintheequationbyitsSI
unit,weget1J=1Nx1m=1Nm.So,ajouleisa
newtonmetre.
KEYWORD
joule(J):theSIunitoftransferredenergy(orwork
done);workdoneistheforceofonenewton(1N)
whenappliedthroughadistanceofonemetre
(1m);1J=1Nm
WORKEDEXAMPLE8.1
Acraneliftsacrateupwardsthroughaheightof
20metres.Theliftingforceprovidedbythecraneis
5.0kN,asshowninFigure8.4.
aHowmuchworkisdonebytheforce?
bHowmuchenergyistransferredtothecrate?
Figure8.4
142

156. 8Workandpower
Step1:Writedownwhatyouknow,andwhatyou
wanttoknow.
F=5.0kN=5000N
d=20m
W=?
Step2:Writedowntheequationforworkdone,
substitutevaluesandsolve.
^=Fx</=5000Nx20m
=100000Nm=100000J
Answer
nI'heworkdonebytheforceis100000J,or100kJ.
I>Sinceworkdone=energytransferred,100kJof
energyistransferredtothecrate.
Workdoneandmgh
linkedExample8.1illustratesanimportantidea.
Ilitforceprovidedbythecranetoliftthecratemust
i|iliilthecrate’sweight,mg.Itliftsthecratethrougha
hiIght,h.Thentheworkitdoesisforcexdistance,ormg
hI'hegaining.p.e.ofthecrateismgh.Thisexplains
*
heretheequationforg.p.e.comesfrom.
InIigure8.5,thechildslidesdowntheramp.Gravity
nil*,herdownwards,andmakesherspeedup.To
*ikulatetheworkdonebygravity,weneedtoknowthe
Wlticaldistance,h,becausethisisthedistancemovedin
(Indirectionoftheforce.Ifwecalculatedtheworkdone
nweightxdistancemoveddowntheramp,wewouldget
hinnswerthatwastoolarge.Nowyoushouldunderstand
*
hywewritethedefinitionofworkdonelikethis:
>orkdone=forcexdistancemovedinthedirection
oftheforce
Forcesdoingnowork
Ifyousitstillonachair(Figure8.6),therearetwoforces
actingonyou.Theseareyourweight,mg,actingdownwards,
andtheupwardcontactforce,C,ofthechair,whichstops
youfromfallingthroughthebottomofthechair.
Neitheroftheseforcesisdoinganyworkonyou.The
reasonisthatneitheroftheforcesiscausingmovement,
soyoudonotmovethroughanydistance,d.FromW=F
xd,theamountofworkdonebyeachforceiszero.When
yousitstillonachair,yourenergydoesnotincreaseor
decreaseasaresultoftheforcesactingonyou.
Figure8.6:Whenyousitstillinachair,therearetwoforces
actingonyou.Neithertransfersenergytoyou.
1l(|ure8.5:Itisimportanttousethecorrectdistancewhen
*
-lieulatingworkdonebyaforce.Gravitymakesthechild
llhIedowntheslope.However,tocalculatetheenergy
|i'"ferredbygravity,wemustusetheverticalheightmoved.
contact
forceof
chair,C
Figure8.7:Thespacecraftstaysataconstantdistancefrom
theEarth.Gravitykeepsitinitsorbitwithouttransferringany
energytoit.
weight,
Figure8.7showsanotherexampleofaforcethatisdoing
nowork.AspacecraftistravellingaroundtheEarthina
circularorbit.TheEarth’sgravitypullsonthespacecraft
tokeepitinitsorbit.Theforceisdirectedtowardsthe
centreoftheEarth.However,sincethespacecraft’sorbit
iscircular,itdoesnotgetanyclosertothecentreofthe
Earth.Thereisnomovementinthedirectionoftheforce,
andsogravitydoesnowork.Thespacecraftcontinues
atasteadyspeed(itsk.e.isconstant)andataconstant
heightabovetheEarth’ssurface(itsg.p.e.isconstant).
Ofcourse,althoughtheforceisdoingnowork,thisdoes
notmeanthatitisnothavinganeffect.Withouttheforce,
thespacecraftwouldescapefromtheEarthanddisappear
intothedepthsofspace.
gravity
ONTINUED
IYEQUATION

157. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
WORKEDEXAMPLE8.2
Agirlcanprovideamaximumpushingforceof200N.
Tomoveaboxweighing400Nontoaplatform,she
usesaplankasaramp,asshowninFigure8.8.
aHowmuchworkdoesshedoinraisingthebox?
bHowmuchg.p.e.doestheboxgain?
400N
Figure8.8
Step1:Writedownwhatyouknow,andwhatyou
wanttoknow.
pushingforcealongtherampF=200N
distancemovedalongrampd=2.5m
weightofboxdownwardsmg=400N
verticaldistancemovedh=0.75m
workdonealongtherampW=?
workdoneagainstgravityW=?
Step2:Calculatetheworkdone,W,bythepushing
forcealongtheramp.
W=pushingforcealongrampxdistance
movedalongramp
-F^d
=200Nx2.5m
=500J
Step3:Calculatethegaining.p.e.ofthebox.Thisis
thesameastheworkdoneagainstgravity,W.
W'=weightofboxxverticaldistancemoved
=mgxh
=400Nx0.75m
=300J
Answer
aThegirldoes500Jofworkinraisingthebox.
bTheboxgains300Jing.p.e.
Note:only300Jistransferredtothebox.The
remaining200Jistheworkdoneagainstfrictionas
theboxispushedalongtheslope.
ACTIVITY8.1
Theefficiencyofaramp
TheAncientEgyptiansarefamousfortheir
pyramids.Thepyramidsaremadefromstone
blockswithamassofatleastonetonne.The
usefulworkdoneinliftingablockistheweight
oftheblockmultipliedbytheverticaldistance
ismoved.Whethertheblockisdraggedorlifte
workhastobedoneagainsttheforceofgravit
TheAncientEgyptiansdidnothavecranes.It
isbelievedtheyusedrampstodragtheblocks
intoplace.Therampisanexampleofasimple
machine.Itreducestherequiredforce(sometir
calledtheeffort)butthisforcehastobeapplie
overabiggerdistancethanifitweremoved
vertically.Iftherewasnofriction,therampwou
be100%efficient.However,additionalworkne
tobedoneagainstfriction.
Yourtaskistodesignanexperimenttoworkoi
howtheefficiencyofarampdependsontheai
oftheslope.
Youneedtorecallwhatefficiencyisandhowtc
calculateit.
Youneedtodecideon:
theequipmentyouwillneed(sketch
alabelleddiagramofyourassembled
apparatus)
yourindependent,dependentandcentre
variables(thatis,whatyouwillchange,wl
youwillmeasureandwhatyouwillkeept
same)
themeasurementsthatyouwilltake(desi
atabletorecordthem)
whatyouneedtocalculate(whatequatio
youwillneed)
thegraphthatyouwillplotandpredictw
yourgraphshouldlooklike.
Thenjointogethertoformdesignteamsofthn
First,peerassesseachother'swork.Thendeve
acombinedplan,puttingtogetherthebestof
individualplans.
Discussyourplanwithaneighbouringgroupai
bepreparedtoshareyourideaswiththeclass.
144y

158. 8Workandpower
0.80m
Figure8.9:Movingawashingmachine.
Questions
IExplainwhynoworkisbeingdonebytheSun’s
gravityontheEarth.
'Aboypullsasled(ataconstantspeed)withaforce
of50Nforadistanceof250metres.
Howmuchworkdoestheboydo?
1A100gapplefallsfromatreeandlandsonthe
ground6metresbelow.
aWhatistheforcethatispullingtheapple,and
howlargeistheforce?
bCalculatehowmuchworkgravitydoesonthe
appleasitfalls.
cWhatenergytransferistakingplace?
4Amanistryingtomovehiswashingmachineinto
thebackofaremovalvan.Thewashingmachinehas
aweightof650Nandtheflooroftheremovalvanis
0.80metresabovetheground(Figure8.9).
aCalculatetheworkthemandoeswhenhe
pushesthewashingmachineuptherampinto
thebackofthevanagainstafrictionalforce
of440N.
bSupermanhappenstobepassingandhelifts
thewashingmachineverticallyintothevan
(withoutusingtheplank).Calculatethework
Supermandoes.
cExplainwhyaandbgivedifferentanswers,
dCalculatetheefficiencyoftheramp.
8.3Power
Exercisinginthegym(Figure8.10)canputgreat
demandsonyourmuscles.Speedingupatreadmill
meansthatyouhavetoworkhardertokeepup.
Equally,yourtrainermightaskyoutofindouthow
manytimesyoucanliftasetofweightsinoneminute.
Theseexercisesareatestofhowpowerfulyouare.
Thefasteryouwork,thegreateryourpower.
Figure8.10:Atthegym,itiseasiertoliftsmallloads,andto
liftthemslowly.Thegreatertheloadyouliftandthefaster
youliftit,thegreaterthepowerrequired.Itisthesame
withrunningonatreadmill.Thefasteryouhavetorun,the
greatertherateatwhichyoudowork.
WhenyouchecktheplaninActivity8.1,gothrough
thislist.
Doestheplanincludethecorrect
equipment?Isanythingmissing?
Doestheplanincludeaclearsketchofthe
experimentalsetup?
Aretheindependent,dependentandcontrol
variablesidentifiedcorrectly?
Theexperimentrequiresthatoneendof
therampisraised.Isthereasensiblerange
ofheights?Doestheplanexplainhowthe
angleoftheslopeiscalculated?
Doesitsuggestrepeatingmeasurements
sothatanaveragevalueofthedependent
variablecanbecalculatedforeachvalueof
theindependentvariable?
Doestheplanexplainhowtodoanycalculations?
Doestheplanexplainwhattoplotoneach
axisofthegraph?
Doestheplanpredictwhatthegraphshould
looklike?
Canyoucarryouttheexperimentsuccessfully
basedonwhatyourclassmateshavewritten?
Improveyourownworkusingthefeedbackonit
bornanothergroup.
PEERASSESSMENT

159. }CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Inphysics,thewordpowerisusedwithaspecial
meaning.Itmeanstherateatwhichyoudowork(that
is,howfastyouwork).Themoreworkyoudo,andthe
shorterthetimeinwhichyoudoit,thegreateryour
power.Poweristherateatwhichenergyistransferred,or
therateatwhichworkisdone.
KEYWORD
power:therateatwhichworkisdone,ortherate
atwhichenergyistransferred
Fastworking
Powertellsyouabouttherateatwhichaforcedoeswork,
thatistherateatwhichittransfersenergy.Whenyoulift
anobjectup,youaretransferringenergytoit.Itspotential
energyincreases.Youcanincreaseyourpowerby:
liftingaheavierobjectinthesametime
liftingtheobjectmorequickly.
Itisnotjustpeoplewhodowork.Machinesalsodo
work,andwecantalkabouttheirpowerinthesameway.
Acranedoesworkwhenitliftsaload.Thebigger
theloadandthefasteritliftstheload,thegreater
thepowerofthecrane.
Alocomotivepullingatrainofcoachesorwagons
doeswork.Thegreatertheforcewithwhichit
pullsandthegreaterthespeedatwhichitpulls,the
greaterthepowerofthelocomotive.
Question
5Yourneighbourisliftingbricksandplacingthem
ontopofawall.Heliftsthemslowly,oneatatime.
Statetwowaysinwhichhecouldincreasehispower
(therateatwhichheistransferringenergytothe
bricks).
8.4Calculatingpower
WeknowfromSection8.3,thatpoweristherateat
whichworkisdone.Sinceworkdoneisequaltoenergy
transferred,wecanwritetheseideasaboutpoweras
equations,asshown.
KEYEQUATIONS
_
workdone
power
timetaken
_
W
P-^
energytransferred
power=
timetaken
ACTIVITY8.2
Themeaningofworkandpower
Workandpowerhaveveryspecificmeaningsin
physics.Youwillcreatearesourcethathelpspeople
understandtheircorrectmeaningsinphysics.Youcould
startbycollectingdefinitionsthatarewronginphysics.
Forexample,collectimagesofpowerfulpeopleor
cars,oranimageofagymnastperformingthe'iron
cross'(Figure8.11).Ifyouarefeelingcreative,you
couldwriteapoemorcreateasongorpodcast.
Workforafewminutesinsmallgroupsto
discussideasandchooseoneideatodevelop.
Developyourideawithinthetimeframegiven
byyourteacher.
Iftheclassisdividedintotwoorthreelarge
groups,youwillperformorpresentyourideato
theotherpairsorthreesinyourgroup.
Voteontheotherpresentations.Apairorthree
cannotrepresenttheirgroupunlessthephysics
iscorrectsohelpcorrectanyphysicsmistakes.
Thepairorthreechosenbyeachgroupwill
presenttotherestoftheclass.
Figure8.11:Thisgymnastisperformingthe'ironcross'
onrings.Itisamovethatrequirestremendousstrengthin
thecore,armsandwrists.Butishedoinganyworkfroma
physicspointofview?
146>

160. 8Workandpower
watt(W):theunitofpowerwhen1Jofworkis
doneperunittime;1W=1J/s
k.e.=±mv2
2
=4x800kgx(25m/s)2
=250000J
Step2:Calculatethepower.
_
workdone
power ; —
timetaken
_
250000J
10s
=25000W=
25kW
Answer
Theenergyisbeingtransferredtothecar(fromits
engine)atarateof25kW,or25kJpersecond.
Carenginesarenotveryefficient.Inthisexample,
thecar’senginemaytransferenergyattherateof
100kWorso,althoughmostofthisiswastedas
thermalenergy.
Questions
6aHowmanywattsarethereinakilowatt?
bHowmanywattsarethereinamegawatt?
7Onehorsepoweristhepoweroutputofonehorse
whenitliftsamassof33000lbofwaterthrough
aheightofonefootinoneminuteasshownin
Figure8.12.
Doyoufindtheproblem-solvingstrategyinthe
workedexamplesuseful?
Doyouuseitallthetime?
Doyouhaveabetterapproach?
Ah=1foott
33000lb
Figure8.12:Agraphicalrepresentationofhorsepower.
Unitsofpower
I'owerismeasuredinwatts(W).Onewatt(1W)isthe
powerwhenonejoule(1J)ofworkisdoneperunittime.
Moonewattisonejoulepersecond.
IW=1J/s
1000W=1kW(kilowatt)
1000000W=1MW(megawatt)
IikecarenottoconfuseIVforworkdone(orenergy
Ifansferred)withWforwatts.Inbooks,thefirstoftheseis
ihowninitalictype(ashere),butyoucannotusuallytell
thedifferencewhentheyarehandwritten.SIunitsare
alienrelatedtoeachother.Itisusefultoremembersome
oltheconnections,suchas1J=1Nmand1W=1J/s.
Acarofmass800kgacceleratesfromresttoaspeed
of25m/sin10s.Whatisitspower?
Step1:Calculatetheworkdone.Thisistheincrease
inthecar’skineticenergy.
Poweringeneral
Wecanapplytheideaofpowertoanytransferof
energy.Forexample,electriclightbulbstransferenergy
suppliedtothembyelectricity.Theytransferenergy
bylightandheating.Mostlightbulbsarelabelled
withtheirpowerrating-forexample,40W,60W,
100W-totelltheuserabouttherateatwhichthebulb
tuinsfersenergy.
Wecanexpresstheefficiencyofalightbulboranyother
energy-changingdeviceintermsofthepoweritsupplies:
„. usefulpoweroutput,„
percentageefficiency= x100%
powerinput
Hemember,fromChapter6,howthiscompareswiththe
<quationforenergyefficiencyintermsofenergy:
_
. usefulenergyoutput,
percentageefficiency= x100%
energyinput
I’hcreismoreaboutelectricalpowerinChapter18.
CONTINUED
KEYWORD
REFLECTION
WORKEDEXAMPLE8.3

161. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Youneedtoknowthat1lb=0.453592kgandone
foot=0.3048metres.
aCalculatethemassofwaterthehorseliftsin
oneminute.
bCalculatetheweightofwaterthehorseliftsin
oneminute.(Assumethatg=10N/kg)
cCalculatetheworkthehorsedoesinoneminute.
dCalculatethepoweroutputofthehorse.
8Anaveragemanneedstoeatfoodcontainingabout
2500kcalofchemicalpotentialenergyperday
(1kcal=4.18kJ).
ACTIVITY8.3
Whatisthepowerofaworld-classsprinter?
Workonthisproblemonyourownforthree
minutes.Shareyourideaswithapartnerfora
furthertwominutes.Bepreparedtoshareyour
resultsandthinkingwiththerestoftheclass.
The100metresprintworldrecordof9.58seconds
wassetinthefinaloftheWorldAthletics
ChampionshipsinBerlinin2009.
Workouttheaveragespeedforasprinter
running100min9.58s.
Usingtheaveragespeed,workoutthe
sprinter'skineticenergyfortherace.Assume
thathehadamassof86kg.
Workouthispoweroutput.
Thispoweroutputismuchsmallerthanthefigure
workedoutbyateamofMexicanscientists.They
workedoutthathehadamaximumpoweroutput
ofabout2600Wandhistotalmechanicalwork
was80kJ.
Whatismissingfromyourcalculation?
Apartfromthefactthathedidnotrun
ataconstantspeed,arethereanyother
assumptionsyoumadeinyourcalculation?
Toreachamorerealisticvalue,thinkaboutthe
forcesinvolvedwhenheisacceleratingand
evenwhenheisrunningataconstantspeed.
aWhatis2500kcalexpressedinjoules?
bCalculatethepoweroutputoftheaverageman,
evenwhenheisdoingnowork.
9Itisestimatedthatthehumanbrainhasapower
requirementof40W.Howmuchenergydoesituse
inanhour?
10Alightbulbtransfers1000Jofenergyin10s.
Whatisitspower?
11Anelectricmotortransfers100Jin8.0s.Itthen
transfersthesameamountofenergyin6.0s.
Hasitspowerincreasedordecreased?
ACTIVITY8.4
Revisionboomerang
Youwillbeinagroupofthree.Yourteacherwill
letyouknowhowmuchtimeyouhavetodrawa
mindmaponasheetofA3.Itshouldincludeall
thekeyterms,conceptsandequationsrelatedto
energy(fromChapters6,7and8).Usepicturesto
illustratetheideaswherepossible,butyourmind
mapshouldinclude:
theprincipleofconsen/ationofenergyand
thevariousenergystoresandenergytransfers
thevariousenergyresources(whichonesdepend
onsunlight,whicharerenewable,andsoon)
anyequations(bothwordandsymbol)you
needtoknow(includeunitsinpencilora
differentcolour)
Whenyourteachertellsyouto,passyourmind
maptothepersononyourleft.Youreceiveamind
mapfromthepersononyourright.Yourjobisto
correctanymistakesandaddmissinginformation.
Whenyourteachertellsyou,themindmapswill
changehandsasecondtime.Correctandadd
asbefore.
Thenexttimeyourteacherasksyoutopassonthe
mindmaps,yourownmindmapshouldarriveback
withyou.
SELFASSESSMENT
DidyouknowmostoftheinformationforActivity8.4,
ordidyourclassmatesneedtoaddalotofmissing
information?
Ifyoucouldnotrememberasmuchasyouthought
youwould,youneedtodevelopastrategytohelp
youlearnthematerial(forexample,bydevelopinga
setofflashcards).
148>

162. 8Workandpower
Howdoesthestoppingdistanceofavehiclevary
Withitsspeed?
Figure8.13:Acollisionbetweenacarandacrashtest
dummyinordertoraiseawarenessofroadsafety.
Youaregoingtoapplysomeofthephysicsyou
havelearnedtokeepyouandothermotoristssafer.
Table8.1showshowthestoppingdistanceforan
emergencystopisaffectedbythespeedofthecar.
Anemergencystopiswhenadriverattemptsto
Stopintheshortestpossibledistanceinorderto
avoidanaccident.
Notethatstoppingdistance=thinkingdistance+
brakingdistance.Thedrivercannotapplythebrakes
instantly.Thinkingdistanceisthespeedofthecar
multipliedbythedriver'sreactiontime.Thisisthe
distancetravelledbetweenthedriverbecoming
awareofahazardandapplyingthebrakes.
Speed/
m/s
Thinking
distance
/m
Braking
distance
/m
Stopping
distance
/m
8.9 6 6 12
13.4 9 14 23
17.9 12 24 36
22.3 15 38 53
26.8 18 55 73
31.3 21 75 96
Table8.1:Stoppingdistancesfordifferentspeeds.
Whenacarcomestoanemergencystopallits
kineticenergyistransformedintothermalenergy
becauseoftheworkdonebythebrakes,W,which
applyabraking(frictional)force,F,throughout
thebrakingdistance.So,W=Fd,whereFisthe
frictionalforceanddisthestoppingdistance.
Wecanwrite:|mv2=Fd.
1Assumethatthemassofthecaris1500kg
andthebrakingforceis10000N.Showthat
yougetnearlythesamebrakingdistanceasin
Table8.1.
2Findtheguidelinespublishedinyourcountry.
Thoughthephysicsisthesame,different
assumptionsmighthavebeenmadetoarriveat
differentnumbers.Ifyouknowhowto,develop
aspreadsheetsothatcalculationsforthe
differentspeedscanbedoneatthesame.time.
3Whatcouldreducethebrakingforce?
4Workoutthebrakingdistancewhenthe
brakingforceishalved.
5Workoutthereactiontimeforthethinking
distancesinTable8.1.
6Workoutthethinkingdistancewhenthe
thinkingtimeisdoubled.
7Listthewaysinwhichreactiontimeforadriver
couldincrease.
8Designoneoftwosafetycampaignposters.
Usephysics(andgraphs)tobackupyour
claims.
Designaposterurgingpeopletodrive
moreslowly.Alongerstoppingdistance
reducesthechancethatanaccidentcan
beavoidedandincreasestheimpact
speed.Emphasisethatdamage(and
injuries)dependsonthekineticenergyof
thecar,notitsspeed.
Designasafetycampaignposter
urgingpeoplenottousetheirphone
whendrivingasitcouldincreasetheir
reactiontime.
PROJECT
149)

163. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
SUMMARY
Energytransferredbyaforceiscalledworkdone.
Theamountofworkdoneistheamountofenergytransferred.
Theamountofworkdonedependsonthesizeoftheforce-thegreatertheforce,themoreworkisdone.
Theamountofworkdonedependsthedistancemovedinthedirectionoftheforce-thefurtheritmoves,
themoreworkisdone.
Tocalculatetheenergytransferredbytheforceofgravity,wemustusetheverticalheightmoved.
Workdone=forcexdistanceinthedirectionoftheforce.
Theworkdonetomoveaweightverticallyupwardsisequaltog.p.e.
Poweristherateatwhichenergyistransferred,ortherateatwhichworkisdone.
Powercanbeincreasedbyincreasingtheworkdoneinagiventime.
Powercanbeincreasedbyreducingthetimeoverwhichthesameworkisdone.
Ar
RecallandusetheequationP=—p
insimplesystems.
EXAM-STYLEQUESTIONS
1Workdoneismeasuredinwhichofthefollowingunits? [1]
Anewton Bwatt Ckilogram Djoule
2Powerismeasuredinwhichofthefollowingunits? [1]
Anewton Bwatt Ckilogram Djoule
3Amanpushesa25kgpramupaslopeasshowninthediagram.
Hepusheswithaforceof150Nalongthe20mslope.Howmuchenergyis
dissipatedasthermalenergy? [2]
A250J B500J C2000J D2500J
150
>

164. 8Workandpower
b
d
d
workdonebythebrakesandthemotor. [2]
iiCalculatethemaximumvelocityofthetrain.
CONTINUED
COMMAND WORDS
[1]
[1]
[2]
Hl
calculate:workout
fromgivenfacts,
figuresorinformation
state:expressin
clearterms
explain:setout
purposesor
reasons;make
therelationships
betweenthings
evident;provide
whyand/orhowand
supportwithrelevant
evidence
TheEmpireStateBuildinginNewYorkisthevenueforanannualrunning
competition.Competitorsraceup86floors(1576stairs)or320metresto
finishclosetothetop.Therecordfastesttimeis9minutes33secondsby
PaulCrakein2003.Hismasswas62.5kg.
Thediagramshowsasectionthroughasubwaystation.Thetrackatthe
stationplatformisdesignedtobehigherupthanthetrackinthetunnels.
Thedriverusesbrakestostopthetrainattheplatformandamotorto
makethetrainsetoff.Whatistheadvantageofhavingtheplatformhigher
upthanthetrackinthetunnels?Explainyouranswerintermsofthe
a
b
bAsubwaytrainhasamassof270000kg(includingpassengers)anda
maximumkineticenergyof81.7MJ.
iStatetherelationshipbetweenkineticenergy(k.e.),massandvelocity.[1]
CalculatePaulCrake’sweight.
Statetherelationshipbetweenworkdone,forceanddistance.
CalculatetheworkdonebyPaulCrake.ExpressyouranswerinkJ
(1kJ=1000J).
Statetherelationshipbetweenpower,workdone,andtime.
CalculatePaulCrake’saveragepoweroutputduringhis
record-breakingrun. [2]
[Total:7]
[3]
[Total:6]
Foreachsentence,selectthecorrectwordfromthelist,
workenergymoreless
aWhenitmovesanobject,asmallerforcedoes_
workthana
biggerforce. [1]
Thegreaterthedistanceanobjectismovedbytheforce,the
workitdoes. [1]
Poweristherateatwhich istransferred. [1]
Poweristherateatwhich isdone. [1]
[Total:4]

165. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
SELF-EVALUATIONCHECKLIST
Afterstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutosee
anygapsinyourknowledgeandhelpyoutolearnmoreeffectively.
Ican
See
Topic...
Needs
morework
Almost
there
Confident
tomoveon
Understandthatworkdoneequalsenergytransferred. 8.1
Relateworkdonetotheforcemultipliedbythedistance
movedinthedirectionoftheforce.
8.2
RecallanduseW=Fd.=hE. 8.2
Understandtherelationshipbetweenpower,workdone
andtime.
8.3
AU
RecallandusetheequationP=forsimpleproblems. 8.4
152

166. Chapter9
Thekinetic
particlemode
ofmatter
INTHISCHAPTERYOUWILL:

167. >
CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
melting
liquid
gas
Figure9.1:Makinglinksbetweenwords.
GETTINGSTARTED
Workwithapartner.Takealargesheetofpaper
andwritethewords'solid','liquid'and'gas'on
thepaper.
Aroundeachword,writeasmuchasyoucanabout
thatstateofmatter.Youcanincludedrawings.
Usingadifferentcolouredpen,makeasmany
linksasyoucanbetweenthethreewords.
MELTINGICELEADSTOMAGMAFLOW
Figure9.2:ScientistsinArizonaandOxfordhaveshown
acorrelationbetweenthemeltingofglaciersandan
increaseinvolcaniceruptionssuchasthisoneinIceland
whicheruptedin2010.
ThevolcanoinFigure9.2iserupting.High
temperaturesinsidetheearthhavemeltedtherock
creatingmagma.Scientistsbelievetheeruption
mayhavebeentriggeredbythemeltingofthe
glaciersituatedabovethevolcano.Theglacier
meltingmeanttherewaslessicepressingdownon
therocks.Thisreducedthepressureonthemagma
underneaththerocks.Thismadeiteasierfor
magmatoflow.Themeltingoftheglaciersislinked
toglobalwarming,whichiscausedbj^thechanges
togasesintheatmosphere.Changesofstate,such
asthemeltingofglaciers,canhavedramaticeffects.
Wearefamiliarwiththechangesth<thappen
whenicemelts.Aglass-likesolidchangesintoa
transparent,colourless,runnyliquid.Heattheliquid
andit'vanishes'intothinair.Althoughthissounds
likeamagictrick,itissofamiliarthatitdoesnot
surpriseus.Itismoresurprisingwhenweseesolid
rockheatupandbecomemagma.
Inthischapter,wewilllookatmaterialsandtheir
differentstates-solid,liquidandgas.Wewill
considerhowtheparticlesinmatterbehaveand
howthiscanhelpusexplainsomeofthethingswe
observewhenmaterialschangefromonestateto
another.
Discussionquestions
1Listtensolids,tenliquidsandtengases.
Arethereanysubstanceswhicharehardto
categorise?
2TheEarthisdistinctiveamongtheplanetsof
theSolarSysteminbeingtheonlyplaneton
whichwaterisfoundtoexistnaturallyinall
threeofitsphysicalstates.Discusshowlifeon
Earthwouldbeaffectedifoneofthestatesof
matterdidnotexist.
154>

168. 9Thekineticparticlemodelofmatter
9.1Statesofmatter
Figure9.3:Icecubeshaveafixed
shape.
n
A.
Figure9.4:Thiscolouredwater
takestheshapeoftheflasks.
Figure9.5:Thesteamleavingthe
potquicklycondensestoform
waterdroplets.
State solid liquid gas
Size rigid,fixedshape,fixed
volume;cannotbesquashed
notrigid,nofixedshape,
fixedvolume;cannotbe
squashed
notrigid,nofixedshape,
nofixedvolume;canbe
squashed
Shape takestheshapeofits
container
takestheshapeofits
container
expandstofillitscontainer
Table9.1:Thedistinguishingpropertiesofthethreestatesofmatter.
Matterexistsinthreestates:solid,liquidandgas.
Anexampleofthisiswaterwhichcanexistassolidice,
liquidwaterorsteamwhichisaninvisiblegas.Steam
quicklycondensesinairtoformtinywaterdroplets,
whicharewhatwesee.Wecandescribethesestatesby
describingtheirshapeandvolume(size).Table9.1shows
howthesehelpustodistinguishbetweensolids,liquids
andgases.
Figure9.6:Thevolumeofaliquidstaysthesame.
Figure9.6showsafamouspsychologyexperiment.A
youngchildwillusuallythinkthetallerglassholdsmore
waterevenwhentheyseeitbeingpouredfromthewider
glass.Thechilddoesnotrealiseyetthataliquidhasa
fixedvolume.Althoughthedrinkchangesitsshapewhen
youpouritfromoneglasstotheother,itsvolumestays
thesame.
Changesofstate
Heatasolidanditmeltstobecomealiquid.Heattheliquid
anditboilstobecomeagas.Coolthegasanditbecomes
firstaliquidandthenasolid.Thesearechangesofstate.
ThenamesforthesechangesareshowninFigure9.7.
GAS
evaporation
orboiling
condensing
melting
LIQUID
solidifying
SOLID
Figure9.7:Namingchangesofstate.
155>

169. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Anotherwordforaliquidchangingtoagasis
evaporation.Wewillseethedifferencebetween
evaporationandboilinglater.
KEYWORDS
statesofmatter:solid,liquidorgas
changesofstate:changingfromonestateof
mattertoanother
evaporation:changingfromaliquidtoagasat
anytemperature
boiling:changingfromliquidtogasatafixed
temperaturecalledtheboilingpoint
melting:changingfromsolidtoliquid
condensing:changingfromgastoliquid
solidifying/freezing:changingfromliquid
tosolid
Questions
1Copyandcompletethesesentences:
Thethreestatesofmatterare , and
Asolidhasadefiniteshapeand .Aliquid
hasadefinite buttakestheshapeofits
container.Agaswillexpandtofillallthe
available.
Whenasolidisheatedit tomakea .
Thetemperaturethishappensatiscalledthe
Theboilingpointisthetemperatureatwhicha
turnstoa .
2aWhatnameisgiventothetemperatureatwhich
agascondensestoformaliquid?
bWhatnameisgiventotheprocessduringwhich
aliquidchangesintoasolid?
cWhatnameisgiventothetemperatureatwhich
thishappens?
3Tomeasurethevolumeofaliquid,youcanpour
itintoameasuringcylinder.Measuringcylinders
comeindifferentshapesandsizes-tall,short,wide,
narrow.Explainwhytheshapeofthecylinderdoes
notaffectthemeasurementofvolume.
9.2Thekineticparticle
modelofmatter
Inthissection,wewilluseamodeltohelpusexplaint
behaviourofmaterials.Scientistsoftenusemodelsto
explainthingswhichwecannotseedirectly.Usingthis
modelwillhelpusanswerquestionssuchas:
whydoesanicecubechangeshapeasitmelts?
howcanwesmellperfumefromacrossaroom?
whydoesittaketimetomeltasolid?
Themodeliscalledthekineticparticlemodelofmatter.
Theword‘kinetic’meansrelatedtomovement.Allmat
ismadeupoftinyparticles-atoms,moleculesorions.
Whenasubstanceisheated,itsparticlesgainenergyan
movefaster.Thehigherthetemperature,thefasterthe
particlesmove.
KEYWORDS
model:awayofrepresentingasystemwhichwe
cannotexperiencedirectly
kineticparticlemodelofmatter:amodelin
whichmatterconsistsofmovingparticles
atom:thesmallestpartofanelementthatcanexis
molecule:twoormoreatomsjoinedtogetherby
chemicalbonds.
Therearemanydifferenttypesofparticleswithdiffen
chemicalproperties.Inthischapterwewilllookathoi
theseparticlesmoveratherthanhowtheyreact.Wew
drawalltheatomsandmoleculesasspheresandrefer
themallasparticles.
carbondioxidemolecule
W-
&
**
nitrogenmolecule
X.
ww*
oxygenmolecule
<A»
*
**
W*
watermolecule w
Mu
***
Mw <
argonatom **
Figure9.8a:Airisamixtureofelementsandcompounds
156>

170. 9Thekineticparticlemodelofmatter
9.8b:Forthismodelwewillassumeallatomsand
*
>lmulesareidenticalsphericalparticles.
ili"Ideathatmatterismadeupofidentical,spherical
mi'Iiculesisagreatsimplification,butusingthismodel
willhelptoexplainthebehaviourofmaterials.
Describingtheparticlestructure
ofsolid,liquidsandgases
I
"
me9.9showshowwepicturetheparticlesinasolid,a
liquidandagas.Eachstateofmattercanbedescribedby
iI*mribingthearrangement,separationandmotion
ilITparticles.
Particlemovementand
temperature
1metictheoryusestheideathatasparticlesheatup
ihcvgainmorekineticenergyandsomovefaster.Asa
substancecoolsdownitsparticleslosekineticenergyand
slowdown.Thissuggeststhatthereisatheoreticallower
limittohowcoldanythingcanbe.Iftheparticlesloseall
theirkineticenergyandstopmoving,itisnotpossiblefor
thesubstancetocoolanyfurther.
Thelowestpossibletemperatureanythingcanreachis
-273°C.Thisisalsoknownasabsolutezero.
KEYWORDS
absolutezero:thetemperatureatwhichparticles
havenokineticenergy
Figure9.9:Representationsofa:gas,b:liquid,c:solid.The
arrangement,separationandmotionoftheparticleschange
asthegascoolstobecomealiquidandthenasolid.
Table9.2:Thearrangementandmotionofparticlesinthethreedifferentstatesofmatter.Comparethesestatementswith
thediagramsshowninFigure9.9.
ArrangementandseparationofparticlesMotionofparticles
Solid Theparticlesarepackedcloselytogether,in
aregularpattern.Noticethateachparticleis
inclosecontactwithallitsneighbours.
Becausetheparticlesaresotightlypacked,they
cannotmovearound.However,theydomoveabit.
Theycanvibrateaboutafixedposition.Thehotter
thesolid,themoretheyvibrate.
IIquid Theparticlesarepackedslightlyless
closelytogetherthaninasolid.Theparticles
arearrangedrandomlyrather
thaninafixedpattern.
Becausetheparticlesareslightlylesstightly
packedthaninasolid,theycanmovearound.So
theparticlesarebothvibratingandmovingfrom
placetoplace.Thehottertheliquidis,thefasterits
moleculesmove.
Gas Theparticlesarewidelyseparatedfrom
oneanother.Theyarenolongerincontact,
unlesstheycollidewitheachother.Inair,the
averageseparationbetweentheparticlesis
abouttentimestheirdiameter.
Theparticlesmovefreelyabout,bouncingoffone
anotherandoffthewallsofthecontainer.Inairat
roomtemperature,theaveragespeedoftheparticles
isabout500m/sandthisincreaseswithtemperature.
157>

171. yCAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
Evidenceforthekineticmodel
Atomsandmoleculesarefartoosmalltosee,evenwitha
microscope,butexperimentsshowtheeffectsofmoving
atomsandmolecules.Theseexperimentsdonotprove
therearemovingparticles,buttheydoprovidesupport
fortheidea.
In1827,ascientistcalledRobertBrownwasusinga
microscopetostudypollengrainswhenhenoticedtiny
particlesjigglingabout.Atfirsthethoughtthatthey
mightbealive,butwhenherepeatedhisexperimentwith
tinygrainsofdustsuspendedinwater,hesawthatthe
dustalsomovedaround.Thismotionisnowknownas
Brownianmotion,andithappensbecausethemoving
particlesareconstantlyknockedaboutbythefast¬
movingparticlesoftheair.
KEYWORDS
Brownianmotion:themotionofsmallparticles
suspendedinaliquidorgas,causedbymolecular
bombardment
observationswhatyouseehappeninginan
experiment
Wecandoasimilarexperimentusingsmokeparticles.
Theoxygenandnitrogenmoleculesthatmakeuptheair
arefartoosmalltosee,sowehavetolookatsomething
bigger,andlookfortheeffectoftheairmolecules.
Wecanuseasmokecel)(Figure9.10a).Thisisasma
glassboxwhichcontainsairwithasmallamountof
smoke.Thecellislitfromtheside,andthemicroscoj
usedtoviewthesmokeparticles.
Thesmokeparticlesshowupastinyspecksoflight,
buttheyaretoosmalltoseeanydetailoftheirshape
Whatisnoticeableisthewaytheymove.Ifyouwatcl
asingleparticle,youwillseethatitfollowsrandom
path,frequentlychangingdirection.Thisisbecausee
moleculesrepeatedlyhitthesmokeparticle.
Explanationsusingthe
kineticmodel
Thekineticmodelofmattercanbeusedtoexplainn
observations.Herearesomeofthem:
Liquidstakeuptheshapeoftheircontainer
becausetheirparticlesarefreetomoveaboutwi
theliquid.
Gasesfilltheircontainerbecausetheirparticles।
moveaboutwithcompletefreedom.
Solidskeeptheirshapebecausetheparticlesare
packedtightlytogether.
Gasesdiffuse(spreadout)fromplacetoplace,si
that,forexample,wecansmellperfumeacrosst]
room.Theperfumeparticlesspreadaboutbecat
theyarefreetomove.
coverslip
smokecell
smoke
Figure9.10a:AnexperimentalarrangementforobservingBrownianmotion.Thesmokeparticlesarejustlargeenought<
showupunderthemicroscope.Theairmoleculesthatcollidewiththemaremuchtoosmalltosee.b:Theinvisiblysmall.
moleculesrepeatedlyhitthesmokeparticlemakingitchangedirection.Thedottedlineshowsthepathofthesmokepari
158
>

172. 9Thekineticparticlemodelofmatter
1Placesomeidenticalsmallballsormarbleson
ashallowtray.Theyshouldcoveraboutone-
quarteroftheareaofthetray.
2Tipthetrayslightlysothattheballsallrollto
thelowerend.Thepatterntheyformislike
thearrangementofparticlesinasolid.
3Keepthetrayslightlytippedandshakeit
gentlysothattheballscanmoveabout.This
islikealiquid.
4Keepshakingthetrayandtipitsothatit,,
becomeshorizontal.Theballsmovearound
freely,collidingwitheachotherandthesides
ofthetray.Thisisliketheparticlesinagas.
Dissolvedsubstancesdiffusethroughoutaliquid.
Sugarcrystalsinadrinkdissolveandmolecules
spreadthroughouttheliquid,carriedbythemobile
particles.Inahotterdrink,theparticlesaremoving
fasterandthesugardiffusesmorequickly.
Mostsolidsexpandwhentheymelt.Theparticles
areslightlyfurtherapartinaliquidthaninasolid.
Liquidsexpandalotwhentheyboil.Theparticles
ofagasaremuchfurtherapartthaninaliquid.We
canthinkaboutthistheotherwayround.Gases
contractalotwhentheycondense.Ifalloftheair
intheroomyouarenowinwascooledenough,it
wouldcondensetoformathinlayerofliquid,two
orthreemillimetresdeep,onthefloor.
Forcesandthekineticmodel
Wehaveseenthatthekineticmodelofmattercanexplain
IIhedifferencesbetweensolids,liquidsandgases.We
<'nnexplainsomeotherobservationsifweaddanother
scientificideatothekinetictheory:weneedtoconsider
Iheforcesbetweentheparticlesthatmakeupmatter.
iWhydotheparticlesthatmakeupasolidoraliquidstick
jtogether?Theremustbeattractiveforces(forcespulling
themtogether)betweenthem.Withoutattractiveforces
Ioholdtogethertheparticles,therewouldbenosolidsor
liquids,onlygases.Nomatterhowmuchwecooledmatter
<|own,itwouldremainasagas.
Anotherwaytorefertotheseforcesistosaythatthere
.trebondsbetweentheparticles.Eachparticleofasolid
IInstronglybondedtoitsneighbours.Thisisbecausethe
iforcesbetweenparticlesarestrongestwhentheparticlesare
dosetogether.Inaliquid,theparticlesareslightlyfurther
apartandsotheforcesbetweenthemareslightlyweaker.
Inagas,theparticlesarefarapart,sothattheparticlesdo
notattracteachotherandcanmovefreelyabout.
MoreonBrownianmotion
Themoleculesinairhaveanaveragediameterofabout
4x1O“10metres.Thismakesthemimpossibletoseewith
ilaboratorymicroscope.Thesmokeparticlesconsistof
inanymoleculesandsoaremanytimeslargerthanthe
Airmolecules.Asmokeparticlehasadiameterofabout
Ix10"7metres,whichisabout250timesthediameter
i>ftheairmolecules.Theairmoleculesarelightbutfast
movingandsohaveenoughkineticenergytocausethe
tfnokeparticlestochangedirectiononimpact.
Demonstratingthekineticmodel
Thekinetictheorycanbemodelledusingsmall
ballsormarblesinatray.
Figure9.11:Modellingkinetictheoryusingmarblesin
atray.
attractiveforces:forcesbetweenparticleswhich
holdtheparticlesinfixedpositionsinasolid
bonds:anothernamefortheforcesbetween
particles
KEYWORDS
ACTIVITY9.1

173. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
Yourtaskistomakeavideo,orapresentationto
explainthekineticmodel,usingthetrayofballs.
Youcould:
showthearrangementandmovementof
particlesineachofthethreestates
demonstrateabsolutezero
demonstrateandexplainchangesofstate-
melting,boiling,condensingandfreezing
modelBrownianmotion(hint:usealarger,
different-colouredballforthesmokeparticle)
addsomedifferent-colouredballstoone
cornerofthetraytorepresentperfume;show
howtheperfumediffuses(spreadsout).
Whydoesagascausepressureonthewallsofits
container?Figure9.12showstheparticlesthatmak<
upagas.Theparticlesofagasmovearoundinside
itscontainer,bumpingintothesides.Thegascauses
pressureonthewallsofthecontainerbecausetheg;
particlesareconstantlycollidingwiththewalls.
Questions
4Sketchthreediagramstoshowthearrangementof
moleculesinasolid,aliquidandagas.
5aInwhichstateofmatteraretheparticlesmost
closelypacked?
bInwhichstateofmatteraretheymostwidely
separated?
cInwhichstatedotheparticlesmovefastest?
6aDescribewhatismeantbyBrownianmotion.
bHowcanthekineticmodelbeusedtoexplain
Brownianmotion?
cAstudentdidanexperimenttoobserve
Brownianmotion.Shethenrepeatedthe
experimentinamuchcolderroom.
Describeandexplainhowherobservations
wouldchange.
7Usethekineticmodelofmattertoexplainwhywe
canwalkthroughairandswimthroughwaterbut
wecannotwalkthroughasolidwall.
9.3Gasesandthe
kineticmodel
Thekineticmodelcanhelpusunderstandhowgases
behave.Thinkingabouttheparticlesinagashelpsus
answerquestionsaboutthegas.
Figure9.12:Movinggasparticlesinacontainer.
Whathappenstoagaswhenitisheated?Figure9.1
showsthesamegasatahighertemperature.Thehig
thetemperatureofagas,thefasteritsparticlesare
moving.Theparticleswillhitthewallsmoreoftena
withmoreforce.Thisincreasesthepressure.
Figure9.13:Heatingthegasinacontainer.
Whathappenswhenagasiscompressed(squashed
Figure9.14showsthesamegasagain.Thistimeth
volumeofthecontainerhasbeendecreased.Theg
hasbeencompressedintoasmallerspace.Thepar
don’tmoveasfarbetweencollisions,sotheycollid
thewallsmoreoften.Decreasingthevolumeofaj
increasesitspressure.
160>

174. 9Thekineticparticlemodelofmatter
gasmolecule container
lujure9.14:Decreasingthevolumeofagasinacontainer.
Figure9.17:Thediaphragmmovesdownasyoubreathein
andupasyoubreatheout.
Questions
8Copyandcompletetheparagraph.
Themoleculesina areconstantlymoving
freelyandrapidlyandsohitthewallsofthe
container.Thiscreates onthewalls.When
thegasisheated,themoleculesmove and
thepressure
'
Whenthegasiscompressed,
themoleculeshitthewallsmoreoftenandthe
pressure .
9Aballoonisinflatedbyblowingairintoit.Explain
whatwouldhappenifaninflatedballoonwasputin
afreezer.
10Atincancontainingairistightlysealedsonoair
canescape.Thecanisthenheated.Describewhat
happensto:
athespeedoftheairmoleculesinsidethecan
bhowoftentheairmoleculeshitthewallsof
thecan
ctheforcewithwhichtheairmoleculeshit
thewalls
dthepressureonthewallsofthecan.
ToproducealoudsoundthegirlinFigure9.16needsto
blowoutwithasmuchpressureasshecan.Musicians
learntobreathefromtheirdiaphragm.Figure9.17shows
howthegirl’sdiaphragmmovesdownsoalargevolume
ofaircanenterherlungs.Herdiaphragmthenmovesup,
reducingthevolumeandsoincreasingthepressureof
theairsheblowsoutandtheloudnessofhermusic.
Iijure9.15:Pumpinguptyresmeanssquashingalotmore
particlesintoafixedspace.Thisincreasesthepressure.
IIncyclistinFigure9.15needstounderstandgas
pressure.Hepumpsairintohistyrestomakethemhard.
IlIlietemperaturerises,thepressureinhistyreswill
teaseevenmore.Thiscouldbursthistyres.
1
n|ure9.16:Youneedairpressuretoplaythetrumpet.

175. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
9.4Temperatureandthe
Celsiusscale
Temperatureisameasureofhowhotorcoldsomething
is.Temperatureismeasuredusingathermometer.
Mostthermometerstakeaminuteorsotomeasure
temperature.Thisisbecausethermalenergyhastobe
transferredtoorfromthethermometeruntilitisatthe
sametemperatureasthethingitismeasuring.
Figure9.18a:Thermalenergytransfersfromthegirlto
thethermometeruntiltheyareatthesametemperature,
b:Thermalenergytransfersfromthethermometertothe
iceuntiltheyareatthesametemperature.
Temperatureandinternalenergy
Athermometertellsusabouttheaverageenergyof
theparticlesintheobjectwhosetemperatureweare
measuring.Itdoesthisbysharingtheenergyofthe
particles.Iftheyaremovingrapidly,thethermometer
willindicateahighertemperature.Placinga
thermometerintoanobjecttomeasureitstemperature
isratherlikeputtingyourfingerintosomebathwater
todetecthowhotitis.Yourfingerdoesnothaveascale
from0to100,butitcantellyouhowhotorcoldthe
wateris,fromuncomfortablycoldtocomfortablywarm
topainfullyhot.
Thetemperatureofanobjectisameasureoftheaverage
kineticenergyofitsparticles.Becauseitistheaverage
kineticenergyofaparticle,itdoesnotdependonthe
sizeoftheobject.
Internalenergyisthetotalenergyofallofthe
particles.
Temperatureisameasureoftheaveragekinetic
energyoftheindividualparticles.
Abathofwaterat50°Gcanhavethesametemperati
asacupoftea,butithasmoreinternalenergythant
cupofteabecauseithasfarmoremolecules.
KEYWORDS
temperature:ameasureofhowhotorcold
somethingis;ameasureoftheaverageenergyc
theparticlesinasubstance
Thetemperatureofafireworkmayreach1500°C,wl
meansitsparticleshaveveryhighkineticenergy,but
haslesstotalenergythanthebathbecauseithasvery
fewparticles.
Figure9.19:Thefireworkhasthefastestparticlesandsi
thehighesttemperature.Thebathwaterhasmanymore
particlesandmuchmoreinternalenergythanthetea.
162y

176. 9Thekineticparticlemodelofmatter
TheKelvintemperaturescalestartsfromabsolutezero,
or-273°C.Temperaturesmeasuredonthisscaleare
calledabsolutetemperatures.Scientistsoftenmeasure
temperaturesusingtheKelvinscale.Achangein
temperatureofonedegreeisthesameforbothscales.
TheKelvintemperature(T)canbecalculatedfromthe
Celsiustemperature(0)usingtheequation:
T(K)=0(°C)+273
Figure9.20:Amodemliquid-in-glassthermometer.
ThermometersliketheoneshowninFigure9.20areused
inschoollaboratories.Thebulbcontainsaliquidwhich
expandswhenitgetshot.Theliquidmovesintothetube
andwecanreadthetemperatureonthescale.
Theliquidusedisusuallyalcohol.Thisexpandsalot
whenheatedanditissafe.
Somethermometersusemercury.Mercurythermometers
canbeusedatverylowtemperatures.Mercuryis
poisonoussomercurythermometersarenotusedin
ichools.
ThescalewasdevisedbytheSwedishscientistAnders
Celsius.
11isscaleisknownastheCelsiusscale.Ithastwofixedpoints:
0°C:themeltingpointofpureiceatatmospheric
pressure
100°C:theboilingpointofpurewaterat
atmosphericpressure.
Bachtimehemadeanewthermometer,Celsiuswould
Calibrateit.Heputthethermometerinmeltingiceand
marked0°Conthethermometer.Hethenputitin
boilingwaterandmarked100°C.Hethendividedthe
ipacebetweenthesepointsinto100parts.Eachpart
representsonedegreeCelsius.
TheKelvintemperaturescale
1inetictheorysuggeststhereisalimittohowlow
temperaturescango.Thelowestpossibletemperature
(ihcpointatwhichmoleculeshavenokineticenergy)is
273°C.Thisiscalledabsolutezero.
KEYEQUATION
conversionbetweenKelvintemperatureanddegrees
Celsius:
T(K)=0(°C)+273
WORKEDEXAMPLE9.1
Calculatetheabsolutetemperatureofthehuman
body.Assumethatthetemperatureofthehuman
bodyis37°C.
T(K)=0(°C)+273
T=37+273=310K
KEYWORDS
fixedpoints:knownvaluesusedtocalibratea
measuringinstrument
calibrate:tomarkastandardscaleontoa
measuringinstrument
Kelvintemperaturescale:(ortheabsolute
temperaturescale)thetemperaturemeasured
fromabsolutezero.Adifferenceintemperatureof
1kelvinisthesameasadifferenceof1°C.0Kis
approximately-273°C
Questions
11Copyandcompletethesesentences.
Athermometerisusedtomeasure ,.whichis
ameasureofhowhotsomethingis.
Itismeasuredin or
Temperaturedependsonhowfastthe are
moving.
12Alaboratorythermometerhasnotemperature
markings.Describehowyoucoulduseiceand
boilingwatertocalibratethethermometer.
TheCelsiustemperaturescale
Themeltingandboilingpointsofwaterareusedto
definetheCelsiustemperaturescale.

177. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
13Calculatethetemperatureinkelvinof:
aa classroomat20°C
blavaat800°C
cthesurfacetemperatureofminorplanetPluto
at-233°C.
9.5Thegaslaws
Thetemperature,pressureandvolumeofagasallaffect
eachother.Thegaslawsexplainmathematicallyhowthe
threeaffecteachother.Therearelawsdescribinghoweach
ofthesequantitiesaffectstheothers,butwewilllookjust
atthelawconnectingthepressureofagasanditsvolume.
Itisimportanttobeclearaboutwhattheterms
‘temperature’,‘pressure’and‘volume’mean.Thegas
lawsallrefertoafixedmassofgas.Imaginethegasina
sealedcontainerwhichcanbesquashedorheated.The
numberofmoleculesdoesnotchange.
Thetemperatureofagasisameasureoftheaverage
kineticenergyofthemolecules.Inahotgas,the
moleculesmovefasterthaninacoldgas.
Thepressureofagasiscausedbyatomsormolecules
hittingthewalls,changingmomentumandsocausinga
force.Thepressureistheforceperunitareaonthewalls
ofthecontainer.
Pressureandvolume
In1662,RobertBoyle,aphysicistandchemist,
investigatedtherelationshipbetweenthepressureona
gas,p,anditsvolume,V.
Figure9.21:Increasingthepressureonagasdecreasesits
volume.
TheeffectofincreasingthepressureonagascanI
investigatedusingthesimpleapparatusshowninF
9.21.Addingweightsincreasesthepressureandca
thevolumeofthegastodecrease.
Figure9.22showsamoreaccuratemethod.Inthis
apparatus,someairistrappedinsidetheverticalgl
tube.Theoilinthebottomoftheapparatuscanbe
compressedwithapump,sothatitpushesupinsid
tube,compressingtheair.Thevolumeoftheairca
readfromthescale.Thepressureexertedonitbytl
canbereadfromthedialgauge.
Increasingthepressureonthegasdecreasesitsvoh
Table9.3showssometypicalresults.Boylefounda
mathematicalrelationshipbetweenthepressure,p,
thevolume,V,ofthegas.
Figure9.22:Apparatusforincreasingthepressureon
gas.Afixedmassofairistrappedinsidethetube,and
pressureonitisincreased.
Table9.3:Typicalresultsforanexperimentintopressu
andvolumeofgas.Thetemperatureofthegasdoesm
change.
Pressure,
p/Pa
Volume,
V/cm3
Pressurexvolunr
pV/Pacm3
WO 60 6000
125 48 6000
150 40 6000
200 30 6000
250 24 6000
300 20 6000
164y

178. 9Thekineticparticlemodelofmatter
Volume,V
e.
d
KEYWORDS
WORKEDEXAMPLE9.2
KEYEQUATION
ure
es
he
ne
oil
Hoyle’sexperimentsshowedthatincreasingthepressure
decreasedthevolume.ThisisshowninFigure9.23a.
Hoylealsofoundthatwhenhemultipliedpressureby
Volume,healwaysgotthesameresult.Youcanseethisin
thelastcolumnofTable9.3.
inverselyproportional:twoquantitiesare
inverselyproportionalwhenincreasingone
quantitydecreasestheotherbythesamefactor;
doublingonequantityhalvestheother
WorkedExample9.2showshowtousetheequation
Pi
“Pi^2toAndhowthevolumeofagaschanges
whenthepressureonitischanged.Youcanusethesame
equationtoworkouthowthepressurechangeswhenthe
volumeischanged.
relationshipbetweenpressureandvolumeforgasat
aconstanttemperature:
pV=constant
Answer
Thevolumeoftheairincreasesto10cm3.
Ibiscanbewrittenmathematicallyas:
pV=constant
Thisrelationshipcanbeexpressedindifferentways.We
canwritethesameideainawaythatisusefulfordoing
Calculations:
initialpressurexinitialvolume=finalpressurex
finalvolume
Figure9.23bshowsthatplottingpagainst
-p
givesa
straight-linegraphpassingthroughtheorigin.
Finally,wecanwritetherelationshipinwords:the
volumeofafixedmassofgasisinverselyproportionalto
itspressure,provideditstemperatureremainsconstant.
Figure9.23:TwographstorepresenttheresultsofaBoyle's
Idwexperiment,a:Thegraphofpressureagainstvolume
ihowsthatincreasingthepressurecausesadecreaseinthe
Volume,b:Themathematicalrelationshipbetweenpand—
inbeseenfromthisgraph.Itisastraightlinethroughthe
ijrigin,whichmeansthatpressureisinverselyproportionalto
Volume.
Or:PiVi-p2V2
WherepxandVxareonepairofreadingsofpressureand
Volume,andp2andV2areanotherpair.
Doublingthepressurehalvesthevolume.Thismeans
thatpressureisinverselyproportionaltovolume.Using
thesymbol
*
(‘isproportionalto’),wecanwrite:
or
Ascubadiverreleasesabubbleofair.Thebubblehas
avolumeof2cm3.Hewatchesitrisetothesurface,
expandingasitrises.Thediverisatadepthwherethe
pressureis5atmospheres.Whatwillthevolumeof
thebubblebewhenitreachesthesurface,wherethe
volumeis1atmosphere?Assumethatthetemperature
doesnotchange.
Step1:Writedowntheinitialandfinalvaluesofthe
quantitiesthatweknow.
Pi=5atmospheres
K]=2cm3
p2=1atmosphere
/2=?
Step2:WritedowntheBoyle’slawequationand
substitutevalues.
P\V\=P2V2
5atmospheresx2cm3=1atmospherexU,
Step3:Thereisonlyoneunknownquantityinthis
equation(V2).Rearrangeitandsolve.
P2
_5atmospheresx2cm3_
3
1atmosphere

179. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Units
IntheequationPi^-P2^2>itdoesnotmatterwhat
unitsweuseforpandV,aslongasweusethesameunits
forbothvaluesofpandthesameunitsforbothvalues
ofV.
ThestandardunitofpressureisthePascal(Pa).
1Pa=1N/m2.PressurecanalsobemeasuredinkPa,
N/cm2oratmospheres.Oneatmosphereisapproximately
100kPa.
Volumeisusuallymeasuredinm3,dm3,cm3orlitres.
Questions
14Whatdoeachofthetermsintheequation
Pi^i=Pi^2represent?
15Thepressureon6dm3ofnitrogengasisdoubledata
fixedtemperature.Whatwillitsvolumebecome?
16Aflaskholds6litresofairatapressureof
2atmospheres.Calculatethevolumewhenthe
gasiscompressedbyincreasingthepressureto
6atmospheres.Assumethatthetemperature
remainsconstant.
REFLECTION
Physicsinvolvesalotofcalculations.Presenting
yourcalculationsclearlyhelpsyoucheckyour
work.Italsohelpsyouidentifyanyerrors.
Lookbackoveryourcalculations.Whichstepsare
youusing?
Identifywhichvaluesaregiveninthequestion.
Checkthattheunitsareconsistentand
changethemifnecessary.
Writedowntheequation.
Substitutevaluesintotheequation.
Rearrangetheequationifneeded.
Calculatetheanswer.
Giveunitsfortheanswer.
Canyoumakeyourcalculationanswersmore
clear?Makeanoteofwhatyoucouldimproveand
refertothisnexttimeyouaredoingcalculations.
PROJECT
Makingagame
Yourtaskistomakeagametocheckhowwellyou
haveunderstoodthistopic.Youcaninventyourown
gameoruseoneoftheideasbelow.
Pairs
Makepairsofcards,onewithakeywordandtheother
withitsdefinition.Dothisforasmanykeywordsorideas
asyoucanfind.Becreative-forexample,youcould
drawBrownianmotionratherthanwriteadefinition.
Putallthecardsfacedownonatable,makingsure
theyaremixedup.
Playerstaketurnstoturnovertwocards.Ifthey
match,theplayerkeepsthemandtakesanothergo.
Iftheydon'tmatch,theplayerturnsthemfacedown
againandthenextplayertakesaturn.
Thewinneristheplayerwithmostcardsattheend.
Bannedwords
Inthisgameaplayertakesacardwhichhasakey
wordonitandalistofbannedwords.
Anexampleisshownhere.
Theplayerhastodescribe
theword'condensation'totheir
teamwithoutusinganyofthe
bannedwords.Theycouldsay,
'itisthechangeofstatethat
happenswhenwatervapour
turnstowater'.
Condensation
liquid
gas
cool
particle
166>

180. 9Thekineticparticlemodelofmatter
1Whichoneofthesestatementsisdescribingagas?
CONTINUED
EXAM-STYLEQUESTIONS
B
C
D
Itexpandstofillthevolumeofitscontainer.
Ithasafixedsizeandshape.
Ithasafixedvolumebuttakesuptheshapeofitscontainer.
Ithasstrongforcesbetweentheparticles.
Youcanmakeyourgameharderoreasierbythe Whenyouhavemadeyourcards,swapwithanother
wordsyouchooseforyourbannedlist. groupandplaythegame.Playerstaketurns
choosingacardanddescribingthewordtotheir
Makeasetofcardsforthefollowingwords
team.Theaimistogetthroughallthecardsas
andphrases:solid,liquid,gas,vapour,melting,
evaporation,condensation,freezing,meltingpoint,
boilingpoint,kineticmodelofmatter,Brownian
motion,temperature,diffuse,expand,changeof
state,Boyle'slaw,Pascal.
quicklyaspossible.
SUMMARY
Mattercanexistinthreestates-solid,liquidandgas.
Solidshaveafixedshapeandvolume.Liquidshaveafixedvolumebuttaketheshapeoftheircontainer.Gases
expandtofilltheircontainer.
Thekineticmodelexplainsthebehaviourofmaterialsbydescribingwhatishappeningtotheparticlesofwhich
theyaremade.
Accordingtothekineticmodel,matterismadeofmovingparticlesthatareclosetogetherinsolidsandliquids,
andfarapartingases.
Thereareattractiveforcesbetweenparticlesthatactstronglywhentheparticlesareclosetogether.
InBrownianmotion,themovementofwaterorairparticlesisrevealedbytheireffectonvisiblegrainsofpollen
orsmokeparticles.
Asthetemperatureofasubstanceincreases,thekineticenergyofitsparticlesincreases.
Theparticlesofagasbombardthewallsofitscontainer.Thiscausespressure.Increasingtemperatureincreases
pressure.Decreasingthevolumeincreasesthepressure.
TemperaturecanbemeasuredusingtheKelvintemperaturescalewhichhasitszeroat-273°C.
PressureandvolumearerelatedbytheequationpV=constant.

181. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
2Brownianmotioncanbeobservedbylookingatsmokeparticlesundera
microscope.Whatcausesthemovementofthesmokeparticles? [1]
AThesmokeparticlesaremovedbyaircurrents.
BThesmokeparticleshitthewallsofthecontainer.
CAirmoleculesbitthesmokeparticles.
DTherearestrongforcesbetweenthesmokeparticles.
3Describethedifferencesbetweensolids,liquidsandgasesintermsofthe
arrangementandmovementoftheirparticles. [6]
4Astudentobservessmokeparticlesunderamicroscope.Thesmokeislit
upsosheseesthesmokeparticlesasdotsoflight.
aDescribethemovementofthesmokeparticles. [1]
bNametheparticleswhicharecausingthismovement. [1]
cExplainwhyobservationofthismovementprovidesevidenceforthe
kineticmodelofmatter. [2]
[Total:4]
5Thediagramshowsasyringecontainingasmallamountofair.
Theendofthesyringeissealedsonoaircanenterorleave.
aDeterminetheamountofairinthesyringefromthediagram. [1]
bAstudentpushesintheendofthesyringe,compressingthegas.
Stateandexplainwhathappenstothepressureoftheair. [2]
cThesyringeisplacedonaradiator.Afterawhile,thestudentnotices
thatthevolumeoftheairhasincreased.Explainintermsoftheair
moleculeswhythishashappened. [2]
[Total:5]
6aBeforegoingforabikeride,acyclistpumpshistyresupsotheyhavea
pressureof2.5atmospheres.Straightafterhisride,thecyclistcheckshis
tyrepressure.Itisnow4atmospheres.Explainwhyithasincreased.[2]
bAfterafewminutes,thepressurereturnsto2.5atmospheres.Hereleases
theairsothepressuredropsto1atmosphere.Thetyrehasavolumeof
1200cm3.Calculatethevolumeofairthatwillbereleasedfromthetyre.[4]
[Total:6]
COMMAND WORD
describe:statethe
pointsofatopic;givi
characteristicsand
mainfeatures
explain:setout
purposesor
reasons;make
therelationships
betweenthings
evident;provide
whyand/orhowand
supportwithrelevant
evidence
determine:establish
ananswerusingthe
informationavailable
Kstate:expressin
clearterms
calculate:workout
fromgivenfacts,
figuresorinformation
168>

182. 9Thekineticparticlemodelofmatter
SELF-EVALUATIONCHECKLIST
i’terstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutosee
lygapsinyourknowledgeandhelpyoutolearnmoreeffectively.
1can
See
Topic...
Needs
morework
Almost
there
Confident
tomoveon
Describetheshapeandvolumeofsolids,liquids
andgases.
9.1
Drawandexplainparticlediagramsshowingthe
separationandarrangementofparticlesinsolids,liquids
andgases.
9.2
Describethemovementofparticlesinallthreestates
ofmatter.
9.2
DescribeandexplainBrownianmotion. 9.2
Usethekineticmodeltoexplaingaspressure. 9.3
DescribeandusetheCelsiustemperaturescale. 9.4
DescribeandusetheKelvintemperaturescale. 9.4
ConverttemperaturesfromCelsiustoKelvinandfrom
KelvintoCelsius.
9.4
Pelategaspressuretothechangeinforcecreatedper
unitareaofparticleshittingthewallofacontainer.
9.5
1:ecallandusetheequationpV=constant. 9.5

183. explainchangesofstateusingthekineticmodelofmatter.
describehowandwhysolids,liquidsandgasesexpandwhentheirtemperaturesrise
explainsomeeverydayusesandconsequencesofthermalexpansion
relateenergysuppliedtoincreaseintemperaturewhenanobjectisheated
>
measurethespecificheatcapacityofsomematerials
INTHISCHAPTERYOUWILL:
>Chapter10
Thermal
properties
ofmatter

184. 10Thermalpropertiesofmatter
GETTINGSTARTED
THESTRANGEBEHAVIOUROFWATER
Mostsubstancesexpandwhentheygethotand
iOntractwhentheygetcold.Waterdoesnotalways
1'lllowthisrule.Thinkabouthowapondcoolsdown
incoldweather.Astheairtemperaturedrops,the
wateratthetopofthepondiscooledbythecold
nraboveitandcontracts.Thismakesthewaterat
Figure10.2:Thewaterbelowtheiceiscold,butnot
ioldenoughtofreeze.
thetopofthepondmoredense,soitsinkstothe
bottom.Thecoldestwaterwillbeatthebottomof
thepond.Ifthiscontinued,icewouldformatthe
bottomofthepond.
Fortunatelyforthefish,waterdoesnotkeep
contracting.Asitcoolsfrom4°Cto0°C,thewater
expands.Thismeansthatthecolderwaterisless
densethanthewaterbelowitandsoitremainsat
thetop.Thiswatercontinuestocoolandeventually
freezes.Theiceformsatthetopofthepond,not
thebottom.Thismeansfishcansurviveinthewater
beneaththeice.Thispropertyofwateriscalledthe
anomalousexpansionofwater.
Unlikemostsubstances,waterislessdenseasa
solid(ice)thanitisasaliquid.Thismeansthatice
floatsinwater.Mostsubstancesaremoredense
whentheyaresolidandsodonotfloat.Thisstrange
behaviouriscausedbythehydrogenbondsinwater
whichcauseittoformacrystalstructurewiththe
particlesinicemorespacedoutthaninwater.

185. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
2DiscusswhatwouldhappentolifeonEarth
ifwaterfrozelikeotherliquidssothatponds
frozefromthebottomup.
Discussionquestions
1Sketchagraphtoshowhowthedensityof
waterchangesfrom0to10°C.
10.1Thermalexpansion
Mostsubstances-solids,liquidsandgases-expandwhen
theirtemperaturerises.Thisiscalledthermalexpansion.
Thermalexpansionhappensbecausetheparticlesgain
energyandmovefaster,pushingeachotherfurtherapart.
Theexpansionofsolids
Figure10.3showsanexperimentthatdemonstratesthat
ametalballexpandswhenitisheated.
Whentheballiscold,itjustfitsthroughthering.
Theball,butnotthering,isheatedstrongly.Itnow
willnotpassthroughthering.Ithasexpanded.
Whentheballcoolsdown,itcontractsandreturns
toitsoriginalsizeandwillonceagainpassthrough
thering.
Figure10.3a:Themetalballiscoldandhaspassedthrough
thering,b:Themetalballishot.Ithasexpandedandwillno
longerfitthroughthering.
KEYWORDS
thermalexpansion:theincreaseinvolumeofa
materialwhenitstemperaturerises
Usesofexpansion
Rivetsareusedinshipbuildingandotherindustriesto
joinmetalplates.Ared-hotrivetispassedthroughholes
intwometalplatesandthenhammereduntiltheend
rounded(Figure10.4).Astherivetcools,itcontracts
pullsthetwoplatestogethertightly.
Ametallidorcapmaystickonaglassjarorbottle,£
behardtounscrew.Heatingthelid(forexample,by
runninghotwateroverit)causesittoexpand.Thegl
expandsmuchlessthanthemetallid,meaningthattl
lidloosensandcanberemoved.
hotrivet coldrive
Figure10.4:Joiningtwometalplatesusingarivet.
Figure10.5:Steeltyresareheatedsotheyexpandand<
befittedtotrainwheels.Theycontractandfitverytighth
Asteel‘tyre’canbefittedontothewheelofatrainw
thetyreisveryhot.Itthencoolsandcontracts,soth:
fitstightlyontothewheel.
heat
Figure10.6:Abimetallicstrip.Invarisametalalloywhic
expandsverylittlewhenheated.Copperexpandsmore,
differenceinexpansioncausesthestriptobend.
Abimetallicstrip(Figure10.6)isdesignedtobendasi
getshot.Thestripismadeoftwometalsjoinedfirmly
together.Onemetalexpandsmuchmorethantheothe
172>

186. 10Thermalpropertiesofmatter
id
i
cylinder piston
ii
ACTIVITY10.1
lei
t
piston
rises
Glasscontainersmaycrackwhenhotliquidisplaced
inthem.Thisisbecausetheinnersurfaceoftheglass
expandsrapidly,beforethethermalenergyhaspassed
throughtotheoutersurface.Theforceofexpansion
crackstheglass.Toavoidthis,glasssuchasPyrexhas
beendevelopedthatexpandsverylittleonheating.An
alternativeistoughenedglass,whichhasbeentreated
withchemicalstoreducethechanceofcracking.
\Ihestripisheated,thismetalexpands,causingthestrip
iibend.Themetalthatexpandsmoreisontheoutsideof
llucurve,becausetheoutercurveislongerthantheinner
MlThesestripsareusedindevicessuchasfirealarms
"idthermostats.Thermostatsareusedtocontrolthe
iini|)eratureofdevicessuchasovensandirons.
Figure10.9:Agasexpandswhenitisheatedatconstant
pressure.
Figure10.8:Asthetemperaturedrops,sodoesthe
volumeoftheliquidinthethermometer.Thealcoholinthis
thermometerremainsliquidatverylowtemperatures.
gas
molecules
Consequencesofexpansion
Iheexpansionofmaterialscancauseproblems.For
p*ample,metalbridgesandrailwaylinesexpandonhot
Tnandthereisadangerthattheymightbend.Toavoid
llll'.bridgesaremadeinsections,withexpansionjoints
I'tweenthesections(Figure10.7).Onahotday,thebridge
pandsandthegapsbetweensectionsdecrease.Railway
linesarenowusuallymadefromametalalloythatexpands
i'iylittle.Onaconcreteroadway,youmaynoticethatthe
madsurfaceisinshortsections.Thegapsbetweenarefilled
Hhsofttar,whichbecomessquashedastheroadexpands.
Diesofabimetallicstrip
Abimetallicstripcanbeusedinanelectriccircuit
b।completeorbreakacircuitdependingonthe
Inmperature.
Designafirealarmcircuitwhichwillsound
abuzzerwhenitgetshot.Designaposter
topromoteyourinvention.Includeacircuit
diagramandanexplanationofhowthe
deviceworks.
Considerhowyoucouldchangeyourcircuit
tomakeanalarmtowarngardenerswhen
temperaturesaredroppingandallowthem
toprotectdelicateplantsfromfrost.Designa
posterforafrostalarm.
Theexpansionofliquids
Manythermometersusetheexpansionofaliquidto
measuretemperature.Asthetemperatureoftheliquid
rises,itexpandsandthelevelofliquidinthetuberises.
Theexpansionofgases
Gasesexpandwhentheyareheated,justlikesolidsand
liquids.Wecanexplainthisusingthekineticmodelof
matter(seeChapter9).Figure10.9showssomegasina
cylinderfittedwithapiston.Atfirst,thegasiscoldand
itsparticlespressweaklyonthepiston.Whenthegas
isheated,itsparticlesmovefaster.Nowtheypushwith
greaterforceonthepistonandpushitupwards.Thegas
hasexpanded.
IIgure10.7:Thislorryisabouttocrossanexpansionjoint
..ii,>roadbridge.Onahotday,thebridgeexpandsandthe
mhirlockingteethofthejointmoveclosertogether.

187. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Theexpansionofgasesastheyheatupmeansthatthe
densityofahotgasislowerthanthedensityofthesame
gaswhenitiscold.Thisiswhyhotairrises(youwill
learnmoreaboutthisinChapter11).
Figure10.10:Theexpansionofairasitheatsupcreates
thermalcurrents,whichthishanggliderisusing.
Comparingsolids,liquids
andgases
Whichexpandsmost,asolid,aliquidoragas,fora
givenriseintemperature?
Solidsexpandleastwhentheyareheated.Some,
suchasPyrexglassandinvarmetalalloy,havebeen
designedtoexpandaslittleaspossible.
Liquidsgenerallyexpandmorethansolids.
Gasesexpandevenmorethanliquids.
Therearesomeexceptionstothis.Forexample,liquid
paraffinexpandsveryrapidlyonheating.Petrol
(gasoline)isstoredincoolundergroundtanks.Ifa
motoristfillstheirtankonahotday,thepetrolwillheat
upandexpand.Thiscancausethefueltooverflowwhen
itexpands.
Whenamaterialexpands,itsparticles(atomsor
molecules)donotgetanybigger.However,theyhave
moreenergy,sotheycanmovearoundmoreandtakeup
morespace.Itisdifficultfortheparticlesofasolidto
pushtheirneighboursaside,soasoliddoesnotexpand
much.Whenagasisheated,itsparticlesmoveabout
morerapidly,anditiseasyforthemtopushthewalls
oftheircontainerfurtherapart,sothatthegastakesup
morespace.
Questions*
1Copyandcompletethesesentences:
Whenanobjectisheatedit .Whenitc
Liquidsexpandmorethan butlesstha
Abimetallicstripismadeoftwo whic
expandbydifferentamountswhenheated.
Thiscausesthestripto .
2LookatthebridgeinFigure10.11.Oneendo:
bridgeisonrollersratherthanbeingfixed.Ex]
howthishelpsitcopewithextremetemperatui
3Alcoholhasafreezingpointof-115°C.Explai
whycolouredwatercouldnotbeusedinthe
thermometerinFigure10.8.
4Figure10.12showsanexperimenttocomparet
expansionofliquids.Equalvolumesofethanol
waterandmercurywereheatedinawaterbath.
liquids.
aWhichliquidexpandsthemost?
bExplainwhythisliquidisusedinmostsch
laboratorythermometers.

188. Isit
tie
tin
10.2Specificheatcapacity
Energyandtemperature
Mlobjectsstoreenergy,calledinternalenergy.Internal
iuergyisameasureofthetotalenergyofalltheparticles
intheobject.Thisincludesboththekineticenergyof
theparticlesandchemicalpotentialenergyofthebonds
betweenthem.Supplyingthermalenergytoanobject
Willraiseitstemperature.Theparticleswillmovefaster
ritheygainkineticenergy.
Ilowever,energyandtemperaturearenotthesamething.
Iheinternalenergyofanobjectisthetotalenergyofall
Ilsparticles.Acupofteamayhaveahighertemperature
thanabathofwarmwater,buttheenergystoredin
thebathwaterismuchgreaterastherearemanymore
Waterparticles.
Figure10.13:Theteaishotterthanthebathsoitsparticles
movefaster.However,thebathhasfarmoreparticlesand
toresmoreenergy.
Ileatingwatercausesthewaterparticlestogainkinetic
energyandspeedup.Ittakesmoreenergytoraisethe
temperatureofalargeamountofwaterbecausemore
particlesneedtohavetheirspeedchanged.Thissection
dealswithincreasingthekineticenergyofparticles.We
willlookatthepotentialenergyinSection10.3.
Supposethatyouwanttomakeahotdrinkforyourself
andsomefriends.Youneedtoboilsomewater.You
willbewastingenergyifyouputtoomuchwaterinthe
kettleorpan.Itissensibletoboiljusttherightamount.
Also,ifthewaterfromyourtapisreallycold,itwilltake
longer,andrequiremoreenergy,toreachboilingpoint.
10Thermalpropertiesofmatter
So,theamountofenergyyouneedtosupplytoboilthe
waterwilldependontwofacts:
themassofthewater
theincreaseintemperature.
Inordertocalculatehowmuchenergymustbesupplied
toboilacertainmassofwater,weneedtoknowathird
fact:
ittakes4200Jtoraisethetemperatureof1kgof
waterby1°C.
Letusassumethatthecoldwaterfromyourtapisat
20°C.Youhavetoprovideenoughenergytoheatitto
100°C,soitstemperaturemustincreaseby80°C.Letus
alsoassumethatyouneed2kgofwaterforallthedrinks.
Theamountofenergyrequiredtoheat2kgofwaterby
80°Cis,therefore:
energyrequired=2kgx4200J/(kg°C)x80°C
=672000J=672kJ
Anotherwaytoexpressthethirdfactaboveistosaythat
thespecificheatcapacityofwateris4200Jperkgper°C
or4200J/(kg°C).Ingeneral,thespecificheatcapacity
ofasubstanceisdefinedastheenergyneededtoraise
thetemperatureof1kgofthesubstanceby1°C.Itis
definedbytheequation:
c=^mA#
KEYEQUATION
.c
,,
.t
energyrequired
specificheatcapacity=
massxtemperatureincrease
mA#
wherec=specificheatcapacity,AE=energyrequired,
m=massandA#=increaseintemperature.Aisthe
Greekletterdelta;itmeans‘changein’.
Theenergyneededcanbecalculatedbyrearrangingthe
equation:
energyrequired=massxspecificheatcapacity
xincreaseintemperature.■
AE=mcAO
KEYWORDS
specificheatcapacity:theenergyrequiredper
unitmassperunittemperatureincrease
I

189. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
WorkedExample10.1showshowtousethisequationin
moredetail.
WORKEDEXAMPLE10.1
Akettleheats1.5kgofwater.Howmuchenergyis
neededtoraisethetemperatureofthewaterfrom
20°Cto100°C?Assumethatspecificheatcapacityof
water=4200J/(kg°C).
Step1:Calculatetherequiredincreasein
temperature.
increaseintemperature=100°C-20°C
=80°C
Step2:Writedowntheotherquantitiesneededto
calculatetheenergy.
massofwater=1.5kg
specificheatcapacityofwater
=4200J/(kg°C)
Step3:Writedowntheequationforenergy
required,substitutevalues,andcalculatethe
result.
energyrequired=massxspecificheat
capacityxincreasein
temperature
=1.5kgx4200J/(kg°C)x80°C
=504000J
=504kJ
Answer
504kJofenergyisrequiredtoboilthewater.
Themeaningofspecific
heatcapacity
Energyisneededtoraisethetemperatureofanymaterial.
Theenergyisneededtoincreasethekineticenergyofthe
particlesofthematerial.Insolids,theyvibratemore.In
gases,theymoveaboutfaster.Inliquids,itisabitofboth.
Wecancomparedifferentmaterialsbyconsidering
standardamounts(1kg),andastandardincreasein
temperature(1°C).Differentmaterialsrequiredifferent
amountsofenergytoraisethetemperatureof1kgby
1°C.Inotherwords,theyhavedifferentspecificheat
capacities.Table10.1showsthevaluesofspecificheat
capacityforavarietyofmaterials.
Table10.1showsthatthereisquiteawiderangeof
values.Thespecificheatcapacityofsteel,forexampli
isone-tenththatofwater.Thismeansthat,whenyou
supplyequalamountsofenergyto1kgofsteelandt
1kgofwater,thetemperatureofthesteelrisestentir
asmuchasthetemperatureofthewater.
Table10.1:Specificheatcapacitiesofavarietyofmateri
Typeof
material
Material
Specificheat
capacity/J/(kg°i
metals steel 420
aluminium 910
copper 385
gold 300
lead 130
non-metals glass 670
nylon 1700
polythene 2300
ice 2100
liquids water 4200
seawater 3900
ethanol 2500
oliveoil 1970
gases air 1000
watervapour2020(at100°C)
methane 2200
Thespecificheatcapacity
ofwater
Waterisanunusualsubstance.Asyoucanseefrom
Table10.1,ithasahighvalueofspecificheatcapacity
(s.h.c.)comparedwithothermaterials.Thishas
importantconsequences:
ittakesalotofenergytoheatupwater
hotwatertakesalongtimetocooldown.
Theconsequencesofthiscanbeseeninourclimates.
Inthehotmonthsofsummer,thelandwarmsupquit
(lowspecificheatcapacity)whiletheseawarmsupon
slowly.Inthewinter,theseacoolsgraduallywhilethe
landcoolsrapidly.Peoplewholivealongwayfromth
sea(inthecontinentalinteriorofNorthAmericaor

190. 10Thermalpropertiesofmatter
Method
powerpack
joulemeter
-
-
heater
thermometer
metalblock
insulation
Figure10.14
1
Gettingstarted
1
2
6
8
y
EXPERIMENTAL SKILLS10.1
Iufasia,forexample)experiencefreezingwintersand
«0ryhotsummers.Peoplewholiveonislandsandin
t<lastaiareas(suchaswesternEurope)areprotected
11ninclimaticextremesbecausetheseaactsasastore
2
3
Safety:Theelectricheatercanbecomeextremely
hot.Leaveittocoolinsidetheblockwhenyouhave
finishedyourexperiment.
ofheatinthewinter,andstaysrelativelycoolinthe
summer.
4
5
-
-
-
Measuringthespecificheatcapacityofametal
Inthisexperimentyouwillheatametalblock.You
willmeasuretheenergyyousupply,themassofthe
metalandthechangeintemperature.Youwillthen
beabletocalculatethes.h.c.ofthemetal.
Youwillneed:
ablockofmetalwithholesforthe
thermometerandheater
insulationfortheblock
electricheater
powerpack
joulemeter(yourteachermayshowyouan
alternativewaytofindtheenergyused)
thermometer
accesstobalance.
Shouldyouinsulatethemetalblock?Explain
youranswer.
Whyshouldyoumeasurethemassofthe
blockatthestartoftheexperimentrather
thantheend?
-
Measureandrecordthemass,m,oftheblock
inkg.
SetuptheexperimentasshowninFigure10.14.
Measureandrecordtheinitialtemperature(0J
oftheblock.
Turnonthepowersupply.
Whentherehasbeenatemperatureriseof
10°C,turnoffthepowersupplyandrecordthe
Joulemeterreading.
Watchthethermometerforafewminutesand
recordthehighesttemperature(02)itreaches.
Calculatethechangeintemperatureusing
=02
~^1-
Calculatethespecificheatcapacityofthe
metalusingtheequation:
c=
^
'
mA0

191. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
EXPERIMENTAL SKILLS10.2
Measuringthespecificheatcapacityofwater
Inthisexperimentyouwillheatamassofwaterand
finditsspecificheatcapacity
Youwillneed:
abeakerofwaterwithalidwithholesfor
thethermometerandheater
insulationforthebeaker
electricheater
powerpack
joulemeter(yourteachermayshowyouan
alternativewaytofindtheenergyused)
thermometer
accesstobalance.
Safety:Theheaterandwaterwillgethot.Allow
themtocoolbeforeclearingawayyourapparatus.
1Put0.25kgofwaterintothebeaker.
2SetuptheexperimentasshowninFigure10.15.
3Measureandrecordtheinitialtemperature(0,)
oftheblock.
4Turnonthepowersupplyandleaveuntilthe
temperaturechangesbyabout50°C.
5Turnoffthepowersupply.Recordthefinal
temperature(02).
6Calculatethechangeintemperatureusing
=02-Oy.
7Recordthejoulemeterreading.
8Calculatethespecificheatcapacityofthe
metalusingtheequation:
Questions
Howwillyoumeasurethemassofthewater?2
Method
powerpack
Gettingstarted
1Whyisitimportanttohavealidonthebeaker?
1Compareyouranswerswiththevaluesof
specificheatcapacitygiveninTable10.1.
Aretheyhigherorlower?
2Thethermalenergysuppliedbytheheater
heatstheheateritself,the hermometer
andthesurroundingairaswellastheblock.
Explainwhateffectthiswillhaveonthe
accuracyofyouranswer.
Figure10.15
178

192. 10Thermalpropertiesofmatter
Questions
a
*
b
Table10.2
a
b
c
d
A
/
Time/min
c
Figure10.16:Anelectrickettle.
Thespecificheatcapacityofcopperis385J/kg°C.
Calculatetheenergyneededtoheat3kgofcopper:
Fable10.2givesthespecificheatcapacitiesofsome
materials.
Ittakesenergytochangeasolidintoaliquid.The
temperaturestaysthesameastheicemelts.Similarly,
whenaliquidbecomesagas,itstemperaturestaysthe
sameeventhoughenergyisbeingsuppliedtoit.Thisis
duetochangesinthechemicalpotentialenergyinthe
bondsbetweenthemoleculesoratoms.Energy-must
beprovidedtobreakbondsandchangeasubstance
fromsolidtoliquid.Energyisalsoneededtoovercome
theattractionbetweentheparticleswhenasubstance
changesfromliquidtogas.Whenthesechangesare
reversed,energyisgivenout.
a
b
Figure10.17:Atemperatureagainsttimegraphtoshow
thechangesthatoccurwheniceisheateduntiliteventually
becomessteam.A:iceisheatingup.B:iceismelting.
C:waterisheatingup.D:waterisboiling.E:steamis
heatingup.
a
b
<D
100
Acookheats500gofoliveoilinasteelpanwhich
hasamassof300g.Theoilneedstobeheated
from20°Cto190°C.UsingdatafromTable10.1,
calculatethethermalenergyneeded:
TheelectrickettleinFigure10.16hasapowerrating
of2000W.Ittakes90secondstoheat500gwater
from20°Ctoboiling.
toheatthepan
toheattheoil
intotal.
Namethesubstancethatrequirestheleast
amountofenergytoraiseitstemperature.
Thetabledoesnotgiveunits.Whatistheunit
forspecificheatcapacity?
Statetheamountofenergyneededtoheat1kg
ofglassby1°C.
Calculatetheenergyneededtoheat1kgof
glassby10°C.
Usethisinformationtocalculatean
approximatevalueforthespecificheatcapacity
ofwater.
Compareyouranswertothespecificheat
capacityofwatergiveninTable10.1.Comment
onwhyitisdifferent.
by1°C
from20°Cto50°C.
10.3Changingstate
Figure10.17showswhathappenstothetemperature
whenyoutakesomeicefromthefreezerandheatitat
asteadyrate.Inafreezer,iceisatatemperaturewell
belowitsfreezingpoint,maybeaslowas-20°C.From
thegraph,youcanseethattheicewarmsupto0°C,
thenstaysatthistemperaturewhileitmelts.Aslumps
oficefloatinwater,bothareat0°C.Whenalloftheice
hasmelted,thewater’stemperaturestartstoriseagain.
At100°C,theboilingpointofwater,thetemperature
againremainssteady.Thewaterisboilingtoformsteam.
Eventually,allofthewaterhasturnedtosteam.Ifyou
continuetoheatthesteam,thetemperatureofthesteam
willriseagain.
Material Specificheatcapacity
water 4200
gold 300
glass 840
air 1000

193. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Condensationiswhenagaschangestoaliquid.The
particlesslowdownandtheparticlesaredrawntogether.
Solidification,orfreezing,iswhenaliquidchangesto
asolid.Asaliquidlosesenergy,itsparticlesslowdown
andthebondsholdingtheparticlestogetherre-form.
Theboilingandmeltingpointsofasubstancechange
iftheairpressurechanges.Waterhasameltingpoint
of0°Candaboilingpointof100°Catstandard
atmosphericpressure.Thisisthepressureatsealevel
whichisabout101.32kPaor1atmosphere.Ataltitude,
waterboilsatalowertemperature.
Figure10.18:OntopofMountEverestwaterboilsat71°C
Investigatingachangeofstate
Time/min
Figure10.19a:Thethermometermeasuresthetemperatureofthewaxasitcools,b:Thegraphshowshowthe
temperaturechanges.
Figure10.19ashowsonewaytoinvestigatethebehaviour
ofaliquidmaterialasitsolidifies.Thetesttubecontains
awaxysubstancecalledstearicacid.Thisiswarmedup,
anditbecomesaclear,colourlessliquid.Itisthenleft
tocooldown,anditstemperatureismonitoredusinga
thermometer(oranelectronictemperatureprobe)and
recorded.
Figure10.19bshowstheresults.
Atfirst,theliquidwaxcoolsdown.Itstemperaturedrops
gradually.Thewaxishotterthanitssurroundings,so
thermalenergyistransferredtothesurroundings.The
graphisslightlycurved.Asthetemperaturedrops,there
islessdifferencebetweenthetemperatureofthewaxand
itssurroundings,soitcoolsmoreslowly.
Nowthetemperatureofthewaxstaysthesamefor
afewminutes.Thetubecontainsamixtureofclear
liquidandwhitesolid.Thewaxissolidifying.Thewax
isstilltransferringthermalenergytothesurroundings,
becauseitisstillwarmerthanitssurroundings,butits
temperaturedoesnotdecrease.
Thewax’stemperaturestartstodropagain.Allthewax
isnowsolid.Itcontinuescoolinguntilitreachesthe
temperatureofitssurroundings.
Thedashedlineonthegraphhasbeendrawntofindth
temperatureatwhichthestearicacidchangesfroma
liquidtoasolid.
180)

194. 10Thermalpropertiesofmatter
Breakingfree
«l
l»«l
ID
Figure10.20
Questions
KEYWORDS ACTIVITY9.2
PEERASSESSMENT
«
Iamsoboredinthissolid.
It'ssosquashedandInever
gettoseeanyoneexceptthese
particlesoneachsideofme.
IfonlyIhadtheenergytochange...
Ihestearicacidexperiment(Figure10.19)showsthat
ipuresubstancechangesfromsolidtoliquidatafixed
lumperature,knownasthemeltingpoint.Similarly,a
liquidchangestoagasatafixedtemperature,knownas
Hiboilingpoint.Table10.3showsthemeltingandboiling
pointsofsomepuresubstances.
bible10.3:Themeltingandboilingpointsofsomepure
instances.Mercuryisinterestingbecauseitistheonly
nittalthatisnotsolidatroomtemperature.Tungstenisa
unital,andithasthehighestboilingpointofanysubstance.
Iloliumhasthelowestmeltingandboilingpointsofany
nlument.Infact,heliumwillonlysolidifyifitiscompressed
quashed)aswellascooled.
\puresubstancehasaclearmeltingpointandaclear
indboilingpoint.Amixtureofsubstancesmaymelt
Uiboiloverarangeoftemperatures.Candlewaxisan
।xnmple.Itisnotasingle,puresubstance,andsome
nlthesubstancesinitmeltatlowertemperaturesthan
others.
1Hssolvingthingsinwaterchangestheboilingpointand
Iniczingpointofthewater.Forexample,saltywater
boilsatahighertemperaturethanpurewaterandfreezes
illulowertemperature.
Notallsubstancesmeltorboilwhentheyareheated,
burn,andothersdecompose(breakdown)into
nnplersubstancesbeforetheyhaveachanceto
।hnngestate.
meltingpoint:thetemperatureatwhichasolid
meltstobecomealiquid
boilingpoint:thetemperatureatwhichaliquid
changestoagas(atconstantpressure)
Createacomicstriptotellthestoryofwhat
happenstotheparticlewhenitgetswhatitwants
(energy)andchangesfromasolidtoaliquidand
thentoagas.
Yourcomicstripshouldbe:
engaging(usepictures,colourandhumour)
scientificallycorrect(checkbackthroughthe
chaptersofar;usethecorrectscientificwords
andexplainwhatishappening).
Swapcomicstripswithanothergroup.Studytheir
workandgivethemfeedback.Include:
threewaysitworkswell
onesuggestionforhowitcouldbeeven
better;thiscouldbesomethingtheyneedto
makeclearer,orsomethingtheycouldadd.
9LookatFigure10.17.Sketchthegraphandaddnotes
explainingwhatishappeningduringeachsection.

195. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
10UseFigure10.19btodeterminethemeltingpointof
stearicacid.
11UsetheinformationinTable9.2toexplainwhy
airdoesn’thaveafixedmeltingorboilingpoint.
(Hint:airisroughly80%nitrogenand20%oxygen).
Evaporation
Figure10.21:Inwarmweatherpuddlesevaporatequickly.
Aliquidcanchangestatewithoutboiling.Afterit
rains,thepuddlesdryupeventhoughthetemperature
ismuchlowerthan100°C.Thewaterfromthepuddles
hasevaporated.Theliquidwaterhasbecomeagas
calledwatervapourintheair.Thisistheprocessof
evaporation.Wecanthinkofavapourasagasata
temperaturebelowitsboilingpoint.
Aliquidevaporatesmorequicklyasitstemperature
approachesitsboilingpoint.Thatiswhypuddles
disappearfasteronahotdaythanacoldday.
gasparticle
particlewith
higherkinetic
energyleaves
theliquidsurface
liquidparticle
Figure10.22:Fast-movingparticlesleavethesurfaceofa
liquid.Thisishowtheliquidevaporates.
Howcanweusethekineticmolecularmodelofm.
toexplainevaporation?Imagineabeakerofwater,
waterwillgraduallyevaporate.Figure10.22shows
particlesthatmakeupthewater.Theparticlesoft
wateraremovingaround,andsomearemovingfa:
thanothers.Somemaybemovingfastenoughtoe.
fromthesurfaceofthewater.TheybecomeparticL
ofwatervapourintheair.Inthisway,allofthewa
particlesmayeventuallyescapefromthebeaker,an
waterwillhaveevaporated.
Ifthetemperatureoftheliquidishigher,moreofi'
particleswillhaveenoughenergytoescape.Thismt
theliquidwillevaporatemorequickly.Thehottest
particlesaremostlikelytoescapeastheyhavemost
energy.Whentheyescape,theaverageenergyofthe
remainingparticlesisless,sotheliquidcoolsdown.
Coolingbyevaporation
Ifyougetwet,perhapsintherainorafterswimmin;
youwillnoticethatyoucanquicklygetcold.Thewt
onyourbodyisevaporating,andthiscoolsyoudow
Whydoesevaporationmakethingscooler?
LookagainatFigure10.22.Theparticlesthatare
escapingfromthewaterarethefastest-movingones.
Theyaretheparticleswiththemostkineticenergy.
Thismeansthattheparticlesthatremainarethosev
lessenergy.Nowtheparticlesoftheliquidhaveless
energy(onaverage)andsothetemperatureofthewi
decreases.Thewatercoolsdown.
Comparingevaporation
andboiling
Evaporationandboilingbothinvolvealiquidturnin
intoagas.Evaporationisdifferentfromboiling.
Boilingonlyhappensattheboilingpointofthe
substance.Evaporationoccursatalltemperatur
Foraliquidtoboil,ithastobeheated-the
kineticenergyofitsparticlesmustbeincreased.
Evaporationhappenswhenthemostenergetic
particlesescape,soevaporationtakesenergyfro
thesubstance.
Boilinghappensthroughouttheliquid.Evapora
onlyhappensatthesurface.
Aboilingliquidbubbles.Aliquidcanevaporate
withoutbubbles.
182y

196. 10Thermalpropertiesofmatter
Hvvasingthetemperature
Increasingthesurfacearea
I
insulator
Blowingairacrossthesurface
COLD
door
UWARM
pump
183>
er
:a
Adraughtismovingair.Whenparticlesescapefromthe
water,theyareblownawaysothattheycannotfallback
intothewater.Thishelpstheliquidevaporatequickly.
narrow
hole
cooling
fins
it
r
ha
Asaliquidevaporates,theremainingliquidcools.This
meansthatthermalenergywillflowtotheliquidfrom
anyobjectsincontactwithit.Whenwegethotwesweat.
Thesweatevaporates,causingthermalenergytoflow
fromtheskin.Thishelpsuscooldown.
Fridgesusethecoolingeffectofevaporation(seeFigure
10.27).Aliquidiscompressedthensquirtedthrougha
narrowholesoitspressureisreducedanditevaporates.
Thisdrawsthermalenergyfrominsidethefridgeinto
theliquid.Theliquidisthenpumpedoutofthefridge
tothepipesonthebackofthefridgewhereitis
compressedandcondenses,releasingthethermalenergy
tothesurroundings.
InIigure10.24,theliquidhasagreatersurfacearea
oftheparticlesareclosetothesurface,andsothey
mu।tcapemoreeasily.Thismeanstheliquidevaporates
miHquickly.
Figure10.26:Sweatinghelpsusregulateourbody
temperatureinhotconditions.
Iiqure10.25:Adraughtblowingacrossthesurface
uhmasestherateofevaporation.
Ilir"-
10.24:Increasingthesurfaceareaincreasestherate
Jiporation.
Imii.rungthetemperatureoftheliquidmeansthe
I lesonaveragehavemorekineticenergy.Moreof
du।mitideswillhaveenoughenergytoescape.This
fWuutheliquidwillevaporatemorequickly.
Mumn10.23:Increasingthetemperatureincreasestherate
.1>.q
nnation.
low-pressurehigh-pressure
pipe pipe
Figure10.27:Therefrigerantliquidabsorbsthermalenergy
fromthefridgeasitevaporates.
oo°i
ooo
Ipwedingupevaporation
11miusethekineticmodeltoexplainsomewaysof
*M*<ln>i|upevaporation.

197. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Questions
12Copyandcompletethefollowingsentences.
a isthechangefromaliquidtoagasata
temperaturebelowtheboilingpoint.
bEvaporationcausesaliquidtocoolbecausethe
moving,hotterparticlesarethemost
likelytoescape.Thismeanstheparticlesleft
behindaretheslowermoving, ones.
13Explainintermsofmovementandpositionof
particleswhathappenstoanicecubeasitisheated
andmelts.
14Tungstenmeltsa|amuchhighertemperaturet
iron.Whatcanyousayabouttheforcesbetwec
tungstenatoms,comparedtotheforcesbetwee
ironatoms?
15Asolidmaterialisheatedbutitstemperaturec
notrise.
aWhatishappeningtothesolid?
bWhathappenstotheenergythatisbeing
suppliedtothematerial?
16Usethekinetictheorytoexplainwhyawettov
willdrymuchfasterifhungoutsideonawarm
windydaythanifleftfoldedupinabag.
17Explainhowcoveringabottleofmilkwithad
clothwillhelptocoolthemilk.
PROJECT
Hotstuff
Figure10.28:Heatinggoldcausesittomelt.Liquidgold
canbepouredintomoulds,whereitwillcoolandsolidify.
Option1:Changingstate
Createavideoorarticleforarevisionguideto
explainchangesofstate.Showobjectschanging
state,suchasanicecubemeltingorapuddle
ofwaterevaporating(youcouldusetimelapse
photography).Youshouldshowwhatishappening
totheparticles.Youmaywanttousemarblesor
drawingstorepresenttheparticles.Youmustinclu
thefollowingkeywords:
melting solidifying
boiling internalenergy
condensing *temperature.
Writethreemultiplechoicequestionsforthestude
totesttheirunderstandingtogowithyourvideo.
Option2:Specificheatcapacityquestions
Specificheatcapacityisimportantinengineering
andinthechoiceofmaterialfordifferentproducts
Investigatetheapplicationsofspecificheatcapaci
inansweringquestionssuchas:
Whichisthebestmetalforapan?
Whatmakesagoodcoolantforthecooling
systemforanengine?
Whydothedifferentspecificheatcapacities
sandandwatercreateseabreezes?
Whichtypesoffoodstoremostthermalener
andsostayhotthelongest?
Researchoneormoreofthesequestions.Present
yourresultsasawebsiteentryforasitewhichansw
scientificquestionsposedbysciencestudents.Wri
aquestiontotestthestudents'understanding.
Includeacalculationandanswer.
184

198. 10Thermalpropertiesofmatter
Solids,liquidsandgasescontractwhentheirtemperaturesdrop.
Solidsexpandleastbecausetheirparticlesareheldtogetherbystrongattractiveforces.
HlWhichoneofthefollowingstatementsistrue?
A
PEERASSESSMENT
SUMMARY
EXAM-STYLEQUESTIONS
B
C
D
n
tha
ha
Thefireworkhasthehighesttemperatureanditcontainsthemostinternal
energy.
Thebathhasthehighesttemperatureandcontainsthemostinternalenergy.
Thefireworkhasthehighesttemperaturebutcontainstheleastinternalenergy.
Thebathhasthelowesttemperatureandcontainstheleastinternalenergy.
Presentyourworktoanothergroup.Askthemtoansweryourquestionsandcommentonwhatthey
havelearnt.
Solids,liquidsandgasesexpandwhentheirtemperaturesrise.
Whenaliquidevaporatesthemostenergeticparticlesescapefromthesurface,sotheliquidcools.
Evaporationoccursattemperaturesbelowtheboilingpointoftheliquid.Ithappensfasterathigher
temperatures,withalargersurfaceareaorifthereisadraughtacrossthesurface.
Gasesexpandmostastheirparticlesarefreetomove.
Expansioncancauseproblemssuchasthebendingoftraintracks,butcanalsobeuseful,forexamplein
thermometers.
Meltingisthechangefromasolidtoaliquidwithoutanychangeintemperature.
Boilingisthechangefromaliquidtoagaswithoutachangeintemperature.
Supplyingheatenergyincreasestheinternalenergyofasubstanceanditstemperaturerise.
Theamountofenergyneededtoraisethetemperatureof1Kgofasubstanceby1°Ciscalledthespecificheat
capacityofthesubstance.
TheenergyneededtoheatamassmofasubstancebyA0degreescanbecalculatedusingtheequation
mM
'
Duringachangeofstate,energyissuppliedorgivenoutbutthetemperaturedoesnotchange.

199. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
2Whichstatementaboutthedifferencebetweenevaporationandboiling
istrue? [1]
AEvaporationhappensatanytemperature.
BEvaporationhappensthroughoutaliquid.
BEvaporationhappensataspecifictemperature.
CEvaporationandboilingarenotdifferent.
3Whatisthermalexpansion? [1]
AAnincreaseindensityduetoanincreaseintemperature.
BAdecreaseindensityduetoadecreaseintemperature.
CAdecreaseindensityduetoanincreaseintemperature.
DAdecreaseinvolumeduetoanincreaseintemperature.
4Ajewellerismakingasilverring.Sheheatsthesilveruntilitmelts,andthen
continuestoheatitforanotherminuteafterithasmelted.Shethenpoursthe
molten(melted)silverintoamouldandleavesittocooldown.
aDescribewhathappenstothearrangementandmovementofthe
silveratomsasthesilverisheatedfromroomtemperatureandpoured
intothemould. [3]
bThegraphshowshowthetemperatureofthesilvervariesasitisheated.
Usethegraphtofindthemeltingpointofsilver. [1]
cStatehowlongittakesforthesilvertomelt. [1]
[Total:5]
5Thediagramshowsthecircuitforafirealarmusingabimetallicstrip.
aBrassexpandsmorethaniron.Whichmetalshouldbeatthetopof
thestrip? [1]
bDescribewhathappensasthetemperaturerisesincaseofafire. [3]
[Total:4]
COMMAND WORDS
describe:statethe
pointsofatopic;give
characteristicsand
mainfeatures
state:expressin
clearterms
186>

200. 10Thermalpropertiesofmatter
1min
1min
Adjusttimesaccordingtoyourparticularoven.
'NTINUED
[11
b
[1]
11udafollowsthecookinginstructionsforatinofbakedbeans.
COMMAND WORDS
420g
©Microwave
[i]
[3]
[2]
\cookspillswater.Somefallsonthefloor,someonthehotstovetop.
Ilietemperatureofthestovetopisabout80°C.Hegoestogetaclothto
wipeup.Whenhereturns,heseesthestoveisdrybutthefloorisstillwet.
750watt
CategoryD
Heatonfullpower
Totaltime
Emptycontentsintoanon-metallicbowlandcover.
Stirwellbeforeserving.
650watt
CategoryB
850watt
CategoryE
a
b
11uda’smicrowaveovenhasapowerof650W.Sheheatsthebeansforthe
recommendedtimeandchecksthetemperature.Thebeanshaveheatedfrom
10°Cto90°C.
Usetheequationenergy=powerxtimetocalculatetheenergysupplied.
Calculatethespecificheatcapacityofthebeans.Giveyouranswerto
twosignificantfigures.
Shenoticesthatthedishsheheatedthebeansinhasalsobecomehot
andrealisesthiswillaffecthercalculation.Explainwhetherherresult
willbetoohighortoolow.
Hudaalsomeasuresthespecificheatcapacityoftinnedspaghetti.
Shecalculatesitas3100J/kg°C.Ifsheheatsboththebeansand
spaghettitothesametemperature,whichwillcooldownquicker?
Explainyouranswer.
Nametheprocessbywhichwaterbecomesagasatatemperature
belowitsboilingpoint.
Explainwhyenergymustbesuppliedtoaliquidtoturnitintoagas.
Inyouranswer,refertotheparticlesoftheliquidandtheforces
betweenthem.
Explainwhythefloorwilldrymorequicklyifthecookturnsonan
electricfan.
explain:producean
answerfromagiven
sourceorrecallI
memory
calculate:workout
fromgivenfacts,
figuresorinformation
[2]
[Total:81
[1]
[Total:3]
Heatonfullpower 2mins 2mins
Removecoverandstir.Re-cover,then:
2mins 2mins
4mins 3mins 2mins

201. CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
SELF-EVALUATION CHECKLIST
Afterstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutosee
anygapsinyourknowledgeandhelpyoutolearnmoreeffectively.
Ican
See
Topic...
Needs
morework
Almost
there
Confident1
tomoveorl
Describeeverydayexamplesofthermalexpansion. 10.1
Statewhichexpandsmost,solid,liquidorgas,and
explainthisintermsoftheparticles.
10.1
Statetheeffectofraisinganobjects,temperatureonits
internalenergy.
10.2
A77
Recallandusetheequationc=. 10.2
Describeexperimentstomeasurethespecificheat
capacityofsolidsandliquids.
10.2
Describechangesofstateasachangeininternalenergy
withoutachangeintemperature.
10.3
Knowwhyevaporationcausescooling. 10.3
Explainthefactorsthatincreasetherateofevaporation.10.3
Explainhowanevaporatingliquidcausesobjectsin
contactwithittocool.
10.3
188y

202. Chapter11
carryoutexperimentstodemonstrateconduction,convectionandradiation
researchapplicationsandconsequencesofthermalenergytransfer.
INTHISCHAPTERYOUWILL:
explainwhysomematerialsconductandothersdonot
investigatethedifferencesbetweengoodandbademittersofradiation
describeandexplainconvectioncurrents
explainthermalenergyradiation
"hermalenergy
transfers

203. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
GETTINGSTARTED
Figure11.1showsitemsthatareusedtotransfer
thermalenergyorpreventthermalenergytransfer.
Workinagrouptodecidewhateachitemisforand
howitworks.Yourteacherwillstopyouandpicka
memberofthegrouptoreportbackononeofthe
photos.Iftheycandothistheyscoreapointforthe
team.Iftheyuseanyofthefollowingtermscorrectly,
yourteacherwillawardbonuspoints.
conductionconvectionradiationconductor insulator
convection
current
infrared
radiation
Rules:
Allmembersofthegroupneedtobeabletoanswer.
Youcannotwritedownyourideas.
Youcanwriteamaximumofthreewordsforeachpictureasaprompt.
Figure11.1:Aselectionofitemsthattransferthermalenergy,orpreventthermalenergytransfer.
WHATCANWELEARNFROMAREINDEER'SNOSE?
Figure11.2:Reindeeraresuperblyadaptedtoprevent
thermalenergylosses.
Reindeerliveinsomeofthemostdifficultconditions
ontheplanet.Theyneedtocopewithtemperatures
droppingto-50°Candbelow.
Reindeerareadaptedsotheyloseverylittlethermal
energyfromtheirbodies.Theyhavetwolayersof
fur:softfurnearthebody,thenalayerofguard
hairswhicharehollowtubescontainingair.Airisan
excellentinsulator.Thehairsaredenselypacked,
insulatingthebodysowellreindeercansleepon
snowwithoutmeltingit.
190y

204. 11Thermalenergytransfers
CONTINUED
Reindeerhaveotheradaptationsthatmakethem
suitedtolifeintheArctic.Theyneedtostick
togetherinblizzards,becausetheycannotseeeach
other.Theirfeetmakeaclickingsoundastendons
moveacrossbones.Thismeanstheycanheareach
other.Theyalsohaveaglandinthelegthatleavesa
scentontheground,whichhelpsthemlocateeach
other.
Reindeer'seyeschangeinwinter.Insummer,they
haveagoldenreflectivelayeratthebackoftheeye,
likeacat'seye.Inwinter,thislayerturnsdarkblue,
whichmeanslesslightescapesandtheeyesare
moresensitive.
Inthischapteryouwillstudythewaysinwhich
thermalenergyistransferredandhowwecanuse
thistoouradvantage.
Discussionquestions
1Humannasalpassagesalsotransferthermal
energyfromexhaledair.Trybreathingonto
yourhand,firstfromyourmouth,thenfrom
yournose.Youwillnoticetheairfromyour
noseiscooler.Discusshowweusethistohelp
uscooldownortokeepwarm.
2Thereindeer'snasalpassagescanbereferred
toasaheatenergyexchangesystem.They
exchangethethermalenergyfromthewarm
hairbreathedouttothecoldairbreathedin.
Thermalenergyexchangerscanreducethe
amountoffossilfuelsweuse,helpingreduce
theamountofenergyweuseandcutting
costs.Discussanyotherusesofthermalenergy
exchangers.Thesemaybeexamplesyouhave
seen,orsituationswhereyouthinktheycould
beuseful.
Ihereindeer'snasalpassagesserveasacomplex
thermalenergyexchangesystem.Theyhavea
।omplexsystemoftubeswhichallowwarmair
Ifeingbreathedouttopassovercoldairbeing
Ireathedin.Thermalenergyistransferredtothe
incomingairsothattheairleavingareindeer's
bodyiscold,andthereindeerkeepsthethermal
nergy.Thewarmairiscooledbyabout21°C
beforeleavingthereindeer'snose.
Ihereindeeraresowellinsulatedthattheyarein
lungerofoverheatingwhenrunningfrompredators
aswolves.
Iheylosethermalenergyfromtheirlargetongues.
Alsotheycirculatemorebloodtotheirlegs,whichare
lessinsulatedsocangiveoutthethermalenergy.
Itisimportantthatareindeerdoesnotlosefeelingin
itsnosewhilefindingfoodinthesnow.Tokeepthe
nosewarmandsensitive,ithasextrabloodvessels.
Thismeansthatwhenareindeerisphotographed
Withatemperature-sensitivecamera,thenoseseems
toglow.
Figure11.3:Areindeer'snosehasacomplexthermal
exchangesystem.Reindeerlosethermalenergythrough
theirlargetongues.
11.1Conduction
AswediscussedinChapter6,thermalenergytransfers
fromahotterplacetoacolderplace,thatis,froma
highertemperaturetoalowertemperature.Thermal
energyrequiresatemperaturedifferenceifitistobe
transferred.Inthischapter,welookatthevariousways
inwhichthermalenergyistransferred.Westartwith
thermalconduction.
Lyingonthetablearetwospoons:oneismetal,theother
iswooden(Figure11.4).Youpickupthemetalspoon
-itfeelscold.Youpickupthewoodenspoon-itfeels
warm.Infact,bothareatthesametemperature(room
temperature)asathermometerwouldprovetoyou.

205. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Howcanthisbe?Whatyouaredetectingisthefactthat
metalisagoodconductorofthermalenergy,andwood
isapoorconductorofthermalenergy.
Whenyourfingertouchesametalobject,thermalenergy
isconductedoutofyourfingerandintothemetal.
Becausemetalisagoodthermalconductor,thermal
energyspreadsrapidlythroughthemetal.Thermal
energycontinuestoescapefromyourfinger,leavingit
colderthanbefore.Thetemperature-sensitivenervesin
yourfingertiptellyourbrainthatyourfingeriscold.So,
youthinkyouaretouchingsomethingcold.
Whenyoutouchawoodenobject,thermalenergy
conductsintotheareathatyourfingerisindirectcontact
with.However,becausewoodisagoodthermalinsulator,
thethermalenergytravelsnofurther.Yourfingerloses
nomorethermalenergyandremainswarm.Themessage
fromthenervesinyourfingertipisthatyourfingeris
warm.So,youthinkyouaretouchingsomethingwarm.
Notethatthenervesinyourfingertellyouhowhotyour
fingeris,nothowhottheobjectthatyouaretouchingis!
Thisissimilartoourdiscussionofthermometersin
Chapter10.Athermometerinwaterindicatesitsown
temperature,andwehavetoassumethatthetemperature
ofthewateristhesameasthis.
Figure11.4a:Touchingametalspoon,b:Touchinga
woodenspoon.
KEYWORDS
thermalconduction:thetransferofthermal
energybythevibrationofmolecules
thermalconductor:asubstancethatconducts
thermalenergy
thermalinsulator:asubstancethatconductsvery
littlethermalenergy
EXPERIMENTAL SKILLS11.1
Demonstratingconduction
Inthisseriesofexperiments,youwillinvestigate
howmaterialsconduct,andwhicharethebest
conductors.Thisinformationisvitalforengineers
anddesignerswhoseworkinvolvesplanninghowto
keeptemperaturessafeandcomfortable.
Safety:Theseexperimentswillinvolveheating
materialstohightemperatures.Plancarefullyso
thatyoucanleavehotmaterialstocoolsafely.Wear
safetygoggleswhileusingtheBunsenburner.
Carryoutariskassessmentandmakesureyour
teacherhascheckeditbeforebeginningany
practicalwork.
Gettingstarted
LookattheapparatusinFigure11.5.Explainhow
attachingpaperclipsusingpetroleumjellywillallow
youtoobserveconduction.
Part1:Howisthermalenergyconductedalonga
metalbar?
Youwillneed:
Bunsenburner
heatproofmat
clampandstand
copperrod
paperclips
petroleumjelly.
192
>

206. 11Thermalenergytransfers
MethodMethod
clampstand
paperclips
rod
Bunsenburner
hi’4resistantmat
Figure11.6:Experimentalset-upforpart2.IIflure11.5:Experimentalset-upforpart1.
11
2
2
3
Questions
1
I'urt2:Whichmetalisthebestconductor?
Questions
1
2
ONTINUED
Writethemetalsinorderfromthebest
conductortotheworstconductor.
Describewhathappenedtothepaperclips.
Whatdoesthistellyouabouthowthermal
energyisconductedalongtherod?
Doyouthinkthiswasafairtest?Explainwhy
orwhynot.Describeanypossiblesourcesof
experimentalerror.
Usesmallblobsofpetroleumjellytoattach
paperclipsalongthecopperrod,asshown
inFigure11.5.
Securetherodintheclampandheatthe
otherend.
Watchcarefullywhathappenstothepaperclips.
Usesmallblobsofpetroleumjellytoattacha
papercliptotheendofeachmetalrod.
Placetherodsonthetripod,asshownin
Figure11.6,andheattheends.
Useatablelikethistorecordthetimetaken
foreachpapercliptofall.
Youwillneed:
Bunsenburner
heatproofmat
tripod
rodsmadefromdifferentmetals
paperclips
timer
petroleumjelly.
Metal Timetakenforcliptofall/
seconds

207. >
CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
CONTINUED
Part3:Iswateragoodconductorofthermal
energy?
Youwillneed:
Bunsenburner
heatproofmat
boilingtube
smallpieceofwiregauze
testtubeholder
coldwater
icecube.
Method
Figure11.7:Experimentalset-upforpart3.
1Fillthree-quartersoftheboilingtubewith
water.Addasmallicecubeandthenasmall
pieceofwiregauzetoholdtheiceinplace,as
showninFigure11.7.
2Lookattheboilingtube.Touchittocheckthe
temperature.
3Holdthetubewiththetesttubeholderand
gentlyheatthewateratthetopofthetube.
4Watchcarefullytoseewhathappenstothe
wateratthetopofthetubeandtotheice.
Questions
1Describeyourobservations.
2Explainwhythisexperimentshowsthatwate
apoorconductorofthermalenergy.
Part4:Whatmaterialsmakegoodinsulators?
Youwillneed:
4equalsizedbeakerswithcardboardlids
4thermometers,pushedthroughthe
cardboardlids
timer
arangeofinsulatingmaterials
electrickettle.
Method
Figure11.8:Experimentalset-upforpart4.
194y

208. 11Thermalenergytransfers
JUNUED
Wi.ipthreeofthebeakersindifferentinsulating
in.lterials.Leavethefourthbeakerwithout
insulation.
I><"jignatabletorecordyourresults.

Boilthekettleandcarefullypourequal
Iamountsofwaterintoeachofthebeakers.
’
M<osurethetemperatureofthewaterineach
IK'.ikereveryminutefortenminutes.
Questions
1Whywasthebeakerwithnoinsulationincluded
intheexperiment?
2Drawagraphofyourresults.Plottemperature
onthey-axisagainsttimeonthex-axis.Plotall
foursetsofresultsonthesameaxis.
3Whichinsulatorkeptthewaterthehottest?
4Listtheinsulatorsinorderfrombesttoworst.
energyflow
coldhot
worstinsulatoritnductor diamond
glassrod
Bunsenburner
lead
rubber,wood
>.I<onductor bestinsulator
195)
CTION
Bothmetalsandnon-metalsconductthermalenergy.
Metalsaregenerallymuchbetterconductorsthannon-
metals.Weneeddifferentexplanationsofconductionfor
thesetwotypesofmaterial.
Figure11.9:Conductionofthermalenergyinnon-metals.
Aglassrodisheatedatoneend.Thermalenergytravels
fromthehotendtothecoldend.
polyethene,
nylon
Wewillstartwithnon-metals.Imaginealongglassrod
(Figure11.9).Oneendisbeingheated,theotherendis
cold.Thismakesatemperaturedifferencebetweenthe
twoends,andsothermalenergyflowsalongtherod.
Whatisgoingoninsidetherod?
ice,marble,
glass
IIIIcomparesconductorsandinsulators.In
11metalsaregoodconductorsofthermalenergy
ihInunmetalsarepoorconductors.Airandwaterare

।>mconductorsofthermalenergy.
II-iiljouttheexperimentsyouhavedone
iIyuleashortsentencetosumupwhatyou
.irntineach.Whatwasthemostpowerful
niilngmomentforyouintheseexperiments?
Ilunliboutwhythiswasandhowithelpedyour
ynilmstanding.
111:Comparingconductorsofthermalenergy,from
IlK'.l'onductorstotheworst.Abadconductorisagood
||Almostallgoodconductorsaremetals;polymers
ini).ireatthebottomofthelist.Glasswoolisan
h4Huntinsulatorbecauseitismostlyair.
Explainingconductionin
metalsandnon-metals
silver,copper
aluminum,
steel
polystyrene
glasswool

209. )CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
LookattheparticlesinsidetherodinFigure11.9.Atthe
hotendoftherod,theatomsarevibratingmuchmore
thantheyareatthecoldend.Astheatomsvibrate,they
collidewiththeirneighbours.Thisprocessresultsineach
atomsharingitsenergywithitsneighbouringatoms.
Atomswithalotofenergyendupwithless,andthose
withalittleendupwithmore.Thecollisionsgradually
transferenergyfromtheatomsatthehotendtothoseat
thecoldend.Energyissteadilytransferreddowntherod,
fromthehotendtothecoldend.
Thisishowpoorconductors(suchasglass,iceand
plastic)conductthermalenergy.Itisnotaveryefficient
methodofthermalenergytransfer.
Metalsaregoodconductorsforanotherreason.Manyof
theelectronsinmetallicconductorsarefreetomove(they
aredelocalised).Thesearetheparticleswhichcarryelectric
current.Theyalsocarrythermalenergyastheygethotand
movethroughthemetal.Figure11.10showsacopperrod,
heatedinthesamewayastheglassrod.Noticethefree
electrons,whichcarrythermalenergythroughthemetal.
KEYWORD
electron:anegativelychargedparticle,smaller
thananatom
Figure11.10:Conductionofthermalenergyinmetals.
Metalshavefreeelectronswhichcarrythermalenergy,
makingmetalsgoodconductors.
Liquidscanalsoconductthermalenergy,becausethe
particlesofwhichtheyaremadeareinclosecontact
withoneanother.However,astheparticlesarefreeto
move,vibrationsarenotpassedonaseasilyasinasolid.
Theparticlesingasesareveryspreadout,makinggases
verypoorconductorsofthermalenergy.
Questions *
1Copythesesentences.Choosethecorrectwordfroffl
eachsetofbrackets.
Conductionhappensmostlyin{solids/liquids/
gases}.Thermalenergyflowsfromthe{hotterI
cooler}partsofanobjecttothe{hotter/cooler}part
Amaterialwhichdoesnotconductthermalenergy
welliscalled{aconductorIaninsulatorIaresistor}.
Anexampleis{copper/polystyrene/gold}.
2Explainwhyawoodenspoonisbetterthanametal
onetostirasaucepanofhotsoup.
3UsetheinformationinTable11.1toexplainwhy
walkingonamarblefloorinbarefeetwouldfeel
colderthanwalkingonawoodenfloor.
4Explainwhytwothinlayersofclothingareoften
warmerthanonethicklayer.
5Explainwhy:
acopperisabetterconductorthanwood
bwoodisabetterconductorthanair.
11.2Convection
Aswehaveseen,liquidsandgasesarenotusuallygood
conductorsofthermalenergy.Thermalenergytransfer
inliquidsandgasesismainlybyanothermethodknown
asconvection.
Figure11.11:Theheaterisheatingtheairgoingintothe
balloon.Thismakestheairexpandsoitbecomeslessdeni!
andmakestheballoonfloatup.
196y

210. 11Thermalenergytransfers
IInirungofhotairisjustoneexampleofconvection.
ILdiiucanrisebecauseairisafluid,andconvectioncan
inil<)rvedinanyfluidany(liquidorgas).Convection
iln'thappeninsolidsastheparticlesareinfixed
l*"illonssotheycannotflow.
ITilli'11.13showshowaconvectioncurrentcanbe
11uivcdinwater.Abovetheflame,waterisheated
*mlexpands.Nowitsdensityislessthanthatofthe
|mmuildingwater,anditfloatsupwards.Thepurpledye
Ie .howitmoves.Colderwater,whichismoredense,
>intoreplaceit.
BIadvectioncurrentisamovementofafluidthat
ilienergyfromawarmerplacetoacoolerone.This
hlightsanimportantdifferencebetweenconvection
midconduction.
Inconvection,energyistransferredthrougha
materialfromawarmerplacetoacoolerplaceby
themovementofthematerialitself.
Inconduction,energyistransferredthrougha
materialfromawarmerplacetoacoolerplace
withoutthematerialitselfmoving.
tiii'ii”11.12:Coldairsinksbelowanyobjectwhichis
Mi<'ithanitssurroundings.
Figure11.13:Becausewaterisclearandcolourless,itcan
bedifficulttoseehowthewatermovestoformaconvection
current.Crystalsofpotassiummanganate(VII)actasapurple
dyetoshowthemovementofthewater.
convection:thetransferofthermalenergy
throughamaterialbythemovementofthe
materialitself
fluid:asubstancewhichcanflow;liquidsand
gasesarefluids
density:theratioofmasstovolumefora
substance
convectioncurrent:thetransferofthermal
energybythemotionofafluid
Mltinilies’isapopularsaying-oneofthethings
H"lullsrememberfromtheirsciencelessons.Figure
||ihowsthissciencebeingputtouse.Whenairis
'
IIIexpandsandsoitsdensitydecreases.The
Mllvmislessdensethanitssurroundings,soitfloats
"I<nlFortheballoontorise,thedensityofthehot
NUn I'.illoonitself,plusthebasketanditsoccupants,
Mu'
iIIogetherhaveadensitylessthanthatofthe
IMi*
1"Hidingcoldair.
theppositeeffectcanbeseeninFigure11.12.Coldair
Ek

Ihisphotoistakenwithatechniquewhichshows
Ifnsofair.Youcanseecoldairsinkingdownbelow
liiiinpizza.
KEYWORDS

211. >
CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
EXPERIMENTAL SKILLS11.2
Demonstratingconvection
Convectioninairandwaterisdifficulttosee
asairandwateraretransparentandcolourless.
Theseexperimentswillletyouobserveconvection
currentsinwaterandair.
Youwillneed:
Bunsenburnerthincard
heatproofmat scissors
tripod thinthread
beaker potassium
tweezers
manganate(VII)
crystals.
Safety:WeareyeprotectionwhenusingaBunsen
burner.Wearprotectivegloveswhenhandling
potassiummanganate(VII).Donotholdthecard
spiralnearaflame.
Gettingstarted
Whatisthepurposeofthepotassium
manganate(VII)crystalinthisexperiment?
Methodpart1:Convectioninaliquid
Figure11.14:Set-upforexperiment.
1Fillthree-quartersofabeakerwithwater.
2Whenthewaterissettled,usetweezersto
placetwoorthreesmallcrystalsofpotassium
manganate(VII)onthebottomofthebeaker,at
oneside.(SeeFigure11.14.)
3UseaBunsenburnertoheatthewatergently,
justbelowthecrystals.
4Observewhathappenstothecolourofthe
potassiummanganate(VII)asitmoves.This
showshowthewaterisflowing.
Methodpart2:Convectioninair
Figure11.15:Cuttingaspiralandusingittodemonstrati
convection.
1Cutaspiralfromthincard.
2Attachathinthreadtotheceptreofthespiral.
3Holdthethreadaboveaheatsource(suchasa
radiator)andobservehowitmoves.
Questions
1Drawthebeakerofwaterandarrowstoshow
themovementofthepurplecolourwhenyou
heatedit.
2TheexperimentshowninFigure11.16uses
smokefromburningpapertoshowconvection
currentsinair.Explainwhythesmokemoves
downonechimneyanduptheother.
198>

212. 11Thermalenergytransfers
Ilgut«11.18:Inarefrigerator,coldairsinksfromthefreezing
>i|aitment.Ifthefreezerwasatthebottom,coldairwould
*
iinthere,andthefoodatthetopwouldnotbecooled.
Figure11.20:Iceinglassofwater.
Questions
6
heat
Figure11.19:Apanofwater.
7
Iililobjectsalsoproduceconvectioncurrents.Youmay
wVnoticedcoldwatersinkingbelowanicecubeina
dindInarefrigerator,thefreezingsurfaceisusually
rIuoncdatthetopandtheback,sothatcoldairwill
mlIothebottom.Warmairrisestoreplacethecoldair
nillinks.Thewarmairisthencooled,(seeFigure11.18).
CopyFigure11.19.Addarrowslabelled‘cold
densewater’and‘hot,lessdensewater’toshowthe
convectioncurrentinthepan.
TheicecubeinFigure11.20floatsatthetopofthe
water.Drawandlabeladiagramtoshowhowthe
icecubecreatesaconvectioncurrentwhichcoolsall
ofthewater.
ceiling
24°C
Theparticlesinthehotterfluidhavemorekineticenergy
sotheymovearoundfaster.Astheyflowfromplaceto
place,theytakethisenergywiththem.
Convectionisthemainmethodofthermalenergy
transferinfluids.Thermalenergycanbeconducted
throughaliquidbutthisisgenerallyaslowprocess
comparedwithconvection.
Explainingconvection
Wehavealreadyseenthatconvectionresultsfromthe
expansionofafluidwhenitisheated.Expansionmeans
anincreaseinvolumewhilemassstaysconstant.This
meansthatdensitydecreases.Alessdensematerialis
lighterandsoispushedupwardsbythesurrounding
densermaterial.
IConvectioncurrentsatwork
clioncurrentshelptoshareenergybetweenwarm
mloldplaces.Ifyouaresittinginaroomwithan

»<।liKheater,thermalenergywillbemovingaround
III*momfromtheheaterasaresultofconvection
I
>n.ji|,whichrisefromtheheater.Youarelikelyto
1»lIksourceofconvectioncurrentsyourself,since
pinibodyisusuallywarmerthanyoursurroundings
Iigure11.21).Manybitinginsectsmakeuseofthis
pinIForexample,bedbugscrawlacrossthebedroom
RI
111rTheycandetectasleepingpersonbelowby
dingthewarmestspotontheceiling.Thentheydrop
*li>ii«'hldownontothesleeper.Thisisaloteasierthan
>1mlingaboutontopofthebedding.
floor
18°C
hum#11.17:Convectioncurrentsriseabovethewarm
Mi’llinaroom.

213. >
CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
8Aninventormakesanelectrickettlewiththeheating
elementatthetop.Explainwhyitwillnotwork.
9aDrawdiagramstoshowthedifferenceinthe
arrangementofparticlesinahotgasanda
coldgas.
bUseyourdiagramstoexplainwhyhotgases
rise.Usethewords‘expand’and‘density’in
youranswer.
10Explainwhyconvectiondoesnothappeninsolids.
11.3Radiation
Atnight,whenitisdark,youcanseemuchfurther
thanduringtheday.Inthedaytime,themostdistant
objectyouarelikelytobeabletoseeistheSun,about
150millionkilometresaway.Atnight,youcanseemuch
further,tothedistantstars.Themostdistantobject
visibletothenakedeyeistheAndromedagalaxy,about
20millionmillionmillionkilometresaway.
ThelightthatreachesusfromtheSunandotherstars
travelstousthroughspaceintheformofelectromagnetic
radiation.Thisradiationtravelsaselectromagnetic
waves.Ittravelsovervastdistances,followinga
straightlinethroughemptyspace.Radiationistheonly
formofthermalenergytransferwhichdoesnotinvolve
themovementofparticles.Thermalradiationdoesnot
needamediumtotravelthrough.Itcantravelthrougha
vacuum.Aswellaslight,theEarthreceivesotherformsof
electromagneticradiationfromtheSun,includinginfrared
andultravioletradiation.(Youwilllearnmuchmoreabout
electromagneticradiationinChapter15.)
Allobjectsemitinfraredradiation.Thehotteranobject,
themoreinfraredradiationitgivesout.Youcanusethis
ideatohelpyoutodoabitofdetectivework.Imagine
youarriveatacrimescene.Thesuspectsayshehasjust
arrivedbycar.Yoususpecthehasbeenatthescenefor
anhour,givinghimtimetocommitthecrime.Holding
yourhandovertheenginecompartmentwillquicklytell
youiftheengineisradiatingthermalenergy.
KEYWORDS
infraredradiation:electromagneticradiationwith
awavelengthgreaterthanthatofvisiblelight;
sometimesknownasthermalenergyradiation
Ourskindetectstheinfraredradiationproducedbya
hotobject.Nervecellsjustbelowthesurfacerespondto
thermalenergy.Younoticethisifyouareoutdoorsona
sunnyday.
Tosummarise,infraredradiation:
isproducedbywarmorhotobjects
isaformofelectromagneticradiation
travelsthroughemptyspace(andthroughair)inthe
formofwaves
travelsinstraightlines
warmstheobjectthatabsorbsit
isinvisibletothenakedeye
canbedetectedbynervecellsintheskin.
Figure11.21:Usinganinfrared-sensitivecamera.Slight
variationsintemperatureshowupasdifferentcolours.The
scaleshowshowthecolourrelatestotemperature.
Figure11.21showsanotherwayofdetectinginfrared
radiation:byusingatemperature-sensitivecamera.The
photographofawomansittingatadeskistakenwitha
camerawhichdetectsinfraredradiationinsteadoflight.
Itisverysensitivetoslightdifferencesintemperature.
Questions
1 1Whichstatementaboutinfraredradiationistrue?
AInfraredradiationtravelsslowerthanlight.
BInfraredradiationcannotbereflected.
CInfraredradiationcantravelthroughavacuum.
DInfraredradiationistransferredbythe
movementofparticles.

214. 11Thermalenergytransfers
IxplainwhythermalenergyfromtheSuncanonly
icachusbyradiation,notconductionorconvection.
Whatevidenceisthereinthisinfraredphotograph
(Iigure11.22)tosuggestthatthecarhasonlyjust
brokendown?
Figure11.22:Infraredphotographofacar.
ACTIVITY9.1
Figure11.23:Thermalimageofafingerprint.
Asciencemuseumispreparingadisplayabout
infraredphotography.Youarearesearcherand
Ii.ivebeengiventhefollowingbrief.
Investigateoneoftheusesofthistechnologyand
।mpareaninformationboardaboutit.Yourboard
shouldbeonesideofA4paper.Itmustinclude
itleastoneeye-catchingpicturetakenwithan
Infraredcamera,andamaximumof150words
Inscribingthescienceandhowitisused.
CONTINUED
Theboardshouldbeinterestingtothegeneral
publicandalsodetailedenoughtoberelevantto
highschoolphysicsstudents.
Usesofinfraredtechnologyinclude:
medicine
military
detectionofdrugfarms
astronomy
wildlifephotography
forensicscience
studyingthermalenergylossfrombuildings.
PEERASSESSMENT
Displayeachgroup'sworkfromActivity9.1around
theclassroom.Takeaclassvoteonwhichboard
bestmeetsthebrief.Discusswhatmakesitagood
pieceofwork.Adaptyourboardtoaddsomeof
thefeaturesyouidentified.
Goodabsorbers,goodemitters
Figure11.24:Asunshieldreflectsunwantedradiation,which
wouldotherwisemakethecarveryhot.
Onahot,sunnyday,cardriversmayparktheircarswith
asunshieldbehindthewindscreen(Figure11.24).Sucha
sunshieldisusuallyshiny,becausethisreflectslightand
infraredradationfromtheSun.Thisstopsthecargetting
uncomfortablyhot.Theblackplasticpartsofthecar
(suchasthesteeringwheelanddashboard)areverygood
absorbersofinfraredradaition,andtheycanbecometoo
hottotouch.
201)

215. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Itisthesurfacethatdetermineswhetheranobject
absorbsorreflectsinfraredradiation.Asurfacethatisa
goodreflectorisapoorabsorber.Onahotday,youmay
havenoticedhowtheblacksurfaceofatarred(metalled)
roademitsthermalenergy.Blacksurfacesreadilyabsorb
infraredradiation.Theyarealsogoodemitters.
Youmayalsohavenoticedthatyoustaycooleronahot
dayifyouwearlightcolouredclothes.Lightsurfacesdo
notabsorbmuchthermalradiation.Instead,theyreflect
theradiation.ThehouseinFigure11.25ispaintedwhite
toreflectinfraredradiation.
Shinyorwhitesurfacesarethebestreflectors(the
worstabsorbers).
Matteblacksurfacesarethebestabsorbers(the
worstreflectors).
Matteblacksurfacesarethebestemitters.
Figure11.25:Thishouseisbuiltfrombottlesfilledwithsand.
Itispaintedwhitetoreduceabsorptionofinfraredradiation.
Questions
14ExplainwhytheworkerinFigure11.26iswearinga
shinysuit.
Figure11.26:Aworkersupervisingtheflowofhot
moltenmetal.
15Whichwillstayhotlonger:teainashinysilverteapot
orteaindarkbrownone?Explainyouranswer.
Factorsaffectihginfrared
radiation
Allobjectsemitradiationandabsorbradiationfrom
theirsurroundings.Thehotteranobjectis,themore
radiationitemitseachsecond,orthemorepowerit
radiates.Anyobjectwhichishotterthanitssurrounding
radiatesmoreenergypersecondthanitabsorbsand
sowillcooldown.Anobjectwhichiscoolerthan
itssurroundingsabsorbsmoreenergypersecond
thanitradiatesuntilitreachesthetemperatureofits
surroundings.Anobjectwithatemperaturethatremain
constantabsorbsthermalenergyatthesamerateasit
emitsthermalenergy.Anobjectwithalargesurfaceare:
emitsthermalenergyatfasterrate.
Figure11.27showsthreebeakersofwater.The
roomtemperatureis20°C.Allthreebedkersradiate
andabsorbenergy,buttheamountorradiationand
absorptiondependsonthetemperature.
0°C
Figure11.27:BeakerAwillwarmup,beakerBwillcool
downandbeakerCwillremainataconstanttemperature.
202>

216. 11Thermalenergytransfers
Questions
1
2
3
woodenbench
Figure11.29:Set-upfortheexperiment.
Method
1
boardtoisolatecansthermometer
4
woodenbench
Questions
Figure11.28:Set-upfortheexperiment.
IKPERIMENTALSKILLS11.3
2
3
1
2
3
1
2
matteblack
lurface
matteblack
surface
PlotasimilargraphtoexperimentA.
Whatcanyouconcludefromyourresults?
SetuptheexperimentasshowninFigure11.28.
Fillthetwocanswithequalvolumesofhotwater.
Usethermometersorelectronictemperature
probestomeasurethetemperatureofthe
waterineachcaneveryminutefortenminutes.
Safety:Takecarewhenusinghotwater.Donot
touchormovethecanswhiletheyarefullofvery
hotwater.Weareyeprotectionwhenusinga
llunsenburner.
Gettingstarted
lookatFigure11.28.Usewhatyouknowabout
thermalenergytransferstoexplainwhythecans
mustbefittedwithlids,andwhytheyshouldstand
onawoodenorplasticsurface.
SetuptheexperimentasshowninFigure11.29;
usethesamecansasinexperimentA.
Fillthecanswithequalvolumesofcoldwater.
Placethecansatequaldistancesfromthe
Bunsenflameorelectricheater.
Usethermometersorelectronictemperature
probestomeasurethetemperatureofthe
waterineachcaneveryminutefortenminutes.
ExperimentA:Whichsurfaceradiatesbetter,black
orshiny?
Method
Whatfeaturesoftheexperimentaldesign
ensurethatthisisafairtest?
Plotagraphoftemperatureonthey-axis
againsttimeonthex-axis.Plotbothsetsof
resultsonthesamegraph.
Whatcanyouconcludefromyourresults?
Investigatingtheemissionandabsorptionof
Infraredradiation
Inthisinvestigationyouwillusecansofwaterwith
Jlfforentsurfacestoinvestigatewhichemitsmost
mfraredradiationandwhichabsorbsmostinfrared
i.idlation.
theamountofinfraredradiationemittedalso
li'pendsonthesurfaceareaandtemperatureof
tinobjectssothesemustbecontrolledinyour
experiment.
Youwillneed:
1shinysilvercanand1dullblackcan
2lidswithholesforthermometers
2thermometersorelectronictemperature
probes
timer
kettle
boardtoisolatethecansfromeachother
Bunsenburnerorelectricalheater.
ExperimentB:Whichsurfaceabsorbsthermal
energybetter,blackorshiny?
thermometerBunsenburner
J—shinysilver
surface
—shinysilver
surface

217. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
11.4Consequencesof
thermalenergytransfer
Inthissection,wewillseehowwecanuseideasabout
thermalenergytransferstounderstandalotofdifferent
situations.Rememberthat:
Thermalenergytravelsfromahotterplacetoa
colderplace.Itisthetemperaturedifferencethat
makesitflow.
Conductionisthemainwayinwhichenergycan
passthroughasolid.Energytravelsthroughthe
solidbutthesoliditselfcannotmove.
Convectionisthemainwayinwhichenergyis
transferredinafluid.Warmfluidmovesaround,
carryingenergywithit.
Radiationistheonlywayinwhichthermalenergy
cantravelthroughemptyspace.Infraredradiation
canalsopassthroughsometransparentmaterials
suchasair.
Hotobjectshavealotofinternalenergy.Aswehave
seen,energytendstoescapefromahotobject,spreading
toitscoolersurroundingsbyconduction,convection
andradiation.Thiscanbeagreatproblem.Wemayuse
alotofenergy(andmoney)toheatourhomesduring
coldweather,andtheenergysimplyescapes.Weeatfood
tosupplytheenergyweneedtokeepourbodieswarm,
butenergyescapesfromusatarateofabout100watts
(100W=100J/s).
Tokeepenergyinsomethingthatishotterthanits
surroundings,weneedtoinsulateit.Knowingabout
conduction,convectionandradiationcanhelpusto
designeffectiveinsulation.
Rememberthatallthreemechanismsofenergytransfer
(conduction,convectionandradiation)maybeinvolved
whenanobjectwarmsuporcoolsdown.
Homeinsulation
Awell-insulatedhousecanavoidalotofenergywastage
duringcoldweather.Insulationcanalsohelptoprevent
thehousefrombecominguncomfortablyhotduring
warmweather.Figure11.30showswherethermalenergy
islostfromahouse,andsomewaystoreducethermal
energylosses.MoredetailsarelistedinTable11.2.
26%roof:
*
installloftinsulation
33%walls:
fitcavitywall
insulation
3%doors
installa
curtainor
fitdraugh
excluders
8%floors:
fitcarpetsor
underfloor18%windows:
12%draughts:
fitdraught
excluders
insulationfitdoubleglazing
Figure11.30:Insulatingahousereducesthermalenergy
lossandheatingcosts.
Method
_
Whyitworks
I
...
1
|thickcurtains,
draught
excluders
1
stops
convection
currents,and
soprevents
thermalenerg
transfer
MKMa
loftand
underfloor
insulating
materials
| \
prevents
conductionof
thermalenerg
throughfloors
andceilings
j
mi
1i
r
doubleand
tripleglazing
ofwindows
vacuum
betweenglass
panescutsou
lossesorgain:
byconduction
andconvectio
ki
cavitywallsreduces
thermalenerg
lossorgainbj
conduction
MM
foamor
rockwoolin
wallcavity
furtherreduce
thermalenerg
transferby
convection
Table11.2:Methodsofretainingenergyinahouseinacc
climate,andofkeepingahousecoolinahotclimate.
204y

218. 11Thermalenergytransfers
stopper
Figure11.33:ThiscouplearehopingtohaveababybyIVF
Theflasktheyareholdingcontainstheirfrozenembryos.
metalor
plasticcase
insulating
supports
-
Ihmble-glazedwindowsusuallyhaveavacuumbetween
Uhtwopanesofglass.Thismeansthatenergycanonly
?1ipebyradiation,sinceconductionandconvection
iUhrequireamaterial.Modernhousesareoftenbuilt
HUItcavitywalls,withanairgapbetweenthetwo
Ims<srsofbricks.Itisimpossibletohaveavacuumin
ducavity,andconvectioncurrentscantransferenergy
Hitossthegap(seeFigure11.31a).Fillingthecavitywith
hininmeansthatasmallamountofenergyislostby
induction,althoughthefoamisaverypoorconductor.
Ilowever,thisdoesstopconvectioncurrentsfromflowing
silveredinner
surfaces
vacuum
betweenwalls
figure11.31a:Acavitywallreducesthermalenergyloss
।,onductionbecauseairisagoodinsulator.However,a
Bnvectioncurrentcantransferenergyfromtheinnerwall
।।theouterwall,b:Fillingthecavitywithfoamormineral
fljfissorrock)woolpreventsconvectioncurrentsfrom
<>rming.
Figure11.32:Avacuumflaskiscleverlydesignedtokeep
hotthingshotbyreducingthermalenergylosses.Italso
keepscoldthingscold.Althoughwemightsay'itstopsthe
coldgettingout',itismorecorrecttosaythatitprevents
thermalenergyfromgettingin.Thefirstsuchflaskwas
designedbyJamesDewar,aScottishphysicist,inthe1870s.
Heneededflaskstostoreliquefiedairandothergasesat
temperaturesaslowas-200°C.Soonafter,peoplerealised
thataflasklikethiswasalsousefulfortakinghotorcold
drinksonapicnic.
Keepingcool
\lcuumflasksareusedtokeephotdrinkshot.They
inalsobeusedtokeepcolddrinkscold.Giantvacuum
linksareusedtostoreliquidnitrogenandheliumatvery
lowtemperatures,readyforuseinsuchapplicationsas
bodyscannersinhospitals.Theyalsohavemedicaluses,
1UchasforstoringfrozenembryosforIVFtreatment.
IIgure11.32showstheconstructionofavacuum
flunk.Glassisgenerallyused,becauseglassisagood
uliulator.However,someflasksaremadeofsteelfor
nddedstrength.Airisremovedfromthegapbetweenthe
doublewalls,creatingavacuum.Thisreduceslossesby
onductionandconvectionbecausebothofthemneeda
materialtotravelthrough.Thesilvercoatingontheglass
induceslossesbyradiationbyreflectinganyinfrared
inflation.Thestopperismadeofplasticanditprevents
lossesbyconvectionandevaporation.
double-walled—fl-
glassflask

219. CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
ACTIVITY11.2
Marketingavacuumflask
Imagineyouhavejustinventedthevacuumflask.
Youwanttogointobusinessproducingandselling
flasksbutneedfinance.Prepareapresentationfor
potentialinvestors.Youwillneedtoexplainthe
technicaldetailsofhowitworks.Youmustusethe
words'conduction','convection'and'radiation'.
Theinvestorswanttoknowyouareanexpertand
understandthescience.Youshouldalsoexplain
whyyourproductwillbepopularwithcustomers.
Yourpresentationcanincludeaninformation
leaflet,avideooranadvertisingposter.
Acarenginebumsfuelandsogetsveryhot.Thecooling
system(Figure11.34a)transferssomeofthisthermal
energytothesurroundingssotheenginedoesnot
overheat.Thissystemusesmanyofthethingsyouhave
learnt:
Specificheatcapacity:waterflowsaroundtheblock
toabsorbthermalenergy.Waterisagoodchoiceas
ithasaveryhighspecificheatcapacity.
Convection:asthewaterisheated,aconvection
currentflowsinthedirectionshownbythearrows.
Thepumpisusedtospeedupthisflow.
Conduction:theradiatorhasmetalfins(Figure
11.34b)sothethermalenergyisconductedtoall
partsoftheradiator.
Radiation:thefinshavealargesurfaceareaandare
blacktoincreasetherateofthermalenergyradiation.
water
Figure11.34a:Transferofthermalenergyinacarcooling
system,b:Theblackmetalradiatorcoolerfinsinacar.
Woodburnsonabonfireto’producethermalenergy.
Theheatingeffectyouexperiencewhensittingbyafireis
almostentirelyduetoradiation.Airisabadconductor
andthermalenergytransferduetoconvectionwillheat
theairabovethefire.
Figure11.35:Theolderchildwillfeelhotterthanthe
youngerchildashisblackT-shirtabsorbsinfraredradiation
whereasthewhiteT-shirtreflectstheinfraredradiation.
f
1
Thermalenergytransfer,
climateandweather
RadiationfromtheSunisessentialforlifeonEarth.
TheSun’sradiationwarmstheEarth.ThewarmEarth
emitssomeinfraredradiation.GasesintheEarth’s
atmosphere,suchascarbondioxide,absorbsomeofthis
thermalenergyandthiswarmsouratmosphere.Thisis
thegreenhouseeffect(Figure11.36)andwithoutitlife
onEarthwouldbeimpossible.However,theamount
ofgreenhousegasesintheatmosphereisincreasing,
trappingmorethermalenergy.ThismeansthatEarth
anditsatmosphereareabsorbingmoreinfraredradiation
thantheyemit.Thisisthecauseofglobalwarming.
Figure11.36:FortheEarthtomaintainaconstant
temperature,infraredradiationmustbeemittedatthesame
rateasitisabsorbed.Upsettingthisbalanceiscausing
globalwarming.
206

220. 11Thermalenergytransfers
Iflure.11.37:Convectioncurrentsintheoceansflowinpredictabledirections.Theredlinesshowwarmsurfacewaterand
lh<bluelinesshowtheflowofthedeeper,colderwater.Thewarmcurrentsaffecttheclimate.Forexample,theBritishIsles
inwarmedbytheGulfsteam(aflowofwarmwaterfromtheGulfofMexico).

onvectioncurrentsexplaintheoriginsofwindsand
oisancurrents,whicharetwoofthemajorfactorsthat
iontrolclimatepatternsaroundtheworld.Forexample,
warmairrisesabovetheEquator,andcolderairsinksin
Mibtropicalareas.Thiscreatesthepatternoftradewinds
thatareexperiencedinthetropics.
<)ceancurrents(Figure11.37)helptospreadthermal
nergyfromequatorialregionstocoolerpartsofthe
Iarth’ssurface.Warmwateratthesurfaceofthesea
flowstowardsthepoles.Inpolarregions,colderwater
MinksandflowsbacktowardstheEquator.Providedthis
Iutternremainsconstant,thishelpstomaketemperate
legionsoftheworldmorehabitable.However,thereis
thatthepatternofoceancurrentsischanging,
perhapsasaconsequenceofglobalwarming.
Questions
16Inarollingmill,ironisheatedtomakeitmalleable
anditisthenpassedthroughrollerstoproduce
thinsheetsofthemetal.Explainhowthefollowing
becomehotinthisprocess:
atherollerswhichpressthemetal
bthefaceofaworker
ctheairinthebuilding.
17ThecoatinFigure11.38isdesignedforacold
climate.
Figure11.38
Describethefeaturesofthecoatwhichprevent
thermalenergylossby:
aconduction
bconvection
cradiation.
207y

221. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
18Figure11.39showsasolarwaterheater.Coldwater
flowsthroughthepipesandisheatedbytheSun.
Suggestreasonswhy:
atheinsideofthepanelispaintedblack
bthebackofthepanelisinsulated
cthecoldwaterentersatthebottomofthe
panel,andleavesatthetop.
Figure11.39:Asolarwaterheater.
PROJECT
Betheteacher
Astudenthassubmittedtheiranswerstoahomework
question.Asyouwillsee,theyhavenotunderstood
thetopic.Yourtaskisto:
Marktheirworkandidentifyanymistakes.Isthere
anythingthatshowstheyhavebeenlisteningin
class?Writebriefencouragingfeedbacknotes
sothestudentwillbewillingtotryagain.Discuss
withyourgroupwherethestudentisgoing
wrongandwhatyoumightdotohelpthemavoid
makingthismistakeagain.
Writeamodelanswer.Eachmemberofthe
groupshoulddothisindividually,thenasa
group,shareyourideasandmakesureyou
havethebestpossibleanswer.
Planafiveminuterevisionsessioninwhichyou
explainthemainpointsthestudenthasmissed.
Youcandothisusingpresentationsoftware,
ashortvideo,anillustratedtalkoryoucould
demonstratesomeexperiments(checkwith
yourteacher).Includestudyhintsonhowto
rememberkeypointsaswellasthefactual
information.Makeyourpresentationaslively
andrelevantasyoucan.
Deliverthisrevisionsessiontoanothergroupof
students.
Prepareafollowupworksheet,withanswers,to
checkthatthestudentshaveunderstoodyour
presentation.
Homeworkquestions
1Thephotographshowsareusableinsulated
coffeecup.
Explainhowthecupreducesthermalenergy
lossbyconduction,convectionandradiation.
2Astudentheatssomeice.Sherecordsthe
temperatureandplotsagraph.
Describewhatishappeningateachstageofthe
graphandidentifyanyimportanttemperatures.
208)

222. 11Thermalenergytransfers
PEERASSESSMENT
<)nceyourpeershavecompletedyourworksheet,readtheiranswersandgivethemfeedback.Commenton
howwelltheyusescientificterms,andhowcleartheirexplanationsare.
REFLECTION
Whatproblemsdidyouencounterwhileworkingonthisproject?
Didplayingthepartoftheteachertellyouanythingaboutthewayyoulearn?
SUMMARY
1Metalsaregoodthermalconductors.Mostnon-metalsaregoodinsulators.
1Metalsaregoodthermalconductorsbecausetheyhavefreeelectrons.
1Hotfluidsarelessdensethancoldfluids.Thiscausesconvectioncurrents.
Infraredradiationtransfersthermalenergyusingelectromagneticwaves.
1Infraredradiationdoesnotrequireamedium(itcantravelthroughavacuum).
1Shiny,whitesurfacesreflectinfraredradiation.Theyarepooremittersandabsorbersofinfraredradiation.
1Dull,blacksurfacesaregoodabsorbersandemittersofinfraredradiation,butpoorreflectors.
1‘heamountofinfraredradiationemittedalsodependsonthesurfaceareaandtemperatureoftheobject.
209y

223. >
CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
EXAM-STYLEQUESTIONS
1Whichworddescribesamaterialthatdoesnotletthermalenergypass
throughit? [1]
Aconductor
Bvacuum
Cresistor
Dinsulator
2Whichstatesofmattercanconventionhappenin? [1]
Asolidsandliquids
Bliquidsandgases
Cgasesandsolids
Dsolids,liquidsandgases
3Hotfluidsrise.Thiscanbeexplainedbecause,asthefluidgetshotter,
thereisadecreaseinthefluid’s: [1]
Amass
Btemperature
Cdensity
Dvolume
4Thisdiagramshowsanelectricwaterheater.
aThecopperwallishottotouch.Nametheprocessbywhichthermal
energyfromthewaterpassesthroughthewall. [1]
bDescribehowaheateratthebottomofthetankheatsallthewaterin
thetank. [3]
cThehotwallstransferthermalenergytothesurroundings.Suggesta
waythisthermalenergylosscouldbereduced. [1]
[Total:5]
COMMAND WORDS
describe:statethe
pointsofatopic;give
characteristicsand
mainfeatures
suggest:apply
knowledgeand
understanding
tosituationswhere
therearearange
ofvalidresponses
inordertomake
proposals/putforward
considerations

224. 11Thermalenergytransfers
[1]
[2]b
[2]c
[Total:5]
shinyblackLesliecube
shiny'silver'
a
[1]
b
[2]
CONTINUED
COMMAND WORD
64.5°C
71.2°C
60.4°C
65.3°C
Suggestaquestionwhichcanbeansweredusingtheapparatusin
thediagram.
ThediagramshowsaLesliecube.Itisametalboxandeachsidehasa
differentsurface.
explain:setout
purposesor
reasons/make
therelationships
betweenthings
evidentIprovide
whyand/orhowand
supportwithrelevant
evidence
foryou.
shinyblack
matteblack
shinysilver
mattewhite
SarayausesaLesliecubetoinvestigateinfraredradiation.Shefillsthecube
withboilingwatersoallsidesareatthesametemperature.Sheusesthe
infrareddetectortoinvestigatetheradiationfromeachsurface.
Explainwhichrodyouwouldexpectthedrawingpintofallfromfirst.
Describetwofeaturesoftheexperimentwhichmakeitafairtest.
Whyisitimportanttokeepthedetectorthesamedistancefrom
eachside?
Matchthesurfacestothetemperatures.Thefirstonehasbeendone
heatproofmat matteblackmattewhite

225. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
*
COMMAND WORD
state:expressin
clearterms
CONTINUED
cSaraya’sinfrareddetectorgavethetemperaturetothenearest0.1°C.
Zainrepeatstheexperimentusingadifferentinfrareddetectorwhich
givesthetemperaturetothenearestdegree.
SuggesthowZain’sconclusionwilldifferfromSaraya’s. [2]
[Total:5]
7Somedoubleglazedwindowshaveaplasticframeandtwopanesofglass
withavacuumbetweenthem.
aExplainwhyplasticisusedfortheframe. [1]
bNamethetypesofthermalenergytransferwhicharestoppedby
thevacuum. [2]
cDoubleglazingreducesthermalenergylossesfromahouse.Describe
twootherwaystoreducethermalenergylossesandstatethetypeof
thermalenergytransferwhicheachreduces. [4]
[Total:7]
8Twobeakersarefilledwithwaterat20°C.BeakerAisplacedoutsideinthe
snowat-5°C.BeakerBisplacedinaroomat25°C.Thetemperatureof
eachbeakeristakenhalfanhourlater.
aStatewhatyouwouldexpecttohappentothetemperaturesofthetwo
beakers. [2]
bExplainyouranswerintermsoftheenergyradiatedorabsorbedby
thewaterineachbeaker. [2]
cAthirdbeaker,alsofilledwithwaterat20°Cremainsatthistemperature.
Stateandexplainwhatyoucandeducefromthisaboutwherethethird
beakerhasbeenplaced.. [2]
[Total:6]
212>

226. 11Thermalenergytransfers
'.ELF-EVALUATIONCHECKLIST
XIrstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutosee
iingapsinyourknowledgeandhelpyoutolearnmoreeffectively.
Ican
See
Topic...
Needs
morework
Almost
there
Confident
tomoveon
Describeexperimentstoshowthepropertiesof
conductorsandinsulators.
11.1
Iixplainthermalconductioninmetallicconductorsin
termsoffreeelectrons.
11.1
Statethatthermalenergyistransferredbyconvectionin
lluids.
11.2
Explainhowchangesindensitycauseconvection
currents.
11.2
Describeexperimentstodemonstrateconvection. 11.2
Describeexperimentstodemonstrateconvection. 11.2
Statethatinfraredradiationisapartofthe
electromagneticspectrumandcanpassthrougha
vacuum.
11.3
Describehowthesurfaceofanobjectaffectshowit
emits,reflectsorabsorbsinfraredradiation.
11.3
Describeexperimentstoshowwhichsurfacesabsorb
andemitmostinfraredradiation.
11.3
Describeandexplainsomepracticalapplicationsof
thermalenergytransfer.
11.4
213)

227. INTHISCHAPTERYOUWILL:
III
describehowsoundsareproducedandhowtheytravel
measurethespeedofsound
describehowtheamplitudeandfrequencyofasoundwavearelinkedtoitsloudnessandpitch
statetherangeofhumanhearing
definetheterm'ultrasound'anddescribesomeofitsapplications.
>Chapter12
Sound

228. 12Sound
Drawatablelikethis.YourtableshouldfillapieceofA4paper.
1
2
.ETTINGSTARTED
THESOUNDOFSILENCE
EvelynGlennie(Figure12.2)isoneofthe
world'stoppercussionistsdespitebeing
profoundlydeaf.Canyouworkouthowshe
experiencessound?(Hint:sheplaysbarefoot.)
Discusswithapartnerallthewaysinwhich
youhaveusedsoundtoday.Whatadaptations
couldyoumaketogetthroughyourdaily
activitiesifyoucouldnothearsounds?
Itisimportantforourwellbeingtoreduceunnecessary
rloise,butsoundreductionisnotalwaysentirely
positive.Silentelectriccarsraisefearsofmore
accidentsaspeopleareunawareoftheirapproach.
Ourworldisincreasinglynoisyasweaddmore
machinesandtechnology.Silenceisnotsomething
citydwellershearoften.Psychologistsbelievenoise
r.amajorcauseofstress.Acousticengineersstudy
^aystomakeabettersoundenvironment.Mobile
phonenoiseisalleviatedbyusingvibrationrather
thansoundtoinformusofacall.
Wearesurroundedbysounds.Naturalsounds
includebirds,animalsandthewind.Othersounds
includecars,computersandmusicalinstruments.
'Joundswakeus,soundsalertusthatourfoodis
।ooked,ourplaneisreadytoboardorthatacaris
approaching.Itishardformostofustoimaginea
Worldwithoutsound.Howwouldwecommunicate
ouremotions?Howwouldababyattractattention
Whenitishungry?Howdifferentwouldourformsof
ontertainmentbe?
Figure12.1:Outerspaceisa
vacuum.Thismeanssounds
cannottravelthroughit.
Astronautscommunicateby
usingradiowaves.
Figure12.2:Somepeoplearebornorbecomedeaf,
ortheyareonlyabletohearalimitedrangeofsound.
Theylearnhowtonavigatetheworldusingother
senses.
Writeordrawsomethingineachcellofthetable.Makeitasdetailedasyoucan.
Pairupwithanotherstudentandcompareyourtables.Makeanyadditionsorchangesyouwanttobasedon
yourdiscussion.
Joinwithanotherpairandshareyourideas.Again,makeanychanges.
Inthischapterwewilllookathowsoundsare
created,howtheytravelandhowwehearsounds.
Wewillalsolookatwhysoundscanbesovaried.
Whyaresomesoundshighorlowpitched?Whyare
somesoundsloudandothersquiet?
Discussionquestions
Howaresoundsmade? Howdoessoundtravel? Howfastissound?
Howdowedetectsound?Howdosoundsdifferfromeachother?Aretheresoundswecannothear?

229. AMlllIlllll
iintiv
>
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
12.1Makingsounds
Allsoundsarecausedbysomethingvibrating.The
vibrationsarenotalwaysvisiblebecausetheymaybetoo
smallortoofast.Vibratingsourcescausetheairaround
themtovibrate.Thesevibrationsarepassedthroughthe
airtoourearswheretheycausetheeardrumtovibrate
andwehearsound.
Figure12.3:Thewaterparticlesonthisspeakerare
vibratingasthespeakermakesasound.Thevibrations
ofthespeakerandthewaterwillchangeifthepitchor
loudnessofthesoundischanged.
Figures12.4-12.7linksoundstothevibratingsources
whichcausethem.
Figure12.4:Hittingthetuningforkcausestheprongsto
vibrate.
Figure12.5:Hittingthegongwithaharrimercausesitto
vibrate.
Figure12.6:Thecicadahasribbedmembranesatthebas
ofitsabdomenwhichvibratecausingaparticularlyloud
sound.
Figure12.7:Vocalfoldsinthehumanthroatvibrateto
createspeech.
216>

230. 12Sound
Questions
3
12.2Howdoes
1
2
IIgure12.9:Themusician'slefthandvariesthelengthofthe
itringswhilsthisrightplucksthestringstocausevibrations.
Windinstrumentsareblowntocausethecolumnof
airinsidetovibrate.Playerscoveranduncoverholesto
changethelengthoftheaircolumn.Thischangesthe
pitchofthenote.
Siringinstrumentsarepluckedorbowedtocausethemto
>ibrate.Thelengthofthestringcanbechanged(usually
byholdingthestringdownwithafinger)andthischanges
Ihenoteproduced.Thebodyoftheinstrumentandthe
urinsideitalsovibrateandthisgivestheinstrumentits
ililtinctivesound.Thisiswhyanoudandaviolincan
playthesamenotebutsoundverydifferent.
Figure12.10:Thesystemofholesonafluteallowsarange
ofnotestobeplayed.
figure12.8:Djembedrumsaremadeinarangeofsizes
i।producedifferentnotes.Theskinvibrateswhenhitand
producessoundwaves.
Whatareallsoundscausedby?
Adrum,afluteandaviolinallplayanote.Foreach
instrumentstatewhatvibratestocreatethesound.
Describehowadrummerhittingadrumleadstoa
listenerhearingsound.
Anorchestraproducesvibrationsbyacomplex
combinationofhitting,plucking,bowingandblowing.
Thesevibrationspassthroughtheairtotheaudience,
causingtheireardrumstovibrate,sotheyhearthesound.
Ifthesoundisveryloud,suchasatarockconcert,the
audiencemayfeelvibrationsthroughouttheirbodiesand
throughthefloor.
Musicalsounds
Musicalinstrumentsareengineeredtovibrateinways
thillmakearangeofinterestingandbeautifulsounds.
soundtravel?
Soundisaseriesofvibrationspassingthroughairor
anothermaterial.Thesourceofthesoundvibratesand
thismakestheairparticlesarounditvibratebackand
forwardinthedirectionthesoundistravelling.These
vibrationsmakeasoundwave.Thistypeofwaveiscalled
alongitudinalwave.

231. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
YouwilllearnmoreaboutwavesinChapter14.
Themovementoftheairparticlescanbedemonstrated
usingaslinkyspring,asshowninFigure12.11.
Figure12.11:Movingtheendofthespringbackandforwards
causesthecoilsofthespringtobesquashedtogetherand
thenstretchedout.Thesquashedupareamovesalongthe
springbutthespringremainsinitsoriginalplace.
Moreaboutsoundwaves
z
Whenatuningforkishit,ittheprongsstarttovibrate.
Figure12.12showshowthiscreatesasoundwave.As
theright-handprongmovesforward,itsquashestheair
particlestogether.Thismakesadenseregionintheair
calledacompression.Asitmovesback,theairparticles
aremorespreadout,makingalessdenseregioncalled
ararefaction.Airdoesnotmovefromthetuningforkto
thelistener-thevibrationspassthroughtheairbythe
patternsofcompressionsandrarefactionscausedbythe
airmoleculesbeingpushedbackandforth.
Thevibrationsoftheprohgsarenoteasytosee,butth
effectonwatercanbedramatic,asseeninFigure12.13
Figure12.13:Thenumberonthetuningforktellsusthat
theforkvibrates440timesasecond.
;
I
KEYWORDS
compression:aregionofasoundwavewhere
theparticlesarepushedtogether
rarefaction:aregionofasbundwavewherethe
particlesarefurtherapart
Whatcansoundtravelthrough
Soundwavesarevibrationscausedbyparticlesmoving
backandforth.Inavacuumtherearenoparticles,soi
isimpossibleforsoundtotravel.
Soundneedsamedium(material)totravelthrough.T1
canbeshownusingabellinaglassjar.Figure12.14
showsanexperimenttodemonstratethis.
Figure12.12:Thetuningfork'sprongsmovebackandforthcreatingcompressionsandrarefactionsintheair.
218
>

232. 12Sound
figure12.14:Whenthebatteryisconnected,thebellcan
I
"■seenandheard.Vibrationsfromthebellpassthroughthe
inthejar,throughtheglassandthenthroughtheairto
yourear.Whenthepumpremovestheairfromthejar,the
bellcanstillbeseenvibrating,butcannotbeheard.
Soundcantravelthroughsolids.Youmaybeableto
hearsoundsfromoutsidetheclassroomasyoureadthis.
Soundvibrationscantravelthroughthewalls.
Boundscanalsopassthroughliquids.Manyseaanimals
luchasdolphinsandwhalesusesoundtocommunicate
Witheachotherandtonavigate.
*
bC
V
e<approximatesizeofearth
Figure12.15:Thisisasolarflare,whereahugeamountof
matterisemittedbytheSun.AnexplosionlikethisonEarth
wouldcreateadeafeningsound.
TheSunisveryactiveandyetwehearnosoundfromit.
Thisisbecausetherearenoparticlestocarrythehuge
disturbancescausedbyexplosionsandsolarflares.We
canseetheSunbecause,unlikesoundwaves,lightwaves
cantravelthroughavacuum.
Anothermajordifferencebetweensoundandlightwaves
istheirspeed.Lighttravelsat300000000m/s,abouta
milliontimesfasterthansound.Thismeansweseethe
lightningalmostasithappensbuthearthesoundlater.
Tocalculatehowmanykilometresawaythestormis,
measurethetimebetweenthelightningandthethunder
anddividebythree.Thisworksbecausesoundtravels
1kminabout3seconds.
Questions
4Explainwhysoundcannotbeheardinavacuum.
5Describeandexplainwhatisheardwhenthe
vacuumpumpinfigure12.14isswitchedon.
6Aboyseeslightningandhearsthethunderclap9
secondslater.Calculatehowfarawayisthestormis.
12.3Thespeedofsound
Soundtravelsatbetween330m/sand350m/sinair.The
speedchangesslightlydependingonthetemperatureand
humidityoftheair.Thisismuchslowerthanlight,but
stillsofastthatweareusuallynotawareofthetimeit
takesforsoundstoreachus,unlessthedistanceittravels
islarge.
Anechoisareflectedsoundwave.Ifyoubangtwo
woodenblockstogetherintheclassroomyouwillhear
onlyonebang.Thesoundwillreflectfromthewalls,but
theechowillbesoclosetotheoriginalbangthatyou
willnothearitasaseparatesound.Bangingtheblocks
outsidewillmeanthesoundhasfurthertotravelsoyou
mayhearanecho.Thiscanbeusedtomeasurethespeed
ofsound.
219)

233. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
1
2
3
4
5
6
Method
largeflatwall
Questions
1
2
atleast50m
Figure12.16:Measuringthespeedofsoundinair.
EXPERIMENTAL SKILLS12.1
REFLECTION
Whywillitbedifficulttomeasurethetimefrom
hittingtheblockstogethertohearingtheecho?
Whichmeasurement(distanceortime)doyou
thinkwasleastaccurate?Explainyouranswer.
Discusstheexperimentwithapartner.Suggest
waystoimproveyourexperimenttoincreasethe
accuracyofthismeasurement.
Safety:Youwillbecreatingveryloudsounds.Avoid
doingthisnearanyone'searsasloudsoundscan
damagetheear.
Calculatethetotaldistancetravelledbythe
sound.Eachbanginvolvesthesoundtravelling
tothewallandback,sothetotaldistanceis
20x2xdistancefromwall.
Gettingstarted
Whyisitnecessarytostandalongdistancefrom
thewall?
Youwillneed:
2woodenblocks
stopwatch
longtapemeasureortrundlewheel.
Measuringthespeedofsoundinair
Youcancalculatethespeedofsoundinairby
measuringthetimetakenforanechotobeheard.
Whatistheactualspeedofsoundinair?
Howdoesthevalueyoucalculatedcompare
tothis?
Standameasureddistance(ideallyatleast
50metres)fromalargeflatwall.Bangthe
blockstogetherandlistenfortheecho.
Nowtrytobangtheblocksinanevenrhythm
sothateachclapcoincideswiththeechoofthe
previousbang.Thiswillmeanyoudon'thear
theechoseparatelyfromthenextbang.This
maytakesomepractice.
Yourpartnershouldthenmeasurethetimefor
20ofyourbangs.
Recordthetimetakenfor20bangsandthe
distancefromthewall.
Calculatethespeedofsoundinair.
Usetheequation:
,distancetravelled
speed=;:
timetaken
Amoreprecisevalueofthespeedofsoundcanbe
obtainedusinganelectronictimerandmicrophones.
Figure12.17showsthismethod.Thewoodenblocksand
thetwomicrophonesarearrangedinastraightline.
Whenthestudentsbangsthetwoblocksofwood
together,itcreatesasudden,loudsound.Thesound
reachesmicrophone1andapulseofelectriccurrent
issenttothetimer.Thetimerstartstiming.Afraction
ofasecondlaterthesoundreachesmicrophone2.
Asecondpulseofcurrentissenttothetimerandstops
it.Thetimernowindicatesthetimeittookforthesour
totravelfrommicrophone1tomicrophone2.Ifthe
distancebetweenthetwomicrophonesismeasured,the
speedofsoundcanbecalculatedusingtheequation:
j_
distance
speed
time
220y

234. 12Sound
5000
424000
3000
2000
<D
1000
0
airwaterbrickiron
Material
Figure12.18:Thespeedofsoundindifferentmaterials.
Questions
7Aspectatoratacricketmatchseesthebatsmanhit
theball,then1.2secondslaterhehearsthestrike.
Howfarawayisthespectator?Thespeedofsound
inairis330m/s.
8Soundtravelsat1500m/sinfreshwaterandat
1530m/sinsaltwater.Explainthedifferencein
speeds.
9ExplainwhythemethodshowninFigure12.17
ismoreaccuratethantheechomethodwhen
measuringthespeedofsound.
12.4Seeingandhearing
sounds
Seeingsounds
Acathoderayoscilloscopeandmicrophonecanbeused
torepresentsoundsonadisplayscreen(Figure12.19).
Themicrophonepicksupthesoundandconvertsittoan
electricalsignal.Theoscilloscopeconvertsthistoaline
whichrepresentsthevibrationsthatmakeupthesound
wave.
Thevibrationfromamusicalinstrumentiscomplicated
becauseitisproducedbyvibrationsoftheairandthe
instrumentitself.Asignalgeneratorcanbeusedto
produceapuresoundwave.Purenotesareeasierto
measure,butnotsomusical.
Theoscilloscopetracethatrepresentsapurenote
isasimplecurveasshowninFigure12.20a.When
representingamusicalnotefromaparticularmusical
instrument,thepatternismorecomplicated.Figures
12.20bandcshowthis.
=1244m
Therockfaceis612metresawayfromtheman.
Amanblowsawhistleandhearstheechofromarock
faceafter3.6seconds.Howfarawayfromtherockis
he?Assumespeedofsoundinair=340m/s.
Step1:Calculatethedistancetravelledbythesound,
speed=
distance
time
so,distance=speedxtime
=340m/sx3.6s
Step2:Halvethisdistance.(Thedistanceyouhave
alreadycalculatedisthetotaldistance
travelledbythesound,totherockfaceand
back.Thedistancetotherockfaceishalfof
this.)
1224
2
=612m
Answer
Soundtravelsatdifferentspeedsindifferentmaterials.
Figure12.18showsthespeedindifferentmaterials.
Thiscanbeexplainedbyconsideringthespacingof
particles.Particlesareclosertogetherinsolidsthanin
liquids,sothevibrationscanbepassedonmoreeasily
insolids.Thismeansthatthesoundwavetravelsfasterin
usolid.
figure12.17:Anothermethodformeasuringthespeed
"Isound.
microphone2
timer
microphone1
WORKEDEXAMPLE12.1

235. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Figure12.19:Asthestudentblows,hecreatesvibrationsin
theair.Thevibrationsaredetectedbythemicrophoneand
convertedtoelectricalsignals,whicharedisplayedonthe
oscilloscope.
Signalgenerator
Piano
Figure12.20:Thethreerepresentationsofsoundwaves
shownhereareallthesamenote.Eachwavehasarepeating
pattern.Thedesignoftheinstrumentaddsextravibrations
calledovertoneswhichgiveeachinstrumentitsdistinctive
sound.Allthreewaveshavefourrepeats,meaningthatthey
allarethesamenote.
Theoscilloscopecanbeusedtoobservetwoimportant
thingsaboutthewave(Figure12.21).
Theamplitudeisthefurthestdistancetheparticles
movefromtheirundisturbedposition.Thisisshow
bytheheightordepthoftheoscilloscopetrace.
Thefrequencyisthenumberofvibrationseach
second.Themorewavesonthescreen,thehigher
thefrequency.Frequencyismeasuredinhertz.
Onehertzmeansonewavepersecond.
KEYWORDS
amplitude:thegreatestheightordepthofa
wavefromitsundisturbedposition
frequency:thenumberofcompletevibrationsor
wavesperunittime
hertz:theunitoffrequency;1Hz=1waveper
second
Figure12.21:Thefrequencyofthenotecanbecalculated
fromtheoscilloscopetrace.Thisoscilloscopeissetat
20ms/div.Thismeanseachdivisiononthegridrepresents
20ms(0.02s).Thetimeforonewave(markedasT)istwo
divisionsor0.04seconds.Onewavetakes0.04seconds,so
thenumberofwaveseachsecondis1+0.04=25Hz.
Connectingasignalgeneratorandaspeakertoan
oscilloscopeallowsyoutobothseearepresentationof
thesoundwaveandhearthesound.
Increasingtheloudnessofthesoundproducestaller
waves-theyhaveabiggeramplitude.Increasingthe
frequencymeansmorewaveswillbeseenonthescreen-
theyhaveahigherfrequency,andthesoundheardwillt
higherpitched.Remember:
highfrequencymeanshighpitch
largeamplitudemeansloudsound.
222y

236. 12Sound
Hearingsounds
Younghumanscanhearsoundsfrom20Hzupto20000Hz.
A*wegrowolder,thesensorycellsintheearwhichdetect
ibrationsdeteriorate.Thismeansthattherangeofsounds
hichcanbehearddecreaseswithage.Thesecellscanalso
I"damagedbyrepeatedexposuretoveryloudnoise.
Soundswhichhaveahigherfrequencythan20000Hz
uretoohighpitchedtobeheardbythehumanear.
Ihesesoundsareknownasultrasound.
ultrasound:anysoundwithafrequencyhigher
than20000Hz
Manyanimalscanhearhighpitchedsoundsthatwe
cannot.Manyanimalssuchasdolphinscommunicate
Usingultrasound.Whistlescreatingultrasoundcanbe
usedtotrainanimals(Figure12.22).
Figure12.22:Thiswhistlewasinventedbyascientistcalled
FrancisGaltoninaround1900tohelphiminvestigatehuman
hearing.Similarwhistlesareusedinanimaltraining.
Swapleafletswithanotherstudent.Completeagridlikethistogivethemfeedback:
Question ©@® Comments
Doestheleafletexplainwhatthe
frequencyofasoundis?
Doesitexplainthedifferencein
hearingrangebetweenteenagers
andadults?
*
Doesitexplainwhythemosquito
devicewillstopyoungpeople
hangingaroundwheretheyare
notwanted?
Anannoyingnoise
Themosquitosoundalarmisadeviceintendedto
stopyoungpeoplecongregatinginareaswherethey
arenotwanted.Thedeviceemitsahighpitched
pulsingsoundwhichyoungpeoplecanhearbut
olderpeoplecannot.Itisnotharmfulbutisannoying
tothosewhocanhearit.
Preparealeaflettoinformyoungpeopleaboutthe
deviceandthesciencebehindit.Usethewords
'frequency'and'ultrasound'inyourexplanation.
KEYWORD
ACTIVITY12.1
PEERASSESSMENT

237. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Applicationsofultrasound
Sonar
Sonarisamethodusedtomeasurethedepthofwateror
tolocateanunderwaterobject.Figure12.23showshow
thisworks.
Figure12.23:Usingsonartomeasuredepth.Whateffect
mightthefishhave?
Apulseofultrasoundissentdownfromaboatandreflects
fromtheseabed.Thetimetakenforthereflectedpulseto
bereceivedismeasured.Thisisused,withthespeedof
soundinwatertocalculatethedepthofthewater.
WORKEDEXAMPLE12.2
Ashipsendsoutanultrasoundpulseandreceivesthe
echoafter3seconds.Thespeedofsoundinwateris
1500m/s.Calculatethedepthofthewater.
Step1:Calculatethedistancetravelledbythepulse:
distance=speedxtime=1500m/sx3s
=4500m
Step2:Halvethistogetthedepth.Rememberthe
pulsegoesdownandbackupagain.
A-4500m
depth-—
-—-2250m
Answer
Thedepthofthewateris2250m.
Materialtesting
Ultrasoundcanbeusedtodetectflawsinsidematerials.
Asmallcrackinametalgirdercouldcauseabuilding
tocollapse.Figure12.24showsultrasoundbeingpassed
throughuncracked(A)andcracked(B)metal.The
originalandreflectedpulsesareshownonoscilloscope
traceA.OscilloscopetraceBhasanextrapeak.This
indicatestotheengineersthatsomeultrasoundisbeing
reflectedfromacrackorflawinsidethemetal.
Figure12.24:Theexactlocationoftheflawcanbe
calculatedfromthespeedofsoundint(iemetalandthe
timeforthereflectedpulsetobedetected.
.:
Ultrasoundisalsousedinmedicine(Figure12.25).
Ultrasonicwavesarepartiallyreflectedfromboundaries
betweendifferentmaterials,suchasthechambersof
apatient’sheart,orafetus.Computeranalysisofthe
reflectedwavesproducesanimage.
Figure12.25:Theultrasoundimageshowsthedoctorand
themotherhowthisfetusisdeveloping.
Questions
10Statetherangeofsoundayoungpersoncan
typicallyhear.
11Describewhathappenstothisrangeastheperson
getsolder.Whatelsecanhavethiseffectonhearing?
224y

238. 12Sound
Howdomusicalinstrumentsproducearange
ofnotes?
Yourtaskistoinvestigatehowdifferentinstruments
createarangeofsounds.
Windinstruments
Windinstrumentsproducesoundsbymaking
columnsofairvibrate.Puttingdifferentamountsof
airintotesttubescreatescolumnsofairabovethe
water.Theaircanbemadetovibratebytappingthe
glassorbyblowingoverthetopofthetube.
Investigatetheeffectofchangingthelengthofthe
aircolumn.Howdoesitaffectthepitchofthenote
produced?
Pluckedinstruments
Pluckingastretchedstringcreatesasound.Usea
stringedinstrumentorahomemadeelasticband
guitartoinvestigatetherelationshipbetweenthe
lengthofthestringandthepitchofthesound.
Optional
Extendyourinvestigationtoansweroneofthe
following:
Whatothermethodsareusedtoproduce
differentnotesinmusicalinstruments?
Isthenoteproducedbyastringaffectedby
otherfactorssuchasthematerialitismade
from,itsthicknessorthetension(howtightlyit
isstretched)?
Howisthefrequencyofanotefromastring
relatedtolength?Youcanmeasurethe
frequencyofthesounddirectlyorbyusinga
microphoneandoscilloscope.Youcandrawa
graphoffrequencyagainstlength.
Presentyourresultstotheclass.Thiscouldbe
asaposter,atalk,apresentation,orplayinga
recognisabletuneonanimprovisedinstrument.
Youshouldincludeevidencesuchasdiagrams,
tablesorgraphs.
Figure12.28:Amusicalinstrumentthatisplucked.
12
Figure12.26:Threesoundsproducedonanoscilloscope.
Figure12.27:Usingultrasoundtofinddepth.
a
b
a
b
Calculatethedepthofthesea.
Calculatethedepthoftheshoaloffish.
Explainwhythefirstreflectedpulselastsfor
longerthanthesecond.
Whichsoundisquietest?Explainyouranswer.
Whichtwosoundshavethesamepitch?
Explainyouranswer.
13Ashippositionedaboveashoaloffish(Figure
12.27)sendsoutanultrasoundpulseandreceives
tworeflectedpulses,oneafter0.2secondsandthe
otherafter0.5seconds.Thespeedofsoundinwater
is1500m/s.
Figure12.26showsthetracesproducedonan
oscilloscopebythreedifferentsounds.
PROJECT

239. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
SUMMARY
Allsoundsarecausedbyvibratingsources.
Soundwavesarelongitudinalwaves.
Soundwavesconsistofaseriesofcompressionsandrarefactions.
Soundwavesneedamediumtotravelthrough.
Soundtravelsatbetween330m/sand350m/sinair.
Soundtravelsfastestinsolidsandslowestingases.
Anechoisareflectedsound.
Thegreatertheamplitudeofasound,thelouderthesound.
Thegreaterthefrequencyofasound,thehigheritspitch.
Humanscanhearsoundsbetween20Hzand20000Hz.
Ultrasoundcanbeusedinmedicalscans,testingmaterialsanddepthcalculation.
226
>

240. 12Sound
CONTINUED
Whatismeantbytheterm‘frequencyofasound’? [1]
Whatunitsisfrequencymeasuredin? [1]
Whatnameisgiventosoundswithafrequencyofmorethan
20000Hz? [1]
irummerisplayingasolo.
Describehowthesoundproducedtravelsthroughtheair. [1]
Whattypeofwaveisasoundwave? [1]
Describeanexperimenttoshowthatsoundneedsamediumto
travelthrough. [2]
[Total:4]
6Ashipusesechosoundingtofindthedepthofthewater.Itsendsoutan
ultrasoundpulseanddetectstheechoafter3seconds.
aCalculatethedepthofthewater.Assumethatthespeedofsoundin
wateris1500m/s. [2]
bTheship’ssirenisheardbyaboywindsurfingnearby.Heis400metres
awayfromtheboat.Howlongwillthesoundtaketoreachhim.
Assumethatthespeedofsoundinairis340m/s. [2]
cThesoundofthesirenalsotravelsthroughthemetalbodyoftheship.
Whichofthesecouldbethespeedofsoundinthesteel? [1]
200m/s 1200m/s 5800m/s
[Total:5]
4a
b
c
d
5A
a
b
COMMAND WORD
describe:statethe
pointsofatopic;give
characteristicsand
mainfeatures
calculate:workout
fromgivenfacts,
figuresorinformation
227>

241. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
SELF-EVALUATIONCHECKLIST
Afterstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutose<
anygapsinyourknowledgeandhelpyoutolearnmoreeffectively.
1can
See
Topic...
Needs
morework
Amost
there
Confident
tomovecl
Describehowsoundsareproduced. 12.1
Describesoundwaves. 12.1
Usetheterms‘compression’and‘rarefaction’. 12.2
Statethatsoundneedsamedium. 12.2
Statethespeedofsoundinair. 12.3
Describeamethodtomeasurethespeedofsoundinair.12.3
Comparethespeedofsoundinsolids,liquidsandgases.12.3
Describehowtheamplitudeofasoundaffects
itsloudness.
12.4
Describehowthefrequencyofasoundaffectsitspitch. 12.4
Explainhowanechoismade. 12.3
Definetheterm‘ultrasound’. 12.4
Statetherangeofsoundswhichhumanscanhear. 12.4
Describemedicalandengineeringapplications
ofultrasound.
12.4
228>

242. Light
INTHISCHAPTERYOUWILL:
usethelawofreflectionoflighttoexplainhowanimageisformedinaplanemirror
constructraydiagramsforreflection
investigatetherefractionoflight
drawraydiagramstoshowhowlensesformimages
describethedifferencebetweenrealandvirtualimages
describetotalinternalreflectionandhowitisused
describehowthevisiblespectrumisformed.
I

243. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
GETTINGSTARTED
Thestatementsoppositearealltrue,butthere Lighttravelsinstraightlines.
aresomeexceptionstoeachstatement.Discuss ,., ., , ,,
eachstatementandidentifywhenlightbehaves *
L,9htPassesstra,9htthrou9h9^.
differently.Rewriteeachstatementtoinclude.Ljghtalwaystravelsatthesamespeed.
theseexceptions.
THEWORLDINADROPOFRAIN
Figure13.1:Thelightisbentbytheraindrop,formingan
upsidedownimageofthescene.
Thisraindroponabladeofglassshowsanimage
ofitssurroundings.Thelightrayscausingthishave
travelledabout150millionkilometresfromthe
SuntotheEarth.Someraysstrikingthegrassare
absorbedandsupplyenergyforphotosynthesis,
whileothersarereflected,allowingyoutoseethe
grass.Thoserayspassingthroughthewaterdropare
refracted(bent)toformtheimagethatyousee.
NoticethattheSunisatthebottomandthetrees
appeartogrowdownwards.Theimageisupside
down.Youreyescontainlenseswhichproduce
imagesinaverysimilarwaytothewaterdrop.
LightfromtheSunreflectsfromdifferentobjectsand
entersyoureyes.Thelensformsanupside-down
image,likethewaterdrop.Yourbrainknowsthe
imageisupsidedownsoautomaticallyturnsit
upright.Ithashadyearsofpractice,soyouarenot
awareofthisprocess.
TheimageinFigure13.1hasbeencapturedusing
acamerawhichalsocontainsalenstoproduce
animage.
Lighttravelsat300millionmetrespersecond,soit
takeseightminutesand20secondstotravelfrom
theSuntotheEarth.ThismeansthatiftheSun
stoppedproducinglight,wewouldnotknowfor
eightminutesand20seconds.
Inthischapteryouwilllearnmoreaboutthe
processesofreflectionandrefraction,whichcanhelp
usexplainhowlightbehaves.
WhenApolloastronautsvisitedtheModn,theyleft
behindreflectorsonitssurface.Theseareusedto
measurethedistancefromtheEarthtotheMoon.
Alaserbeamisdirectedfromanobservatoryon
Earthsothatitreflectsbackfromthesereflectors
leftonthelunarsurface.Thetimetakenbythelight
totravelthereandbackismeasuredand,because
thespeedoflightisknown,thedistancecanbe
calculated.
Asimilarmethodcanbeusedwithsoundwaves
tomeasurethedepthofwater.Soundisreflected
fromtheseabedandthetimetakenismeasured.
Knowingthespeedofsoundinwaterallowsthe
depthtobecalculated.
Discussionquestions
1Howarethesetwomethodssimilar?
2Whatarethedifferences?
3Whyislightsuitableforoneandsoundfor
theother?
230

244. 13Light
Figure13.2a:Arayboxproducesabroadbeamoflight,
whichcanbenarroweddownusingametalplatewithaslit
init.b:Markingthelineoftheraywithcrossesallowsyouto
recorditsposition.
13.1Reflectionoflight
Ijghtusuallytravelsinstraightlines.Itchangesdirection
ifithitsashinysurface.Thischangeindirectionata
shinysurfacesuchasamirroriscalledreflection.Wewill
lookatreflectioninthissection.
Youcanseethatlighttravelsinastraightlineusingaray
box,asshowninFigure13.2.Alightbulbproduceslight,
whichspreadsoutinalldirections.Arayboxproducesa
broadbeam.Byplacinganarrowslitinthepathofthe
beam,youcanseeasinglenarrowbeamorrayoflight.
Therayshinesacrossapieceofpaper.Youcanrecordits
positionbymakingdotsalongitslength.Layingaruler
alongthedotsshowsthattheylieinastraightline.
Youmayseedemonstrationsusingadifferentsource
oflight,alaser.Alaser(Figure13.3a)hasthegreat
advantagethatallofthelightitproducescomesoutin
anarrowbeam.Alloftheenergyisconcentratedinthis
beam,ratherthanspreadingoutinalldirections(aswith
alightbulb).Thetotalamountofenergycomingfrom
thelaserisprobablymuchlessthanthatfromabulb,but
itismuchmoreconcentrated.Thatiswhyitisdangerous
ifalaserbeamgetsintoyoureye.
KEYWORDS
reflectionthechangeofdirectionofaraywhen
itstrikesasurfacewithoutpassingthroughit
raybox:apparatususedtoproducearayoflight
ray:anarrowbeamoflight
laser:adeviceforproducinganarrowbeam
oflightofasinglecolour(monochromatic)or
wavelength
Figure13.3a:Laserbeamsareveryintense.Itisimportantto
protectyoureyeswhenusingalaser,b:Scarring(justleftof
centre)totheretinaofachildafterlookingatalaserpointer.
231
>

245. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Lookinginthemirror
Mostofuslookinamirror,atleastonceaday,tocheck
onourappearance(Figure13.4).Itisimportanttousto
knowhowpeopleseeus.
Archaeologistshavefoundbronzemirrorsmorethan
2000yearsold,sothedesiretoseeourselvesclearlyhas
beenaroundforalongtime.
Figure13.4:Psychologistsusemirrorstotestthe
intelligenceofanimals.Dotheyrecognisethattheyare
lookingatthemselves?Chimpanzees'reactionsshowthey
clearlydorecognisetheirimages.Otheranimals,suchas
catsanddogs,donot-theymayeventrytoattacktheir
ownreflection.
Modernmirrorsgiveaveryclearimage.Whenyoulook
inamirror,raysoflightfromyourfacereflectoffthe
shinysurfaceandbacktoyoureyes.Youseemtoseean
imageofyourselfbehindthemirror.Tounderstandwhy
thisis,weneedtousethelawofreflectionoflight.
Whenarayoflightreflectsoffamirrororotherreflecting
surface,itfollowsapathasshowninFigure13.5.Theray
bouncesoff,ratherlikeaballbouncingoffawall.Thetwo
raysareknownastheincidentrayandthereflectedray.
ray
Figure13.5:Thisraydiagramshowsthereflectionoflight.
Thenormalisdrawnat90°tothemirror.Thentheanglesare
measuredbetweentheraysandthenormal.Theangleof
incidenceandtheangleofreflectionareequal:i=r.
Theangleofincidence,i,andtfteangleofreflection,r,are
foundtobeequaltoeachother.Thisisthelawofreflection:
angleofincidence=angleofreflection
i=r
Anglesofincidenceandreflectionarealwaysmeasured
betweentherayandthenormaltothesurface.Theangles
betweentheraysandthemirrorarealsoequal,butit
wouldbehardtomeasurethemifthemirrorwascurved.
Thelawofreflectionalsoworksforcurvedmirrors.
KEYWORDS
incidentray:arayoflightarrivingatasurface
reflectedray:arayoflightwhichhasbeen
reflectedfromasurface
raydiagram:adiagramshowingthepathofrays
oflight
angleofincidence:theanglebetweenthe
incidentrayandthenormaldrawnatthepoint
wheretherayhitsthesurface
angleofreflection:theanglebetweenthe
reflectedrayandthenormaldrawnatthepoint
wheretherayhitsthesurface
normal:thelinedrawnatrightanglestoasurface
atthepointwherearayhitsthesurface
ACTIVITY13.1
Investigatingthelawofreflection
Figure13.6:Investigatingthelawofreflection.
ThestudentinFigure13.6isinvestigatingthelaw
ofreflection.
Writestep-by-stepinstructionsforhisinvestigation.
Includealistoftheapparatusneeded,andthe
measurementsthatheshouldtake.
232y

246. 13Light
Theimageinaplanemirror
Whydoweseesuchaclearimagewhenwelookina
planemirror?Andwhydoesitappeartobebehindthe
mirror?
Figure13.7a:Lookinginthemirror,theobserverseesan
Imageofthecandle.Theimageappearstobebehindthe
mirror,b:Theraydiagramshowshowtheimageisformed
Raysfromthecandleflamearereflectedaccordingtothe
lawofreflection.
Figure13.7ashowshowanobserverseesanimageof
ucandleinaplanemirror.Lightraysfromtheflame
arereflectedbythemirror.Someofthementerthe
observer’seye.Theobserverhastolookforwardand
slightlytothelefttoseetheimageofthecandle.
Figure13.7bshowshowtheimageisformed.Thesolid
linesshowthelightfromthecandlereflectingfrom
themirror.Thegirl’sbrainassumesthattherayshave
travelledinstraightlinesfromapointbehindthemirror,
shownbythedashedlines.(Ourbrainsassumethatlight
travelsinstraightlines,eventhoughweknowthatlight
isreflectedbymirrors.)Inreality,nolightiscomingfrom
behindthemirror.Thedashedlinesappeartobecoming
fromapointbehindthemirror,atthesamedistance
behindthemirrorasthecandleisinfrontofit.Youcan
seethisfromthesymmetryofthediagram.
Theimagelooksasthoughitisthesamesizeasthe
candle.Also,itis(ofcourse)amirrorimage-itis
left-rightreversed,orlaterallyinverted.Youwillknow
thisifyouhaveseenwritingreflectedinamirror.Ifyou
couldplacetheobjectanditsimageside-by-side,you
wouldseethattheyaremirrorimagesofeachother,in
thesamewaythatyourleftandrighthandsaremirror
imagesofeachother.
Theimageofthecandleinthemirrorisnotarealimage.
Arealimageisanimagethatcanbeformedonascreen.
Ifyouplaceapieceofpaperatthepositionoftheimage,
youwillnotseeapictureofthecandleonit,becauseno
raysoflightfromthecandlereachthatspot.Thatiswhy
wedrewdashedlines,toshowwheretheraysappearto
becomingfrom.Wesaythatitisavirtualimage.
KEYWORDS
image:whatweseewhenweviewanobjectby
meansofreflectedrays
planemirror:(oraflatmirror)amirrorwithaflat,
reflectivesurface
laterallyinverted:animageinwhichleftand
righthavebeenreversed
realimage:animagethatcanbeformedona
screen
virtualimage:animagethatcannotbeformed
onascreen
Tosummarise,whenanobjectisreflectedinaplane
mirror,itsimageis:
thesamesizeastheobject
thesamedistancebehindthemirrorastheobjectis
infrontofit
laterallyinverted
virtual.
233>

247. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Questions
1aWhyisthewordambulancewritteninreverse
onthefrontofthisvehicle?
Figure13.8:Anambulance.
bWritethewordPOLICEinthesameway.
2Astudentinvestigatedthe^lawofreflection.She
increasedherangleofincidenceby20°eachtime.
AngleofincidenceAngleofreflection
20° 20°
40° 39°
60° 30°
aWhichangleofreflectiondidshemeasure
incorrectly?
bSuggestwhatshemayhavedonewrong.
3Drawadiagramtoshowarayhittingamirrorwith
anangleofincidenceof40°.Drawthereflected
ray.Labelbothrays,thenormalandtheanglesof
incidenceandreflection.
i
4Whatanglemustrayhitamirroratforthedirection
oftheraytobeturnedthrough90°?Drawa
diagramtoillustrateyouranswer.
Raydiagrams
Lightraysfollowstrictrules.Byfollowingthesamerules
wecanconstructdetailedraydiagrams.Theseshowwhere
animageisformedandwhattypeofimageitis.Worked
Example13.1showsthestepsinconstructingaraydiagram
toshowtheformationofanimage
in^a
planemirror.
WORKEDEXAMPLE13.1
Asmalllampisplaced5cminfrontofaplanemirror.
Drawanaccuratescalediagram,anduseittoshowthat
theimageofthelampis5cmbehindthemirror.
Step1:Drawalinetorepresentthemirror.Indicateits
reflectingsurfacebydrawingshortlinesonthe
backoftheline.Markthepositionoftheobject
(thelamp)withacrossandlabelitO.
Thestepsneededtodrawtheraydiagramarelisted
belowandshowninFigures13.9a-d.(Ithelpstowork
onsquaredpaperorgraphpaper.)
234

248. 13Light
Figure13.9b
Answer
235>
CONTINUED
FromthediagramforStep4,itisclearthattheimageis
5cmfromthemirror,directlyoppositetheobject.The
linejoiningOtoIisperpendiculartothemirror.
Step2:Drawtworaysfromtheobjecttothemirror.
Drawinthenormallineswheretheystrike
themirror.
Step4:Extendthereflectedraysbackbehindthe
mirror.Drawthisusingdottedlines.Thepoint
wheretheycrossiswheretheimageisformed.
LabelthispointI.
Step3:Usingaprotractor,measuretheangleof
incidenceforeachray.Marktheequalangleof
reflection.Drawinthereflectedrays.

249. }CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Questions
5Figure13.10showsanobjectplaced6cmfroma
planemirror.
Figure13.10:Objectandmirror.
aCopyFigure13.10andusethelawofreflection
tofindthepathoftheraysaftertheyhit
themirror.
bTracetheseraysbacktofindthepositionof
theimage.
cMeasuretheperpendiculardistancefromthe
imagetothemirror.
6Figure13.11showshowaperiscopeusesmirrorsto
allowtheusertoseebeyondobstacles.
mirror
Figure13.11:Periscope.
aCopyandcompletethediagraminFigure13.11
toshowhowthelightreachestheobserver.
bTheimagetheobserverseesisnotlaterally
inverted.Explainwhy.
13.2Refractionoflight
Figure13.12:Theripplesseenonthebottomofthepool
arecausedbywaterbendingthelight.
Ifyoulookdownatthebottomofaswimmingpool,you
mayseepatternsofshadowyripplesasinFigure13.12
Thesurfaceofthewaterisnotflat.Thereareoftensmall
ripplesonthewater,andthesecausetheraysofsunlight
tochangedirection.Wherethepatternisdarker,raysof
lighthavebeenbentaway,producingasortofshadow.
Thisbendingofraysoflightwhentheytravelfromone
mediumtoanotheriscalledrefraction.
Therearemanyeffectscausedbytherefractionoflight.
Someexamplesarethesparklingofdiamonds,theway
thelensinyoureyeproducesanimageoftheworld
aroundyou,andthetwinklingofthestarsinthenight
sky.Theimageofabentstrawinliquid(Figure13.13)is
anotherconsequenceofrefraction.
Figure13.13:Thestrawispartlyimmersedinthedrink.
Becauseoftherefractionofthelightcomingfromthepart
ofthestrawthatisunderwater,thestrawappearsbent.
236>

250. 13Light
Refractionoccurswhenarayoflighttravelsfromone
mediumintoanother.Therayoflightmaychange
direction.Refractionhappensattheboundarybetween
thetwomaterials.Therayapproachingtheboundaryis
calledtheincidentrayandtherayleavingtheboundary
iscalledtherefractedray.Theangleofincidence,i,
andangleofrefraction,r,aremeasuredtothenormal
drawnatthepointwheretherayhitstheboundary
(seeFigure13.14).
KEYWORDS
refraction:thebendingoflightwhenitpasses
fromonemediumtoanother
angleofrefraction:theanglebetweena
refractedrayandthenormaltothesurfaceatthe
pointwhereitpassesfromonemediumtoanother
angleof
incidence
normal
angleof
reflection
refracted
ray
incident
ray
air
glass
Figure13.14:Refractionofalightrayenteringglass.
EXPERIMENTAL SKILLS13.1
Investigatingrefraction
Youcaninvestigaterefractionbyshiningarayoflight
intoaglassorPerspex®blockandtracingthepathof
theray.
Youwillneed:
raybox
powerpack
rectangularglassorPerspex®block
glassorPerspex®ofdifferentshapes
plainpaper
opticalpins.
Safety:Youwillbeworkinginadarkenedarea,so
keepyourworkareatidytoavoidtriphazards.
Gettingstarted
Howwillyoumarkthepathoftheraysgoinginand
outoftheblock?
Howwillyoudeterminethepathoftherayinsidethe
glassblock?
Method
1Placetheblockonpaperanddrawroundit.
2ShinearayintotheblockasshowninFigure
13.15.Lookcarefullyattherayinsidetheglass.
Youwillseeittravelsinastraightline.Itonly
bendsatthesurface.
air
glass
Figure13.15:Rayhittingtheglassblock.
3Marktheraygoingintotheblockandthe"
raycomingoutoftheblockwithcrossesor
opticalpins.
4Removetheblockanddrawtheraygoinginto
theblockandtheraycomingout.
237)

251. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
5Jointhesetworaystoshowthepathofthe 12Recordyourobservationsinsketches.Noteany
lightthroughtheglass. unexpectedeffectsyouobserve-thesewillbe
explainedlaterinthischaoter.
6Drawthenormalatthepointswheretheray
entersandleavestheblock.
7Measureanglesiandrfortherayenteringthe
blockandrecordtheminatable. a
AngleofincidenceinAngleofrefractionin
/\ \
air,i glass,r /\
Figure13.16:Lightraysincidentonglassblocks.
Questions
1Copyandcompletethesesentences:
8Repeatthisforraysenteringtheblockat Whenaraygoesfromairtoglassitbends
differentangles. thenormal.
9Ifyouhaveblocksofdifferentmaterials,repeat Foraraygoingfromairtoglass,theangleof
theexperimenttofindoutwhichmaterial issmallerthantheanqleof
bendslightthemost.
Whenaraygoesfromglasstoairitbends
10Investigatewhathappensifarayoflighthits thenormal.
theglassblockat90°.
2Describewhathappenswhenarayhitsthe
11Investigatetherefractionoflightasitpasses blockat90°. \
throughdifferentshapedblocks,suchasthose
inFigure13.16.
Changingdirection
Figure13.17showslightpassingthrougharectangular
block.Noticethatthelightonlybendsatthepoint
whereitentersorleavestheblock,soitisthechangeof
materialthatcausesthebending.
FromFigure13.17,youcanseethatthedirectionin
whichtheraybendsdependsonwhetheritisenteringor
leavingtheglass.
Theraybendstowardsthenormalwhenentering
theglass.
Theraybendsawayfromthenormalwhenleaving
theglass.
Oneconsequenceofthisisthat,whenaraypasses
throughaparallel-sidedblockofglassorPerspex®,it
returnstoitsoriginaldirectionoftravel,althoughitis
shiftedtooneside.
Figure13.17:Demonstratingtherefractionofarayof
lightwhenitpassesthrougharectangularblockofglassor
Perspex®.Theraybendsasitenterstheblock.Asitleaves,
itbendsbacktoitsoriginaldirection.
238>

252. 13Light
Whenwelookattheworldthroughawindow,weare
lookingthroughaparallel-sidedsheetofglass.Although
theraysoflightareshiftedslightlyastheypassthrough
theglass,wedonotseeadistortedimagebecausethey
allreachustravellingintheiroriginaldirection.
Thismatcheswhatwehaveseenwithlight.Lightslows
downwhenitgoesfromairtoglass,anditbendstowards
thenormal(Figure13.14).
Figure13.19:Toexplainwhyachangeinspeedexplains
thebendingcausedbyrefraction,pictureatruck'swheels
slippingofftheroadintothesand.Thetruckturnstothe
sidebecauseitcannotmovesoquicklythroughthesand.
Figure13.18:Whenarayhitsaboundaryat90°itisnot
refracted.
Arayoflightmaystrikeasurfaceatanangleof
incidenceof0°,asshowninFigure13.18.Inthiscase,
itdoesnotbend-itsimplypassesstraightthroughand
carriesoninthesamedirection.Usuallywesaythat
refractionisthebendingoflightwhenitpassesfromone
mediumtoanother.However,weshouldbearinmind
that,whenthelightisperpendiculartotheboundary
betweenthetwomaterials,thereisnobending.
Explainingrefraction
Lightisrefractedbecauseittravelsatdifferentspeeds
indifferentmaterials.Lighttravelsfastestinavacuum
(emptyspace)andalmostasfastinair.Ittravelsmore
slowlyinglass,waterandothertransparentsubstances.
Figure13.19showshowachangeofspeedcancausea
changeofdirection.Imagineatruckisdrivingalonga
roadacrossthedesert.Thedriveriscarelessandallows
thewheelsonthelefttodriftofftheroadontothesand.
Here,theyspinaround,sothattheleft-handsideofthe
truckmovesmoreslowly.Theright-handsideisstillon
theroad,andkeepsmovingquickly,sothatthetruck
startstoturntotheleft.
Theboundarybetweenthetwomaterialsistheedgeof
theroad.Thenormalisatrightanglestotheroad.The
truckhasturnedtotheleft,soitsdirectionhasmoved
towardsthenormal.
Questions
7Describewhathappenstoarayoflightthatpasses
from:
aairtoglass
bglasstoair.
8Whyaretheangleofincidenceandtheangleof
refractionalwaysmeasuredbetweentherayand
thenormal?
9Drawadiagramtoshowalightraypassingfrom
glasstoair.Marktheincidentandrefracted
rays,thenormalandtheanglesofincidence
andrefraction.
10Aswimmingpoolislitfromthebottom.
UseFigure13.20toexplainwhytheswimmingpool
appearstobeshallowerthanitis.
Figure13.20:Lightinaswimmingpool.
239>

253. I
)CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Refractiveindex
Lighttravelsveryfast.Asfarasweknow,nothingcan
travelanyfasterthanlight.Thespeedoflightasittravels
thoughemptyspaceisexactly299792458m/s.Thisis
usuallyroundedto300000000m/sor3x108m/s.
Whenarayoflightpassesfromairintoglass,itslows
downandbendstowardsthenormal.Thishappenswhen
lightpassesfromonetransparentmediumtoanother,or
whenlightpassesbetweendifferentregions,suchasfrom
hotwatertocoldwater.
Therefractiveindexofamaterialisameasureofhow
muchthelightslows,orhowmuchitisbent.Ifthespeed
oflightishalvedwhenitentersamaterial,therefractive
indexis2,andsoon.Therefractiveindexistheratioofthe
speedsoflightintwodifferentmediaordifferentregions.
KEYWORDS
speedoflight:thespeedatwhichlighttravels
(usuallyinavacuum:3.0x108m/s)
refractiveindex:theratioofthespeedsofalight
waveintwodifferentmedia
Table13.1:Thespeedoflightinsometransparent
materials.Notethatthevaluesareonlyapproximate.
MaterialSpeedoflight/m/s
speedinvacuum
speedinmaterial
vacuum 2.998x108 1exactly
air 2.997x108 1.0003
water 2.308x108 1.33
Perspex®2.000x108 1.5
glass (1.800-2.000)x108 1.5-1.7
diamond1.250x108 2.4
Calculatingrefractiveindex
ACTIVITY13.2
Interpretingdata
Youcanusethemeasurementsyoutookin
Experimentalskills13.1toinvestigatetherelationship
betweentheanglesofincidenceandrefraction.
Thevalueof—
-isalwaysthesamefora,particular
sinr
material.Thisvalueistherefractiveindex®fthematerial.
Therefractiveindexofglassisabout1.5andtherefractive
indexofwaterisabout1.33.
Refractiveindex(ri)canbecalculatedu^ingtheequation:
WORKEDEXAMPLE13.2
Arayoflighthitsthesurfaceofwateratanangle
ofincidenceof30°.Itisrefractedatanangleof22°.
Findtherefractiveindex,n,ofwater.
Step1:Writedownwhatyouknowandwhatyou
wanttofindout.
i=30°
r=22°
n=?
Step2:Writedowntheequationandsubstitute
thesevalues.
n=
llUi
=
sin30
sinrsin22
240)

254. 13Light
15
16
Answer
n=1.33
13.3TotalinternalQuestions
11
reflection
12
13a
b
c
14
materialX
19°
Figure13.21:Rayoflightpassingintoair.
a
b
CONTINUED
Whenyouinvestigatedrefractionyoumayhavenoticed
thatnotallthelightisrefracted.Someisreflectedback
fromthesurface,asshowninFigure13.17.Youcanalso
seethatasthelightemergesfromtheglass,somelight
isreflectedbackinsidetheglass.Thisiscalledinternal
reflection.
Likeallreflectedlight,thesereflectedraysobeythelaw
ofreflection:
a
b
Figure13.21showsarayoflightpassingfromair
intoanunknownmaterial(X).Therayisdeflected
by19°.
a
b
Figure13.22:Usingarayboxtoinvestigatereflectionwhen
arayoflightstrikesaglassblock.Therayenterstheblock
withoutbending,becauseitisdirectedalongtheradiusof
theblock.
Arayoflightentersablockofglassatanangleof
incidenceof40°.Theangleofrefractionintheglass
is25°.Calculatetherefractiveindexoftheglass.
Calculatetheangleofrefraction.
Calculatethespeedoflightintheglass.
Determinetheanglesofincidenceand
refraction.
CalculatetherefractiveindexofmaterialX.
Giveyouranswertotwosignificantfigures.
Describewhathappenstothespeedoflightas
itpassesfromairtoglass.
Useyouranswertopartatoexplainwhylight
enteringglassatanangleisrefracted.
Explainintermsofthespeedoflightwhya
rayenteringtheglassalongthenormalisnot
refracted.
Define‘refractiveindex’.
Explainwhyrefractiveindexdoesnothave
aunit.
angleofincidence=angleofreflection
Raysreflectedbackinsidematerialssuchasglasscan
beveryuseful.Figure13.22showsinternalreflection.
Arayoflightisincidentonasemicircularglassblock.
Therayentersthecurvedsideoftheblockatrightangles
andsopassesstraightthroughtothemidpointofthe
straightside.
aRefractioncanhappenwhenlightpassesfrom
onetransparentsolidtoanother.Usethe
informationinTable13.1todeducewhatwill
happentoarayoflightpassingfromdiamond
toglass.Sketchadiagramtoshowwhatwill
happen.
Arayoflightentersglasswitharefractiveindexof
1.52atanangleofincidenceof60°.
Step3:Useacalculatorortablestofindthesinesand
calculaten.
_
0.5
n
0.375
=1.33

255. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Whathappensnextdependsontheangleofincidence
oftherayatthemidpoint.Figure13.23showsthe
possibilities.
InFigure13.23a,theangleofincidenceissmall,somost
ofthelightemergesfromtheblock.Thereisafaint
reflectedrayinsidetheglassblock.Therefractedray
bendsawayfromthenormal.
InFigure13.23b,theangleofincidencehasincreased,so
morelightisreflectedinsidetheblock.Therefractedray
bendsevenfurtherawayfromthenormal.
InFigure13.23c,therefractedrayemergesalongand
paralleltothesurfaceoftheblockforaparticularangle
ofincidence.Thisangleiscalledthecriticalangle.Most
ofthelightisreflectedinsidetheblock.
InFigure13.23d,theangleofincidenceisevengreater
andallofthelightisreflectedinsidetheblock.No
refractedrayemergesfromtheblock.
Figure13.23:Howarayoflightisreflectedorrefracted
insideaglassblockdependsontheangleofincidence.
Wehavebeenlookingathowlightisreflectedinsidea
glassblock.Wehaveseenthat,iftheangleofincidence
isgreaterthanthecriticalangle,thelightisentirely
reflectedinsidetheglass.Thisisknownastotalinternal
reflection(TIK):
total,because100%ofthelightisreflected
internal,becauseithappensinsidetheglass
reflection,becausetherayisentirelyreflected.
Fortotalinternalreflectiontohappen,theangleof
incidenceoftheraymustbegreaterthanthecritical
angle.Thecriticalangledependsonthematerialbeing
used.Forglass,itisabout42°,dependingonthetypeof
glass.Forwater,thecriticalangleisgreater,about49°.
Fordiamond,thecriticalangleissmall,about25°.
Raysoflightthatenteradiamondareverylikelytobe
totallyinternallyreflected,sotheybouncearoundinside,
eventuallyemergingfromoneofthediamond’scutfaces.
Thatexplainswhydiamondsaresuchsparklyjewels.
KEYWORDS
internalreflection:whenarayoflightstrikesthe
innersurfaceofamaterialandsomeofitreflects
backinsideit
criticalangle:theminimumangleofincidenceat
whichtotalinternalreflectionoccurs
totalinternalreflection(TIR):whenarayoflight
strikestheinnersurfaceofamaterialand100%of
thelightreflectsbackinsideit
EXPERIMENTAL SKILLS13.2
Totalinternalreflectionandthecriticalangle
Youwillshinearayoflightintoablockand
investigatewhenandhowitisreflected.
Youwillneed:
rayboxwithaslit
powerpack
glassorPerspex®semi-circularblock
plainpaper
rulerandprotractor.
Safety:Asyouwillbeworkinginadarkenedarea,
youshouldkeepyourworkareatidytoavoidtrip
hazards.
242y

256. 13Light
Method
1
2
raybox
3
4
5
Observethepathoftheray.Youshouldsee
tworays.Oneisrefractedasitemergesfrom
theblockandtheotherisreflectedbackinto
theblock.
Figure13.24:Shiningtherayoflightontothe
glassblock.
Gettingstarted
Howwillyourecordthepathoftheraysoflightthat
youobserveastheypassthroughtheblock?
Movetherayboxtoincreasetheangleof
incidenceuntiltherefractedrayemergesalong
thesideoftheblock.Theangleofincidenceat
whichthishappensiscalledthecriticalangle(c).
Marktherayswithcrosses.Removetheblockand
drawtherayasitpassesthroughtheblock.
Placethesemi-circularblockonthepaperand
drawaroundit.Marktheexactcentreofthe
straightsidewithanormalline.
Shinearayoflightintotheblockalongthe
curvedsidesothatithitsthemidpointofthe
straightside.Noticethattherayenterstheblock
atrightangles.
6
7
Figure13.26:Totalinternalreflection.
Measurethecriticalangle,c.
Drawroundtheblockagain,andshinearayinto
theblockatananglegreaterthanthecritical
angle.Theraywillbetotallyinternallyreflected.
8Marktherays,removetheblockandmeasure
angles/andr.
Questions
1Whyistheraynotbentwhenitenterstheblock?
2Whatwasthecriticalangleforyourblock?
3Whatconclusioncanyoudrawfromyour
measurementsofanglesiandrforthetotally
internallyreflectedray?
refracted
ray
Figure13.25:Criticalangle.
internally
reflected
ray
CONTINUED

257. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Questions
17Explainthemeaningofthewords‘total’and
‘internal’intheexpression‘totalinternalreflection’.
18Thecriticalangleforwateris49°.Ifarayoflight
strikestheuppersurfaceofapondatanangle
ofincidenceof45°,willitbetotallyinternally
reflected?Explainyouranswer.
19LookatFigure13.27.
Figure13.27
aNameanglesx,yandz.
bWritedownanyrelationshipsyouknow
betweentheangles.
cDescribewhatwillhappenwhenanglexis
increased.
Criticalangleand
refractiveindex
Aswehaveseen,thecriticalangledependsonthe
materialthroughwhicharayistravelling.Thegreaterthe
refractiveindexofthematerial,thesmallerthecritical
angle(seetheexampleofdiamondinWorkedExample
13.3).Wecanusetheequationn= toseehow
sinr
criticalanglecandrefractiveindexnarerelated.Todo
thisweneedtobeawarethantherefractiveindexforglass
relatestolightpassingfromairtoglass,notfromglass
toair.Duringyourreflectionandrefractionexperiments
youalwaysmarkedtherayswitharrows.Thisisbecause
alllightraysarereversible.Whenyoushinelightfromthe
oppositedirection,youwillgetthesamelines,onlythe
arrowstellyouwhichwaytheraywastravelling.
Figure13.28showswhathappenswhenwereverse
therays.
Figure13.28:Reversingtherays.
YoucanseefromFigure13.28thattheangleof
incidenceis90°,andtheangleofrefractionastheray
goesintoglassisthecriticalangle.Substitutinginthe
equationgives:
_
sinz_
sin90°
n=—
:
sinrsinc
Sincesin90°=1
_
1
n
sine
KEYEQUATION
criticalangle:
_
1
n=
sinc
Notethatyouneedtoknowthisequation,butyoudo
notneedtorememberhowitisdrived.
Notealsothatthelettercisusedbothforcriticalangle
andthespeedoflight.Readquestionscarefullyand
makesureyouknowwhichmeaningisbeingused.
WORKEDEXAMPLE13.3
Findthecriticalangle,c,fordiamond.Assumethat
refractiveindexn=2.40.
Step1:Substitutethevalueofnintheequation:
Step2:Rearrangetomakecthesubject:
c=sin-10.417
Step3:Useacalculator(remembertocheckyour
calculatorissettodegreesnotradians)tofind
sin-10.417.
Answer
c=24.6°
244>

258. 13Light
Opticalfibres
Arevolutionintelecommunicationshasbeenmade
possiblebytheinventionoffibreoptics.Telephone
messagesandotherelectronicsignalssuchasinternet
computermessagesorcabletelevisionsignalsarepassed
alongfineglassfibresintheformofflashinglaserlight,
whichisadigitalsignal.Figure13.29ashowshowfine
thesefibrescanbe.Eachofthesefibresiscapableof
carryingthousandsoftelephonecallssimultaneously.
Figure13.29:Theuseoffibreopticshasgreatlyincreased
thecapacityandspeedoftheworld'stelecommunications
networks.Withoutthistechnology,cabletelevisionand
theInternetwouldnotbepossible,a:Eachofthesevery
finefibresofhigh-purityglasscancarrymanytelephone
messagessimultaneously,b:Lighttravelsalongafibreby
totalinternalreflection.
Insideafibre,lighttravelsalongbytotalinternal
reflection(seeFigure13.29b).Itbouncesalonginsidethe
fibrebecause,eachtimeitstrikestheinsideofthefibre,
itsangleofincidenceisgreaterthanthecriticalangle.
Thismeansnolightislostasitisreflected.Thefibrecan
followacurvedpathandthelightbouncesalonginside
it,followingthecurve.Forsignalstotraveloverlong
distances,theglassusedmustbeofaveryhighpurity,so
thatitdoesnotabsorbthelight.
Opticalfibresarealsousedinmedicine.Anendoscope
isadevicethatcanbeusedbydoctorstoseeinsidea
patient’sbody,forexample,toseeinsidethestomach
(Figure13.30).Onebundleoffibrescarrieslightdown
intothebody,whileanotherbundlecarriesanimage
backuptotheuser.Theendoscopemayalsohaveasmall
probeorcuttingtoolbuiltin,sothatminoroperations
canbeperformedwithouttheneedformajorsurgery.
Figure13.30:Passingafibreopticcabledownthe
oesophagustothestomachallowsthedoctortoseeinside
thepatientwithouttheneedformajorsurgery.
Lightcanbeusedinsensoryplaytosoothorstimulate
childrenwithsensoryprocessingissues.Usingoptical
fibresmeanschildrencanplayfreelywithlightswithno
riskofelectrocutionorburning.
Questions
20Listthreeusesoftotalinternalreflection.
21aThecriticalangleforamaterialis38°.
Calculateitsrefractiveindex.
bCalculatethecriticalanglefordiamond.
UsedatafromTable13.1.
22Sketchadiagramtoshowhowarayoflightcan
travelalongacurvedglassfibre.Indicatethepoints
wheretotalinternalreflectionoccurs.

259. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
23Figure13.31showsabicyclereflector.Itreflects
lightbytotalinternalreflection.
redplastic
reflector
lightraysfromcar
Figure13.31:Howlightisreflectedinabicycle
reflector.
aWhyislightnotrefractedwhenitenters
theplastic?
bWhatistheangleofincidenceatthe
diagonalsurface?
cCopyandcompletethediagraminFigure13.31
toshowthepathoftheraysfromthecar.
dWhatcanyoudeduceaboutthecriticalangleof
theplastic?
13.4Lenses
Lensesareallaroundus,forexampleinspectaclesand
cameras.Lensesareparticularlyimportanttoscientists
ininstrumentsincludingmicroscopesandtelescopes.
Figure13.32showstwousesoflenseswhichhelpedto
revolutionisescience.
Convergingand
diverginglenses
Lensescanbedividedintotwotypes,accordingtotheir
effectonlight(Figure13.33):
converginglensesarefatterinthemiddlethanat
theedges
diverginglensesarethinnerinthemiddlethanat
theedges.
KEYWORDS
converginglens:alensthatcausesraysof
lightparalleltotheaxistoconvergeatthe
principalfocus
diverginglens:alensthatcausesraysof
lightparalleltotheaxistodivergefromthe
principalfocus
Figure13.32a:Morethan400yearsago,Galileogroundhisownglasslensestomaketelescopeslikethis.Whathesaw
throughthemsuggestedtheEarthwasnotthecentreoftheuniverse,b:Afewyearslater,VanLeeuwenhoekusedasimple
singlelensmicroscopetoobservebacteriafromhisteeth,providingaclueastohowinfectiousdiseasesarespread.
246)

260. 13Light
Converginglenses
Diverginglenses
Figure13.33:Converginglensesarefattestinthemiddle.
Diverginglensesarethinnestinthemiddle.Theyaregiven
thesenamesbecauseoftheireffectonparallelraysoflight.
Usually,wesimplydrawthecross-sectionofthelens.
parallelbeam(Figure13.34b).Thisdiagramisthesame
asFigure13.34a,butinreverse.
Aconverginglensisso-calledbecauseitmakesparallel
raysoflightconverge.Theprincipalfocusisthepoint
wheretheraysareconcentratedtogether,andwherea
pieceofpaperneedstobeplacedifitistobeburned.
Thedistancefromthecentreofthelenstotheprincipal
focusiscalledthefocallengthofthelens.Thefatterthe
lens,theclosertheprincipalfocusistothelens.Afat
lenshasashorterfocallengththanathinlens.Figure
13.35showsthis.
focuslength
parallelrays
b
principalfocusparallelrays
vprincipalfocus V
focuslength
Figure13.34:Theeffectofaconverginglensonraysoflight,a:Aconverginglensmakesparallelraysconvergeatthe
principalfocus,b:Raysfromtheprincipalfocusofaconverginglensareturnedintoaparallelbeamoflight.
Youhaveprobablyusedamagnifyingglasstolookat
smallobjects.Thisisaconverginglens.‘Converging’
meansbringingtogether.Youmayevenhaveuseda
magnifyingglasstofocustheraysoftheSunontoa
pieceofpaper,tosetfiretoit.(Morethanathousand
yearsago,anArabscientistdescribedhowpeopleused
lensesforstartingfires.)
Figure13.34ashowshowaconverginglensfocusesthe
parallelraysoftheSun.Ononesideofthelens,the
raysareparalleltotheprincipalaxisofthelens.After
theypassthroughthelens,theyconvergeonasingle
point:theprincipalfocus(orfocalpoint).Afterthey
havepassedthroughtheprincipalfocus,theyspread
outagain.Aconverginglenscanbeusedtoproducea
beamofparallelrays.Asourceoflight,suchasasmall
lightbulb,isplacedattheprincipalfocus.Astheypass
throughthelens,theraysarebentsothattheybecomea
Figure13.35a:Afatterlensismorepowerfulsobendsthe
lightmore.Thisgivesashorterfocallength,b:Athinnerlens
isnotaspowerfulanddoesnotbendthelightasmuch.
principalaxis:thelinepassingthroughthecentre
ofalensperpendiculartoitssurface
principalfocus/focalpoint:thepointatwhich
raysoflightparalleltotheprincipalaxisconverge
afterpassingthroughaconverginglens
focallength:thedistancefromthecentreofthe
lenstoitsprincipalfocus
KEYWORDS

261. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Lensesworkbyrefractinglight.Whenaraystrikesthe
surfaceofthelens,itisrefractedtowardsthenormal.
Whenitleavestheglassofthelens,itbendsawayfrom
thenormal.Thecleverthingabouttheshapeofa
converginglensisthatitbendsallraysjustenoughfor
themtomeetattheprincipalfocus.
Formingarealimage
WhentheSun’sraysarefocusedontoapieceofpaper,
atinyimageoftheSuniscreated.Itiseasiertoseehow
aconverginglensmakesanimageifyoufocusanimage
ofalightbulboradistantwindowontoapieceofwhite
paper.Thepaperactsasascreentocatchtheimage.
(Becarefulifyoutrythisyourself-youcouldsetfireto
thepaperorburnyourself.Andremember:neverlook
directlyattheSun.)
Figure13.36showsanexperimentinwhichanimageof
alightbulb(theobject)isformedbyaconverginglens.
TheimageintheraindropinFigure13.1wasformedin
thesameway.
Figure13.36:Formingarealimageofalightbulbusinga
converginglens.Theimageisupsidedownonthescreenat
thebackright.
Imagescanbedescribedintermsoftheirsize(enlarged,
thesamesize,ordiminished),whichwayuptheyare
(invertedorupright),andwheretheyareformed.
KEYWORDS
enlarged:usedtodescribeaninnagewhichis
biggerthantheobject
diminished:usedtodescribeanimagewhichis
smallerthantheobject
inverted:usedtodescribeanimagewhichis
upsidedowncomparedtotheobject
upright:usedtodescribeanimagewhichisthe
samewayupastheobject
TheimageofthelightbulbinFigure13.36is:
inverted
diminished
nearertothelensthantheobject
real.
Wesaythattheimageisreal,becauselightreallydoes
fallonthescreentomaketheimage.Iflightonly
appearedtobecomingfromtheimage,wewouldsay
thattheimagewasvirtual.Thesize
^f
theimagedepends
onhowfatorthinthelensis.
Drawingraydiagrams
forlenses
Wecanexplainhowthisrealimageisformedusinga
raydiagram.Theseraydiagramsaredrawntoscale
andshowthepathoftwoparticularrays.Thesteps
neededtodrawanaccurateraydiagramareshownin
WorkedExample13.4.Rememberthatithelpstowork
onsquaredpaperorgraphpaperwhendrawingaray
diagram.
248y

262. 13Light
WORKEDEXAMPLE13.4
Drawaraydiagramtofindtheimageformedofa3cmtallobjectplaced12cmfromaconverginglenswhichhasa
focallengthof5cm.
Step1:Drawthelens(asimpleoutlineshapewilldo)
withahorizontalaxisthroughthemiddleofit.
Step2:Markthepositionsoftheprincipalfocus(F)on
eitherside,at5cmfromthecentreofthelens.
Markthepositionoftheobject,O,alongwith
a3cmarrowstandingontheaxis,.Placethe
arrow12cmfromthelens.
Step3:Drawray1,astraightlinefromthetopofthe
objectarrowwhichpassesundeflectedthrough
themiddleofthelens.
Step4:Drawray2fromthetopoftheobjectarrow
paralleltotheprincipalaxis.Asitpasses
throughthelens,itisrefractedthroughthe
principalfocus.Tomakethingseasierwhenwe
drawraydiagrams,weonlyshowraysbending
once,atthecentreofthelens.
Lookforthepointwherethetworayscross.Thisisthe
positionofthetopoftheimage(I).
Withanaccuratelydrawnraydiagram,youcanseethat
theimageisinverted,diminishedandreal.
cXs
1
Figure13.37a
A
I
SXrs
c -
?
Figure13.37b
Figure13.37c
Figure13.37d
So,toconstructaraydiagramlikethis,drawtworays
startingfromthetopoftheobject:
ray1:unrefractedthroughthecentreofthelens
ray2:paralleltotheaxisandthenrefractedthrough
theprincipalfocus.
249y

263. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
ACTIVITY13.3
Raydiagrams
Imagineyouworkforacompanywhichmakes
camerasandprojectors.Youhavebeenaskedto
produceshort,technicalguidesexplainingtheuse
oflensesintheseproducts.
Drawingraydiagramsletsuspredictwhatan
imagewilllooklike.Alenscanmakedifferent
imagesdependingonitsposition.Drawthe
followingraydiagrams:
Alenswithafocallengthof3cmwiththe
object8cmfromthelens.
Thesamelensbutwiththeobject5cmfrom
thelens.
Describetheimageformedineachcase.Whichis
foracamera,andwhichforafilmprojector?
Drawdiagramstoinvestigatethedifferencethe
thicknessofalensmakestotheimage.Remember,
afatlenshasashortfocallength,athinlenshasa
longerfocallength.
PEERASSESSMENT
Swapraydiagramswithapartnerandcheckifthey
arecorrectlydrawn.
Givethemasmileyface,straightfaceorsadface
foreachofthefollowingfeatures:
axisdrawnhorizontallythroughthemiddleof
thelens
focalpoint,F,markedoneachsideofthelens
rayfromthetopoftheobjectpassingthough
thecentreofthelenswithoutbeingdeflected
rayfromthetopoftheobjectparalleltothe
axisstrikingthelensthenpassingthrough
theprincipalfocus
raysdrawnasstraightlinesusingaruler
correctarrowsonbothrays
imageclearlymarked,includingwhichway
upitis.
Discusswhat,ifanything,wasmissingandhowyou
canbothimproveyourdiagrams.
Questions
24Copyandcompletethesesentences:
Aconverginglensrefractsparallelraysoflighttoa
pointcalledthe .
Thedistancefromthispointtothelensiscalledthe
Thefatterthelensthe thisdistancewillbe.
25CopyandcompletetheraydiagramsinFigure13.38
toshowwhathappenswhentherayshitthelenses.
Figure13.38:Raydiagrams.
Magnifyingglasses
Amagnifyingglassisaconverginglens.Youholditclose
toasmallobjectandpeerthroughittoseeamagnified
image.Figure13.39showshowamagnifyingglasscan
helptomagnifyprintforsomeonewithpooreyesight.
Figure13.39:Aconverginglensproducesamagnified
imagemakingsmallprinteasiertoread.
Theobjectviewedbyamagnifyingglassisclosertothe
lensthantheprincipalfocus.Wecandrawaraydiagram
usingthesametworaysasinFigure13.40.
Ray1isunrefractedasitpassesthroughthecentre
ofthelens.
250y

264. 13Light
Ray2startsoffparalleltotheaxisandisrefracted
bythelenssothatitpassesthroughtheprincipal
focus.
Rays1and2donotcrossovereachother.Theyare
diverging(spreadingapart)aftertheyhavepassed
throughthelens.However,byextendingtherays
backwards,asshownbythedashedlines,wecanseethat
theybothappeartobecomingfromapointbehindthe
object.Thisisthepositionoftheimage(I).Wedraw
dashedlinesbecauselightdoesnotactuallytravelalong
thesepartsoftherays.Wecannotcatchtheimageona
screen,becausethereisnolightthere.
Figure13.40:Araydiagramtoshowhowamagnifying
glassworks.Theobjectisbetweenthelensandthefocus.
Theimageproducedisvirtual.Tofinditsposition,therays
havetobeextrapolatedback(dashedlines)tothepoint
wheretheycross.
Fromtheraydiagram(Figure13.40),wecanseethatthe
imageproducedbyamagnifyingglassis:
upright
enlarged
furtherfromthelensthantheobject
virtual.
So,ifyoureadapageofabookusingamagnifying
glass,theimageyouarelookingatisbehindthepage
thatyouarereading.Thisimageisavirtualimage.
Usinglensestocorrect
eyesightproblems
Oureyescontainconverginglenseswhichformanimage
ontheretinaatthebackoftheeye.Thelensesinour
eyesareflexibleandmusclescanchangetheshapeand
strengthofthelens.Thisallowsustofocusonobjectsat
differentdistances.Figure13.41ashowsaneyeforming
animageofadistantobject.Figure13.41bshowsthe
eyefocusingonamuchcloserobject.Thelightfromthe
closerobjectisdivergingsothelensneedstobethicker
andstrongertoformanimageontheretina.
Figure13.41a:Parallellightfromadistantobjectisfocused
byaweaklens,b:Diverginglightfromacloseobjectneeds
astrongerlens.
Someeyesareunabletochangetheirstrengthenough
tofocusoneithercloseordistantobjects.Anextralens,
wornasspectaclesorcontactlenses,canworkwiththe
eyelenstoletitfocusasneeded.
Shortsight
Apersonwithshortsightcanseecloseupobjectsclearly,
butcannotformaclearimageofdistantobjects.The
imageisformedinfrontoftheretina.Thisisusually
becausetheeyeballisslightlytoolongsothattherays
meetinfrontoftheretina.Tocorrectthis,adiverging
lensisusedtomaketheraysfromthedistantobject
diverge.Theeyelensisthenabletoformafocused
image.Figure13.42showstheproblemandhowitis
corrected.

265. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Figure13.42a:Withshortsight,theimageisformedbefore
theretina,b:Usingadiverginglenstohelpthelensinthe
eyetoformanimageontheretina.
Figure13.43:Withoutglasses,distantobjectsappear
blurredtoashort-sightedperson.Thediverginglensinthe
glassesletsthepersonfocusondistantobjects.
Longsight
Along-sightedpersonhastheoppositeproblem.Along¬
sightedeyecanfocusondistantobjectsbutnotclose
objects.Thiscanbebecausetheeyeballistooshort,or
thelenscannotbecomestrongenoughsotheraysfrom
acloseobjectcannotbeconvergedenoughtoaforman
imageontheretina.Aconverginglenscausestheraysto
converge,allowingtheeyelenstoformafocusedimage
ofcloseobjects,asshowninFigure13.44.
Figure13.44a:Withlongsight,theimageisformedbehind
theretina,b:Aconverginglensworkswiththelensinthe
eyetoformanimageontheretina.
Questions
26LookatFigure13.40.Howcanyoutellfromthe
diagramthattheimageformedbythemagnifying
glassisavirtualimage?
27aAconverginglenshasfocallength3cm.An
object,2cmtall,isplaced5cmfromthecentre
ofthelens,ontheprincipalaxis.Drawan
accurateraydiagramtorepresentthis.
bUseyourdiagramtodeterminethedistanceof
thevirtualimageformedfromthelens,andthe
heightoftheimage.
252y

266. 13Light
28Figure13.45showsaneyelookingatadistant
object.Thepersonseesablurredimage.
Figure13.45:Eyelookingatadistantobject.
aNametheeyesightproblemthispersonhas.
bDescribehowtheshapeoftheeyecausesthis
problem.
cDrawadiagramtoshowhowalenscanbeused
tohelpthispersontoseeafocusedimageofthe
distantobject.
13.5Dispersionoflight
Whenwhitelightpassesthroughglass,itrefractsas
itentersandleavestheglass,andcanbesplitintoa
spectrumofcolours.Figure13.46ashowslightbeing
refractedasitpassesthroughaglassprism.Youcansee
thatthecoloursmergeintooneanother,andtheyare
notallofequalwidthsinthespectrum.Arainbowisa
naturallyoccurringspectrum.WhitelightfromtheSunis
splitupintoaspectrumofcoloursasitentersandleaves
dropletsofwaterintheair.Itisalsoreflectedbacktothe
viewerbytotalinternalreflection,whichiswhyyoumust
havetheSunbehindyoutoobservearainbow.
Figure13.46a:Whitelightisdispersedbyaprism.
b:Raindropstaketheplaceoftheprismtoformarainbow.
Thissplittingupofwhitelightintoaspectrumisknown
asdispersion.IsaacNewtonsetouttoexplainhowit
happens.Ithadbeensuggestedthatlightiscoloured
bypassingitthroughaprism.Newtonshowedthat
thiswasthewrongideabyarrangingforthespectrum
tobepassedbackthroughanotherprism.Thecolours
recombinedtoformwhitelightagain.Heconcludedthat
whitelightisamixtureofallthedifferentcoloursofthe
spectrum.Newtondescribedthevisiblelightspectrum
asbeingmadeupofsevencolours-red,orange,yellow,
green,blue,indigoandviolet.Youmaywonderwhy
indigoandvioletareseparateratherthanjustbeing
purple.Thisisbecause,inthe17thcentury,seven
wasconsideredtobeamysticalnumber.So,Newton
wantedsevencoloursnotjustsix.Thecolourscanbe
rememberedasaname,RoyGBiv.
Figure13.47:Thefirstprismrefractsthelightcausingitto
disperse.Thesecondprismrefractsthelightback,causing
thecolourstorecombine.
So,whathappensinaprismtoproduceaspectrum?
Asthewhitelightenterstheprism,itslowsdown.We
saythatitisrefractedand,aswehaveseen,itsdirection
changes.Dispersionoccursbecauseeachcolouris
refractedbyadifferentamount(Figure13.47).Violet
lightslowsdownthemost,andsoitisrefractedthe
most.Redlightisleastaffected.
KEYWORDS
spectrum:(plural:spectra)waves,orcolours,of
light,separatedoutinorderaccordingtotheir
wavelengths
dispersion:theseparationofdifferent
wavelengthsoflightbecausetheyarerefracted
throughdifferentangles
Lightofasinglecolourisnotdispersedbyaprism.Itis
refractedsothatitchangesdirection,butitisnotsplitup
intoaspectrum.YoucanseethisinFigure13.48.Thisis
becauseitislightofasinglecolour.Thislightisdescribed
asmonochromatic(mono=one,chromatic=coloured).
Monochromaticlightislightofasinglefrequency.

267. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Figure13.48:Monochromaticlightfromalaserisnot
dispersedbyaprismasthelightisallonefrequency.
KEYWORD
monochromatic:describesarayoflight(orother
electromagneticradiation)ofasinglewavelength
Questions
29Copyandcompletethesesentences.
Whenlightentersglassitslowsdown(causingitto
changedirection.Thisiscalled .
Redlightchangesdirection thanvioletlight.
Thelightissplitintodifferentcolours.Thisiscalled
30Drawadiagramtoshowhowwhitelightcanbe
dispersedintoaspectrumusingaglassprism.Label
theredandvioletlight.
31Listthecoloursofthevisiblespectruminorder
startingwiththecolourwhichisrefractedtheleast.
REFLECTION
Whenwestudylight,weuseverysimilarwordsand
diagramsforreflectionandrefraction.Haveyou
foundthisdifficult?Howhaveyoulearnttherules
forraydiagrams?Howsuccessfulhaveyoubeen?
PROJECT
Lightfantastic
Inthischapteryouhaveseenlotsofopticaleffects
causedbythereflectionandrefractionoflight.
Someareuseful,someareanuisance.Yourtaskisto
investigate,illustrateandexplainoneoftheseeffects.
Yourresultsshouldbepresentedasaposterand
shouldinclude:
aphoto-preferablyoneyouhavetaken-of
theeffect
araydiagramtoshowwhatishappening
adescriptionofthescientificlawsandprinciples
whichexplaintheeffect.
Youcanuseaneffectyouhaveseeninclass,suchas
theformationofanimageinaplanemirror,oryou
canapplywhatyouhavelearnttoanewsituation.
Forexample:
HowdoestheturtleinFigure13.49seeitself?
(Withawaterproofcamerayoucouldtakea
similarphotoofyourselfinaswimmingpool.)
Whydodiamondssparklesomuch?
Whydolensescreateupsidedownimages?
Howcaninteriordesignersusemirrorstomake
aspaceseembigger?
Howdoestheeyeformanimage(andhowcan
glasseshelpsolvevisionproblems)?
Howdoesaperiscopework?
Figure13.49:Reflectionatthesurfaceofthewaterallows
thisturtletoseeitself.
254>

268. 13Light
SUMMARY
Thelawofreflection:angleofincidence=angleofreflection.
Theimageinaplanemirrorisupright,asfarbehindthemirrorastheobjectisinfrontandswappedroundleft
toright.
Theimageinaplanemirrorisvirtual.
Refractionisthebendingoflightasitgoesfromonesubstancetoanother.
Refractioniscausedbylighttravellingatdifferentspeedsindifferentmaterials.
Whenlightpassesfromairtoglassitbendstowardsthenormal.Whenitpassesfromglasstoairitbendsaway
fromthenormal.
Whenlighttravellingthroughglasshitsaboundarywithair,somelightpassesfromglasstoair,somelightis
internallyreflectedbackintotheglass.
Whentheangleofincidenceinglassisequalto,orgreaterthan,thecriticalangle,allthelightisreflectedback
intotheglass.Thisistotalinternalreflection.
Therefractiveindexisameasureofhowmuchlightisslowed,orbent,byamaterial.
Refractiveindexcanbecalculatedusingtheequationn=
S1-
sinr
Refractiveindex=———-—
.
criticalangle
Opticalfibrescantransmitinformationrapidlyandefficientlyusingtotalinternalreflection.Thisisusefulin
telecommunicationsandmedicine.
Converginglensesbendparallelraystogethersotheymeetatapointcalledtheprincipalfocus.
Drawingarayparalleltotheaxis,andaraywhichstrikesthecentreofalensallowsustodrawaraydiagram
andfindthetypeofimageformed.
Amagnifyingglassproducesavirtualimage.
Oureyesuseaflexibleconvexlenstoformimages.
Ashort-sightedeyehasalenswhichistoopowerful.Thiscanbecorrectedusingadiverginglens.
Along-sightedeyehasalenswhichistooweak.Thiscanbecorrectedusingaconverginglens.
Whitelightcanbedispersedbypassingitthroughaglassprism.Thiscreatesthevisiblespectrum.
255>

269. >CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
EXAM-STYLEQUESTIONS
1Thediagramshowsarayoflightstrikingaplanemirror.
Whichrowgivesthecorrectlabelsforthediagram? [1]
X Y Z
normal incidentray angleofrefraction
angleofincidence incidentray normal
reflectedray normal angleofreflection
normal incidentray reflectedray
2Whichoneofthefollowingstatementsabouttheimageformedinaplane
mirroristrue? [1]
ATheimagecanbeformedonascreen.
BTheimageisdiminished.
CTheimageisupright.
DTheimageisonthesurfaceofthemirror.
3Arayoflightpassesfromairtoglassatanangleof30°.Whichstatement
correctlydescribeswhathappens? [1]
ATheraybendstowardsthenormal.
BTheraypassesthroughtheglasswithoutbending.
CTherayistotallyinternallyreflected.
DTheraybendsawayfromthenormal.
4Thisdiagramshowslightbeingrefractedbyaconverginglens.
aWhatnameisgiventothedistancemarkedX? [1]
256>

270. 13Light
object
upright inverted diminished
nottoscale
40°'1
a
b
[Total:7]
CONTINUED
COMMAND WORDS
7a
bTheimageformedisvirtual.Explainwhatthismeans.
[1]
[1]
[1]
[2]
[2]
[2]
[1]
Thediagramshowsarayof
lightstrikingtheinnersurface
ofaglassblock.Thecritical
anglefortheglassis42°.
calculate:workout
fromgivenfacts,
figuresorinformation
Thisdiagramshowsonerayoflightpassingthroughthelens.Thelenscan
beusedtoformanimageofanobject.
Construct
araydiagramtoshowhowamagnifyingglasswithafocal
lengthof6cmproducesanimageofanobjectplaced4cmfromthelens.[5]
explain:setout
purposesor
reasons;make
therelationships
betweenthings
evident;provide
whyand/orhowand
supportwithrelevant
evidence
Thisdiagramshowsarayoflightentering
aglassprism.Therefractiveindexofthe
glassis42°.
a
b
c
d
Copythediagram.Withoutcalculating,continuetherayfromthepoint
whereitstrikesthesurfaceuntilitleavestheblock.
Whatcanyousayaboutthespeedoflightintheglassblock?
Calculatetherefractiveindexoftheglass.
Writedowntheequationyouuse.
Calculatetheangleofincidenceatwhichtherayenterstheglassblock.
[1]
[Total:6]
enlarged
[Total:6]
Explainwhytherayisnotrefractedasitenterstheglass.
Copyandcompletethediagramtoshowwhathappenstotheray.
Whatnameisgiventotheeffectyouhavedrawn?
Iftheprismwasremoved,whatotherpieceofapparatuscouldproduce
thesameeffectonthelightray? [1]
[Total:6]
Copythediagramanddrawanotherraytocompleteit.Drawanarrow
torepresenttheimage. [3]
Choosetwoofthesewordstodescribetheimageinb. [2]

271. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
SELF-EVALUATIONCHECKLIST
Afterstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutosee
anygapsinyourknowledgeandhelpyoutolearnmoreeffectively.
1can
See
Topic...
Needs
morework
Almost
there
Confident
tomoveon
Describehowanimageisformedinaplanemirror. 13.1
Recallandusetheequation:angleofincidence=
angleofreflection.
13.1
Drawaraydiagramtoshowtheformationofa
virtualimageinaplanemirror.
13.1
Describeanexperimenttoinvestigatehowlightis
refractedwhenitpassesthroughaglassblock.
13.2
Drawalabelleddiagramoflightpassingthrougha
glassblock,labellingthenormalandtheanglesof
incidenceandrefraction.
13.2 1
Describeinternalreflectionandrecallthatifthe
angleofincidenceisgreaterthanthecriticalangle,
totalinternalreflectionoccurs.
13.3
Recallandusetheequations:
_
sinz
n
smr
1
n~
:—criticalangle
13.2,13.3
Describeandexplainhowtotalinternalreflectionis
usedintelecommunicationsandmedicine.
13.3
Drawafullylabelledraydiagramtoshowthe
formationofarealimagebyaconverginglens.
13.4
Describethenatureoftheimageformed. 13.4
Drawafullylabelledraydiagramtoshowthe
formationofavirtualimagebyaconverginglens
andexplainhowthisisusedinamagnifyingglass.
13.4
Drawdiagramstoshowthecorrectionoflongand
short-sightednessusinglenses.
13.4
Namethesevencoloursofthevisiblespectrumin
thecorrectorder.
13.5
258
>

272. describediffractionofwaves.
describeawaveintermsofspeed,amplitude,frequencyandwavelength
identifydifferencesbetweentransversewavesandlongitudinalwaves
calculatewavespeed
describereflectionandrefractionofwaves
>Chapter14
Properties
ofwaves

273. F
)CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
GETTINGSTARTED
Inagroup,discussallthedifferenttypesofwaveyoucanthinkof.
Whatdothesewaveshaveincommon?Whatdowavesdo?
MAKINGWAVES
Figure14.1:Einstein'slastpredictionwasconfirmedafter
100years.
Einsteinhypothesizedthattheuniverseismadeofa
fabricthathecalledthespace-timecontinuum.He
predictedthatinteractionsbetweenmassiveobjects
wouldcreateripplesinspace-timeinmuchthe
samewaythatthrowingastoneintoapondcauses
ripplesinthewater.
Theripplesareveryhardtodetectastheyaretiny.
Thespace-timecontinuumisverystiffandonly
reallyhugeeventscauseanymeasurableripples.
Lasersareusedtopickuptinyvariationsinspace¬
timethatcouldbecausedbyapassinggravitational
wave.In2016,scientistsworkingattheLaser
InterferometerGravitational-Waveobservatory
(LIGO)detectedarippleinspace-timecausedby
twoblackholesspirallingtowardseachotherand
mergingintoone.
Thediscoveryofgravitationalwavesisimportantfor
tworeasons:
Itprovidesfurtherevidencetosupport
Einstein'stheoryofrelativity.
Gravitationalwavestravelhugedistances
withoutthesignalbeingaffected.Thiscould
allowastronomerstoinvestigateqreasofthe
universewhichwecannotaccessusinglight.
ThiscouldprovideinformationabouttheBig
Bang,andtheexistenceofdarkmatter.
Theideaofawaveisaveryusefulmodelinphysics.
Physiciststalkaboutlightwaves,soiJndwaves,
electromagneticwaves,andsoon.Itiseasytosee
howwavesbehaveinwaterandsprings.Itisnot
obviousthatlight,gravityandsoundarewavesin
thewaywethinkofwavesinthesea.Inthischapter,
wewillinvestigatewavesinwaterandsprings,and
seehowthesewavescanacta^agoodmodelfor
bothlightandsound.
Discussionquestions
1Doesthediscoveryofgravitationalwavesprove
thatEinsteinwasright?
2AmemberoftheLIGOteamcommentedthat
thediscoveryofgravitationalwavesmeanswe
areatthestartoftheeraofgravitationalwave
astronomy.Gravitationalwaveswillprovideus
withinformationthatwillhelpustounderstand
theuniverse.Discussotherwaysinwhich
humanshaveusedwavestogatherinformation
abouttheearthandbeyond.
14.1Describingwaves
Physicistsusewavesasamodeltoexplainthebehaviour
oflight,soundandelectromagneticradiation.Water
wavescanhelpusunderstandthebehaviourofwavesas
theyareeasytoobserve.Wavesarewhatweseeonthe
seaoralake,butphysicistshaveamorespecialisedidea
ofwaves.Wecanbegintounderstandthismodelinthe
laboratoryusingarippletank(Figure14.2).
260y

274. 14Propertiesofwaves
Arippletankisashallowglass-bottomedtankcontaining
asmallamountofwater.Alightshiningdownwards
throughthewatercastsashadowoftheripplesonthe
floorbelow,showingthepatternthattheymake.
Figure14.2:Theripplesonthesurfaceofthewaterinthis
rippletankareproducedbythebar,whichvibratesupand
down.Thepatternoftheripplesisseenbyshiningalight
downwardsthroughthewater.Thiscastsashadowofthe
ripplesonthescreenbeneaththetank.
Figure14.3showstwopatternsofripples,straightand
circular,whichareproducedindifferentways.
Figure14.3:Twopatternsofripplesonwater,a:Straight
ripplesareamodelforabroadbeamoflight,b:Circular
ripplesareamodelforlightspreadingoutfromalamp.
Onewayofmakingripplesonthesurfaceofthewaterin
arippletankistohaveawoodenbarthatjusttouchesthe
surfaceofthewater(asinFigure14.2).Thebarvibrates
upanddownatasteadyrate.Thissendsequallyspaced
straightripplesacrossthesurfaceofthewater.
Asphericaldippercanproduceadifferentpatternof
ripples.Thedipperjusttouchesthesurfaceofthewater.
Asitvibratesupanddown,equallyspacedcircularripples
spreadoutacrossthesurfaceofthewater.
Ineachcase,theripplesareproducedbysomething
vibratingupanddownvertically,buttheripplesmove
outhorizontally.Thevibratingbarordipperpushes
watermoleculesupanddown.Eachmoleculedrags
itsneighboursupanddown.Thesethenstarttheir
neighboursmoving,andsoon.Eachmoleculesimply
movesupanddown.Energyistransferredbythewave,
butthewatermoleculesremaininthesameplaceafterthe
wavehaspassed.Awavetransfersenergybutnotmatter.
Howcanthesepatternsofripplesbeamodelforthe
behaviouroflight?Thestraightripplesarelikeabeam
oflight,perhapscomingfromtheSun.Theripplesmove
straightacrossthesurfaceofthewater,justaslightfromthe
Suntravelsinstraightlines.Thecircularripplesspreading
outfromavibratingdipperarelikelightspreadingoutfrom
alamp.(Thedipperisthelamp.)Inthischapterwewilluse
wavesintherippletankasamodeltohelpusunderstand
thebehavioroflightandsoundwaves.
IKEYWORDS
model,awayofrepresentingasysteminorderto
understandhowitfunctions
rippletank:ashallowwatertankusedto
demonstratehowwavesbehave
ripple:asmalluniformwaveonthesurfaceofwater
Wavelengthandamplitude
Wavesareoftenrepresentedbyawavyline,asshownin
Figure14.4.Wehavealreadyusedthisideaforsound
waves(inChapter12)andwewilldosoagainfor
electromagneticwaves(inChapter15).Thiswavyline
islikeadownwardslicethoughtheripplesintheripple
tank.Itshowsthesuccessionofcrests(alsocalledpeaks)
andtroughsofwhichtheripplesaremade.
261)

275. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Figure14.4b:Thesamewaveasarepresentedasa
smoothlyvaryingwavylineonagraph.Thisshapeisknown
asasinewave.Ifyouhaveagraphicscalculator,youcan
useittodisplayagraphofy=sinx,whichwilllooklikethis
graph.
ThegraphinFigure14.4bshowsawavetravelling
fromlefttoright.Thehorizontalaxis(x-axis)shows
thedistance,x,travelledhorizontallybythewave.The
verticalaxis(y-axis)showshowfar(distancey)the
surfaceofthewaterhasbeendisplacedfromitsnormal
level.Wecanthinkofthex-axisasthelevelofthe
surfaceofthewaterwhenitisundisturbed.Theblueline
onthegraphshowshowfarthesurfaceofthewaterhas
beendisplacedfromitsundisturbedlevel.
Twomeasurementsaremarkedonthegraph.These
definequantitiesusedtodescribewaves.
ThewavelengthAofawaveisthedistancefrom
onecrestofthewavetothenextorbetweenany
twopointsonthewavewhichareinstep.Sincethe
wavelengthisadistance,itismeasuredinmetres,m.
Itssymbolis2,theGreekletterlambda.
Theamplitude,A,ofawaveisthemaximum
distancethatthesurfaceofthewaterisdisplaced
fromitsundisturbedlevel,thatis,theheightof
acrest(orthedepthofatrough).Forrippleson
thesurfaceofwater,theamplitudeisadistance,
measuredinmetres,m.ItssymbolisA.
Forripplesinarippletank,thewavelengthmightbea
fewmillimetresandtheamplitudeamillimetreortwo.
Wavesontheopenseaaremuchbigger,withwavelengths
oftensofmetres,andamplitudesvaryingfromafew
centimetresuptoseveralmetres.
KEYWORDS
crest/peak:thehighestpointofawave
trough:thelowestpointofawave
wavelength:thedistancebetweentwoadjacent
crests(ortroughs)ofawave
Frequencyandperiod
Asthebarintherippletankvibrates,itsendsout
ripples.Eachup-and-downmovementsendsoutasingle
ripple.Themoretimesthebarvibrateseachsecond,the
moreripplesitsendsout.Thisisshowninthegraph
inFigure14.5.Takecare!Thislooksverysimilarto
thepreviouswavegraphinFigure14.4,butherethe
horizontalaxisshowstime,t,notdistance,x.Thisgraph
showshowthesurfaceofthewaterataparticularpoint
movesupanddownastimepasses.
FromtherepresentationofthewaveinFigure14.5,we
candefinetwoquantitiesforwavesingeneral:
Thefrequency,/,ofawaveisthenumberofwaves
sentouteachsecond.Frequencyismeasuredin
hertz,Hz.Onehertz(1Hz)isonecompletewaveor
ripplepersecond.
Theperiod,T,ofawaveisthetimetakenfor
onecompletewavetopassapoint.Theperiodis
measuredinseconds,s.
Figure14.5:Agraphtoshowtheperiodofawave.Notice
thatthisgraphhastime,t,onitshorizontalaxis.
Wehavealreadydiscussedthefrequencyandperiodof
asoundwaveinChapter12.Itisalwaysimportantto
checkwhetherawavegraphhastime,t,ordistance,x,
onitshorizontalaxis.
262>

276. 14Propertiesofwaves
Thefrequencyofawaveisthenumberofwavesmade
eachsecond,orthenumberofwavespassingapointper
second.Theperiodisthenumberofsecondsittakesfor
eachwavetopassapoint.Frequency,/,andperiod,T,
areobviouslyrelatedtoeachother.Waveswithashort
periodhaveahighfrequency.
frequency(Hz)=-
period(s)
/=1J
T
period(s)=
frequency(Hz)
Wavesontheseamighthaveaperiodof10seconds.
Theirfrequencyisthereforeabout0.1Hz.Asoundwave
mighthaveafrequencyof1000Hz.Itsperiodis
therefore—
-—,whichmeansthatawavearrivesevery
1000s
7
1ms(onemillisecond).
Wavespeed
Thewavespeedistherateatwhichthecrestofawave
travels.Forexample,itcouldbethespeedofthecrestof
arippletravellingoverthesurfaceofthewater.Speedis
measuredinmetrespersecond(m/s).
Wavescanhaveverydifferentspeeds.Ripplesinaripple
tanktravelafewcentimetrespersecond.Soundwaves
travelat330m/sthroughair.Lightwavestravelatabout
300000000m/sthroughair.
KEYWORDS
wavespeed:thespeedatwhichawavetravels
Wavesandenergy
Wecanalsothinkofthespeedofawaveasthespeedat
whichittransfersenergyfromplacetoplace.
ThinkoftheSun.Itisasourceofenergy.Itsenergy
reachesusintheformofradiation-lightwavesand
infraredwaves-whichtravelsthroughthevacuumof
spaceandwhichisabsorbedbytheEarth.
Thinkofaloudspeaker.Itvibratesandcausestheair
nearbytovibrate.Thesevibrationsspreadoutinthe
airasasoundwave.Whentheyreachourears,our
eardrumsvibrate.Energyhasbeentransferredbythe
soundwavestoourears.
Thebiggertheamplitude,themoreenergythewave
transfers.Alargeamplitudemeansabrightlightora
loudsound.
Ifyouhaveeverbeenknockedoverbyawaveinthesea,
youwillknowthatwaterwavesalsocarryenergy.Itis
importanttorememberthat,whenawavetravelsfrom
oneplacetoanother,itisnotmatterthatismoving.
Thewaveismoving,anditiscarryingenergy.Itmay
movethroughmatteroreventhroughavacuum,butthe
matteritselfisnottransferredfromplacetoplace.A
wavetransfersenergywithouttransferringmatter.
Earthquakesshowthehugeamountsofenergywhich
wavescantransfer.VibrationspassingthroughtheEarth
cancausebuildingstocollapse.Aseismometerrecords
thevibrationscausedbyanearthquake.
Figure14.6a:VibrationscausedbyshiftsintheEarth's
tectonicplatescancausedevastation,b:Theseismometer
recordstheamplitudeandfrequencyofthevibrations.
263>

277. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Transverseand
longitudinalwaves
Ripplesinarippletankareonewayoflookingatthe
behaviourofwaves.Youcanmodelwavesinotherways.
AsshowninFigure14.7,astretchedslinkyspringcanbe
usedtomodelwaves.Fixoneendofthespringandmove
theotherendfromsidetoside(Figure14.7a).Youwill
seethatawavetravelsalongthespring.(Youmayalso
noticeitreflectingfromthefixedendofthespring.)You
candemonstratethesamesortofwaveusingastretched
ropeorpieceofelastic.Youcanseethelinkbetween
frequencyandwavelengthbychangingtherateatwhich
youmovetheendofthespringupanddown.Increasing
thefrequencydecreasesthewavelength.
Asecondtypeofwavecanalsobemodelledwitha
stretchedslinkyspring.Insteadofmovingthefreeend
fromsidetoside,moveitbackwardsandforwards
(Figure14.7b).Aseriesofcompressionstravelsalong
thespring.Theseareregionsinwhichthesegments
ofthespringarecompressedtogether.Inbetween
arerarefactions,regionsinwhichthesegmentsofthe
springarefurtherapart.Thistypeofwavecannotbe
demonstratedonastretchedrope.
Thedirectionofpropagationofawaveisthedirection
inwhichittravels.Youcanobservethisbywatchingthe
movementofcrestsandtroughs,orcompressionsand
rarefactions.
ThedemonstrationsinFigure14.7showthetwodifferent
typesofwave:
Transversewaves:theparticlescarryingthewave
movefromsidetoside,atrightanglestothe
directionofpropagationofthewave.
Longitudinalwaves:theparticlescarryingthe
wavemovebackandforth,alongthedirectionof
propagationofthewave.
directionofpropagation
*primaryorP-wave
undisturb
secondaryorS-wave undisturbe
Figure14.8:Primaryandsecondaryseismicwaves.
Arippleonthesurfaceofwaterisanexampleof£
transversewave.Theparticlesofthewatermoveu
downasthewavetravelshorizontally.
Inanearthquaketherearetwotypesofseismicwa
primaryorP-wavesandsecondaryorS-waves.P-w
arelongitudinal.Thesewavesarecalledprimaryw;
theytravelfasterthanslowersecondarywavesand
feltfirst.Secondarywavesaretransverse.
Asoundwaveisanexampleofalongitudinalwave,
asoundtravelsthroughair,theairmoleculesmove
andforthasthewavetravels.Table14.1listsexamp
transverseandlongitudinalwaves.
------
»
Transversewaves Longitudinalwaves
ripplesonwater sound
lightandallother
electromagneticwaves
primaryseismicwavt
(P-waves)
secondaryseismicwaves
(S-waves)
Table14.1:Transverseandlongitudinalwaves.
Figure14.7:Modellingwavesusingaspring,a:Atransversewaveonastretchedspring.Itismadebymovingthefree
fromsidetoside,b:Alongitudinalwaveonastretchedspring.Itismadebypushingthefreeendbackandforth,along
lengthofthespring.
264>

278. 14Propertiesofwaves
KEYWORDS
transversewave:awaveinwhichthevibrationis
.itrightanglestothedirectionofpropagationof
thewave
longitudinalwave:awaveinwhichthevibration
Isforwardandback,paralleltothedirectionof
propagationofthewave
waves:wavescausedbyearthquakes
P-waves:fastmoving,longitudinalseismicwaves
S-waves:slowmoving,transverseseismicwaves
ind|Questions
1Copyandcompletethefollowingsentences.
i: aAwavetransfers fromplacetoplace
esI withouttransferring.
!Sas
|| bIna wavethevibrationsareatright
are
fl anglestothedirectioninwhichthewavetravels.
Ina wavethevibrationsarebackand
lSI forthalongthedirectionofthewave,
ck
of 2ThetwowavesinFigure14.9representtwolight
waves,XandY.
lightY
Figure14.9:Twolightwaves.
Copythesesentences,selectingthecorrectanswers
fromthebrackets.
aThetwowaveshave{thesame/different}
wavelengths.
bThetwowaveshave{thesameIdifferent}
amplitudes.
cLightXwillbe{brighterIdimmer}than
lightY.
3Drawawaveandlabeltheamplitudeandthe
wavelength.
4Giveoneexampleofatransversewaveandone
exampleofalongitudinalwave.
5DeducethewavelengthofthewavesinFigure14.10.
Figure14.10:Waves.
6Thebarofarippletankvibratesfivetimesper
second.
aStatethefrequencyofthewavesitmakes.
bFindtheperiodofthewavesitmakes.
7Astudentseesaflashoflightningand,three
secondslater,shehearsthethunder.
aWhydoessheseethelightningbeforeshehears
thethunder?
bLightandsoundarebothtypesofwaves.
Copyandcompletethefollowingsentences,
choosingthecorrectwordsfromthebrackets.
iLightisa{transverse/longitudinal}wave.
Thevibrationsare{parallelto/atright
anglesto}thedirectionoftravelofthe
wave.Thesewaves{can/cannot}travel
throughavacuum.
iiSoundisa(transverseIlongitudinal}wave.
Thevibrationsare{parallelto/atright
anglesto}thedirectionoftravelofthe
wave.Thesewaves{can/cannot}travel
throughavacuum.
cThespeedofsoundinairis330m/s.Calculate
howfarawaythestormisfromthestudent.
265y

279. CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
14.2Speed,frequency
andwavelength
Howfastdowavestravelacrossthesurfaceofthesea?
Ifyoustandontheendofapier,youmaybeableto
answerthisquestion.
Figure14.11:Bytimingwavesandmeasuringtheir
wavelength,youcanfindthespeedofwaves.
Supposethatthepieris60metreslong,andthat
younoticethatexactlyfivewavesfitintothislength
(Figure14.11).Usingthisinformation,youcandeduce
thatthewavelengthis:
wavelength=
=12m
Nowyoutimethewavesarriving.Theintervalbetween
crestsastheypasstheendofthepieris4.0seconds.How
fastarethewavesmoving?Onewavelength(12metres)
passesin4.0seconds.Sothespeedofthewavesis:
=3.0m/s
Thespeed,v,frequency,/,andwavelength,2,ofawave
areconnected.Wecanwritetheconnectionintheform
ofanequation:
speed(m/s)=frequency(Hz)xwavelength(m)
KEYEQUATION
wavespeed=frequencyxwavelength
v=/2
Anotherwaytothinkofthisi§tosaythatthespeedis
thenumberofwavespassingpersecondmultipliedby
thelengthofeachwave.If100wavespasseachsecond
(/■=100Hz),andeachis4.0mlong(2=4.0m),then
400mofwavespasseachsecond.Thespeedofthewaves
is400m/s.
WORKEDEXAMPLE14.1
AnFMradiostationbroadcastssignalsofwavelength
1.5metresandfrequency200MHz.Whatistheirspeed?
Step1:Writedownwhatyouknow,andwhatyou
wanttoknow.
/=200MHz=200000000Hz
=2x108Hz
2=1.5m
v=?
Step2:Writedowntheequationforwavpspeed.
Substitutevaluesandcalculatetheanswer.
v=/2
v=2x108Hzxl.5m
=3xl08m/s
Answer
Theradiowavestravelthroughtheairat3.0x108m/s.
(Youmightrecognisethisnumberas^hespeedoflight.)
WORKEDEXAMPLE14.2
Thehighestnoteonapianohasafrequencyof
4186Hz.Whatisthewavelengthofthesoundwaves
producedwhenthisnoteisplayed?Assumethatthe
speedofsoundinair=330m/s.Giveyouranswerto
twosignificantfigures.
Step1:Writedownwhatyouknow,andwhatyou
wanttoknow.
/=4186Hz
v=330m/s
2=?
Step2:Writedowntheequationforwavespeed.
Rearrangeittomakewavelength2the
subject.
v=/2
266>

280. 14Propertiesofwaves
CONTINUED
Step3:Substitutevaluesandcalculatetheanswer.
A_
330
4186
=0.07166m
To2s.f.thisis0.072m
Answer
Thewavelengthofthenoteinairis0.072metres.
Changingmaterial,changing
speed
Whenwavestravelfromonematerialintoanother,
theyusuallychangespeed.Lighttravelsmoreslowly
inglassthaninair.Soundtravelsfasterinsteelthan
inair.Whenthishappens,thefrequencyofthewaves
remainsunchanged.Thismeansthattheirwavelength
mustchange.ThisisillustratedinFigure14.12,which
showslightwavestravellingquicklythroughair.They
reachsomeglassandslowdown,andtheirwavelength
decreases.Whentheyleavetheglass,theyspeedup,and
theirwavelengthincreasesagain.
Figure14.12:Waveschangetheirwavelengthwhentheir
speedchanges.Theirfrequencyremainsconstant.Here,
lightwavesslowdownwhentheyenterglassandspeedup
whentheyreturntotheair.
Questions
8Fortheequationv=/2,writedownwhateach
symbolrepresentsandgivetheirSIunits.
9Calculatethespeedofwaterwaveswhichhavea
wavelengthof3mandafrequencyof0.5Hz.
10Awaveinairhasafrequencyof1100Hz,an
amplitudeof4cmandawavelengthof30cm.
aCalculatethespeedofthewave.
bSuggestwhattypeofwaveitislikelytobe.
cCalculatetheperiodofthewave.
11Blowingacrosspanpipes(Figure14.13)createsa
soundwaveinthepipe.
Explainwhythelongerpipesproducelower
pitchednotes.
Figure14.13:Panpipes.
12Whichhavethelongerwavelength,radiowavesof
frequency90000kHzor100MHz?
13Soundwavesgetfasterwhentheygofromairinto
water.Whathappenstotheir:
aspeed?
bwavelength?
cfrequency?
dperiod?
14.3Explainingwave
phenomena
Whenwelookatripplesonthesurfaceofwaterina
rippletank,wecanbegintoseewhyphysicistssaythat
lightbehavesasawave.Theripplesaremuchmore
regularanduniformthanwavesonthesea,sothisisa
goodmodelsystemtolookat.
267y

281. CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Reflectionofripples
normal
Figure14.14:Thereflectionofplanewavesbyaflatmetal
barrierinarippletank,a:Thiscriss-crosspatternisobserved
asthereflectedripplespassthroughtheincomingripples,
b:Howtheripplesareproduced,c:Thearrowsshowhow
thedirectionoftherippleschangeswhentheyarereflected.
Theangleofincidenceisequaltotheangleofreflection,
justasinthelawofreflectionoflight.
Figure14.14showswhathappenswhenaflatbarrier
isplacedintherippletank.Figure14.14ashowsthe
patternoftheripplesobserved,andFigure14.14bshows
howtheripplesareproduced.Straightripples(plane
waves)arereflectedwhentheystriketheflatsurfaceof
thebarrier.Thebarrieractslikeamirror,andtheripples
bounceoffit.Thisshowsanimportantfeatureofhow
wavesbehave.Theypassthrougheachotherwhenthey
overlap.
Figure14.14cshowsadrawingofthesamepattern.
Thisisanoverheadviewoftheripples.Thebluelines
representthetopsoftheripples.Theselinesareknown
aswavefronts.Theseparationofthewavefrontsisequal
tothewavelengthoftheripples.Figure14.14calso
showslines(theredarrows)toindicatehowthedirection
oftraveloftherippleschanges.Thisdiagrammay
remindyouoftheraydiagramforthelawofreflection
oflight(seeFigure14.5).
Lookingattheredarrows,youcanseethatthewaves
obeythesamelawofreflectionaslight:
angleofincidence=angleofreflection
KEYWORD
wavefront:alinejoiningadjacentpointsona
wavethatareallinstepwitheachother
Refractionofripples
Refractionoflightoccurswhenthespeedoflight
changes.Wecanseethesameeffectforripplesinaripple
tank(Figure14.15).Aglassplateisimmersedinthe
water,tomakethewatershallowerinthatpartofthe
tank.There,theripplesmovemoreslowlybecausethey
dragonthebottomofthetank(whichisnowactually
theuppersurfaceofthesubmergedglassplate).
Figure14.15a:Therefractionofplanewavesbyaflatglass
plateinarippletank.Asubmergedglassplatemakesthe
watershallowerinthegreyregion.Inthisregion,theripples
movemoreslowly,sothattheylagbehindtheripplesinthe
deeperwater.
268
>

282. 14Propertiesofwaves
Figure14.15b:Thiswavefrontdiagramshowsthesame
patternofripplesasa.Theraysshowthattherefractedray
isclosertothenormal,justaswhenlightslowsdownon
enteringglass.
InthephotographinFigure14.15a,youcanseethatthese
rippleslagbehindthefaster-movingripplesinthedeeper
water.Theirdirectionoftravelhaschanged.Figure14.15b
showsthesameeffect,butasawavefrontdiagram.Onthe
left,theripplesareindeeperwaterandtheymovefaster.
Theyadvancesteadilyforwards.Ontheright,theripples
aremovingmoreslowly.Theright-handendofarippleis
thefirstparttoentertheshallowerwater,soithasspent
longesttimemovingataslowspeed.Thismeansthatthe
right-handendofeachripplelagsbehind.
Therays(theredarrows)markedonFigure14.15bshow
thedirectioninwhichtheripplesaremoving.Theyare
alwaysatrightanglestotheripples.Theyemphasisehow
theripplesturnsotheirdirectionisclosertothenormalas
theyslowdown,justaswesawwiththerefractionoflight
inChapter13.
Diffraction
Figure14.16:Thewavesontheseaarestraight.Asthey
passthroughthegapintherocks,theystarttospreadout.
Aninterestingthinghappenstowaveswhentheygo
throughagapinabarrier.Figure14.16showswhat
happens.Wavespassingthroughthegapintherocks
spreadouttofillthebay.Thisisanexampleofa
phenomenoncalleddiffraction.Figure14.17shows
waterwavesinarippletankbeingdiffractedastheypass
throughagap.
KEYWORD
diffraction:whenawavespreadsoutasittravels
throughagaporpasttheedgeofanobject
Figure14.17:Ripplesarediffractedastheypassthroughagap
inabarrier-theyspreadintothespacebehindthebarrier.
Soundwavesarediffractedastheypassthrough
doorwaysandopenwindows.Thisiswhywecanhear
apersonaroundthecomer,eventhoughwecannotsee
them.Thissupportstheideathatsoundtravelsasawave.
Lightwavesarealsodiffractedwhentheypassthrough
verytinygaps.Youmightnoticethat,onafoggynight,
streetlampsandcarheadlightsappeartobesurrounded
byahalooflight.Thisisbecausetheirlightisdiffracted
bythetinydropletsofwaterintheair.Thesameeffect
canalsosometimesbeseenaroundtheSunduringthe
day(seeFigure14.18).
Figure14.18:LightfromtheSunisdiffractedasitpasses
throughfoggyair(whichisfulloftinydropletsofwater),
producingahalooflight.
269y

283. 1
yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
EXPERIMENTAL SKILLS14.1
Studyingwavesinarippletank
Inthisexperimentyouwillobservereflectionand
refractionofripplesinarippletankanddescribe
thereflection,refractionanddiffractionofwater
waves.
Youwillneed:
rippletankanddipperwithaneccentric
motor
powersupply
light
metalorplasticbarriers
glasssheettoadjustthedepthofwater.
Safety:Itiseasiesttoobservetheshadowof
thewavesinadarkenedroom.Keepthework
areatidytoavoidtriphazards.Clearupanyspilt
waterimmediately.
Gettingstarted
Whateffectdoeschangingthespeedofthemotor
haveonthewavefrontsproduced?
Ifyouputasmallpieceofcorkinthewater,how
doesitmove?
Method
1Adjustthemotorsothatyoucanclearlyseethe
shadowsofthewavesmovingacrossthetank.
2Placeabarrierinthetankatananglesothatit
reflectsthewaves.
3Lookcarefullyatthereflectedwaves.Investigate
whathappenswhenyouchangetheangleofthe
barrier.
4Placetheglasssheetinthetanksothatthe
waveshititstraighton.Adjustthewaterlevelso
thatthewaterabovetheglassisveryshallow.
5Observethewavesastheyenterandleavethe
shallowerwater.
6Investigatetheeffectofturningtheglasssheet
sothatthewavesentertheshallowwateratan
angle.
7Usetwobarrierstocreateawidegapforthe
wavestopassthrough.Observetheeffect.
8Narrowthegapuntilitisaboutthesamesize
asthewavelengthofthewaves.Observehow
thisaffectsthewaves.
Questions
1Drawadiagramtoshowwhathappenstoplane
waveswhentheystrikeaflatreflectorplacedat
45°totheirdirectionoftravel.
2Drawadiagramtoshowwhathappensto
planewaveswhen:
atheyentertheshallowerwaterstraighton
btheyentershallowerwateratanangle.
3Drawdiagramstoshowdiffractionatawide
gapandatanarrowgap.
4Howcanthespeedofripplesinarippletank
bechanged?
SELF-ASSESSMENT
Compareyourrippletankdiagramstoanotherstudent's.Thinkaboutwhatmakesagooddiagram.Areyour
diagramsclear,simpleandcorrectlylabelled?Addnotestoyourdiagramssayingwhatyouhavedonewell
andwhatcouldbeimproved.
270y

284. 14Propertiesofwaves
ACTIVITY14.1
Learningthedefinitionsofkeywords
Youwillhavenoticedthatthistopiccontainsalot
ofkeywords.Toexplainwavesyouneedtobe
familiarwiththesewords,whichdescribewave
characteristicsandhowwavesbehave.
Designandmakearevisionactivitytohelpyou
rememberthekeydefinitions.Youcoulduseoneof
theideasbeloworcreateyourown.
Flashcards:haveakeywordononesideand
itsdefinitionontheother.Thisway,anything
yougetwrong,youwillseethecorrectanswer
straightawaywhichwillhelpforthenexttime.
Questions:writeasetofshortquestionssuch
as,'Whatistheunitoffrequency?'onalarge
sheetofpaper.Giveeachquestionamark(1
foreasy,2fordifficult).Withafriend,taketurns
answeringquestions.Removeanywhichhave
beenansweredcorrectlyasyougo.Thewinner
isthepersonwithmostpoints.
Createself-testquestionsonaquizapp.This
wayyoucantestyourselfregularlyandquickly.
REFLECTION
Learningkeywordsisimportantinphysicsas
wordshaveveryprecisemeaningswhichmaybe
differentfromtheeverydayuseoftheword.Think
abouthowyoulearnthesewords.Whatactivities
aremosthelpfulforyou?Areyouregularly
checkingbackoverkeywordsfromother
chapters?Haveyoulearntrevisiontechniquesin
othersubjectswhichyoucanapplyhere?
Questions
14TheredlinesinFigure14.19showsoundwaves
createdbypersonA.
personB
Figure14.19
aCopyandcompleteFigure14.19toshowhow
thesoundwavesreachpersonB.
bWhattypeofwavesaresoundwaves?
15Figure14.20showsfourripples,a-d,inaripple
tank.Copyandcompleteeachdiagramtoshow
whathappenstotheripples.Labeleachdiagram
withthewavephenomenonitisshowing.
shallowwater
Figure14.20
Diffraction,moreorless
Wavesarediffractedwhentheypassthroughagapor
aroundtheedgeofanobstacle.Thesizeofthegup
affectsdiffraction.Theeffectisgreatestwhenthewidth
ofthegapisequaltothewavelengthoftheripples
(Figure14.21).
271>

285. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Figure14.21:Diffractionisgreatestwhenthewidthof
thegapisequaltothewavelengthofthewavesbeing
diffracted.Whenthegapismuchsmallerthanthe
wavelength,thewavesdonotpassthroughatall.
CompareFigure14.22withFigure14.17.InFigure
14.22,thegapiswider,soitisnowmuchbiggerthan
thewavelengthofthewaves.Thediffractioneffectisnot
sopronounced.Thecentralpartoftherippleremains
straightafterithaspassedthroughthegap.Attheedges,
theripplesarecurved.
Figure14.22:Thewavelengthofthewavesismuchsmaller
thanthegap,sothewavesarediffractedless.
Soundwaveshavewavelengths'betweenabout10mm
and10metres.Thisiswhytheyarediffractedasthey
passthroughdoorwaysandwindows.Lightwaveshave
amuchshorterwavelength-lessthanamillionthofa
metre.Thisiswhyverysmallgapsareneededtoseelight
beingdiffracted.
Diffractionalsohappensaswavespassanedge.The
greaterthewavelengthofthewaves,thegreatertheangle
Figure14.23:Increasingthewavelengthofwavesincreases
theangleofdiffraction.
,
Questions
16ExplainwhypersonBinquestion14.14canhear
personA,butnotseethem.
17Amanwholivesbehindamountainfindshecan
receivewaveswithawavelengthof1500metres,but
notshorterwaveswithawavelengthof2metres.
Explainwhy.
18Drawadiagramtoshowhowaseriesofparallel,
straightwavefrontsarealteredastheypassthrough
agapwithawidththatisequaltothewavelengthof
thewaves.
272)

286. 14Propertiesofwaves
PROJECT
So,whatisawave?
Youworkforatelevisioncompanythatismakinga
programmeaboutthediscoveryofgravitationalwaves.
Theproducerhasaskedyourteamtoworkonafive
minuteintroductiontothegeneralideaofwaves.You
needtoanswerthequestion:'Whatisawave?'.
Thedirectorthinksviewersmaythinkofwavesas
beingjustwaterwaves.Shewantsviewerstohavea
clearideaofwhatawaveisandhowwavesbehave.
Viewersshouldunderstandwhyphysicistsdescribe
lightandsoundaswaves.
Youwillneedtoshowthatwater,lightandsound
wavesbehaveinthesameways,particularlythat
theycanbereflected,refractedanddiffracted.
Youshouldalsoexplainsomeofthedifferences,
forexample,thatsoundwavesneedamedium.
Yoursectionwillintroducetheprogrammesoit
needstogettheaudience'sinterestsotheydon't
reachfortheremotecontrol.(Youcouldpointout
thattheremotecontrolisoperatedbyinfraredor
radiowaves!)Theproducersuggestsyoumight
showacrowddoingaMexicanwaveasawayof
showingenergybeingtransferredbyawave.You
couldalsoincludepicturesorvideoclipsofwater
waves,includingseismicwavesandtsunamis.
Onceyouhavetheaudienceinterested,youwill
needtointroducetheideathatwavestransmit
energybutnotmatter.Illustratethisbyreferringto
wavesinsprings,boatsbobbingonthewavesat
sea,oranyotherexamplesyoucanthinkof.You
shouldthencoverwavebehaviour,givingasmany
examplesaspossibletoshowtheaudiencehow
manyeverydaythingsdependonwaves.
Youmaypresentyourideasasavideo,orasa
storyboard.
Whentheprogrammeisbroadcastallyournames
willbeinthecredits,alongwithyourjobtitle.
Decideearlyonwhateachpersonwilldo.Jobs
couldinclude:researcher,pictureresearcher,editor,
producer,soundtechnician,camerapersonandany
otherrolesyouthinkareneeded.
SUMMARY
Wavestransferenergywithouttransferringmatter.
Wavescanbeclearlyseeninwaterandsprings.
Wavescanbetransverseorlongitudinal.
Wavespeed,frequencyandwavelengtharerelatedbytheequationv
Wavescanbereflectedandrefracted.
Wavescanbediffractedwhentheypassthroughagaporroundanobstacle.
Thewavemodelcanbeusedtoexplainreflection,refractionanddiffraction.

287. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
COMMAND WORD
describe:statethe
pointsofatopic;give
characteristicsand
mainfeatures
274y

288. 14Propertiesofwaves
Thisdiagramshowsawaveinthesea.5
6d/m
[1]
[1]
c
[1]
d
[2]
e
6
[1]
[1]
[Total:4]
7Avibrationgeneratorisusedtomakeawaveonastring.
Whatisthenameofthedistancelabelleda? [1]a
b
[1]
CONTINUED
[2]
Whatisthewavelengthofthewave?
Astudentsaystheamplitudeis40cm.Explainwhatmistaketheyhave
madeandwritedownthecorrectamplitude.
Laterinthedaytheseaiscalmer.Theamplitudeisreducedto10cm.
Ongraphpaper,drawthenewwave.
Thedistancefromthepulleytothevibrationgeneratoris90cm.
Whatisthewavelengthofthewave?
Abirdisfloatingonthesea.Describethemovementofthebirdas
thewavespassit.
Thestudentobservesthewavesclosertothebeach.Thewavelength
hereis0.5metres.Whatcanyoudeducefromthis?
a
b
cExplainwhyamanlivinginahillyareacanreceivethewavesfrom
stationBbutnotthewavesfromstationA.
[1]
[Total:6]
RadiostationAbroadcastsradiowaveswithawavelengthof1500metres
andafrequencyof200kHz.StationBhasafrequencyof100MHz.
aCalculatethespeedatwhichthewavestravel.
bUseyouranswertopartatocalculatethewavelengthofthewaves
fromstationB.
x/cm
20
10-
-10-
-20-
COMMAND WORDS
explain:setout
purposesor
reasons;make
therelationships
betweenthings
evident;provide
whyand/orhowand
supportwithrelevant
evidence
deduce:conclude
fromavailable
information
calculate:workout
fromgivenfacts,
figuresorinformation

289. Calculatethespeedofthewavesinthestring. [2]
dThevibrationofthestringcausesasoundwavetopassthroughtheair.
Whattypeofwaveis:
ithewaveinthestring? [1]
iithesoundwaveintheair? [1]
[Total:6]
8Astudentinvestigateswavesinarippletank.Allthewateristhesamedepth.
Shemeasuresthewavelengthofeachwaveas22cm.Theperiodofeach
waveis0.58s.
Calculatethespeedofthewave. [4]
SELF-EVALUATIONCHECKLIST
Afterstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhglpyoutosee
anygapsinyourknowledgeandhelpyoutolearnmoreeffectively.
Ican
See
Topic...
Needs
morework
Almost
there
Confident
tomoveon
Statewhatistransferredbyawave. 14.1
Recallthatwaves(includingelectromagneticand
seismicwaves),vibrationsinspringsandropes,
light,andsoundareallexamplesofwavemotion.
14.1 A
Illustratewavemotionusingropes,springsand
waterwaves.
14.1
Defineandusetheterms:wavefront,wavelength,
frequency,amplitude,wavespeed,crest,trough,
compression,rarefaction.
14.1
Recallandusetheequationlinkingwavespeed,
frequencyandwavelength.
14.2
Recallthedirectionofvibrationsintransverseand
longitudinalwaves.
14.1
Giveexamplesoftransverseandlongitudinalwaves. 14.1
Drawdiagramstoshowreflection,refractionand
diffractionofwaves.
14.3
Describehowthewavelengthofawaveaffects
diffraction.
14.3
276

290. describethemainfeaturesoftheelectromagneticspectrum
describesomeusesofdifferentelectromagneticwaves
considerhowelectromagneticwavescanbeusedsafely.
spectrum
INTHISCHAPTERYOUWILL

291. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
GETTINGSTARTED
Thinkaboutwhatyouhavelearntsofaraboutwaves.
Writedownfivebulletpointswhichsumupwhatyouknowaboutwaves-includinglightandsound.
Pairupandagreefivepointsfromyourcombinedlists.Youcanchangeorcombinethem.
Joinwithanotherpairandwriteoutfivepointsthatthegroupagreeson.Displaythesepointstotheclass.
RADIATION-FRIENDORFOE?
Figure15.1a:Thisartist'simpressionshowshowhigh
energygammaradiationcanbefinelytargetedtokill
tumourcells,b:JocelynJoyceBurnellwasoneofthe
firstastrophysiciststoobservelightfrompulsars.Pulsars
emitpulsesoflightatveryregularintervals,making
themenormouslyaccurateclocks.Pulsarscanbeusedto
measureastronomicaldistancesveryaccurately.
Thelightwavesweseearepartofabiggerfamily
ofwavescalledtheelectromagneticspectrum.
Theserangefromgammarays(seenkillingacancer
cellinFigure15.1a)totheradiowavesusedinradio
astronomybyastronomerssuchasJocelynJoyce
Burnell(Figure15.1b).Inthischapteryouwilllearn
aboutthesewaves:whattheyhaveincommonand
thedifferencesthatmakethemusefulinahuge
varietyofways.
Alltheseformsofradiationhaveusefulapplications,
butmanycanalsocauseharm.
UltravioletradiationfromtheSuncancauseskin
cancer.However,intryingtoavoidskincancerby
stayingindoorsandcoveringskinwhilstoutside,we
riskvitaminDdeficiency.Ourbodiescreatevitamin
Dwhenexposedtosunlight.Lackofitcancausea
rangeofhealthproblems,includingrickets.
Gammaradiationreleasedinnucleardisastershas
beenshowntocausecancers,andyetwhenused
correctly,thesameradiationcanbedirectedonto
cancercellstokillthem.
Decisionsabouttheuseofpotentiallyharmful
radiationhavetobalanceriskwith
benefit.
Consider
apregnantwomaninvolvedinacarcrashwho
appearstohavesustainedaseriousspinalinjury.
X-rayscoulddamagethedevelopingfetus,but
withoutX-rayimagestoinformtreatment,the
womancouldbeparalysed.
Decisionslikethishavenoeasyanswers.Whichever
decisionismade,thedoctorswifiaimtominimise
risk;forexample,iftheydecidetouseX-rays,they
willshieldthefetuswithleadshieldswhichblock
X-raysasmuchastheycan.
Asyouwillseeinthischapter,electromagnetic
radiationisallaroundus.Decisionsabouthowto
usethesewaveshavetobetakenbyeverybody,not
justthescientists.
Discussionquestions
Alargesportsarenaconcernedaboutsecurity
isconsideringinstallingX-rayscannerstocheck
supportersandtheirbagsfordangerousitems.
1Discusstherisksandbenefitsofinstallingthis
system.
2Considerwhoshouldmakethefinaldecision:
thearenamanagers,supportersorscientists?
278>

292. 15Theelectromagneticspectrum
15.1Electromagnetic
TheSunisaveryhotobject(Figure15.3).Itssurface
temperatureisabout5500°C,soitemitsalotof
ultravioletradiation.Mostofthisisabsorbedinthe
atmosphere,particularlybytheozonelayer.Asmall
amountofultravioletradiationdoesgetthroughtous.
Theozonelayerisdepleted(decreased)bychemicals
usedinaerosolsandrefrigerants.Depletionletsmore
ultravioletthrough,increasingtheriskofskincancer.
In1985,aninternationalagreementcalledtheMontreal
Protocolregulated,thenbanned,theuseofCFCs-
themainchemicalsinvolvedinthedepletion.In2019,
NASAreporteda20%decreaseinozonedepletion.
Whilethereisstillworktobedone,thisisanexampleof
whatglobalcooperationonclimateissuescanachieve.
Figure15.2:AmodernversionofHerschel'sexperiment.
ThespectrumoflightfromtheSunextendsbeyondthe
visibleregion,frominfraredtoultraviolet.Thethermometer
placedbeyondtheredlightdetectsmorethermalradiation
thananyoftheothers.
Figure15.3:TheSunisexaminedbyseveralsatellite
observatories.ThisimagewasproducedbytheSOHO
satelliteusingacamerathatdetectstheultravioletradiation
givenoffbytheSun.YoucanseesomedetailoftheSun's
surface,includinggiantprominencesloopingoutintospace.
Thedifferentcoloursindicatevariationsinthetemperature
acrosstheSun'ssurface.
Bothinfraredandultravioletradiationwerediscovered
bylookingatthespectrumoflightfromtheSun.
However,theydonothavetobeproducedbyanobject
liketheSun.Imaginealumpofironthatyouheatin
aBunsenflame.Atfirst,itlooksdullandblack.Take
itfromtheflameandyouwillfindthatitisemitting
infraredradiation.Putitbackintheflameandheatit
more.Itbeginstoglow,firstadullredcolour,thenmore
yellow,andeventuallywhitehot.Itisemittingvisible
light.Whenitstemperaturereachesabout1000°C,itwill
alsobeemittingultravioletradiation.
Thisexperimentsuggeststhatthereisaconnection
betweeninfrared,visibleandultravioletradiation.A
coolobjectemitsonlyradiationatthecoolendofthe
spectrum.Thehottertheobject,themoreradiationit
emitsfromthehotterend.
Beyondthevioletendof
thespectrum
Thediscoveryofradiationbeyondtheredendofthe
spectrumencouragedpeopletolookbeyondtheviolet
end.In1801,aGermanscientistcalledJohanRitterused
silverchloridetolookfor‘invisiblerays’.Silversaltsare
blackenedbyexposuretosunlight(thisisthebasisof
filmphotography).Ritterdirectedaspectrumofsunlight
ontopapersoakedinsilverchloridesolution.Thepaper
becameblackenedand,tohissurprise,theeffectwas
strongestbeyondthevioletendofthevisiblespectrum.He
haddiscoveredultravioletradiation(‘ultra’meansbeyond).
waves
WilliamHerschelwasanastronomer.In1799hewas
investigatingthespectrumoflightfromtheSun.Heused
prismtocreateaspectrumthenplacedathermometerat
differentpartsofthespectrum.Thethermometerreading
increasedwhenthelightfellonit.Thelightenergywas
absorbed,creatingaheatingeffect.Herschelnoticedthis
effectwasgreatestforredlightandsmallestforviolet.
IIcrschelthenplacedhisthermometerpasttheredend
ofthespectrum.Thetemperaturerisewasevengreater.
Iicconcludedthattheremustbesometypeofradiation,
invisibletothehumaneye,beyondtheredendofthe
ipectrum.Hecalledthisinfraredradiation(‘infra’means
below).Infraredradiationisthethermalradiation
describedinChapter11.Itistheheatyoucanfeel
radiatingoutfromanyhotobject.

293. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Electromagneticwaves
Wehaveseenthataspectrumisformedwhenlightpasses
throughaprismbecausesomecoloursarerefractedmore
thanothers.Thevioletendofthespectrumisrefracted
themost.Nowwecandeducethatultravioletradiation
isrefractedevenmorethanvioletlight,andthatinfrared
radiationisrefractedlessthanredlight.
Toexplainthespectrum,andotherfeaturesoflight,
physicistsdevelopedthewavemodeloflight.
Justassoundcanbethoughtofasvibrationsorwaves
travellingthroughtheair(oranyothermaterial),so
wecanthinkoflightasbeinganotherformofwave.
Soundscanhavedifferentpitches-thehigherthe
frequency,thehigherthepitch.Wecanthinkofapiano
keyboardasbeinga‘spectrum’ofsoundsofdifferent
frequencies.Lightcanhavedifferentcolours,according
toitsfrequency.Redlighthasalowerfrequencythan
violetlight.Visiblelightoccursasaspectrumofcolours,
dependingonitsfrequency.
AScottishphysicist,JamesClerkMaxwell,described
lightassmalloscillationsinelectricandmagneticfields
whichhecalledelectromagneticwaves.Histheory
allowedhimtopredictthattheycouldhaveanyvalue
offrequency.Inotherwords,beyondtheinfraredand
ultravioletregionsofthespectrum,theremustbeeven
moretypesofelectromagneticwave(orelectromagnetic
radiation).Bytheearlyyearsofthe20thcentury,
physicistshaddiscoveredorartificiallyproducedseveral
othertypesofelectromagneticwavetocompletethe
electromagneticspectrum.
Theelectromagneticspectrumisafamilyoftransverse
waves.Likeallwaves,theycanbereflected,refracted
ordiffracted.Theyalltravelatthesamespeedaslight
inavacuum.Thewaveshavedifferentfrequencies,and
thismeanstheyhavedifferenteffectsonthematerials
withwhichtheyinteract.Thesewaveshavemanyvery
importantuses,andsomecanbehazardous.
Thespeedofelectromagnetic
waves
Alltypesofelectromagneticwavehaveonethingin
common:theytravelatthesamespeedinavacuum.
Theytravelatthespeedoflight,whichhasavalue
closeto300000000m/s(3x108m/s)inavacuumand
approximatelythesamespeedinair.Likelight,the
speedofelectromagneticwavesdependsonthematerial
throughwhichtheyaretravelling.
Wavelengthandfrequency
Wecanrepresentlightasatransversewave.Figure15.4
comparesredlightwithvioletlight.Redlighthasagreater
wavelengththanvioletlight.Thismeansthatthereisa
greaterdistancefromonewavecresttothenext.Thisis
becausebothredlightandvioletlighttravelatthesame
speed,butvioletlighthasagreaterfrequency,soitgoesup
anddownmoreofteninthesamelength.
a wavelength
I4—*| lamp
wvwvw
b wavelength
Figure15.4:Comparingredandvioletlightwaves.
Bothtravelatthesamespeed,butredlighthasalonger
wavelengthbecauseitsfrequencyislower.Th£wavelengthis
thedistancefromonecresttothenext(orfromonetroughto
thenext).Thinkofredlightwavesaslonger,lazywaves.Violet
lightismadeupofshorter,morerapidlyvibratingwaves.
Thewavesthatmakeupvisiblelighthaveveryhigh
frequencies:overonehundredmillionmillionhertz,or
1014Hz.Theirwavelengthsareverysmall,from400nm
(nanometres)forvioletlightto700nmforredlight.
(1nmisone-billionthorone-thousand-millionthofa
metre,so400nm=—-—
m.)Morethanone
1000000000’
millionwavesofvisiblelightfitintoametre.
Figure15.5showsthecompleteelectromagnetic
spectrum,withthewavelengthsandfrequenciesof
eachregion.Infact,wecannotbeverypreciseabout
whereeachregionstartsandstops.Thisissimilartothe
lightspectrum:itishardtosaywhereredfinishesand
orangebegins.Eventheendsofthevisiblelightsection
areuncertain,becausedifferentpeoplecanseeslightly
differentrangesofwavelengths,justastheycanhear
differentrangesofsoundfrequencies.
KEYWORDS
ultravioletradiation:electromagneticradiation
withawavelengthshorterthanthatofvisiblelight
electromagneticspectrum:thefamilyof
radiationssimilartolight
280>

294. 15Theelectromagneticspectrum
increasingfrequency
Figure15.5:Theelectromagneticspectrum.Rememberallwavestravelatthesamespeedandbecausev=/A.Asfrequency
increases,wavelengthdecreases.
Questions
1Drawandlabeltwowavestoshowthedifference
betweenredlightandvioletlight.
2Writethenamesofthewavesintheelectromagnetic
spectrum:
ainorderofincreasingwavelength
binorderofincreasingfrequency.
3Describeandexplainwhathappenswhen
monochromaticgreenlightofwavelength540nm
passesthroughaprism.
4Usetheequationv=/Atocalculatethefrequency
ofthegreenlightinquestion15.3.
Usesofelectromagneticwaves
Thedifferentfrequenciesofthewavesinthe
electromagneticspectrumgivethemdifferentproperties
andthismeansthattheyareusefulindifferentways.
Herearesomeimportantexamples.
Radiowaves
Radiowavesareusedtobroadcastradioandtelevision
signals.Thesewavesaresentoutfromatransmitter
afewkilometresawaytobecapturedbyanaerialon
theroofofahouse.Radioastronomycanbeusedto
detectradiowavesfromobjectssuchasstars,galaxies
andblackholes.Radiofrequencyidentification(RFID)
chipsaremicrochipsinsertedunderneaththeskin.These
canstorevitalmedicalinformationandcouldbeused
insteadofapassport,orasacontactlessbankcard.
AnRFIDtagstoresdata.Ittransmitsthedataasradio
waves,whichisreadbyanelectronicreader.
Figure15.6:TinyRFIDtagscanstoreandtransmitdata.
Ataginsertedinyourbodycouldbeusedtoopendoors
orpayforshoppingtotrackforyourmovements.
281>

295. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Microwaves
Usedinsatellitetelevisionbroadcasting,because
microwavespasseasilythroughtheEarth’satmosphere.
Theytraveluptoabroadcastingsatellite,thousands
ofkilometresawayinspace.Thentheyaresentback
downtosubscribersonEarth.Microwavesarealsoused
totransmitmobilephone(cellphone)signalsbetween
masts,whichmaybeupto20kmapart.
Microwavesovensemitmicrowaves,whichareabsorbed
bymoleculesinfood,causingheating.
Infraredradiation
Infraredradiationisusedinremotecontrolsfordevices
suchastelevisions.Abeamofradiationfromtheremote
controlcarriesacodedsignaltotheappliance,which
changesthesettingsonthetelevision;forexample,
changingchannel,increasingthevolume.Youmaybe
abletouseasmartphonecameratoobservethistypeof
radiation,whichwouldotherwisebeinvisibletooureyes.
Pointaremotecontrolatyourphoneandrecordwhat
happensasyoupressbuttons.Grillsandtoastersalso
useinfraredradiationtocookfood.Securityalarmssend
outbeamsofinfraredradiationanddetectchangesin
thereflectedradiation,whichmayindicatethepresence
ofanintruder.Infraredredlightisalsousedtogether
withopticalfibres.InFigure15.9banendoscopeallows
thedoctortoseeinsidethepatient.Visiblelightispassed
downanopticalfibretoilluminatethelungsofthis
COVID-19patient.Reflectedlighttravelsbackupto
thedoctor’seye.Infraredradiationcanalsobeusedin
medicinetodetectheatwhichmayindicateinfection,or
tospeeduphealingandreducepain.
Visiblelight
Visiblelightprovidesuswithinformationaboutthe
world,bothdirectlythroughoureyesandviaoptical
instrumentssuchascameras,telescopesandmicroscopes.
Visiblelightisalsovitalforphotosynthesis.
Figure15.7:Microwavesallowsignalstobetransmitted
aroundthecurveoftheearth'ssurface.
Figure15.8:Usinganinfraredscannerallowsadoctorto
seeapatient'sveins.Thismeansinjectionscanbetargetted
muchmoreprecisely.
Figure15.9a:Thisleafhasusedvisiblelighttogrow.Itis
illuminatedbythemicroscopelamp,andthereflectedlight
isfocusedbythemicroscopelenses,b:Anendoscopeallows
thedoctorstoseeinsidethepatient.Visiblelightispassed
downanopticalfibretoilluminatethelungsofthisCOVID-19
patient.Reflectedlighttravelsbackuptothedoctor'seye.
282y

296. 15Theelectromagneticspectrum
Ultraviolet(UV)light
tJVlightcausessomechemicalstoemitvisiblelight.This
includesbodyfluidssuchassweatandsaliva.Forensic
'lienlistsusethistofindevidenceatcrimesceneswhich
r.invisibletothehumaneye.
(hemicalswhichglowinUVlightcanbeusedfor
'icuritymarkingofvaluableequipmentandbanknotes.
IJltravioletlightcanbeusedtosterilisewater.Exposure
(oUVradiationdestroysDNAwithinanybacteriaand
virusescontainedinthewater.Thismakesthebacteria
midvirusesharmlessandthewatermuchsafertouse.
Figure15.10:Manybanknoteshavemarkingswhichare
onlyvisibleinultravioletlight.Thishelpsinthedetectionof
forgedbanknotes.
X-rays
X-rayscanpenetratesolidmaterialsandsotheyare
usedinsecurityscannersatairports.Theyarealsoused
inhospitalsandclinicstoseeinsidepatientswithout
theneedforsurgery.TheX-raysaredetectedusing
electronicdetectors.BoneabsorbsX-raysmorestrongly
thanflesh,sobonesappearasashadowintheimage.
Similarly,ametalgunwillappearasashadowbecauseit
absorbsX-raysmorestronglythantheitemsarounditin
Figure15.11.
Figure15.11:X-rayscannersallowairportsecuritytoquickly
detecthiddenitems.
Gammarays
Gammarayscandamageorkilllivingcells.Atargeted
beamofgammarayscanbeusedtokillcancerouscells.
Surgicalinstrumentscanbesterilisedbyusinggamma
raystokillanybacteria.
Gammarayscanalsobeusedinthedetectionofcancer.
Youwilllearnmoreaboutgammaraysandhowtheyare
usedinChapter23.
Figure15.12:Anursedemonstratesaradiationhelmet.
Theholesinthehelmetallowthegammaraystobe
targetedexactlyontoatumour.
Questions
5Nametwotypesofelectromagneticradiationthat
areusedtocookfood.
6Namethreetypesofelectromagneticradiationthat
havemedicaluses.
283y

297. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
7Agirlphonesherfriendonhermobilephonewhile
watchingTVbyalogfire.Explainhowinfrared
radiation,microwaves,visiblelightandradiowaves
areinvolvedinthis.
15.2Electromagnetic
hazards
Alltypesofradiationcanbehazardous.Evenbright
lightshiningintoyoureyescanblindyou.Infrared
radiationcancauseburns.UVradiationfromtheSun
candamageskincellswhichmayleadtosunburnand
skincancer.Sunbedscanalsodamageskincellsand
shouldbeusedwithcare.UVradiationcanalsodamage
eyecellswhichiswhyisimportanttoprotectyoureyes
inbrightsunlightbywearingsunglassesorahat.In
general,thehigherthefrequencyofthewavelength,the
greatertheharmitcancause.
X-raysandgammaraysarethemostdangerouspart
oftheelectromagneticspectrum.Theycancausecell
mutationswhichmayleadtocancer.Peoplewhowork
withX-raysandgammaraysareindangerofbeing
exposedtotoomuchradiation.Theycanprotect
themselvesbystandingwellawaywhenapatientisbeing
examined,orbyenclosingtheequipmentinametalcase,
whichwillabsorbrays.
Longerwavessuchasmicrowavesaremuchlessharmful,
butasweusethemalotmorefrequentlyineveryday
life,weareexposedtomuchlargeramountsofthese
waves.Microwavesareusedtocookfoodinmicrowave
ovens.Thisshowsthattheyhaveaheatingeffectwhen
absorbed.Telephoneengineers,forexample,musttake
carenottoexposethemselvestoexcessiveamountsof
microwaveswhentheyareworkingonthemastsofa
mobilephone(cellphone)network.Domesticmicrowave
ovensmustbecheckedtoensurethatnoradiationis
leakingout.
Mobilephonesuseradiowavesandmicrowaves.Many
peopleareconcernedthatthesecouldbeharmful.
Scientistshaveresearchedthisandtheonlyeffectthey
havefoundconsistentevidenceforisaslightheating
effect.Thisisnotbelievedtobeharmful.Ifanyrisksare
found,theywouldhavemoreeffectonchildren,asthey
arestilldeveloping.Anyharmwouldbeincreasedby
usingthephoneforlonger.
Figure15.13a:Theradiographeroperatesthemachine
fromaseparateroom.
Figure15.13b:AllareaswhereX-raysorgammaraysare
usedhavehazardwarninglabels.
Questions
8Theradiographermustleavetheroombefore
operatinganX-raymachine.Whyisitconsidered
toodangerousfortheradiographerbutnotforthe
patient?
9Aheadlinereads,‘Scientistsprovemobilephonesare
safe’.Explainwhythisheadlineismisleading.
284>

298. ACTIVITY15.1
15Theelectromagneticspectrum
15.3Communicating
usingelectromagnetic
waves
Satellites
SatellitesareobjectswhichorbittheEarth.Earth
hasonenaturalsatellite-theMoon.Earthalsohas
manyartificialsatellites,manyofwhichareusedinthe
transmissionofinformationcarriedbyelectromagnetic
waves.Mostartificialsatellitecommunicationuses
microwaves.
Someofthesatellitesusedforcommunicationarein
geostationaryorbits.Thismeansthattheyorbitatthe
samerateastheEarthturnsandsotheystayaboveone
pointontheEarth’ssurface.Theyareabout35000km
abovetheEarth’ssurfaceandarepositionedabovethe
Equator.Thesesatellitesarepowerfulandcantransmit
largeamountsofdata.Thismakesthemsuitablefor
satellitetelevisionandsomesatellitephones.Thewaves
travelalongdistancetothesatellite,meaningthere
isaslightdelay,makingitmoredifficulttohavea
conversation.
LowEarthorbitsaremuchcloser.Theycanbeaslowas
2000kmabovetheEarth’ssurface.Thismeansthereis
nodelayinconversation.ThesesatellitesorbittheEarth
inaslittleastwohours.Alotmoreoftheseareneeded
thanforgeostationaryorbitsastheyonlycoverasmall
areaoftheEarth’ssurface(seeFigure15.15).They
cannottransmitdataasfastasgeostationarysatellites
andsoarenotsuitablefortelevisiontransmission.
Geostationary\
satellitesstay
aboveonepoint
ontheEarth's
surfaceandcover
alargearea.
LowEarthorbitsatellites
movearoundtheEarth.
Theycoveramuch
smallerarea.
Figure15.15:LowEarthorbitsrequiremanymoresatellites.
285y

299. CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Therightwaveforthejob
Mobilephonesandwirelessinternetusemicrowaves,
becausetheycanpassthroughmostwallsandonlya
smallaerialisneeded.
Figure15.16:Thislaptopisconnectedtotheinternetusing
microwaves.
Bluetooth-usedforshortrangecommunication-uses
radiowaves.Thesignalisweakenedwhenitpasses
throughwalls.Thisshortrangesignalisusefulfor
communicationbetweenelectronicdevicessuchasin
hands-freemobilephonesystems.
Figure15.17:Awomanwearingabluetoothheadset.
Opticalfibres(forcabletelevisionandhighspeed
internet)useinfraredradiationandvisiblelight.Optical
fibresaremadeofglasswhichistransparenttolightand
infraredradiation.Thesewaveshaveahigherfrequency
andcancarrymoredata.Thisisvitalforhighspeed
broadbandconnections.
Figure15.18:Opticalfibres.
Analogueanddigitalsignals
Intelephonecommunication,thesignalbeing
transmittedstartsoffasasoundwave.This(variesin
amplitudeandfrequencyasthepeopletalk.Asound
waveisananaloguesignal
-itcanvarycontinuously.
Intraditionaltelephones,thesoundwaveisconverted
byamicrophonetoanelectricalsignal,x^hich
varies
inthesamewayasthesoundwave.Theelectrical
signalistransmittedviacopperwirestoareceiver,
whereaspeakerconvertsitbacktoasoundwave.The
transmissionprocessforanaloguesignalscausesthewave
tobecomedistortedandunwantedvibrationsareoften
heardasnoise.Thismeansthesignalisnotalwaysclear.
Adigitalsystemisasequenceofpulses.Digitalsignalsare
eitheronoroff.Digitalsystemsusitrgopticalfibresgivea
muchclearersignal.Theyarealsofasterandopticalcables
cancarrymuchmoredatathanacoppercables.
Figure15.19:Ananaloguesignalvariescontinuouslywhile
adigitalsignaliseitheronoroff.
KEYWORDS
analoguesignal:asignalwhichvaries
continuouslyinfrequencyandamplitude
digitalsignal,asignalthatconsistsofaseriesof
pulseswhichareeitheronoroff
286y

300. 15Theelectromagneticspectrum
ADC
DAC
'clean'
regenerator
out
Guesswho?
ACTIVITY15.2
Workinagroupofaboutfourstudents.Writethe
namesofdifferentwavesonidenticalcards(radio
laser
diode
digitalto
analogue
converter
(decoder)
photo¬
diode
boostthesignalifithaslostpower.Asecondconverter
switchesthesignalbacktoananaloguesignalwhichcan
beconvertedtoasoundwave.
Digitalsignalscantransmitdatamuchmorerapidlyand
accuratelythananaloguesignals.Digitalsignalscanalso
communicatedirectlywithcomputerswhichonlyuse
digitaldata.
pulsedbeamoflight
(orinfrared)
'noisy'
pulse
in
Figure15.20:Theuseofregeneratorsmeansthatdigitalsignalscanbetransmittedoverhundredsofkilometreswithout
beingdistorted.
waves,microwaves,infrared,visiblelight,ultra
violet,X-rays,gammarays,redlight,violetlight,
sound,ultrasound,waterandseismicwaves).
Placethecardsfacedowninthemiddle.One
playertakesacardandlooksatitwithoutletting
theotherssee.Theotherplayersthentryto,find
outwhatthewaveisbyaskingquestionstowhich
theanswercanonlybe'yes'or'no'.Eachplayercan
onlyaskonequestion.
Whenthewavehasbeenidentified,thenextplayer
takestheirturn.
Makingadigitalphonecall
Theanaloguesoundwavefromacallerisencodedby
aconverterwhichturnsthesoundwavetoaseriesof
pulses.Thedigitalsignaliscarriedalongopticalfibres
byvisiblepulsesorinfraredwaves.Thesignalpasses
throughoneormoreregeneratorswhichcleanupthe
signal,removinganydistortion.Theregeneratorsalso
amplifierloudspeaker
(encoder)^
. perhaps
microphone analogue
todigital
converter
regenerator
(repeater)
cleansupthe
signal

301. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
REFLECTION
Therearetwospectrawhichyouneedto
rememberinorder:visiblelightandthe
electromagneticspectrum.Howeasilycanyou
rememberthem?Whathelpsyouwiththis?
Mnemonicscanbeveryusefulhere.Thecolours
ofthevisiblespectrumcanberememberedusing
RichardOfYorkGaveBattleInVain.Thefirst
lettersofthewordscorrespondtothefirst
lettersofthecolours.Trytomakeamnemonic
toremembertheorderofthewavesinthe
electromagneticspectrum.
Questions
10Therearetwotypesofcommunicationsatellite:
geostationaryandlowearthorbit.Explainwhich
type:
aisbestsuitedtotransmittingTVsignals
bgivestheshortestdelayforphoneconversations
ctakesleasttimetoorbittheEarth.
11Statetwobenefitsofconvertingaphonesignalfrom
analoguetodigital.
radiationare
PROJECT
Theballooncanonlymanagesixwaves,andsooneof
thewavesmustbethrownoverboard,ortheballoon
willcrash.Asaclassyouwilldebatewhichwave
shouldbesacrificed.Eachwavewillhaveateamwho
Ultravioletradiation
X-rays
Gammarays
Rainbowballoondebate
Balloondebatesusuallyinvolvediscussingwhich
personshouldbethrownoutofanoverloadedhot
airballoonbasket,sothatitdoesnotcrash.Each
passengerexplainswhytheyshouldstayandthena
voteistaken.Inthisactivity,thepassengersarethe
differentwavesoftheelectromagneticspectrum.
preparetospeakinfavouroftheirwaveandlookat
argumentswhytheotherwavesshouldgo.
Youwillneedtodividetheclassintose^enteams(you
couldhavefewerteamsandleavesomewavesoutif
necessary).Yourteacherwillassigneachteamawave.
Youwillneedtoprepareargumentswhyyour
givenwaveissoimportantthatitmuststayinthe
balloon.Thinkaboutwhatitisusedfor-medicalor
communicationusesforexample-andwhylosing
thiswouldbebad.Thinkabouttheharmfulaspects
ofyourwave-youwillhavetoanswerquestions
aboutthese,sobepreparedtodefendyourwave.
Youwillalsoneedtopreparequestionsforthe
otherwaves.Youraimistomakethemseemeither
harmfulornotparticularlyusefulsothattheyare
morelikelytobevotedoff.
Yourteacherwilltellyouhowlongeachteamcan
speakforandhowlongthequestionportionwilllast.
Sevendifferentwavelengthsof
travellinginahotairballoon.
Radiowaves
Microwaves
Infraredradiation
Visiblelight

302. 15Theelectromagneticspectrum
PEERASSESSMENT
Aftereveryteamhasspokenandhasbeen
questioned,youwillallvoteforwhichwavemustgo.
Tohelpwiththis,recordyourimpressionsofeach
grouponasheetsimilartothis.
Useyourscoringandnotestodecidehowyou
willvote.
Groupname
Wave
Initialspeech ©©©
Answeringquestions©@©
Notes
SUMMARY
Theelectromagneticspectrumisagroupofwaveswhichhavesimilarpropertiestolight.
Inorderofincreasingfrequency,thewavesintheelectromagneticspectrumare:radiowaves,microwaves,
infrared,visiblelight,ultraviolet,X-raysandgammarays.
Allelectromagneticwavestravelatthesamespeed-thespeedoflight.
Thespeedoflightis300000000m/s.
Electromagneticwaveshavedifferentwavelengthsandthisgivesthemdifferentpropertiesandmakesthem
suitablefordifferentuses.
Highfrequencyelectromagneticradiationcanbehazardous.Itcandamagecellsandcausecellstomutate.
Radiowaves,microwaves,visiblelightandinfraredareusedincommunicationsystems.
Signalscanbeanalogueordigital.Digitalsignalstransmitdatamoreaccuratelyandfaster.

303. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
EXAM-STYLEQUESTIONS
1Whichstatementiscorrectwhencomparinggammaraysandmicrowaves?[1]
AMicrowaveshaveahigherspeedinairandthesamefrequency.
BMicrowaveshaveahigherfrequencyandthesamewavelength.
CMicrowaveshavealongerwavelengthandthesamespeed.
DMicrowaveshaveahigherfrequencyandthesamespeed.
2Thisispartoftheelectromagneticspectrum:
WhichrowcorrectlydescribesthelabelsP,QandR? [1]
radiowavesmicrowavesP
visible
light
ultravioletQ
gamma
rays
IncreasingR
P Q R
X-rays infraredradiation wavelength
infraredradiation X-rays frequency
X-rays infraredradiation frequency
infraredradiation X-rays wavelength
3Whichofthefollowingstatementsistrueforelectromagneticwaves? [1] \
ATheyallhavethesamefrequency.
BTheycannotberefracted.
CTheyalltravelatthesamespeed.
DTheyareharmless.
4Thisdiagramshowswhitelightbeingdispersedbyaprism.Thedispersedlight
hitsascreen.
aWhatwillbeseenatB? [1]
bWhatwillbeseenatC? [1]
290
>

304. 15Theelectromagneticspectrum
5Thesearesomewavesontheelectromagneticspectrum:
CONTINUED
WhatmightbedetectedatA? [1]
WhatmightbedetectedatD? [1]
StateonewayinwhichthewavesatA,B,CandDarealike. [1]
Stateonewayinwhichthefourwavesdifferfromeachother. [1]
COMMAND WORD
state:expressin
clearterms
[Total:6]

305. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
SELF-EVALUATIONCHECKLIST
Afterstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutosee
anygapsinyourknowledgeandhelpyoutolearnmoreeffectively.
1can
See
Topic...
Needs
morework
Almost
there
Confident
tomoveon
Statethespeedofelectromagneticwavesinavacuum. 15.1
Describethepropertiesallelectromagneticwaves
share,includingspeed.
15.1
Listthewavesintheelectromagneticspectrumin
orderofincreasingfrequencyorwavelength.
15.1
Describehowradiowaves,microwaves,visiblelight
andinfraredareusedincommunications.
15.3
DescribehowX-raysareusedinmedicineandsecurity.15.2
1
Describehowgammarayscanbeharmful. 15.2
Considerthesafetyissueswhichmayarisewhen
usingmicrowaves.
15.2
Describetheadvantagesofusingdigitalsignals. 15.3
292
>

306. >Chapter16
Magnetism
INTHISCHAPTERYOUWILL:
describemagneticforcesbetweenmagnets,andbetweenmagnetsandmagneticmaterials
distinguishbetweenhardandsoftmagneticmaterials,andnon-magneticmaterials
describeanexperimenttoidentifythepatternofmagneticfieldlinesaroundabarmagnet
distinguishbetweenthedesignanduseofpermanentmagnetsandelectromagnets
usemagneticfieldpatternsbetweenmagnetstoexplaintheforcesbetweenthemandunderstandhow
toworkouttherelativestrengthofamagneticfield.

307. CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
GETTINGSTARTED
Spendtwominutesproducingamindmapthat Whichelementsaremagnetic?
answersthefollowingquestionsbeforecomparing ,.., ,
r
noteswithyourneighbourforafurthertwominutes,'
Describe
whatJlfe
onEarthwouldbelikeif
addingorcorrectingyourownwork.Bepreparedto
magnetismdidnotexist.Thsmightinclude
shareyourthoughtswiththeclass.
inventionsthatdependonmagnetism.
FLIPPINGFIELDS
AGermanpolarresearchercalledAlfredWegener
(Figure16.1)suggestedthatcontinentslikeSouth
AmericaandAfricausedtofittogetherlikeajigsaw
andthepieceshavedriftedapart.Fewpeople
believedhimbutlaterscientistsprovedhimright.
ChangesintheEarth'smagneticfieldhelpedprove
theEarth'ssurfaceismadeofplatesthatmoveina
processcalledplatetectonics.
Figure16.1:AlfredWegener,themanwhosetheoryof
continentaldriftwasprovedrightbyphysics.
TheEarthhasamagneticfield,asifagiantbar
magnetsitsalongitsaxis(Figure16.2).Butwhere
doesthisfieldcomefrom?TheEarth'scoreismade
ofiron.Likeallmetals,ironcontainsdelocalised
electrons,whicharefreetomove.Thespinof
theEarth,andconvectioncurrentsintheouter
coremovetheseelectrons,whichcreateselectric
currents.Theseelectriccurrentscreatemagnetic
fieldsjustasinelectromagnets,whichyouwill
meetlaterinthechapter.Whilemanyanimalsuse
theEarth'smagneticfieldfornavigation,wehave
mostlyreplacedthecompasswiththeGPS.
Figure16.2:ANASAimageofwhattheEarth'smagnetic
fieldlookslike.
Molten(liquid)rockeruptingatmid-oceanridges
pushesthetectonicplatesapartataboutthe
samespeedasourfingernailsgrow(Figure16.3).
Grainsofmagnetiteinthemoltenrockactlikelittle
compasses,whicharefreetochangedirection.As
therockcools,theselittlecompasseslineupwith
theEarth'smagneticfieldandbecomefixedin
placewhentherockscrystallise(freeze).Theyshow
thattheEarth'smagneticfieldhasreversedinthe
past.Themagneticpatternissymmetrical(amirror
imageoneachsideofthemid-oceanridge),which
showsthatthecontinentswereoncestucktogether,
provingWegenerright.
294
>

308. 16Magnetism
0
1f
2f
3
4
Discussionquestions
2
3
CONTINUED KEYWORDS
magnetic
stripes
mid-ocean
ridge
barmagnet:arectangular-shapedpermanent
magnetwithanorthpoleatoneendandasouth
poleattheother
Figure16.5a:Twolikemagneticpolesrepeloneanother,
b:Twounlikemagneticpolesattracteachother.
TheEarth'smagneticfieldactsasashield,
protectingouratmosphereandlifeonEarthby
deflectingtheharmfulsolarwind(astreamof
chargedparticlesfromtheSun).Italsoprotects
alltheelectriccircuitsdevelopedoverthelast
century.Intheprocessofreversing,thefield
hastodecreasebeforeincreasingagaininthe
oppositedirection.Somescientistsbelievethat
animalspeciescanbecomeextinctduringthis
reversalprocess,whichhashappenedroughly
onceevery300000yearsinthelast20million
years.Thelastreversalwas780000yearsago.
Figure16.4:Afreelysuspendedmagnetturnssothatit
pointsnorth-south.
Figure16.3:Themagneticstripesontheoceanfloor
thatshowthattheEarth'smagneticfieldhasreversed
onceevery300000yearsonaverage.
DescribewhatcausestheEarth'smagnetism
inlessthan20words.
InwhatwaysistheEarth'smagneticfield
beneficial?
Shouldwebeconcernedaboutmagnetic
reversals?
16.1Permanentmagnets
Acompassneedleislikeabarmagnet.Whenitisfree
torotate(Figure16.4),itturnstopointnorth-south.
Oneendpointsnorth-thisisthemagnet’snorth
pole,pointingroughlyinthedirectionoftheEarth’s
geographicalNorthPole.Theotherendisthemagnet’s
southpole.Sometimes,thenorthandsouthpolesof
amagnetarecalledthenorth-seekingpoleandsouth¬
seekingpole,respectively.
Whentwomagnetsarebroughtclosetogether,thereisa
forcebetweenthem.Thenorthpole(Npole)ofonewill
attractthesouthpole(Spole)oftheother.Twonorth
poleswillrepeleachother,andtwosouthpoleswillrepel
eachother(Figure16.5).Thisissummarisedas:
likepolesrepel
unlikepolesattract.
‘Likepoles’meanspolesthatarethesame-bothnorth,
orbothsouth.‘Unlikepoles’meansoppositepoles-one
northandtheothersouth.Peopleoftenrememberthis
rulemoresimplyas‘oppositesattract’.
295y

309. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Sincethenorthpoleofthecompassneedleisattracted
totheEarth’sNorthPole,itfollowsthattheremustbe
amagneticsouthpoleupthere,undertheArcticice.
Itiseasytogetconfusedaboutthis.Infact,foralong
time,mediaevalscientiststhoughtthatcompassneedles
wereattractedtothePoleStar.Eventually,anEnglish
instrument-makercalledRobertNormannoticedthat,
ifhebalancedacompassneedleverycarefullyatits
midpoint,ittilteddownwardsslightly,pointingintothe
Earth.NowweknowthattheEarthitselfismagnetised,
ratherasiftherewasagiantbarmagnetinsideit.
Magneticmaterials
Magneticmaterialsareattractedbyamagnetandcan
bemagnetised.Thoughtheycanbemagnetised,notall
piecesofmagneticmaterialaremagnets.Theyfirstneed
tobemagnetised.Incontrast,non-magneticmaterials
arenotattractedbyamagnetandcannotbemagnetised.
Examplesincludeplasticandrubber.
Acompassneedleisapermanentmagnet.Likemany
barmagnets,itismadeofhardsteel.Youhaveprobably
comeacrossanothertypeofmagneticmaterial,called
ferrite.Thisisaceramicmaterialusedformaking
fridgemagnetsandthemagnetssometimesusedtokeep
cupboarddoorsshut.Therearealsosmallrare-earth
magnetsintheheadphonesusedwithmobilephones,
whicharebasedonelementssuchasneodymium.
Mostmagneticmaterials(includingsteelandferrite)
containiron,themostcommonmagneticelement.
Forthisreason,theyareknownasferrousmaterials
(fromtheLatinwordferrummeaning‘iron’).Other
magneticelementsincludecobaltandnickel.(Ifa
materialcontainsiron,thisisnotaguaranteethatitwill
bemagnetic.Stainlesssteelcontainsalotofiron,but
magnetswillnotalwayssticktoit.)
Magneticmaterialsmaybeclassifiedashard(permanent)
orsoft(temporary).Table16.1summarisesthe
difference.Asoftmagneticmaterialsuchassoftironcan
bemagnetisedanddemagnetisedeasily.
KEYWORDS
permanentmagnetmagnetisedmagnetic
materialthatproducesitsownmagneticfieldthat
doesnotgetweakerwithtime
hard(material):amaterialthat,oncemagnetised,
isdifficulttodemagnetise
soft(material):amaterialthat,oncemagnetised,
iseasytodemagnetise
i
Inducedmagnetism
Abarmagnetisanexampleofapermanentmagnet.
Itcanremainmagnetised.Itsmagnetism(floesnot
disappear.Permanentmagnetsaremadeofhard
'
magneticmaterials.
Apermanentmagnetcanattractorrepelanother
permanentmagnet.Itcanalsoattractother
unmagnetisedmagneticmaterials.Forexample,abar
magnetcanattractsteelpinsorpaperclips,andafridge
magnetcansticktothesteeldoorofthefridge.
Whatisgoingonhere?Steelpinsaremadeofamagnetic
material.Whenthenorthpoleofapermanentmagnet
isbroughtclosetoapin,thepinisattracted(seeFigure
16.6).Theattractiontellsusthattheendofthepin
nearestthemagneticpolemustbeamagneticsouth
pole,asshowninFigure16.6.Thisisknownasinduced
magnetism.Thetwoobjectswillhaveoppositepolarities.
Inthisexamplethepinwillhaveasouthpoleinducedin
Table16.1:Hardandsoftmagneticmaterials.Hardsteelisbothhardtobendanddifficulttomagnetiseanddemagnetise.
Softironisbotheasiertobendandeasiertomagnetiseanddemagnetise.
Typeofmagnetic
material
Description Examples
"" '
Uses
hard
retainsmagnetismwell,but
difficulttomagnetiseinthefirst
place
hardsteel
permanentmagnets,compass
needles,loudspeakermagnets
soft
easytomagnetise,butreadily
losesitsmagnetism softiron
coresforelectromagnets(discussed
laterinthechapter),transformers
andradioaerials
296y

310. 16Magnetism
theendnearestthenorthpoleofabarmagnet.When
thepermanentmagnetisremoved,thepinwillreturnto
itsunmagnetisedstate(oritmayretainasmallamount
ofmagnetism).
KEYWORDS
magnetised:whenamagneticmaterialhasbeen
mademagnetic
unmagnetised:whenamagneticmaterialhasnot
beenmademagnetic
inducedmagnetism:whenamagneticmaterialis
onlymagnetisedwhenplacedinamagneticfield
(forexample,whenbroughtclosetothepoleofa
permanentmagnet)
N
Figure16.6:Asteelpinistemporarilymagnetisedwhena
permanentmagnetisbroughtclosetoit.
Questions
1Namethreemagneticelements.
2Whatistheruleaboutwhethermagneticpolesrepel
orattract?
3CopyandcompleteTable16.2.
Table16.2
Typeof
magnetic
material
DescriptionExamplesUses
Hard
Soft
4Whyisapermanentmagnetmadeofsteelrather
thaniron?
16.2Magneticfields
Amagnetaffectsanypieceofmagneticmaterialthatis
nearby.Wesaythatthereisamagneticfieldaroundthe
magnet.Youhaveprobablydoneexperimentswithiron
filingsorsmallcompassestoillustratethemagneticfield
ofamagnet.Figure16.7showsthefieldofabarmagnet
asrevealedbyironfilings.
Figure16.7:Themagneticfieldpatternofabarmagnet
isillustratedbyironfilings.Theironfilingsclustermost
stronglyaroundthetwopolesofthemagnet.Thisiswhere
thefieldisstrongest.
Figure16.8:Fieldlinesareusedtorepresentthemagnetic
fieldaroundabarmagnet.
Figure16.8showshowwerepresentthemagneticfieldof
asinglebarmagnetusingmagneticfieldlines.Ofcourse,
thefieldfillsallthespacearoundthemagnet,butwecan
onlydrawaselectionoftypicallinestorepresentit.The
patterntellsustwothingsaboutthefield:
Direction:thedirectionofamagneticfieldlineat
anypointisthedirectionoftheforceonthenorth
poleofamagnetatthatpoint.Weuseaconvention
297>

311. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
thatsaysthatfieldlinescomeoutofnorthpolesand
gointosouthpoles.
Strength:linesthatareclosetogetherindicatea
strongfield.
KEYWORDS
magneticfield:aregionofspacearounda
magnetorelectriccurrentinwhichamagnetic
poleexperiences(feels)aforce
magneticfieldlines:representthedirection
themagneticforcewouldhaveonthenorthpole
ofamagnet
plottingcompass:verysmallcompasswitha
needlethatlinesupwithmagneticfieldlines,
allowingchangesinfielddirectiontobeobserved
andplottedoveraveryshortdistance
Plottingfieldlines
Ironfilingscanillustratethepatternofthemagneticfield
aroundamagnet.Placeamagnetunderastiffsheetof
plainpaperor(preferably)clearplastic.Sprinklefilings
overthepaperorplastic.Tapthepaperorplasticto
allowthefilingstomoveslightlysothattheylineupin
thefield.Youshouldobtainapatternsimilartothat
showninFigure16.7.
Analternativemethodofdoingthisusesasmall
compasscalledaplottingcompass.Whenaplotting
compassisplacedinamagneticfield,itsneedleturns
toindicatethedirectionofthefield.Activity16.1
describeshowtouseaplottingcompasstoshowthe
patternofamagneticfield.Whendrawingmagnetic
fieldpatterns,thefieldlinesshouldnevercrossandthey
shouldincludearrows,pointingfrommagneticnorthto
magneticsouth.
ACTIVITY16.1
Plottingfieldlines
Inpairs,designanexperimenttoplotafieldpattern
aroundabarmagnet.Createaworksheetwhich
givesinstructionsonhowtodotheexperiment.
Thereshouldbeatleasttenfieldlines(includingat
leasttwofromeachsideofthemagnet).Inaddition
tothebarmagnet,theonlypiecesofequipment
neededfortheexperimentareaplottingcompass,
apencilandasheetofplainpaper.
Inyourworksheetremembertoinclude:
howacompassworksandhowitcanreveal
magneticfieldlines
aclearstep-by-stepmethodforplottingthe
fieldlines(includinghelpfulsketches).
Youcouldprovideanoptionalextensiontasksuch
as:plotthefieldlinesbetweentwobarmagnets
thatareattractingorrepelling.(Inthiscase,make
sureyousuggestawaytopreventthemagnets
frommoving.)
Youcouldalsoproduceaveryshortpodcast
(maximumtwominutes)todemonstratehowtodo
theexperiment.
Afteryouhavecreatedyourworksheet,youwill
havetheopportunitytolookattheworksheets
writtenbyotherpairs.Youshouldnoteanyphysics
youhavelearned,andanyideasformakingyour
ownworksheetorpresentationclearerormore
engaginginthefuture.
PEERASSESSMENT
Providefeedbackontheworksheetsproducedby
otherpairs.Asyougivefeedbackontheworksheets,
thinkaboutthesequestions:
Isthephysicsofahowacompassworks
explainedclearly?
Couldyoufollowthestepsinthemethod?
Areanystepsmissingorunclear?
Discussanyimprovementsthatcouldbemade.Each
pairshouldnotedownthefeedbackand,ifthereis
time,makeanyimprovements.
298>

312. 16Magnetism
Questions
5aSketchthefieldpatternaroundabar
magnet.
bSomeonehasdrawnthefieldpatternbutforgot
tolabelthepoles.Dothearrowspointtowards
orawayfromthemagneticnorthpole?
cWhenlookingatafieldpattern,howwouldyou
knowwherethemagneticfieldisstrongest?
dWhereisthefieldstrongestonabar
magnet?
6Figure16.9showstheEarth’smagneticfield.
geographic
Figure16.9:TheEarth'smagneticfield.
Acompassneedleinsideacompassisalightweight
barmagnetsuspendedatitscentreofgravityontop
ofatinyverticalcolumn.
aWhatwillthecompassneedlelineupwithwhen
itissuspended(heldatapoint)sothatitcan
movefreely?
bWhatmagneticpoleisatthegeographical
NorthPolewhenthenorthpoleofthecompass
needlepointsinthatdirection?
cIsthemagneticfieldstrongerattheEquator
oratthegeographicNorthPole?Explain
youranswer.
dWhyisacompassverydifficulttousenearto
thenorthmagneticpole?
Interactingmagneticfields
a
b
Figure16.10a:Theattractionbetweentwoopposite
magneticpolesshowsupintheirfieldpattern,b:Thefield
patternfortwolikepolesrepellingeachother.
Wecanshowthefieldpatternsfortwomagnetsattracting
(Figure16.10a)andrepelling(Figure16.10b)eachother.
Noticethatthereisapointbetweenthetworepelling
magnetswherethereisnomagneticfield.
Whentwobarmagnetsareplacedclosetogethertheir
magneticfieldsinteract(affecteachother)andproduce
anewpatternofmagneticlinesofforce.Fromthese
patterns,itispossibletosaywhetherthemagnetswill
attractorrepel.
Noticethat,inFigure16.10b,thefieldlinesfrom
eachmagnetbetweenthesouthpolesareinthesame
direction.Thefieldlinesinsidetheboxareallpointing
downwardsandareclosetogether,sothefieldthereis
strongerthanthefieldatthenorthpoles.Themagnets
willmovefromwherethefieldisstrongertowhereitis
weaker-solikepolesrepel.Asimilarargumentcanbe
usedtoexplainwhyunlikepolesattract.Activity16.2
isanalternativemodeltoexplainmagneticattraction
andrepulsion.
299>

313. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
ACTIVITY16.2
Animaginarymodeltoexplainmagnetic
attractionandrepulsion
Sometimesithelpstohaveanimaginarymodelto
understandthephysicswhichcausessomethingto
happen,forexample,tounderstandthephysicsin
magneticattraction.
Imaginethatmagneticfieldlinesarelikestretched
elasticbandsthatwanttopullthetwoendscloser
together.Imaginealsothatthereispressure
betweenthefieldlinespushingthemapartand
that,thecloserthelines,thebiggerthispressureis.
Sketchthefieldlinesbetweenandaround:
apairoflikepoles
apairofunlikepoles.
Usethisimaginarymodeltoexplaintheattraction
andrepulsion.
Ifyoucanvisualisewhatishappeningitshould
helpyoutorememberhowtosketchthepatterns.
REFLECTION
Modelsareanimportanttoolinphysics.
DidActivity16.2helpyoutovisualisewhat
isgoingonwhenmagnetsandmagnetised
objectsattractandrepel?
Didithelpyoutorememberhowtosketch
thefieldpatterns?
Canyousuggestabettermodel?
Electromagnets
Usingmagneticmaterialsisonlyonewayofmaking
amagnet.Analternativemethodistousean
electromagnet.Atypicalelectromagnetismadefroma
coilofcopperwire.Acoillikethisissometimescalleda
solenoid.Whenacurrentflowsthroughthewire,thereis
amagneticfieldaroundthecoil(Figure16.11).Copper
wireisoftenused,becauseofitslowresistance,though
othermetalswilldo.Thecoildoesnothavetobemade
fromamagneticmaterial.Thepointisthatitisthe
electriccurrentthatproducesthemagneticfield.
KEYWORDS
electromagnet:acoilofwirethatactsasa
magnetwhenanelectriccurrentpassesthroughit
solenoid:anelectromagnetmadebypassinga
currentthroughacoilofwire
Youcanseethatthemagneticfieldaroundasolenoid
(Figure16.11)issimilartothataroundabarmagnet
(Figure16.8).Oneendofthecoilisanorthpole,and
theotherendisasouthpole.InFigure16.11,the
fieldlinesemergefromtheleft-handend,sothisisthe
northpole.
Thereisnowaytochangethestrengthofapermanent
magnet,buttherearethreewaystoincreasethestrength
ofanelectromagnet:
Increasethecurrentflowingthroughit:thegreater
thecurrent,thegreaterthestrengthofthefield.
Increasethenumberofturnsofwir^onthe
coil:thisdoesnotmeanmakingthefoillonger,
butpackingmoreturnsintothesamespaceto
concentratethefield.
Addasoftironcore:anironcorebecomesstrongly
magnetisedbythefield,andthismakesthewhole
magneticfieldmuchstronger.
Figure16.11:Asolenoid.Whenacurrentflowsthrough
thewire,amagneticfieldisproduced.Thefieldissimilarin
shapetothatofabarmagnet.Notethatthefieldlinesgoall
thewaythroughthecentreofthecoil.
300>

314. 16Magnetism
Electromagnetshavethegreatadvantagethattheycan
beswitchedonandoff.Simplyswitchoffthecurrent
andthefieldaroundthecoildisappears.Thisisthe
basisofanumberofapplications.Forexample,the
electromagneticcranesthatmovelargepiecesofmetal
andpilesofscraparoundinascrapyard(Figure16.12).
Thecurrentisswitchedontostartthemagnetandpick
upthescrapmetal.Whenithasbeenmovedtothe
correctposition,theelectromagnetisswitchedoffand
themetalisreleased.
Electromagnetsarealsousedinelectricdoorbells,
loudspeakers,electricmotors,relaysandtransformers.
TheseusesaredescribedindetaillaterinChapters20
and21.
Figure16.12:Usinganelectromagnetinascrapyard.With
thecurrentswitchedon,asteelobjectorpileofscrapcan
beliftedandmoved.Thenthecurrentisswitchedoffto
releaseit.
Questions
7aNamethreewaystoincreasethestrengthofan
electromagnet.
bWhataretheadvantagesofanelectromagnet
overabarmagnet.
cWhatdoesanelectromagnetneedthata
permanentmagnetdoesnot?
Howdoesthestrengthofanelectromagnet
dependonthenumberofturns?
Aruninvestigatedhowthestrengthofan
electromagnetvarieswiththenumberofturns.
Figure16.13showstheequipmentheput
together,andTable16.3showsthedatahe
collected.
Arunmadesurethatthepoleatthebottomof
theelectromagnetwasthesameasthetopofthe
permanentmagnetsothattheywouldrepel.
withmagnetontop
Figure16.13:Apparatusforinvestigatingthestrength
ofanelectromagnet.
Table16.3:Experimentaldata
Numberofturns Mass/g
6 0.21
8 0.29
10 0.34
12 0.38
14 0.43
16 0.47
18 0.54
20 0.59
ACTIVITY16.3
301y

315. I
yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
1Plotagraphofmassagainstthenumber
ofturns.
2Themassofthepermanentmagnetisnot
changing,sowhatiscausingtheapparent
changeinmass?
3WhatdoesthegraphtellArunaboutthe
strengthoftheelectromagnetasthenumber
ofturnsincreases?
4Whyisitimportantthattheelectromagnet
andpermanentmagnetarerepelling(and
notattracting)?HowcouldArunhave
checkedthisbeforecollectingdata?
5HowelsecouldArunincreasethestrengthof
theelectromagnet?
Thefieldaroundasolenoid
Whenanelectriccurrentflowsthroughasolenoid,a
magneticfieldisproducedbothinsideandoutsidethe
coil(seeFigure16.11).Themagneticfieldinsidean
electromagnetisuniform.Thismeansthatthefield
linesareparallelandthesamedistanceapart.Thefield
aroundtheoutsideofasolenoidissimilartothataround
abarmagnet:
Oneendofthesolenoidisthenorthpoleandthe
otherendisthesouthpole.Fieldlinesemergefrom
thenorthpoleandgointothesouthpole.
Thefieldlinesareclosesttogetheratthepoles,
showingthatthisiswherethemagneticfieldis
strongest.
Thelinesspreadoutfromthepoles,showingthat
thefieldisweakerintheseregions.
Thestrengthofthefieldcanbeincreasedbyincreasing
thecurrent.Thefieldcanbereversedbyreversingthe
directionofthecurrent.
Question
8aSketchadiagramofthemagneticfieldpattern
aroundasolenoid.
bHowwouldthepatternchangewhenthe
currentthroughthesolenoidisreversed?
EXPERIMENTAL SKILLS16.1
Whichmagneticpolesitsbelowthe
geographicalNorthPole?
Theredendofacompassneedlepointstowards
theNorthPoleoftheEarth.Ifyoulookonthe
Internet,itisnotclearwhetherthemagneticpole
nearthegeographicalNorthPoleismagnetic
northormagneticsouth.Therefore,youneedto
workoutwhethertheredendofacompassneedle
isanorthorsouthmagneticpole.
Youneedtoensureyouknowwhichway
conventionalcurrentisflowing(fromthepositive
tothenegativeofabatteryorpowerpack).
Youwillneed:
electromagnet
compass.
Method
1Lookalongtheelectromagnetfnpm
eitherend.
Ifconventionalcurrentisflowingclockwise
aroundtheendloop,youarelookingatthe
southpolelikethearrowsoniheS(asshown
inFigure16.14a).
Ifconventionalcurrentisrunning
anticlockwise,likethearrowsontheN(Figure
16.14b),youarelookingatthenorthpole.
ThemagneticpolesinFigure16.11aredrawn
correctly.Imaginestandingattheleft-hand
endoftheelectromagnetlookingalongit,
asifyouwerelookingdownalongtunnel.
Conventionalcurrentwouldbeflowing
anticlockwise,tellingyouthatyouwereatthe
northpole.
a b
Figure16.14
302y

316. 16Magnetism
CONTINUED
2Bringyourcompasstowardseitherpoleof
theelectromagnet.Useyourobservationsto
decidewhethertheredendofyourcompass
needleisnorthorsouth.Usethisinformation
todecidewhichmagneticpolesitsbelowthe
geographicalNorthPole.
KEYWORDS
conventionalcurrent:thedirectionpositive
chargeswouldflowinacompletecircuit,from
thepositivetonegativeterminalsofacell,and
oppositetothedirectionthatelectronsflow
PROJECT
Readthisnewspaperextract.
COMPASSCONFUSION:HIKER I'
ALMOSTKILLEDBYHISCOMPASS
A
nexperiencedhikerfromtheUSA
whobecamelostintheAustralian
Outbackwasrescuedbythepolicelast
week.Hecomplained,‘mycompass
stoppedworking’.Aspokesmanfor
theWesternAustraliaPoliceForcewas
Youareasciencejournalistwhosejobitistoexplain
thesciencebehindthisstory.Youcandecide
whetheryouwriteanarticle,writeascriptfora
televisionnewsitemorproduceapodcast.
Rememberthatyouraudiencearenotscientists.
Sometimesexplainingaproblemneedsideasfrom
twoormoretopics,asisthecasehere.Youwillneed
toexplainwhymagneticcompassesstopworking
whenusedintheoppositehemisphere(forexample,
acompassdesignedforthenorthernhemisphere
willnotworkinthesouthernhemisphere).Youneed
tousediagramsorvideoclipstohelpyouraudience
followyourexplanation.
Whenusingacompass,youshouldholditflat(that
is,paralleltotheground).Thecompassneedle
(sometimescalledthecard)shouldbeparallelto
thebaseplateofthecompass(whenviewedfrom
quotedassaying,‘thisgentlemanis
luckytobealive.Thehikerwasusing
acompassdesignedforuseintheUSA
andhadnotrealiseduntilitwastoo
latethatheneededtobuyacompass
hereinAustralia.’
theside)andneedstobeabletoswingclockwise
oranticlockwisewhenviewedfromabove.However,
youshouldrecallthatacompassneedlelinesupwith
themagneticfieldlines,whicharenotparalleltothe
groundinAustralia.
Youwillneedtoexplainwhythemagneticfieldlines
arenotparalleltothegroundinAustralia.Youneed
tosketchwhatthecompassneedlelookslikewhen
viewedfromtheside.Itlookslikeatinysee-saw
balancedonatinypillaratitsmid-point(lookback
atChapter4ifnecessary).Usetheprincipleof
momentsandtheideaofequilibriumtoexplainhow
acompassneedleismadetobeparalleltothebase
plate(andtheground).Thenexplainwhythisisa
particularproblemwhenacompassdesignedforone
hemisphereistakenandusedintheother.
303y

317. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
SUMMARY 1
Magnetshaveanorthpoleandasouthpole.
Magneticfieldlinesaredrawnwithanarrowfromnorthtosouth,whichisthedirectionofthemagneticforce.
Likepolesrepelandunlikepolesattract.
Magneticelementsincludeiron,cobaltandnickel.Metalalloys(forexample,steel)containingtheseelements
arealsomagnetic.
Magneticmaterialscanbemagnetisedwhilenon-magneticmaterials(likerubberandglass)cannotbemagnetised.
Apermanentmagnetcanattractunmagnetisedmagneticmaterialsbyinducingmagnetisminthem.
Ahardmagneticmateriallikesteelisdifficulttomagnetiseanddemagnetise.
Asoftmagneticmateriallikesoftironiseasytomagnetiseanddemagnetise.
Amagneticfieldisaregionofspacearoundamagnetorelectriccurrentinwhichamagnetwillfeelaforce.
Amagneticfieldpatternarounda(bar)magnetcanbeproducedusingaplottingcompass.
Amagneticfieldlineistheline(direction)offorceonthenorthpoleofthemagnet,whichiswhyamagnet(for
example,acompassneedle)linesupwithit.
Anelectromagnet(orsolenoid)isamagnetcreatedwhenacurrentispassedthroughwire,whichisusually
shapedintoacoil.
Thestrengthofanelectromagnetincreaseswiththenumberofturnsinthecoil,thestrengthofthecurrentand
ifithasasoftironcore.
Theadvantagesofanelectromagnetoverapermanentmagnetarethatitsstrengthcanbechanged,itcanbe
switchedonandoffanditcanbereversed.
Themagneticforcesofattractionandrepulsionbetweenmagnetsarecausedwhentheirmagneticfieldsinteract
(affecteachother)andproduceanewpatternofmagneticlinesofforce.
Theclosertogethermagneticfieldlinesaretoeachother,thestrongerthemagneticfield.

'"
l" J
’
1 — — ——-— —
EXAM-STYLEQUESTIONS
1Somemetalsandalloysaremagnetic.Whichoftheseismagnetic? [1]
Aaluminium
Bcopper
Cgold
Dsteel
304
>

318. 16Magnetism
COMMAND WORD
explain:setout
purposesor
reasons;make
therelationships
betweenthings
evident;provide
whyand/orhowand
supportwithrelevant
evidence

319. 6
[3]
[11
c
[2]
d
[1]
e
306y
COMMAND WORDS
[1]
[1]
state:expressin
clearterms
suggest:apply
knowledgeand
understanding
tosituationswhere
therearearange
ofvalidresponses
inordertomake
proposals/putforward
considerations
a
b
aWhichpoleisatX?
bIfthemagnetsarereleased,inwhichdirectionwilltheymove?
Plotagraphofforceagainstcurrent.
Statehowthestrengthoftheelectromagnetvarieswiththecurrent
passingthroughthecoil.
[Total:2]
Karamveerinvestigatedhowthestrengthofanelectromagnetvariedwith
current.Heusedanarrangementliketheoneinthediagram.Hepasseda
currentthroughanelectromagnetsothatitattractedasmallsteelplate.For
eachcurrenthepassedthroughthecoil,hesuspendedmoremassesfromthe
bottomofthesteelplateuntilthesteelwaspulledawayfromtheelectromagnet.
Statetwootherwaysthatthestrengthofanelectromagnetcanbe
increased.
Anelectromagnetcanbeswitchedonandoff.Suggestonesituation
wherethiswouldbeanadvantageovertheconstantfieldofa
permanentmagnet.
Intermsofforces,statewhythesteelplatefallsfromtheelectromagnet
oncethesuspendedmassesexceedacertainvalue. [1]
[Total:8]
Current/AForce/N
0 0.00
2 0.60
4 1.20
6 1.90
8 2.40
10 3.00

320. 16Magnetism
COMMAND WORD
SELF-EVALUATIONCHECKLIST
Afterstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutosee
anygapsinyourknowledgeandhelpyoutolearnmoreeffectively.
give:producean
answerfromagiven
sourceorrecall/
memory
CONTINUED
7aWhatisthedifferencebetweenamagneticallysoftandamagnetically
hardmaterial? [11
bGiveanexampleofamagneticallyhardmaterialandsuggestwhereit
mightbeused. [11
cGiveanexampleofamagneticallysoftmaterialandsuggestwhereit
mightbeused. HI
[Total:3]
1can
See
Topic...
Needs
morework
Almost
there
Confident
tomoveon
Recallthenamesofthetwopolesofamagnet. 16.1
Recallthedirectionthatthearrowsonafieldline
shouldpoint.
16.2
Recallwhathappensbetweenlikepolesandunlike
poles.
16.2
Identifycommonmagneticmaterials. 16.1
Recallthedifferencebetweenmagneticand
non-magneticmaterials.
16.1
Accountforinducedmagnetism. 16.2
Recallexamplesofhardandsoftmagneticmaterials
andthedifferencebetweenthem.
16.1
Drawthepatternofmagneticfieldlinesarounda
barmagnetanddescribeanexperimenttoplotthem,
includingthedirectionarrows.
16.2
Recallwhyacompassneedleorotherpermanent
magnetlinesupwiththefieldlineswhenitisplaced
inamagneticfield.
16.2
Describeelectromagnets(andsolenoids). 16.2
Describethethreewaysinwhichthestrengthofan
electromagnetcanbechanged.
16.2
Describetheadvantagesofanelectromagnetovera
permanentmagnet.
16.2
V
Explainthatmagneticforcesareduetointeractions
betweenmagneticfields.
16.2
Usethespacingofthefieldlinestoworkoutthe
relativestrengthofthemagneticfield.
16.2

321. >Chapter17
Static
electricity

322. 17Staticelectricity
1
2
GETTINGSTARTED
ASHOCKINGPHENOMENON
Thesimilaritybetweenlightningboltsandthesign
usedforelectricalhazardsisnotacoincidence.
Alargebuild-upofelectricalchargecancausethe
airtobreakdownandconductelectricity,potentially
deliveringafatalelectricshock.InDecember2014,a
potentialdifferenceof1.3billionvoltswasrecorded
duringathunderstormoverOotyinSouthernIndia.
Inasmallgroup,discusswhatyouknowabout
electricity.Organiseyourideasintoamindmap.
Includeasmanyofthefollowingwordsaspossible,
givingdefinitionsandexampleswhereveryoucan:
electron,charge,current,static,shock,voltage,
conductor,insulator.
Onememberofthegroupshouldnowstaywith
themaptoexplainittoothergroups.Therestof
thegroupshouldvisitthemapsofdifferentgroups.
Havingdonethis,returntoyourowngroupandadd
anyadditionalinformationtoyourmap.
Figure17.1a:Lightningandelectricalsparksareboth
causedbyalargebuild-upofelectriccharge,b:electrical
hazardsymbol Franklinmadeimportantdiscoveriesabout
staticelectricity,buthisexperimentwouldbe
consideredtoodangeroustobecarriedout
now.Shouldscientistsbeallowedtoconduct
hazardousinvestigations?
Gupta'sobservationsdidnotaffectthe
situationhewasstudying.Whythisisan
importantfeatureofscientificresearch?
TheUSscientistBenjaminFranklinwasluckyto
survivehisexperimentsintothiskindofelectricity.
Hesuspectedlightningwasaformofelectricitydue
tothesimilaritybetweenalightningflashandthe
sparksheproducedinhislaboratory.Toinvestigate
heflewakiteintoathundercloud.Toavoidbeing
electrocuted,heincludedametalkeyatthebottom
ofthekitestring,andattachedalengthofribbonto
thekey.Holdingtheribbon,hewasrelativelysafe
fromelectrocution(althoughotherpeoplewere
killedwhentheyrepeatedhisexperiment).Asabolt
oflightningstruckthekite,Franklinsawthefibresof
thekitestringstandonendandasparkjumpedfrom
thekeytotheground.
Franklin'smethodwashazardous,andbyflying
akiteintothecloud,hemayhavebroughtona
lightningflashandtherebyaffectedthesituation
hewasinvestigating.SunilGuptaandtheteam
whomeasuredthepotentialdifferenceintheOoty
stormusedaratherdifferentmethod.Theystudied
muons-atypeofsubatomicparticle-asthey
passedthroughthestormcloud.Themuonsgained
energyfromthepotentialdifferenceinthecloud,
increasingthenumberwhichreachedthescientists'
muonsensors.Thisallowedtheteamtocalculatethe
potentialdifferencewithinthecloud.
Discussionquestions

323. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
17.1Chargingand
discharging
Weexperiencestaticelectricityinanumberofwaysin
everydaylife,includinglightningflashes.Youmayhave
noticedtinysparkswhenyoutakeoffclothesmadeof
syntheticfibres.Youmayhavefeltasmallshockwhen
gettingoutofacar.Anelectrostaticchargebuildsuponthe
carandthendischargesthroughyouwhenyoutouchthe
metaldoor.Youmayhaverubbedaballoononyourclothes
orhairandseenhowitwillsticktoawallorceiling.
Whenyourubaplasticobject(forexample,aballoon)
withacloth,botharelikelytobecomeelectrically
charged.Youcantellthatthisissobyholdingthe
balloonortheclothclosetoyourhair-theyattract
thehair.
Figure17.2:Objectswhichhaveelectrostaticchargecan
attractlightobjectssuchashairorpaper.
Youhaveobservedthatstaticelectricityisgeneratedby
friction.Youhavealsoobservedthatachargedobject
mayattractunchargedobjects.
Nowwehavetothinksystematicallyabouthowto
investigatethisphenomenon.First,howdotwocharged
objectsaffectoneanother?Figure17.3showsoneway
ofinvestigatingthis.Aplasticrodisrubbedwithacloth
sothatbothbecomecharged.Therodishunginacradle
sothatitisfreetomove.Whentheclothisbroughtclose
toit,therodmovestowardsthecloth(Figure17.3a).
Whenasecondrodisrubbedinthesamewayand
broughtclosetothefirstone,thehangingrodmovesaway
(Figure17.3b).Nowwehaveseenbothattractionand
repulsion.Thissuggeststhattherearetwotypesofstatic
electricity.Bothrodshavebeentreatedinthesameway,
soweexpectthemtohavethesametypeofelectricity.
Theclothandtherodmusthavedifferenttypes.
Thetwotypesofstaticelectricityarereferredtoas
positivechargeandnegativecharge.Wecanexplainthe
experimentsshowninFigure17.3bysayingthatthe
processofrubbinggivestherodsonetypeofelectric
charge(say,negative),whiletheclothisgiventhe
oppositetype(say,positive).Figures17.3cand17.3d
showthetwoexperimentswiththechargesmarked.
KEYWORDS
staticelectricity:electricchargeheldbya
chargedinsulator
electrostaticcharge:apropertyofanobject
thatcausesittoattractorrepelotherobjects
withcharge
positivecharge:thetypeofelectriccharge
carriedinthenucleusofanatom
negativecharge:thetypeofelectriccharge
carriedbyelectrons
Fromtheseexperiments,wecanalsosaysomething
abouttheforcesthatelectricchargesex^rtoneachother:
Twopositivechargeswillrepeleachother.
Twonegativechargeswillrepeleachother.
Apositivechargeandanegativechargeattract
eachother.
Figure17.3:Twoexperimentstoshowtheexistenceof
two,opposite,typesofstaticelectricity,a:Thecharged
rodandclothattractoneanother,b:Thetwocharged
rodsrepeloneanother,c:Therodandtheclothhave
oppositeelectriccharges,d:Thetworodshavethesame
typeofelectriccharge.
310y

324. 17Staticelectricity
Questions
and
and
Figure17.4
Figure17.5:Set-upforinvestigation.
b
EXPERIMENTAL SKILLS17.1
Twoobjectswiththesamechargewill
twoobjectswithoppositechargeswill
Youcanseethatthisruleissimilartotherulewesaw
formagneticpolesinChapter16.Butdonotconfuse
magnetismwithstaticelectricity!Magnetismarisesfrom
magneticpoles
-staticelectricityarisesfromelectric
charges.Whenyourubaplasticrod,youarenotmaking
itmagnetic.
Figure17.4showsachild’shairstandingonend
afterthechildhasbeenonatrampoline.
Explainwhy,afterwalkingonanyloncarpet,you
maygetasmallshockwhenyoutouchametal
doorhandle.
Copyandcompletethesesentences.
Therearetwotypesofchargecalled
Howdoesbouncingonthetrampolinecause
thechild’shairtobecomecharged?
Theindividualhairsarerepellingeachother.
Whatdoesthistellyou?
Gettingstarted
Investigatehowmuchyouneedtorubtherodsin
ordertochargethem.Testthisbyusingtherodto
pickuplittlepiecesofpaper.
Investigatingstaticelectricity
Inthisexperimentyouwillfindoutaboutstatic
electricitybychargingmaterialsandobserving
howtheybehave.
Youwillneed:
polytheneandacetateorglassrods
clothsofdifferentmaterials
paperstirrupswithfinethread
clampandstand
watchglass
balloons
tinypiecesofthinpaper.
Method
1Youneedtobesurethatyoucanplacethe
rodssothattheycanturnfreely,eitherby
hangingtheminthepaperstirrup,orby
placingthemonanupturnedwatchglass.
Trythisoutwithyourrods.

325. >
CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
CONTINUED
2Rubapolythenerodwithawoollencloth,
makingsurethatyourubthefulllengthof
therod.Hangtherodorplaceitonawatch
glass.
3Rubanotherpolythenerodandbringone
endclosetoanendofthefirstrod.Dothey
attractorrepel?
4Rubanacetateorglassrodandrepeatthe
test.Whatdoyouobserve?
5Trydifferentcombinationsofrods.Tryacloth
ofadifferentfabric.Giventhatapolythene
rod,rubbedwithawoollencloth,gainsa
negativecharge,whatcanyousayaboutthe
chargesgainedbytheclothandbytheother
rods?
6Blowupaballoonandrubit.Canyou
determinewhetheritgainspositiveor
negativecharge?(Hint:polythenebecomes
negativeandacetatebecomespositive.)
Questions
1Somestudentsinvestigatepolytheneand
acetaterods.Theyfindthatthetworods
attracteachother.Onestudentsaysthis
provesthattherodshaveoppositecharge.
Explainwhythestudentiswrong.
2Describewhatthestudentswouldneed
todoinordertoprovethattherodshave
oppositecharges.
17.2Explainingstatic
electricity
TheAncientGreeksknewsomethingaboutstatic
electricity.Amberisaformofresinfromtrees,which
hardensandbecomesfossilised.Itlookslikeclear,
orangeplastic.TheGreeksknewthat,whenrubbed,
ambercouldattractsmallpiecesofclothorhair.
LiketheAncientGreeks,Franklinhadnoideaabout
electrons-theseparticleswerenotdiscovereduntil100
yearslater.However,thatdidnotstopFranklinfrom
developingagoodunderstandingofstaticelectricity.
Inthediscussionthatfollows,wewilltalkabout
electrons.Theymakeitmucheasiertounderstandwhat
isgoingoninallaspectsofelectricity.
Figure17.6:Aninsecttrappedinresinas’ithardened.
Resinisagoodinsulatorandeasilybecomeselectrostatically
charged.
J
i
Frictionandcharging
Itistheforceoffrictionthatcausescharging.Whena
plasticrodisrubbedonacloth,frictiontransferstiny
particlescalledelectronsfpomonematerialtotheother.
Whentherodismadeofpolythene,usuallyelectronsare
transferredfromtheclothtotherod.
Electronsareapartofeveryatom.Theyarenegatively
charged,andtheyarefoundontheoutsideoftheatom.
Thenucleusinthecentreoftheatomhasapositive
charge.Thepositiveandnegativechargesinanatomare
equalsooverallanatomhasnoelectriccharge-wesay
thatitisneutral.Sincetheouterelectronsarerelatively
weaklyheldintheatom,theycanbepulledawaybythe
forceoffriction.Whenanatomhaslostanelectron,it
becomespositivelycharged.
KEYWORD
neutral:havingnooverallpositiveornegative
charge
312>

326. 17Staticelectricity
Figure17.7:Circuitfortestingmaterials.
2
3
4
KEYWORDS
EXPERIMENTAL SKILLS17.2
Sowhycanchargemovethroughconductorsbutnot
throughinsulators?Ininsulators,theelectronsaretightly
boundtotheiratomsandnoteasilyremoved.Inconductors
someoftheelectronsarefreetomovebetweenatoms(these
electronsaresometimesreferredtoasfreeelectrons).
Whenyourubapolythenerod,itgainselectronsfrom
theclothandsobecomesnegativelycharged.The
electronscannotmovethroughthepolythene,sotheend
whichwasrubbedremainscharged.
Whenacopperrodisrubbed,electronsarealsotransferred
byfriction,buttheseelectronsarefreetomove,sotheyflow
throughtherod,throughyourhandandintotheEarth.
Thismeansthecopperroddoesnotbecomecharged.
Ametalobjectcanbechargedifitisheldbyan
insulatinghandle.Inthiscasethechargewillspread
evenlythroughtheconductor.
Sinceapolythenerodbecomesnegativelychargedwhen
itisrubbedwithacloth,wecanimagineelectronsbeing
transferredfromtheclothtotherod(seeFigure17.8).
Itisdifficulttoexplainwhyonematerialpullselectrons
fromanother.Theatomsthatmakeuppolythenecontain
positivecharges,andthesemustattractelectronsmore
stronglythanthoseofthecloth.
Rememberthatittakestwodifferentmaterialsto
generatestaticelectricity.Onematerialbecomespositive,
theothernegative.
Gettingstarted
Connectthecellandlampinasimplecircuittomake
thelamplight.
Makeagapinthecircuitbyremovingawire.Explain
whythelampnolongerlightsandconsiderhow
placingmaterialsinthegapwillhelpyoudecideif
theyareconductorsorinsulators.
electricalconductor:asubstancethatallowsthe
flowofelectrons(electriccurrent)
electricalinsulator:asubstancethatinhibitsthe
flowofelectrons(electriccurrent)
Usingthecrocodileclips,attachamaterialinto
thegapinthecircuit.
Observewhetherthelamplights.Ifitdoes,itis
aconductor,ifnotitisaninsulator.
Recordyourresultsinatable.
Chargingisalwaystheresultofgainingorlosing
electrons.Positivechargeisnottransferred.
Anobjectwhichgainselectronsbecomesnegative.
Anobjectwhichloseselectronsbecomespositive.
Investigatingconductorsandinsulators
Inthisexperimentyouwillfindtestmaterialstofind
outwhichareconductorsandwhichareinsulators.
Youwillneed:
cell
lamp
wireswithcrocodileclips
materialstotest.
Conductorsandinsulators
Youmayhavenoticedthatalltheexamplesofobjects
becomingchargedinvolvenon-metals.Metalsare
electricalconductors,whichmeanselectronscanmove
throughthemandthemetaldoesn’tstaycharged.Gold
andcopperareparticularlygoodelectricalconductors.
Non-metals,suchasglass,plasticandamberare
electricalinsulators.
Method
ConnectthecircuitasshowninFigure17.7.
Question
Thewiresyouusedaremadeofcoppercovered
inplastic.Explainwhythesematerialswere
chosen.

327. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Figure17.8:Whenapolythenerodisrubbedwithacloth,
electronsaretransferredfromtheclothtothepolythene.
Therodnowhasanoverallnegativechargeandthecloth
hasanoverallnegativecharge.
Questions
4Copyandcompletethesesentences.
Whenapolythenerodisrubbed, movefrom
the tothe .
Thismeanstherodbecomes andthecloth
becomes .
5aDrawadiagramsimilartoFigure17.8toshow
howanacetaterodbecomespositivelycharged
bylosingelectrons.
bWritesentencessimilartothoseyoucompleted
inquestion17.4toexplainhowtheacetate
becomescharged.
6Whenyouholdapolythenerodandrubitwith
acloth,itbecomescharged.Iftherodismade
ofmetal,itwillnotbecomecharged.Explain
thisdifferencebydescribingwhathappenstothe
electronsineachcase.
ACTIVITY17.1:WHATHAVEILEARNT?
Taketwominutestosummarisewhatyouhave
learntsofarinthischapterononesheetofpaper.
Now,taketwomoreminutestocondenseyour
notes.Youmayusetenwordsandfourdiagrams.
Comparesummarieswithapartner.Assumethat
youcanuseonlyfourwordsandonediagramasa
reminderinatest.Whichdiagramandfourwords
wouldyouchoose?
REFLECTION
WhichpartofActivity17.1didyoufindmost
useful?Summarising,condensingorsharing?
Considerhowthismighthelpyouwhenrevising
foratest.
17.3Electricfields
Figure17.9:Thecombhasbeenchargedbyrubbingit.
Itattractsthewaterwhenitisheldclosetothewater.
Achargedobjectcanaffectotherobjects,bothcharged
anduncharged,withoutactuallytouchingthem.For
example,achargedplasticrodcanexertaforceon
anotherchargedrodplacedcloseby.
Wesaythatthereisanelectricfieldaroundacharged
object.Anychargedobjectplacedinthefieldwill
experienceaforceonit.
KEYWORDS
electricfield:aregionofspaceinwhichan
electricchargewillexperienceaforce
314y

328. 17Staticelectricity
Whatiselectriccharge?
Inphysics,wefinditrelativelyeasytoanswerquestions
suchas‘Whatisarainbow?’or‘Howdoesanaircraft
fly?’Itismuchhardertoansweranapparentlysimple
questionlike‘Whatiselectriccharge?’.Aswithenergy,
wehavetoansweritbyexplaininghowobjectsbehave
whentheyhaveit(energyorcharge).
Objectswiththesamesignofchargerepeloneanother.
Objectswithoppositechargesattract.Thisisnotavery
satisfyinganswer,becausemagneticpolesbehaveinthe
sameway:northpolesrepelnorthpolesandattract
southpoles.Becauseelectricchargeisafundamental
propertyofmatter,wehavetogetafeelforit,rather
thanhavingacleardefinition.
Theelectricforcebetweentwochargedobjectsisoneof
thefundamentalforcesofnature.(Theforceofgravity
betweentwomassesisanotherfundamentalforce.)The
electricforceholdstogethertheparticlesthatmakeup
anatom.Itholdsatomstogethertomakemolecules,and
itholdsmoleculestogethertomakesolidobjects.Just
think:wheneveryoustandonthefloor,itistheelectric
forcebetweenmoleculesthatpreventsyoufromfalling
throughthefloor.Itisaveryimportantforce.
Chargedparticles
Wehavealreadyseenthatelectronsarethecharged
particlesthataretransferredfromoneobjecttoanother
whentheyarerubbedtogether.Electricchargeisa
propertyoftheparticlesthatmakeupatoms.
Chargeismeasuredincoulombs(C),namedafter
Charles-AugustindeCoulomb,aFrenchphysicist.He
discoveredthattheforcebetweentwochargedobjects
dependsonhowbigtheirchargesareandonhowfar
aparttheyare.
Anelectronisanegativelychargedparticle.Itismuch
smallerthananatom,andonlyweaklyattachedtothe
outsideoftheatom.Itisheldtherebytheattractionof
thepositivelychargednucleusoftheatom.Thenucleus
ispositivelychargedbecauseitcontainspositively
chargedparticlescalledprotons.
Anelectronhasaverytinyamountofelectriccharge.
Theelectronchargeissosmallthatittakesmorethan
6millionmillionmillionelectronstomake1Cofcharge:
electroncharge=-0.00000000000000000016C
or-1.6x1O-19C
Aprotonhasexactlythesamesizeofchargeasan
electron,butpositive,sotheprotonchargeis:
protoncharge=+0.00000000000000000016C
or+1.6xW-19C
Question
7Drawthemagneticfieldaround:
aanisolatednegativecharge
btwoverticalmetalplates-onepositiveandthe
othernegative.
Figure17.10:Theelectricfieldaroundachargedobjectis
representedbylinesofforce,a:Anisolatedpositivecharge,
b:Anegativelychargedsphere,c:Twoparallelplateswith
oppositecharges.
Therearesimilaritiesbetweenelectricfieldsandmagnetic
fields,buttakecarenottoconfuseelectricfieldswith
magneticfields.Amagnetdoesnotattractelectric
charges.Achargedobjectdoesnotattractamagnet.
Representinganelectricfield
Achargedobjectissurroundedbyanelectricfield.Ifa
chargedobjectmovesintotheelectricfieldofacharged
object,itwillexperienceaforce-itwillbeattracted
orrepelled.Figure17.10showshowwerepresentan
electricfieldbylinesofforce(orelectricfieldlines).This
issimilartothewaywerepresentamagneticfieldby
magneticfieldlines.
Thelinesofforceareshowncomingoutofapositive
chargeandgoingintoanegativecharge.Thisisbecause
thelinesindicatethedirectionoftheforceonapositive
chargeplacedinthefield.Apositivechargeisrepelledby
anotherpositivechargeandattractedbyanegativecharge.
Figure17.10cshowsthefieldbetweentwooppositely
chargedparallelplates.Thelinesofforcebetweenthe
platesarestraightandparalleltooneanother(exceptat
theedges).

329. CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
—
No-oneknowswhythesevaluesurcexactlythesame
size(oreveniftheyareexactlythesamesize),butitis
fortunatethattheyarebecauseitmeansthatanatomthat
contains,say,sixprotonsandsixelectronsiselectrically
neutral.Ifalltheobjectsaroundusweremadeofcharged
atoms,wewouldliveinashockingworld!
Question
8Calculatethenumberofelectronsneededtogivea
chargeofonecoulomb.
KEYWORDS
coulomb(C):theSIunitforelectriccharge
proton:apositivelychargedparticlefoundinthe
atomicnucleus
electroncharge:theelectricchargeofasingle
electron=-1.6x10“19C
protoncharge:theelectricchargeofasingle
proton=+1.6xW19C
PROJECT
Shockingshopping
Figure17.11:Acarpetinashopcancaus.eashock.
Walkingacrossacarpetcancausefriction,which
leadstopeoplebecomingelectrostaticallycharged.
Touchinganotherperson,oraconductingobject
suchasametalstairbannister,cancauseashockas
thechargeflowsthroughtoearth,dischargingthe
person.Theseshocksarenotusuallydangerousbut
canbeuncomfortable.
Amanagerofalargestorehasreceivedalotof
complaintsfromcustomerswhoareexperiencing
painfulshocksastheyshop.Themanagerthinksit
maybeduetothecarpetontheshopfloorwhichis
madeofahard-wearingmaterialcalledpolypropylene.
Thestoremanagerwantstoinvestigatewhethershe
shouldinvestinanewcarpetthatisles^'likelytocreate
static.Untilshedoesthis,shewantstobeableto
advisecustomersonhowtoavoidshocks.
Yourtasksasherscientificconsultantare:
1Explainwhatishappening.AsanInterimmeasure,
shewouldlikeleafletstogivetoshopperstohelp
themunderstandwhatishappeningandwhat
theycandotominimiseshocks.Couldcustomers
dischargeregularlybeforetopmuchchargebuilds
up?Orwouldithelpiftheypickeduptheirfeet
ratherthanshuffling?
2Designtheleaflet.Itmusthavenomorethan
50wordsanditshouldbeillustrated.
3Considerhownewcarpetscouldbetestedto
findoutwhichcreatethemoststatic.Testingby
shockingpeopleisnoteasyorethical.Insteadyou
shouldlookatchargingthecarpetsbyfrictionand
investigatingtheforceofattractionorrepulsion
theyproduce.Youwillneedtogivestep-by-step
instructionsforafairtestinvestigation.
PEERASSESSMENT
Exchangeleafletsandinvestigationplanswithanotherstudent.Givethemwrittenfeedbackbyansweringthe
followingpoints:
Doestheleafletmakeitclearhowcharginghappens?
Doestheleafletgiveclearadviceonavoidingshocks?
Willtheinvestigationprovidevaliddata?
Doestheplancontrolallvariablesinordertogiveafairtest?
316y

330. 17Staticelectricity
SUMMARY
Therearetwotypesofcharge:positiveandnegative.
Likechargesrepel,oppositechargesattract.
Conductorsallowchargetoflow.Insulatorsdonotallowchargetoflow.
Insulatorscanbechargedbyfriction,whichcausesthelossorgainofelectrons.
Anelectricfieldistheareaaroundachargedobjectinwhichachargewillexperienceaforce.
Electricfieldscanberepresentedbyfieldlineswhichshowthedirectionoftheforceonapositivecharge.
Insulatorsarematerialsinwhichelectronsarefixedinplace.Conductorshavefreeelectrons.
EXAM-STYLEQUESTIONS
1Whichofthesedescribeshowanobjectbecomespositivelycharged? [1]
AItgainspositivecharge.
BItlosespositivecharge.
CItgainsnegativecharge.
DItlosesnegativecharge.
2Whenanobjectischargedbyfriction,whichparticleistransferred? [1]
Aproton
Belectron
Cneutron
Datom
3Apositivelychargedrodissuspendedsoitcanmovefreely.Anegatively
chargedrodisbroughtclosetoit.Whatwillhappentothepositiverod? [1]
AItwillrepel.
BNothingwillhappen.
CItisn’tpossibletopredictwhatwillhappenfromtheinformationgiven.
DItwillattract.
4Whatdoesthearrowonanelectricfieldlineshow? [1]
ATheforceonapositivecharge.
BTheforceonaneutralparticle.
CTheforceonanelectron.
DTheforceonanegativeparticle.
317>

331. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
5Astudentinvestigateschargingpolythenebyfriction.Shetakestwoidentical
stripsandrubseachwithacloth.Sheholdsthetwostripstogetherandthey
moveapart.
aExplainwhythestripsmoveapart [2]
Thestudentrepeatstheexperimentusingtwostripsofaluminium.
bStateandexplainwhatthestudentwouldseewiththealuminiumstrips.[2]
cAplasticrodischargedbyrubbingitwithacloth.
Whichstatementdescribedwhathashappenedtothecloth?
ATheclothbecomesnegativelychargedbecauseelectronstransfer
fromtheclothtotherod
BTheclothbecomespositivelychargedbecauseelectronstransfer
fromtheclothtotherod
CTheclothbecomesnegativelychargedbecauseprotonstransfer
fromtherodtothecloth
DTheclothbecomespositivelychargedbecauseprotonstransfer
fromtherodtothecloth.
6Astudentcombshishairwithaplasticcomb.Thecombbecomesnegatively
charged.
aExplainhowthishappens. [2]
bNametheparticleswhicharetransferred. [1]
cThestudentnoticeshishairisnowstandingonend.Explainwhythis
happenstohishair. [2]
[Total:5]
>
COMMAND WORDS
state:expressin
clearterms
explain:setout
purposesor
reasons;make
therelationships
betweenthings
evident;provide
why1and/orhowand
supportwithrelevant
evidence
318>

332. 17Staticelectricity
insulator
[2]
[2]
CONTINUED COMMAND
WORD
[1]
[6]
[Total:7]
apositively
charged
sphere
anegatively
charged
sphere
define:giveprecise
meaning
Astudentrubsaballoonsothatitbecomespositivelycharged.Heplacesthe
balloonontopofaninsulatingsheetonanelectronicbalance.
positivelyandnegatively
chargedplates
Definetheterm‘electricfield'.
Copythesediagramsanddrawtheelectricfieldaroundthecharged
objects:
Explain,intermsofmovementofelectrons,howtheballoonbecomes
positivelycharged.
Thestudentnowbringsachargedplasticrodclosetotheballoon.
Thereadingonthebalanceincreasesto11.63g.Stateandexplain
whatthistellsyouabouttheplasticrod.
Thestudentrepeatstheexperiment,butthistimeheplacestheballoon
directlyontothemetalpanofthebalance.Thistimewhenhebringsthe
chargedrodclosetotheballoon,thereadingonthescaledecreasesto
6.94g.Explainwhy. [3]
[Total:7

333. yCAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
CONTINUED
9Apainterisspraypaintingametalfence.Thenozzleofthesprayeris
negativelycharged.
aStateandexplainwhathappenstothepaintdropletsastheypass
throughthenozzle. [2]
bDescribetwowaysinwhichthismakesthespraypaintingmoreefficient.[2]
cWhatchargeshouldbegiventothefencetomaketheprocesseven
moreefficient? [1]
[Total:7]
COMMAND WORD
describe:statethe
pointsofatopic;give
characteristicsand
mainfeatures
SELF-EVALUATIONCHECKLIST
Afterstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutosee
anygapsinyourknowledgeandhelpyoutolearnmoreeffectively.
Ican
See
Topic...
Needs
morework
Almost
there
Confident
tomoveon
Namethetwotypesofcharge. 17.1
Sayhowobjectswithidenticaloroppositecharges
affecteachother.
17.1
Describeexperimentstoshowhowchargescanbe
producedanddetected.
17.2
Explainhowchargingbyfrictionhappensby
describingwhathappenstotheelectrons.
17.2
Defineandgiveexamplesofconductorsandinsulators.17.2
Nametheunitofelectricalcharge. 17.3
Definewhatismeantbyanelectricfield. 17.3
Drawthefieldbetweentwochargedplatesoraround
apointchargeofachargedsphere,usingarrowsto
showthedirectionofthefield.
17.3
Describethedifferencebetweenconductorsand
insulatorsintermsofasimpleelectronmodel.
17.3

334. >Chapter18
Electrical
quantities
INTHISCHAPTERYOUWILL:
learnthatelectronsflowintheoppositedirectiontoconventionalcurrent
sketchandexplaincurrent-voltagecharacteristics
calculateelectriccurrentandpotentialdifference.
learnaboutelectriccurrent,resistanceandvoltage
describeanexperimenttodetermineresistanceusingammetersandvoltmeters
learnhowtheresistanceofawirerelatestoitslengthanddiameter
understandthatenergyistransferredfromthepowersource(forexample,abattery)tothecircuit
components
calculateresistance,electricalpower,energyandthecostofelectricalenergy

335. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
GETTINGSTARTED
Doesanythinggetusedupgoingaroundacircuit?
Whatdobatteriesandcellshavethatgetsusedup?
Whycanbirdsstandonpowerlineswithoutbeing
electrocuted?
Doyouknowwhathappenswhenwechargea
mobile(cell)phone?
INSPIREDBYANEEL?USINGPHYSICSFORLAWENFORCEMENT
JeanRicher,aFrenchastronomer,metanelectric
eel(Figure18.1)duringanexpeditiontothe
Amazonbasinin1671.Hedescribeditasbeing'as
fatasaleg'anditmadehisarmgonumbfor15
minutesafterhetoucheditwithhisfinger.About
80%ofanelectriceelisbasicallyabattery,witha
positiveterminalatitsheadandanegativeterminal
atitstail.Thechargeiscarriedbypositiveions
insteadofelectrons.Ithasabout5000to6000
stackedplates(calledelectroplaques).Eachpair
ofplatesislikeanelectriccellthatcanproducea
voltageofabout0.15Vsothataneelcandelivera
shockof900Vandacurrentofupto1A.Tosave
energyitcansendoutlowvoltagepulses(75V).
Thismakesthemusclesofpotentialpreytwitch,
whichcanbedetectedbytheeel.Ifitdetectsprey,
theeelcansendoutbiggershockswhenitwants
tostunitsvictim.Someeelsweretakenbacktobe
studiedinEuropewheretheyinspiredtheworkof
Italianscientists.LuigiGalviniusedfrogstoshow
thatelectricitymadetheirlegsmove.Alessandro
Voltainventedthefirstbatterybasedonthe
electriceel.
Figure18.1:Anelectriceel.
NASAresearcherJackCoverfinisheddeveloping
theTASERin1974.Henameditafterthebook
TomSwiftandHisElectricRifle.TASERsareused
bylawenforcementofficerstosubduesuspected
criminalswithoutusinglethalforce(Figure18.2).
TheTASERfirestwothininsulatedcopperwiresat
thetarget.Oncethebarbsattheendofthewires
hookintotheskin,thepersonbecomespartofthe
circuit.Whenacurrentispassedthroughthecircuit,
thepersonlosescontroloftheirmusclfes,likethe
victimsoftheelectriceel.
Figure18.2:Evenwithoutthecartridgethatsendsthe
copperwirestoitstarget,aTASERstillpassesanelectrical
charge.
Discussionquestions
1Describehowanelectriceelislikeabattery.
2ATASERislikeanelectriceelbutwhydoesthe
eelnotneedthewiresusedinaTASER?
322)

336. 18Electricalquantities
18.1Currentinelectric
a
circuits
b
filament
Goodconductors
KEYWORDS
Figure18.3bshowsthesamecircuitasrepresentedbya
circuitdiagram.Eachcomponenthasitsownstandard
symbol.Ifyouimaginetheswitchbeingpushedso
thatitcloses,itisclearfromthediagramthatthereisa
continuouspathforthecurrenttoflowaroundthecircuit.
Figure18.3ashowshowasimplecircuitcanbesetup
inthelaboratory.Oncetheswitchisclosed,thereisa
continuousmetalpathforthecurrenttoflowalong.
Currentflowsfromthepositiveterminalofthebattery
(orcell).Inthecircuitsymbolforacell,thelongerline
representsthepositiveterminal(seeFigure18.3b).
Currentflowsthroughtheswitchandthefilamentlamp,
backtothenegativeterminalofthebattery.Acurrent
thatflowsinthesamedirectionallthetimeiscalled
directcurrent(d.c.).Youwillmeetalternatingcurrent
(a.c.)inChapter21whenyoulearnabouttransformers.
Alternatingcurrentiswhencurrentreversesdirection.
Inmanycountries,mainselectricityhasafrequencyof
50Hzsoitchangesdirection100timespersecond.
Weuseelectriccircuitstotransferenergyfromabattery
orpowersupplytocomponentsinthecircuit,whichthen
transfertheenergytotheirsurroundings.Foranelectric
currenttoflow,twothingsareneeded:acompletecircuit
forittoflowaround,andsomethingto‘push’itaround
thecircuit.Thepushmightbeprovidedbyacell,battery
orpowersupply.Abatteryissimplytwoormorecells
connectedend-to-end.Inmostfamiliarcircuits,metals
suchascopperorsteelprovidethecircuitforthecurrent
toflowaround.
Figure18.3a:Asimpleelectriccircuit,setupinalaboratory,
b:Thesamecircuitrepresentedasacircuitdiagram.
ItisobvioushowtheswitchinFigure18.3aworks.
Youpushthespringymetaldownwardsuntilittouches
theothermetalcontact.Thenthecurrentcanflow
throughit.Mostswitchesworkbybringingtwopieces
ofmetalintocontactwithoneanother,thoughyou
cannotusuallyseethishappening.Itisworthhaving
alookinsidesomeswitchestoseehowtheywork.(Of
course,theymustnotbeconnectedinacircuitwhenyou
examinethem!)
Similarly,takealookatsomefilamentlamps,likethe
oneinFigure18.3a.Everylamphastwometalcontacts,
forthecurrenttoflowinandout.Inside,onefinewire
carriesthecurrentuptothefilament(whichisanother
wire),andasecondwirecarriesthecurrentbackdown
again.Noticealsohowthecircuitsymbolsfortheseand
manyothercomponentshavetwoconnectionsforjoining
themintoacircuit.
current:therateatwhichelectricchargepassesa
pointinacircuit
cell:adevicethatprovidesanelectromotiveforce
(e.m.f.)inacircuitbymeansofachemicalreaction
battery:twoormoreelectricalcellsconnected
togetherinseries
directcurrent(d.c.):electriccurrentthatflowsin
thesamedirectionallthetime
alternatingcurrent(a.c.):electriccurrentthat
(periodically)changesdirection
badconductors
Thewiresweusetoconnectupcircuitsaremadeof
metalbecausemetalsaregoodconductorsofelectric
current.Themetalisusuallysurroundedbyplastic,so
that,iftwowirestouch,theelectriccurrentcannotpass
directlyfromonetoanother(causingashortcircuit).
Plastics(polymers)aregoodelectricalinsulators.

337. CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Mostmetals,includingcopper,silver,goldandsteel
aregoodconductors.
Polymers(suchasPerspex®orpolythene),minerals
andglassaregoodinsulators.
Inbetween,therearemanymaterialsthatdoconduct
electricity,butnotverywell.Forexample,liquidsmay
conduct,buttheyaregenerallypoorconductors.
Peoplecanconductelectricity-thatiswhathappens
whenyougetanelectricshock.Acurrentpassesthrough
yourbodyand,ifitisbigenough,itmakesyourmuscles
contractviolently.Yourheartmaystop,andyoumay
getburns.Ourbodiesconductbecausethewaterinour
tissueisquiteagoodelectricalconductor.
Whatiselectriccurrent?
Whenacircuitiscomplete,anelectriccurrentflows.
Currentflowsfromthepositiveterminalofthesupply,
aroundthecircuit,andbacktothenegativeterminal.
Whatisactuallytravellingaroundthecircuit?Theanswer
iselectriccharge.Thebatteryorpowersupplyinacircuit
providesthepushneededtomakethecurrentflow.This
pushisthesameforcethatcauseselectricchargesto
attractorrepeloneanother.
Acurrentisaflowofelectriccharge.Inametal,the
currentisaflowofelectrons.Thesearethenegatively
chargedparticlesyoulearnedaboutinChapter17.
KEYWORDS
conductor:amaterialthatallowsanelectric
currenttoflowthroughit
insulator:amaterialthatmakesitverydifficultfor
anelectricalcurrenttoflowthroughit
charge:carriedaroundacircuitbythecurrent;
negativechargeiscarriedbyelectrons
Figure18.4:Ammetersmeasureelectriccurrent,inamps
(A).Therearetwotypes:analogue(ontheleft)anddigital
(ontheright).
Measuringelectriccurrent
Tomeasureelectriccurrent,weuseanammeter.There
aretwotypes,asshowninFigure18.4.
Ananaloguemeterhasaneedle,whichmoves
acrossascale.Youhavetomakeajudgementofthe
positionoftheneedleagainstthescale.
Adigitalmetergivesadirectread-outinfigures.
Thereisnojudgementinvolvedintakingareading.
Agalvanometerissometimesusedinsteadofanammeter
whentinycurrentsneedtobemeasured.Ithasadifferent
circuitsymbol-anupwardpointingarrowtorepresent
aneedle.Anammeterisconnectedintoacircuitin
series,thatis,betweenothercomponentsinthecircuit.
Thiscircuitiscalledaseriescircuit,wherecomponents
areconnectedinalinebetweenothercomponents.If
themeterisconnectedthewrongwayround,itwillgive
negativereadings.Toaddanammetertoacircuit,the
circuitmustbebroken(seeFigure18.5).
InasimpleseriescircuitliketheoneshoyvninFigure
18.5,itdoesnotmatterwheretheammeterisadded,
sincethecurrentisthesameallthewayroundtheseries
circuit.Itdoesnotgetusedupasitflowsthroughthe
lamporothercomponentsinthecircuit.
Thereadingonanammeterisinamperes(shortenedto
amps(A),whichistheSIunitofcurrent.Smallercurrents
maybemeasuredinmilliamps(mA)ormicroamps(pA):
1mA=0.001A=IO-3A
1pA=0.000001A=10-6A
Figure18.5:AddinganammetertoacircuitTheammeter
isconnectedinseriessothatthecurrentcanflowthroughit.
324>

338. 18Electricalquantities
Questions
a
current
a
b
KEYWORDS
Wecanthinkofconventionalcurrent,aflowofpositive
charge,movingfrompositivetonegative.Conventional
currentisratherlikeafluidmovingthroughthewires,
justlikewatermovingthroughpipes.Thispicturedoes
nottellusanythingaboutwhatisgoingoninsidethe
wiresorcomponentsofacircuit.However,itisperfectly
goodforworkingoutmanythingstodowithacircuit:
Conventionalelectriccurrentflowsfrompositiveto
negative.Figure18.7showsthedirectionoftheflow
ofchargearoundasimplecircuit.Wepicturepositive
chargeflowingoutofthepositiveterminal,aroundthe
circuitandbackintothecellatthenegativeterminal.
Now,weknowthatinametalitisthenegativelycharged
electronsthatmove.Theyleavethenegativeterminalof
thecell,andflowaroundtothepositiveterminal,inthe
oppositedirectiontothecurrent.Hencewehavetwo
differentpicturesofwhatisgoingoninacircuit.
Figure18.6:Inametal,someelectronsarefreetomove
about.Incopper,thereisoneconductionelectronforeach
atomofthemetal.Theatoms,havinglostanelectron,
arepositivelycharged.Abatterypushestheconduction
electronsthroughthemetal.Theforceistheattraction
betweenunlikechargesthatwasdiscussedinChapter17.
Metalsaregoodelectricalconductorsbecausethey
containelectronsthatcanmoveaboutfreely.(Thishas
alreadybeenmentionedinChapters11and17.)Theidea
isthat,inabadconductor,suchasmostpolymers,allof
theelectronsinthematerialaretightlyboundwithinthe
atomsormolecules,sothattheycannotmove.Metals
aredifferent.Whilemostoftheelectronsinametalare
tightlyboundwithintheiratoms,somearefreetomove
aboutwithinthematerial.Thesearecalledconduction
electrons(seeFigure18.6).Avoltage,suchasthat
providedbyabatteryorpowersupply,canstartthese
conductionelectronsmovinginonedirectionthrough
themetal,andanelectriccurrentflows.Sinceelectrons
haveanegativeelectriccharge,theyareattractedtothe
positiveterminalofthebattery.
galvanometer:ameterformeasuringtinyelectric
current
Figure18.7:Twowaysofpicturingwhathappensinan
electriccircuit:conventionalcurrentflowsfrompositiveto
negative;electronsflowfromnegativetopositive.
Acircuitissetupinwhichacellmakesanelectric
currentflowthroughalamp.Twoammetersare
included,onetomeasurethecurrentflowinginto
thelamp,theothertomeasurethecurrentflowing
outofthelamp.
b
c
electronflow
Whatinstrumentisusedtomeasureelectric
current?
Howshoulditbeconnectedinacircuit?
Drawitscircuitsymbol.
Drawacircuitdiagramtorepresentthis
circuit.
Addanarrowtoshowthedirectionofthe
currentaroundthecircuit.
Whatcanyousayaboutthereadingsonthe
twoammeters?
Nametwomaterialsthataregoodelectrical
conductors.
Nametwomaterialsthataregoodelectrical
insulators.
flowofelectrons
flowofconventional
current
ammeter:ameterformeasuringelectriccurrent
ampere,amps(A):theSIunitofelectriccurrent
4 currentflow
Twopictures:current
andelectrons

339. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
whatthevoltagewillbeacrossaparticularcomponent,
forexample,orhowmuchenergywillbetransferredtoa
particularlamp.
Alternatively,wecanthinkofelectronflowas,a
movementofconductionelectrons,fromnegativeto
positive.Thispicturecanallowustothinkaboutwhatis
goingoninsidethecomponentsofacircuit:whyaresistor
getswarmwhenacurrentflowsthroughit,forexample,
orwhyadiodeallowscurrenttoflowinonedirectiononly.
Thesetwopicturesarebothmodels.Theelectronflow
pictureisamicroscopicmodel,sinceittellsuswhatis
goingonatthelevelofverytinyparticles(electrons).
Theconventionalcurrentpictureisamacroscopicor
largescalemodel.
Theelectronsinacircuitflowintheoppositedirectionto
theelectriccurrent.Itisanuisancetohavetoremember
this.Itstemsfromtheearlydaysofexperimentsonstatic
electricity.BenjaminFranklinrealisedthattherewere
twotypesofelectriccharge,whichhecalledpositiveand
negative.Hehadtochoosewhichtypehewouldcall
positive.Hechosetosaythatwhenamberwasrubbed
withasilkcloth,theamberacquiredanegativecharge.
Franklinwassettingupaconvention,whichother
scientiststhenfollowed-hencetheterm‘conventional
current’.Hehadnowayofknowingthatelectronswere
beingrubbedfromthesilktotheamber,buthischoice
meansthatwenowsaythatelectronshaveanegative
charge.Rememberthatconventionalcurrentandelectron
flowmoveinoppositedirectionsaroundacircuit.
Currentandcharge
Anammetermeasurestherateatwhichelectriccharge
flowspastapointinacircuit.Theelectriccurrentis
definedasthechargepassingapointinthecircuitper
unitoftime(usuallypersecond).Wecanwritethis
relationshipbetweencurrentandchargeasanequation
usingthequantitiesandsymbolsgiveninTable18.1:
current(A)=
charge(C)
time(s)
Table18.1:Symbolsandunitsforsomeelectricalquantities.
Quantity
Symbolfor
quantity
Unit
Symbol
forunit
current / amps A
charge Q coulombs C
time t seconds s
Soacurrentof10Apassingapointmeansthat10Cof
chargeflowspastthatpointeverysecond.Youmayfind
iteasiertorecallthisrelationshipinthefollowingform:
charge(C)=current(A)xtime(s)
Q=H
So,ifacurrentof10Aflowsaroundacircuitfor5s,
then50Cofchargeflowsaroundthecircujt.
WorkedExample18.1showshowtocalculatethecharge
thatflowsinacircuit.
WORKEDEXAMPLE18.1
1Acurrentof150mAflowsaroundacircuitfor
oneminute.Howmuchelectric(jhargeflows
aroundthecircuitinthistime?
Step1:Writedownwhatyouknow,andwhat
youwanttoknow.Putallquantitiesin
theunitsshowninTable18.1.
1=150mA=0.15A(or150xIO’3A)
t=1minute=60s
e=?
Step2:Writedownanappropriateformofthe
equationrelatingQ,/andt.Substitute
valuesandcalculatetheanswer.
Q-It
g=0.15Ax60s=90C
Answer
90coulombsofchargeflowaroundthecircuit.
Questions
4aInwhichdirectiondoesconventionalcurrent
flowaroundacircuit?
bInwhichdirectiondoelectronsflowaround
acircuit?
5aWhatistheunitofelectriccurrent?
bWhatistheunitofelectriccharge?
326y

340. 18Electricalquantities
6aHowmanymilliampsaretherein1amp?
bHowmanymicroampsaretherein1amp?
7Whichofthefollowingequationsshowsthecorrect
relationshipbetweenelectricalunits?
1A=1— 1C=14
s s
8Calculatethemissingvaluesa-dinTable18.2.
Showallyourworking.
Table18.2
Charge Current Time
charge current time
220C 2A a
57.6C b 3hours
c 0.5A 9minutes
5.4C 70mA d
18.2Voltagein
electriccircuits
Figure18.8showsacircuitinwhichacellpushesacurrent
througharesistor.Thecellprovidesthevoltageneeded
topushthecurrentthroughtheresistor.Here,‘voltage’
isaratherlooseterm,andweshouldsaythatthereis
apotentialdifference(p.d.)acrosstheresistor.Potential
differenceisdefinedastheworkdonebyaunitcharge
passingthroughacomponent(aresistor,inthiscase).Itis
measuredinvolts(V)usingavoltmeter.Thep.d.isalsothe
differenceinelectricalpotentialbetweentwopoints:the
pointwherethecurrententersacomponentandwhere
itleavesacomponent.Thisisratherlikethedifferencein
heightthatmakesaballrolldownhill.
Voltmetersarealwaysconnectedinparallelwitha
component,thatis,acrossacomponent.Thiscircuitis
calledaparallelcircuit,wherecomponentsareconnected
inbranchesacrossthecircuit.Voltmeterscaneitherhave
ananaloguedisplay(withaneedle)oradigitaldisplay
(withanumericalvalue).Thereadingonavoltmeterisin
volts(V)buttheyhavedifferentranges.Smallervoltages
maybemeasuredinmillivolts(mV)ormicrovolts(pV).
Takecarenottoconfuseitalic,V,whichisusedasthe
symbolforanunknownpotentialdifferenceorvoltage,
withupright,V,whichisusedasthesymbolfortheunit,
volts.Youcantellthedifferenceinbooks,butyoucannot
easilytellthedifferencewhentheyarewritten.
voltmeter
Figure18.8:Thecellprovidesthep.d.neededtopush
thecurrentaroundthecircuit.Theamountofcurrent
dependsonthep.d.andtheresistanceoftheresistor.The
ammetermeasuresthecurrentflowingthroughtheresistor.
Thevoltmetermeasuresthep.d.acrossit.Thiscircuitcan
thereforebeusedtofindtheresistanceoftheresistor.The
ammeterisconnectedinseriesinthecircuit.Thevoltmeter
isconnectedinparallelinthecircuit.
Thereisaspecialnameforthep.d.acrossacell.Itiscalled
theelectromotiveforce(e.m.f.)ofthecell,anditisalso
measuredinvolts.Thetermcanbemisleadingsincee.m.f.
isavoltage,notaforce.Anycomponentthatpushesa
currentaroundacircuitissaidtobeasourceofe.m.f.,for
example,cells,batteries,powersuppliesanddynamos.The
e.m.f.isdefinedastheelectricalworkdonebyasourcein
movingaunitchargearoundacompletecircuit.
KEYWORDS
voltage:theenergytransferredorworkdoneper
unitcharge;itcanbeimaginedasthepushofa
batteryorpowersupplyinacircuit
potentialdifference(p.d.):theworkdoneby
(aunit)chargepassingthroughanelectrical
component;anothernameforthevoltage
betweentwopoints
volts(V):theSIunitofvoltage(p.d.ore.m.f.);
1V=1J/C
voltmeter:ameterformeasuringthep.'d.
(voltage)betweentwopoints
electromotiveforce(e.m.f.)rtheelectricalwork
donebyasource(cell,battery,etc.)inmoving(a
unit)chargearoundacircuit;thevoltageacross
theterminalsofasource
327>

341. >
CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
Questions
9aWhatdothelettersp.d.standfor?
bWhatmeterisusedtomeasurep.d.?
cDrawthesymbolforthismeter.
10aWhatnameisgiventothep.d.acrossacellor
battery?
bWhatunitisthismeasuredin?
Combininge.m.f.s
Manybattery-operatedelectricalappliancesneedmore
thanonecelltomakethemwork.Forexample,aradio
mayneedfour1.5Vcells.Eachhasane.m.f.of1.5V
and,whenconnectedtogetherinseries,theygivea
combinede.m.f.of6V.Youcanseethat,whencellsare
connectedinseries,theire.m.f.saddup.
Figure18.9showssomeexamplesofthis.Ingeneral,if
cellswithe.m.f.sExandE2areconnectedinseries,their
combinede.m.f.Eisgivenby:
E=E\+E2
Youcanunderstandwhye.m.f.sadduplikethisifyou
thinkaboutwhathappenswhenelectricchargepasses
through.Forfour1.5Vcellsinseries,eachcelldoes
electricalworkoneachunitchargeasitpassesthrough,
sotheircombinede.m.f.mustbe6V.
3.0V 4.5V 1.5V
-I-I- -I-I-I- —I-H—
abe
Figure18.9:Thee.m.f.sofcellsorothersuppliesaddup
whentheyareconnectedinseries.Here,eachindividual
cellhasane.m.f.of1,5V.a,b:Morecellsgiveahigher
combinede.m.f.c:Whenonecellisconnectedthewrong
wayround,thecombinede.m.f.isreduced.
Question
11Three12Vbatteriesareconnectedinseries.
aDrawadiagramtoshowhowthesebatteries
couldbeconnectedtoalamp.
bCalculatethecombinede.m.f.ofthebatteries.
Whatisavolt?
Whydoweusehighvoltagesforourmainssupply?The
reasonisthatasupplywithahighe.m.f.doesalotof
workonthechargethatitpushesaroundthecircuit.
A230Vmainssupplydoes230Jofworkoneach
coulombofchargethattravelsroundthecircuit.
Thisgivesusaclueastowhatwemeanbyavolt.A
supplywithane.m.f.of1Vdoes1Jofworkoneach
coulombofchargeitpushesroundacircuit.Inother
words,avoltisajoulepercoulomb.
Thechemicalenergysuppliedbythecelliswhatpushes
electronsaroundacircuit.Thee.m.f.tellsyouhowmuch
workisdoneoneachcoulombofchargeasitpasses
throughthecell.Thisisdescribedbytheequation:
„
z™workdoneonthecharge(J)
e.m.f.(E)= -
charge(C)
Thebiggerthee.m.f.ofacell,themorestrongly
electronsarepushedaroundthecircuit,justlikepressure
determineshowstronglywaterispushedthroughapipe.
Thepotentialdifferenceacrossadevicesuchasalamp
isameasureofhowmuchelectricalworkisdoneby
eachcoulombasitpassesthroughacomponent.Thisis
describedbytheequation:
,__
workdonebythecharge(J)
p.d.(V)- -
charge(C)
Bothwordequationsabovecanbesummarisedasthe
wordequation:
voltage(V)=
workdoneJ
chargeC
Or,thesymbolequation:
-I
Thevoltageorpotentialdifferenceistheworkdone(or
energytransferred)perunitcharge.
Aselectriccurrentflowsthroughacircuit,thechemical
energyofthecellistransferredtothecomponentsas
internalenergy(inaresistor)orkineticenergy(inamotor!
328>

342. >
CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
Questions
9aWhatdothelettersp.d.standfor?
bWhatmeterisusedtomeasurep.d.?
cDrawthesymbolforthismeter.
10aWhatnameisgiventothep.d.acrossacellor
battery?
bWhatunitisthismeasuredin?
Combininge.m.f.s
Manybattery-operatedelectricalappliancesneedmore
thanonecelltomakethemwork.Forexample,aradio
mayneedfour1.5Vcells.Eachhasane.m.f.of1.5V
and,whenconnectedtogetherinseries,theygivea
combinede.m.f.of6V.Youcanseethat,whencellsare
connectedinseries,theire.m.f.saddup.
Figure18.9showssomeexamplesofthis.Ingeneral,if
cellswithe.m.f.sE{andE2areconnectedinseries,their
combinede.m.f.Eisgivenby:
E=E\+E2
Youcanunderstandwhye.m.f.sadduplikethisifyou
thinkaboutwhathappenswhenelectricchargepasses
through.Forfour1.5Vcellsinseries,eachcelldoes
electricalworkoneachunitchargeasitpassesthrough,
sotheircombinede.m.f.mustbe6V.
3.0V 4.5V 1.5V
-I-I- -I-I-I- —I-H—
abe
Figure18.9:Thee.m.f.sofcellsorothersuppliesaddup
whentheyareconnectedinseries.Here,eachindividual
cellhasane.m.f.of1,5V.a,b:Morecellsgiveahigher
combinede.m.f.c:Whenonecellisconnectedthewrong
wayround,thecombinede.m.f.isreduced.
Question
11Three12Vbatteriesareconnectedinseries.
aDrawadiagramtoshowhowthesebatteries
couldbeconnectedtoalamp.
bCalculatethecombinede.m.f.ofthebatteries.
Whatisavolt?
Whydoweusehighvoltagesforourmainssupply?The
reasonisthatasupplywithahighe.m.f.doesalotof
workonthechargethatitpushesaroundthecircuit.
A230Vmainssupplydoes230Jofworkoneach
coulombofchargethattravelsroundthecircuit.
Thisgivesusaclueastowhatwemeanbyavolt.A
supplywithane.m.f.of1Vdoes1Jofworkoneach
coulombofchargeitpushesroundacircuit.Inother
words,avoltisajoulepercoulomb.
Thechemicalenergysuppliedbythecelliswhatpushes
electronsaroundacircuit.Thee.m.f.tellsyouhowmuch
workisdoneoneachcoulombofchargeasitpasses
throughthecell.Thisisdescribedbytheequation:
z™workdoneonthecharge(J)
e.m.f.(E)=
charge(C)
Thebiggerthee.m.f.ofacell,themorestrongly
electronsarepushedaroundthecircuit,justlikepressure
determineshowstronglywaterispushedthroughapipe.
Thepotentialdifferenceacrossadevicesuchasalamp
isameasureofhowmuchelectricalworkisdoneby
eachcoulombasitpassesthroughacomponent.Thisis
describedbytheequation:
,__
workdonebythecharge(J)
p.d.(V)=
charge(C)
Bothwordequationsabovecanbesummarisedasthe
wordequation:
voltage(V)=
workdoneJ
chargeC
Or,thesymbolequation:
-I
Thevoltageorpotentialdifferenceistheworkdone(or
energytransferred)perunitcharge.
Aselectriccurrentflowsthroughacircuit,thechemical
energyofthecellistransferredtothecomponentsas
internalenergy(inaresistor)orkineticenergy(inamotor!
328>

343. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
3Inthecakemonstermodelofaseriescircuit,
comparethe'cakedifference'acrossthecake
monster(ormonsters)andthe'cakedifference'
acrossthecakefactory.
Optional
4Showhowyouwouldchangethecakemonster
modeltorepresentaparallelcircuit.Usingthe
model,explainwhythecurrentpassingthrough
thecell/batteryisthesumofthecurrents
passingthroughtheindividualbranchesofa
parallelcircuit.
REFLECTION
Thecakemonsteractivityisamodelfortheway
electriccircuitswork.Didithelpyouunderstand
howacircuitworksandgivemeaningtothe
differentcomponents?Forexample,didyouget
theideathatthecakerepresentsenergythatis
beingtransferredfromthefactory(cell)tothe
monsters(lamps)?Canyousuggestabetter
modelforthewayelectriccircuitswork?
18.3Electricalresistance
Ifyouuseashortlengthofwiretoconnectthepositive
andnegativeterminalsofacell(abattery)together,
youcandoalotofdamage.Thewireandthecellmay
bothgethot,asalargecurrentwillflowthroughthem.
Thereisverylittleelectricalresistanceinthecircuit,
sothecurrentislarge.Thecurrentflowinginacircuit
canbecontrolledbyaddingcomponentswithelectrical
resistancetothecircuit.Thegreatertheresistance,the
smallerthecurrentthatwillflow.
Definingresistance
Howmuchcurrentcanacellpushthrougharesistor?
Theelectricalresistanceofacomponentismeasuredin
ohms(Q).Itisdefinedasthepotentialdifferenceacross
thecomponentdividedbythecurrentpassingthroughit:
potentialdifference(V)
resistance(Q)=
current(A)
ThecircuitshowninFigure18.11illustrateshowwe
canmeasuretheresistanceofaresistor(orofanyother
component).Weneedtoknowthecurrentflowing
throughtheresistor,measuredbytheammeter.Wealso
needtoknowthep.d.acrossit,andthisismeasuredby
thevoltmeterconnectedinparallelacrossit.
KEYWORDS
resistance:ameasureofhowdifficultitisfor
anelectriccurrenttoflowthroughadeviceor
acomponentinacircuit;itisthep.d.acrossa
componentdividedbythecurrentthroughit
ohm(0):theSIunitofelectricalresistance;
1n=1v/a
Avoltmeterisalwaysconnectedacrosstherelevant
componentbecauseitismeasuringthepotential
differencebetweenthetwoendsofthecomponent.
Ammetersareconnectedinseriessothatthecurrent
canflowthroughthem.
Voltmetersareconnectedinparalleltomeasurethe
p.d.acrossthecomponent.
WorkedExample18.2andFigure18.11showhowto
calculatetheresistanceofaresistorfrommeasurements
ofcurrentandp.d.Noticethatwecanshowthecurrent
asanarrowentering(orleaving)theresistor.Thep.d.
isshownbyadouble-headedarrowtoindicatethatit
ismeasuredacrosstheresistor.Theresistanceissimply
shownasalabelonornexttotheresistor-itdoesnot
haveadirection.
330>

344. 18Electricalquantities
Whatisanohm?
Letusthinkabouttheequationthatdefineswhatwe
meanbyresistance:
Wecanseethatittakesap.d.of10Vtomakeacurrent
of1Aflowthrougha10Qresistor.Ittakes20Vto
make1Aflowthrougha20Qresistor,andsoon.Hence
resistance(inQ)tellsushowmanyvoltsareneededto
make1Aflowthroughthatresistor.Toputitanother
way:oneohmisonevoltperamp.
1Q=1—
A
InthecaseofWorkedExample18.2,itwouldtake500V
tomake1Aflowthroughthe500Qresistor.
Changingcurrent
Youcanthinkofanelectriccircuitasanobstaclerace.
Thecurrent(orflowofcharge)comesoutofthepositive
terminalofthecellandmusttravelaroundthecircuit
tothenegativeterminal.Alongtheway,itmustpass
throughthedifferentcomponents.Thegreatertheir
resistance,theharderitwillbeforthechargetoflow,and
sothecurrentwillbesmaller.
Thegreatertheresistanceinthecircuit,thesmaller
thecurrentthatflows.However,wecanmakeabigger
currentflowbyincreasingthep.d.thatpushesit.A
biggerp.d.producesabiggercurrent.Thegreaterthe
p.d.inacircuit(oracrossacomponent),thegreaterthe
currentthatflows.
Resistanceandthickness
Thisideaofanobstacleracecanhelpustothinkabout
theresistanceofwiresofdifferentshapes.Along,thin
wirehasmoreresistancethanashort,fatone.Imagine
anobstaclecoursethatincludespipesofdifferentsizes
throughwhichtherunnershavetopass.Itiseasytoget
throughashortpipewithalargediameter.Itismuch
harderwhenthepipeislongandnarrow.
Thelongerawire,thegreateritsresistance.
Thegreaterthediameterofawire,theless
itsresistance.
Questions
18aWhatistheresistanceofalampifacurrentof
5.0Aflowsthroughitwhenitisconnectedtoa
240Vsupply?
bWhenthep.d.acrossthelampisincreased,will
thecurrentflowingincreaseordecrease?
19Astudentcutstwopiecesofwire,onelongandone
short,fromareel.
aWhichpieceofwirewillhavethegreater
resistance?
bDrawacircuitdiagramtoshowhowyouwould
checkyouranswerbymeasuringtheresistances
ofthetwopiecesofwire.
331>

345. CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Measuringresistance
ThecircuitshowninFigure18.12canbeusedtofindthe
resistanceofaresistor.Thecircuithasavariablepower
supply,whichcanbeadjustedtogiveseveraldifferent
valuesofp.d.Foreachvalue,thecurrentismeasured,and
resultslikethoseshowninTable18.4arefound.Thelast
columninTable18.4showsvaluesforR,calculatedusing
R=VH.ThesecanbeaveragedtofindthevalueofR.
variable
powersupply
Figure18.12:Acircuitforinvestigatinghowthecurrent
througharesistorchangesasthevoltageacrossitvaries.
Thepowersupplycanbeadjustedtogivearangeofvalues
ofp.d.(typicallyfromOVto12V).
Table18.4:Typicalresultsforanexperimentalmeasurement
ofresistance.
p.d.P/V
$
.
Current//AResistanceR/Q
2.0 0.08 25.0
4.0 0.17 23.5
6.0 0.24 25.0
8.0 0.31 25.8
10.0 0.40 25.0
12.0 0.49 24.5
EXPERIMENTAL SKILLS18.1
Howlengthandthicknessaffectstheresistance
ofawire
Understandinghowtomeasureresistanceisreally
importantbecauseitintroducesthemeasurements
ofvoltageandcurrent,whichareessentialfor
understandingcircuits.Inmoresophisticated
(complicated)circuits,resistancecanchangesothat
adevicecanrespondtotheenvironment(for
example,alightcancomeonwhenitgetsdark).
Youwillneed:
powersupply
5insulated(coloured)wires
2crocodileclips

metreruler
2lengthsofresistancewireofdifferent
diameter
maskingtape
heatproofmattogounderneaththe
resistancewire
ammeter
voltmeter.

346. 18Electricalquantities
CONTINUED
Ifapowersupplyisnotavailablethenuseasuitable
cellorseriesofcellsbutincludeaswitchandonly
closetheswitchwhenyoutakemeasurements(to
avoiddrainingthecellorbattery).Iftwodifferent
gaugesofwirearenotavailable,lightlytwisttwo
lengthsofthesamegaugewiretogethertodouble
theeffectivecross-section.
Safety:Iftheinsulationonthewiresmeltsorgives
offpoisonousfumes,reducethevoltageyouuse.
Hotwireshavethepotentialtoburnskin.Avoid
connectingthepositiveterminalofthepower
supplydirectlytothenegativeterminal(ensurethat
thecurrenthastopassthroughtheresistancewire).
Itmaybenecessarytoplacearesistor(fixedor
variable)inserieswiththeresistancewireinorderto
reducethecurrentthroughit.Thevoltagesinvolved
aretoolowtocauseanelectricshock.
Gettingstarted
Statewhatdevicesyouwillusetomeasure
voltageandcurrentanddescribehowthey
shouldbewiredintothecircuit.
Whatvariableshouldbekeptthesame(control
variable)?
Whyisitimportanttoavoidtaking
measurementswhenthewirehaszerolength?
Predicthowtheresistanceofthewirewillvary
withthelengthofthewire.
Predicthowtheresistanceofthewirewillvary
withthediameterofthewire.
Identifytheindependentanddependent
variables.
Statetworeasonswhyitisimportanttoplota
graphofyourresults.
Youwillbeplottingagraphofyourresults.
Whatshouldbeplottedalongoneachaxis?
Method1
1SetupthecircuitasshowninFigure18.13a.Setthepowersupplyto12V.
voltmeter
Figure18.13a;Circuitdiagramfortheexperiment,b:Howtoattachthewiretotheruler.
333>

347. 2 Draw a table like Table 18.5, but remember to add more rows as you need them.

Length of resistance
wire / cm

Current / A

Voltage / V


Thin
10
20
30
Thick
10
20
30

Table 18.5

3 If your teacher has not already done so, attach the resistance wire along a metre ruler with insulating (or
masking) tape at both ends and at the centre (see Figure 18.13b).
4 Put crocodile clips at the ends of the insulated (coloured) wires that will attach to the resistance wire.
5 Attach the first crocodile clip to the resistance wire where it crosses the 0.0 cm mark on the ruler and leave
this in place throughout the experiment.
6 Attach the second crocodile clip where the resistance wire crosses the ruler at 10.0 cm and record the current
and voltage values in the table.
7 Move the second crocodile clip at 10.0 cm intervals, each time recording the current and voltage values in the
table.
8 Calculate the resistance values using ??????=
??????
??????
and record the results in the table.
















9
Plot a graph of resistance against length of the resistance wire (with the resistance along the horizontal axis)
and label it ‘thin’.
10
Repeat the experiment with thicker resistance wire (or loosely twist two wires of the same diameter together
but, if you do, ensure the teeth of the crocodile clips are in contact with both wires). Plot the graph on the
same axes for easy comparison and label it ‘thick’.
Method 2
If you do not have access to resistance wires of five different diameters,
twist wires of the same diameter together.
Keep the length of wire the same
(say, 50 cm).
1
Use a micrometer to determine the diameter of the wire or look up the wire diameter corresponding to
the SWG (standard wire gauge) value on the reel that the wire comes from.
2
Use this information to work out the cross-sectional area of the wire or wires.
3
Using wires of the same length but increasing cross-section, record the voltage and current values, and
calculate the resistance.
4
Plot a graph of resistance against the cross-sectional area.
5
Describe the relationship between resistance and cross-section.
6
How would you show that there is an inverse relationship?
334>
Resistance
/ Ω

348. 18Electricalquantities
Questions d
e
a
f
2
3
18.4Moreabout
21a
electricalresistance
b
22a
b
V=IR
23
Questions
Table18.6
CONTINUED
Copyeachstatementchoosingthecorrect
answerfromthebrackets.
Whatcurrentflowswhenap.d.of7.5Visconnected
acrossa2kQresistor?
Theresistanceofaresistancewire
{increases/decreases}whenitsdiameter
increases.
Theresistanceofaresistancewire
{doubles/halves}whenitscross-sectional
areadoubles.
Thismeansthattheresistanceis
{inversely/directly}proportionaltoits
cross-sectionalarea.
Whatp.d.isneededtomakeacurrentof2.0A
flowthrougha30Qresistor?
Withoutcalculation,whatp.d.isrequiredwhen
theresistanceisdoubled?
Ap.d.of240Vacrossaresistorcausesa
currentof80mAtoflowthroughit.Whatis
theresistanceoftheresistor?
Whatp.d.wouldcauseacurrentof40mAto
flowthroughtheresistor?
20Calculatethemissingvaluesa-dinTable18.6.
Showallyourworking.
TheequationR=y
isusedtocalculatetheresistanceof
acomponentinacircuit.Wecanrearrangetheequation
intwowayssothatwecancalculatecurrentorp.d.:
Extrapolateyourresistanceagainstlength
graph(Method1)towardstheorigin.Whatis
theresistancewhenthelengthoftheresistance
wireiszero?
,Useyouranswertothepreviousquestionto
suggestwhyadirectconnectionbetweenthe
terminalsofthepowersupplyisnotagoodidea.
Theresistanceofaresistancewire
{increases/decreases}whenitslength
increases.
Theresistanceofaresistancewire
{doubles/halves}whenitslengthdoubles.
Thismeansthattheresistanceis
{inversely/directly}proportionaltoits
length.Astraight-linegraphpassing
throughtheoriginwillshowthis.
So,forexample,wecancalculatethecurrentthatflows
througha20firesistorwhenthereisap.d.of6.0V
acrossit.ThecurrentIis:
^^
=0.30A
20ft
Current-voltagecharacteristics
WecanusethedatashowninTable18.4toplotagraph
ofcurrentagainstpotentialdifferenceforaresistor.
ThisgraphisshowninFigure18.14andisknownasa
current-voltagecharacteristic.
Thep.d.Kisonthex-axis,becausethisisthe
quantitywevary.Itistheindependentvariable.
Thecurrent7isonthej-axis,becausethisisthe
quantitythatvariesaswechangeV.Itisthe
dependentvariable.
Potential
difference/V
Current
/A
Resistance
/0
240 2 a
12 b 3000
c 0.5 15
120 80 d

349. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Figure18.14:Thecurrent-voltagecharacteristicforthedata
inTable18.4.
Inthiscase,thegraphisastraightlinethatpasses
throughtheorigin.Thisiswhatweexpectbecausethe
y
equationI=—
showsthatthecurrent/isproportional
A
tothep.d.,V.
Aresistorwithacurrent-voltagecharacteristiclike
thisiscalledanohmicresistor.Itiseasytopredictthe
currentthatwillflowthroughanohmicresistorbecause
thecurrentisdirectlyproportionaltothep.d.acrossit.
Doublethevoltagegivesdoublethecurrent,andsoon.
Figure18.15showswhathappensifweuseafilament
lampinsteadofanohmicresistor.Youcanseethatthe
current-voltagecharacteristicforthefilamentlamp
iscurved.
Figure18.15:Thecurrent-voltagecharacteristicfora
filamentlamp.Thegraphisnotastraightlinethroughthe
origin,showingthatthelampisnotanohmicresistor.
Atfirst,forlowvoltages,thegraphisstraight,
showingthatthecurrentincreasesatasteadyrateas
thevoltageisincreased.
Athighervoltages,the£raphstartstocurveover.
Thecurrentincreasesmoreandmoreslowlyasthe
voltageisincreased.Currentisnotproportionalto
voltage.
Thegraphshowsthatthelampisnotanohmicresistor.
Whyisthis?Atfirst,whenthevoltageandcurrentare
small,thelampbehaveslikeanohmicresistor.However,
asthevoltageincreases,thecurrentcausesthefilament
togethotandglowbrightly.Athightemperatures,the
filamenthasahigherresistanceandsothecurrentdoes
notincreaseasrapidlyasitwoulddoifthefilamenthad
remainedcool.
Thismeansthattheresistanceofthetungstenfilament
lampincreasesasthecurrentincreases.Butwhydoes
increasingthecurrentmakethetungstenhotter?This
requiresamoredetailedexplanation.
Increasingthecurrentleadstoanincreaseinthenumber
ofelectronsflowingthroughthetungstenwireandthis
increasesthenumberofcollisionsbetweentheelectrons
andthelattice(theregulararrangement^fatomsinthe
metal).Someofthekineticenergyfromtheelectronsis
transferredintointernalenergy,whichmakesthelattice
vibratemore.Thisincreasestheresistancebecauseit
increasesthenumberofcollisionsbetweentheelectrons
andthelattice.Inasimilarway,itismoredifficultto
movethroughacrowdwherepeoplearemovingin
randomdirectionscomparedtoacrowdofpeoplewho
arestandingstill.
'
KEYWORDS
current-voltagecharacteristic:agraphof
currentontheverticalaxisandvoltageonthe
horizontalaxis
ohmicresistor:hasaconstantresistance;itsf-V
characteristicisastraightline,sothatthecurrent
throughitisdirectlyproportionaltothevoltage
acrossit
Figure18.16showsthetypicalshapesofthecurrent¬
voltagecharacteristicsforohmicresistors,forafilament
lampandadiode.InFigure18.16a,resistorQhasa
higherresistancethanresistorP.Wecantellthisbecause
thecurrentflowingthroughQisalwayslessthanthe
currentthroughP,foranyvoltage.
336y

350. 18Electricalquantities
eg.50V
reverse
conventionalcurrent
Figure18.16:Typicalcurrent-voltagecharacteristics,a:Fortwoohmicresistors,b:Forafilamentlamp,c:Foradiode.
Question
26A2.0metrelengthofwirehasaresistanceof4.0Q.
a
b
Questions
24
25LookatthegraphshowninFigure18.16b.Howcan
youtellfromthegraphthatthelamp’sresistance
increasesasthep.d.acrossitincreases?
LookatthegraphshowninFigure18.16a.Howcan
youtellfromthegraphthattheresistorsareboth
ohmic?
Supposethatwehavea4.0metrelengthofwire.
Itsresistanceis100fl.Whatwillbetheresistanceofa
2.0metrelengthofwirewithtwicethecross-sectional
area?Noticethatmakingthewireshorterwillreduce
itsresistance,andincreasingitsareawillalsoreduceits
resistance.
Whatistheresistanceofapieceofthesame
wireoflength20.0metres?
Whatistheresistanceofa4.0metrewirewith
halfthecross-sectionalarea,madeofthesame
material?
Theresistanceofawireisproportionaltoitslength.
Theresistanceofawireisinverselyproportionalto
itscross-sectionalarea.
Halvingthelengthgiveshalftheresistance=50fl.
Doublingtheareahalvestheresistanceagain=25fl.
Noticethatthesegraphsshowbothpositiveandnegative
voltages.Anegativecurrentmeansoneflowinginthe
oppositedirection.Thisisachievedbyconnectingthe
cellorpowersupplytheotherwayround.Figures18.16a
andbaresymmetrical,showingthat,whicheverway
roundthecomponentsareconnected,thecurrentwillbe
thesameforagivenvoltage.
Figure18.16cistheI-Vcharacteristicofadiode,which
isnotsymmetrical.Adiodeactsasaswitchandonly
allowscurrenttoflowinonedirection.Thearrowonthe
diodesymbolindicatesthedirectionthatconventional
currentcanflow.Adiodeisasemiconductorso
itbehaveslikebothaninsulatorandaconductor.
Adiodebehaveslikeaninsulatoruntilitisgivenenough
voltage(energyperunitcharge)tomakeitbehaveas
aconductor.Forasilicondiodethisthresholdvoltage
is0.7V.
18.5Electricalenergy,
workandpower
Weuseelectricitybecauseitisagoodwayoftransferring
energyfromplacetoplace.Inmostplaces,ifyouswitch
onanelectricheater,youaregettingthebenefitofthe
energyreleasedasfuelthatisburnedinapowerstation,
whichmaybeover100kmaway.
Whenyoupluginanappliancetothemainssupply,you
areconnectinguptoquiteahighvoltage,something
like110Vor230V,dependingonwhereyoulive.This
highvoltageisthee.m.f.ofthesupply.Recallthate.m.f.
isthenamegiventothep.d.acrossanelectricalsource
componentsuchasacellorpowersupplythatpushes
currentaroundacircuit.
Lengthandarea
Wehaveseenthattheresistanceofawiredependsonits
lengthanditsdiameter.Infact,itisthecross-sectional
areaofthewirethatmatters.

351. )>CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Bytheprincipleofconservationofenergy,theenergy
giventothechargesbythecellmustequalthesumofthe
energiesgivenbythechargestothevariouscomponents.
Thismeansthatthee.m.f.acrossthecellisequalto
thesumofthepotentialdifferencesacrossallthe
componentsroundthecircuit.Thetotalvoltageacross
thecellequalsthesumofthevoltagesacrossthelamps.
ThisissomethingyouwillmeetagaininChapter19.
Batteriesandpowersuppliesgiveenergytothechargesin
acircuit.Similarly,wecanthinkaboutothercomponents
inacircuit.Forexample,asmalllampmayhaveap.d.of
1.5Vacrossit.Thismeansthateachcoulombofcharge
does1.5Jofelectricalworktopassthroughthelamp
andwilltransfer1.5Jofenergytothelamp.Remember,
workdoneisthesameasenergytransferred.
Electricalpower
Mostelectricalapplianceshavealabelthatshowstheir
powerrating.AnexampleisshowninFigure18.17.
proline
MOD.:ST44
2450MHz
230V~
50HzMICROWAVE INPUTPOWER :1550W
MICROWAVE ENERGYOUTPUT:950W
SERIALNO. 81000138
MADEINKOREA C
WARNING-HIGHVOLTAGE
Figure18.17:Alabelfromthebackofamicrowaveoven.
Powerratingsareindicatedinwatts(W)orkilowatts
(kW).Thepowerratingofanapplianceshowstherate
atwhichittransfersenergy,andindicatesthemaximum
electricalpowertheappliancedrawsfromthemains
supplywhenitisoperatingatfullpower.
Electricalpoweristherateatwhichenergyistransferred:
energytransferred(J)
power(W)=—
-———
timetaken(s)
P=
M
KEYWORDS
electricalpower:power=currentxp.d(P-VI)
Thesymbol,E,representsenergytransferred.You
shouldrecognisethisdefinitionofpowerfromChapter8.
Thisequationalsoremindsusofthedefinitionofthe
unitofpower,thewatt:onewattisonejoulepersecond,
1W=1J/s
Voltageandenergy
Wehaveseenthatthee.m.f.(voltage)ofasupplytellsus
howmuchenergyittransferstochargesflowingaround
thecircuit.Thegreaterthecurrentflowingaroundthe
circuit,thefasterthatenergyistransferred.Hencethe
rateatwhichenergyistransferredinthecircuit(the
power,P)dependsonboththee.m.f.,E,ofthesupply
andthecurrent,I,thatitpushesroundthecircuit.The
followingequationshowshowtocalculatetheelectrical
power:
power(W)=current(A)xp.d.(V)
P=IV
Youmayprefertorememberthisas,anequationrelating
units:
watts=ampsxvolts
Calculatingenergy
Sinceenergytransferred=powerxtime,wecanusethe
equationP=IVtogiveanequationforelectricalenergy
transferredEintermsofcurrentandvoltage:
energytransferred(J)=current(A)xp.d.(V)xtime(s)
E=IVt
WorkedExample18.3showshowtocalculatethepower
ofadeviceandhowmuchenergyistransferredina
giventime.
338y

352. 18Electricalquantities
WORKEDEXAMPLE18.3
Anelectricfanrunsfromthe230Vmainssupply.The
currentflowingthroughitis0.40A.Atwhatrateis
electricalenergytransferredbythefan?Howmuch
energyistransferredinoneminute?
Step1:First,wehavetocalculatetherateatwhich
electricalenergyistransferred.Thisisthepower,
P.Writedownwhatyouknowandwhatyou
wanttoknow.
F=230V
Z=0.40A
P=
f
Questions
27Writedownanequationlinkingwatts,volts
andamps.
28Calculatethemissingvaluesa-dinTable18.7.
Showallyourworking.
Voltage/VCurrent/APower/P
240 2 a
12 b 60
c 0.5 15
120 80 d
Table18.7
29A12Vpowersupplypushesacurrentof6.0A
througharesistor.Atwhatrateisenergytransferred
totheresistor?
30Atropicalfishtankisfittedwithanelectricheater,
whichhasapowerratingof50W.
Theheaterisconnectedtoa240Vsupply.
Whatcurrentflowsthroughtheheaterwhenitis
switchedon?
31Howmuchenergyistransformedbyanelectric
lampinanhourifacurrentof20mAflowsthrough
itwhenitisconnectedtoa120Vsupply?
Step2:Writedowntheequationforpower,which
involvesVandI,substitutevaluesandsolve.
P=IV
P=0.40Ax230V=92W
Step3:Tocalculatetheenergytransferredin
1minute,useE=Pt(orE=IVt).Recallthat
time,t,mustbeinseconds.
E=92Wx60s=5520J
Answer
So,thefan’spoweris92W,andittransfers5520Jof
energyeachminute.
TheequationE=IVtcouldbeusedtocalculatethe
chemicalenergystoredinabattery.Abatterydelivers
acurrentatitsratedvoltageforagiventime(untilit
discharges).Thechemicalenergystoredinabattery
canalsobefoundbymultiplyingitschargebyits
ratedvoltage.
Unitsofelectricalenergy
Likeotherstoresofenergy,wecouldcalculatethe
amountofenergytransferredelectricallyinjoules(J),
butitismuchmoreconvenienttousekWh.Thisis
because1kW=1000Wand1h=3600s,so
1kWh=1000Wx3600s=
3.6x106J(abignumber).
1kWhissometimescalledaunitofelectricity.Itisnot;
itisaunitofenergy.ItiskWhthataremeasuredusing
anelectricitymeter,liketheoneshowninFigure18.18.
Figure18.18a:Anelectricitymeter,b:Atypicalsmart
electricitymeter,whichcanbeaccessedremotelysothat
thereisnoneedforsomeonetocometoyourhousetoread
yourmeter.
339y

353. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Alldevicesinhomeshaveapowerrating.Forexample,
afanheatermighthaveapoweroutputof2kW.Ifleft
runningfortwohoursitwoulduse4kWh(fourunits
ofelectricalenergy).Thisiseasiertoimaginethan
1.44x107J.
Theequationforworkingoutthenumberofunitsof
electricalenergythatarebeingusedis:
KEYEQUATION
energytransferred=powerxtime
(kWh) (kW)(hours)
WorkedExample18.4showshowthenumberofunitsa
deviceusesdependsonhowmuchpowerittransfersand
thelengthoftimeitoperates.
WORKEDEXAMPLE18.4
Marcusswitchesonawaterheaterfortwohours.The
poweroftheheateris3.5kW.Howmuchenergyis
transferredinkWh(units)?
Step1Startbywritingdownwhatyouknow,andwhat
youwanttoknow.
power=3.5kW
time=2hours
energytransferred(kWh)=?
Step2:Nowwritedowntheequationforenergy
transferredinkWh.
energytransferred(kWh)=
power(kW)xtime(hours)
Step3:Substitutethevaluesofthequantities
ontheright-handsideandcalculate
theanswer.
energytransferred(kWh)=
3.5kWx2hours=7kWh
Answer
Thewaterheateruses7kWh(or7units).
WorkedExample18.5showshowtocalculatethecostofelectricity.
WORKEDEXAMPLE18.5
Zarachecksherelectricitybillforathreemonthperiod.
Themeterreadingatthestartwas2531kWhandat
theenditwas2647kWh.Electricitycosts16pperunit.
Whatisherbillforelectricity?
Step1:Startbywritingdownwhatyouknow,andwhat
youwanttoknow.
metrereadingatstart=2531kWh
meterreadingatend=2647kWh
costperunit=16p
unitsused=?
totalcostofelectricity=?
Step2:Workouthowmanyunitswereused.
Unitsused=2647kWh-2531kWh
=116kWh
Step3:Nowwritedowntheequationforthetotal
costofelectricity.
totalcostofelectricity=
numberofunitsxcostperunit
Step4:Substitutethevaluesofthequantitiesonthe
right-handsideandworkouttheanswer.
totalcostofelectricity=
116kWhx16p=£18.56
Answer
Zara’sbillis£18.56.
340>

354. Shiningthelightonwhatlamptouse
Designapublicawarenessandeducationcampaign
topersuadepeopletoswitchtousingLEDlamps.
Yourcampaignmustincludeaneye-catchingand
scientifically-accurateposterorleafletthatcanbe
understoodbythepublic.Onewaytoencourage
peopletoreadyourleafletistoprovideaguidefor
choosingthecorrectbulb,includingwhetherthey
needascreworbayonetfitting.
Youmustincludeatablethatcomparesthecostof
thedifferentlightbulbs.Thiscostshouldincludethe
purchasepriceplustherunningcostover20years.
Ifyouareworkinginpairsorsmallteams,youcould
alsoproduceapodcast.
Background
Manypeoplearealreadyawarethatenergy-saving
lightbulbscanreducecarbonemissionsandsave
themmoney.However,energy-savinglightbulbsare
oftenmoreexpensivetobuyandsomepeoplethink
theypreferthecolouroflightemittedbytungsten
lampsorpreferthelookofthefilamentthatcanbe
seenthroughtheglasswhenthelampisoff(Figure
18.19).Yourcampaignshouldemphasisethatthe
colouroutputofenergy-savingbulbscanmatchthat
ofthetraditionaltungstenfilamentbulb.
Energy-savinglightbulbscomeintwovarieties:
compactfluorescentlamps(CFL)andclustersoflight
emitteddiodes(LED).OlderpeoplerememberLEDs
Figure18.19:Differentlightbulbs.Fromlefttoright:
LED,tungstenfilamentlight,CFL.
whentheyproducedquiteaharshlightthatwas
highlydirectional(sendingoutabeamoflightinone
direction).Thiswasnotsuitableforlightingaroom.
LEDlightshaveimprovedandsomehaveeven
beenmadetolookexactlylikefilamentlightbulbs.
However,energy-savinglightbulbs,particularly
CFLs,containelectroniccircuitrythatisdifficultto
recycle.CFLsalsocontainmercury,whichistoxicand
adangertohealth.
Peopleneedtobeeducatedtolookforbulbsthat
giveoutthebrightnesstheywant(measuredin
lumens)insteadofthepowerconsumed(measured
inwatts).Forexample,a60Wtungstenfilamentlight
bulb,a14WCFLanda10WLEDallemit850lumens.
Aspartoftheeducationcampaign,youwillneedto
comparethecostofusingthedifferentlightbulbsall
producingthesamelightoutput(say850lumens)and
switchedonfor25000hoursoveraperiodof
18Electricalquantities
Questions
32A3kWairconditioningunitused216kWhof
electricity.Calculatehowmanydaysitwasswitched
onfor.
33Eshanrecordedthereadingsonhisfamily’s
electricitymeteratthebeginningandendofone
month.Thereadingswere990987and991013.
Electricitycosts0.5dirhamsperunit.Calculatethe
costoftheelectricalenergyusedbyEshan’sfamily
inthismonth.
34UsetheinformationinTable18.8toworkoutthe
missingvaluesa-j.Assumethatelectricitycosts16p
perunit.
AppliancePower TimekWh/unitCost
desktop
computer
a 3hours 1.3 b
microwave
oven
870W c 0.87 d
televisione 12hours 1.0 f
energy
efficient
lamp
9W 9 4.5x10-2 h
kettle 2kW3minutes i j
Table18.8
PROJECT

355. >
CAMBRIDGfIGCSB™PHYSICS:COURSEBOOK
CONTINUED
20years.InTable18.9,wehaveassumedthatthe ifyoureallydonothavetimetoresearchup-to-date
lightbulbsallemit850lumens.Whenyouresearch numbers.Bythetimeyoureadthisbook,CFLsand
lightbulbs,notethattungstenfilamentlightbulbs LEDsmayhavebecomeevenmoreefficient,theymay
aresometimescalledincandescentlightbulbsor lastlongerandthecostofelectricitywillcertainlyhave
halogenlightbulbs.OnlyusethedatainTable18.9 changed.
Typeofbulb
Quantity TungstenfilamentCFL LED
powerrating 60W 14W 10W
averagecostperbulb(£) 1 2 4
averagelifespan(hours) 1200 8000 25000
numberofbulbsneededfor25000hours
totalpurchasepriceofbulbsover20years(£)
Costofelectricityover20yearswhenelectricitycosts
£0.15perunit(£)
Totalestimatedcostover20years(£)
Table18.9:Atabletoworkouttherelativecostofthethreedifferentlightbulbs.
Optional
Includeexplanationsofhowthelampswork.Youalreadyknowhowatungstenfilamentlightbulbworks.
SUMMARY
Anelectriccurrentwillflowonlyifthereisasupplyofenergy(forexample,abattery)topushitarounda
completecircuit(thatis,iftherearenogapsinit).
Conductors(forexample,metals)allowelectriccurrenttoflowthroughthemwhileinsulators(forexample,
plastic)resisttheflowofcurrent.
Currentisaflowofelectriccharge(forexample,electrons)inacircuit.
Electriccurrentismeasuredinamperesoramps(A).
Currentismeasuredwithanammeterconnectedintoacircuitinseries.
Conventionalcurrentflowsfromthepositiveterminalofacellorbatterytothenegativeterminal.Electrons
flowintheoppositedirection.
Currentistherateatwhichelectriccharge(forexample,electrons)passesapointinacircuit:I=
@
.
Voltageorpotentialdifference(p.d.)islikethedifferenceinheightthatmakesaballrolldownhill.
Thep.d.acrossacelliscalledtheelectromotiveforce(e.m.f.).
Voltage(andp.d.ande.m.f.)ismeasuredwithavoltmeterconnectedinparallelacrosstherelevantcomponent
andtheyareallmeasuredinvolts(V).
342
>

356. 18Electricalquantities
[1]Whichofthesenon-metalscanconductelectricity?1
Aplastic Bchalk Ccarbon Drubber
[1]2Whichofthefollowingcarriesthecurrentinametallicconductor?
3
[11
C1400J D4000JA1.4J B4J
343)
CONTINUED
EXAM-STYLEQUESTIONS
negativelychargedelectrons
negativelychargedprotons
positivelychargedelectrons
positivelychargedprotons
A
B
c
D
Tochargeacameraflash,acurrentof400mAflowsfroma6.0Vbattery
for1.7s.Howmuchenergyistransferredfromthebatterytotheflash,
approximately?
W
VoltageistheworkdoneorenergytransferredperunitchargegivenbytheequationV=
Theresistanceofacomponentismeasuredinohms(Q).
Theresistanceofacircuitcomponentisthep.d.acrossitdividedbythecurrentpassingthroughitgivenby
theequationR=an^canf°undbyexperiment.
Theresistanceofawireincreaseswhenitgetslongerandresistancedecreasesasthediameterofthewiregetswider.
Theresistanceofawireisproportionaltoitslengthandinverselyproportionaltoitscross-sectionalarea.
Acurrent-voltage(I-V)characteristicisagraphwithcurrentplottedontheverticalaxisandthevoltageonthe
horizontalaxis.
WhenthegradientofanI-Vcharacteristicissmaller,theresistanceisbigger.
Theresistanceofanohmicresistorisconstantbecausethecurrentthroughitisdirectlyproportionaltothe
voltageacrossit.TheI-Vcharacteristicofanohmicresistorisastraightlinethroughtheorigin.
Afilamentlampisanexampleofacomponentthatisnon-ohmic.Asthecurrentthroughthefilamentincreases,
itgetshotandsoitsresistanceincreases.
Electriccircuitstransferenergyfromthebatteryorpowersourcetothecircuitcomponentsandthenthe
surroundings.
Electricalpoweriscurrentmultipliedbyvoltage(P=IV)andelectricalenergyis£=IVt.
Theequationforworkingoutthenumberofunitsofelectricalenergybeingusedis:
Energytransferred(kWh)=power(kW)xtime(hours)
Thecostofelectricitycanbeworkedoutusing:totalcostofelectricity=numberofunitsxcostperunit

357. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
4Whichofthecurrentvoltage(/T')characteristicsshownbelowisfora
speed=1x107m/s
separation=5kmp.d.=33MV
I=30kA
5Chargeseparationbetweenthegroundandacloudleadstoasparkcalled
lightning.Whenalightningflashoccurs,acurrentpassesthroughtheair.
Usethedatainthisdiagramtoanswerthequestions.
aStatetherelationshipbetweendistance,speedandtime. [1]
bCalculatethetimeittakesthelightningstriketocrossthegapbetween
thecloudandtheground. [2]
cStatetherelationshipbetweencharge,currentandtime. [1]
dCalculatethechargetransferredbythelightningstrike. [2]
eStatetherelationshipbetweenpotentialdifference,energytransferred
andcharge. [1]
fCalculatetheenergyinonelightningstrikeusingthechargeand
voltagevalues. [2]
gAnalternativeapproachforworkingouttheenergyinthelightning
strikedoesnotinvolveworkingoutthechargetransferred.Showthat
thismethodgivesthesameanswer. [2]
[Total:11]
COMMAND WORDS
state:expressin
clearterms
calculate:workout
fromgivenfacts,
figuresorinformation
344)

358. 18Electricalquantities
COMMAND WORD
SELF-EVALUATIONCHECKLIST
Afterstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutosee
anygapsinyourknowledgeandhelpyoutolearnmoreeffectively.
1can
See
Topic...
Needs
morework
Almost
there
Confident
tomoveon
Recallwhatisrequiredforanelectriccurrentto
flowinacircuit.
18.1
Distinguishbetweenconductorsandinsulatorsfroma
listandgiveexamplesofeach.
18.1
Statewhatacurrentismadeof. 18.1
Recalltheunitsofcurrent,voltageandresistance.
18.1,18.2,
18.3
Recallwhatdevicesmeasurecurrentandvoltageand
knowhowtoconnectthemintoacircuit.
18.1
Workoutthedirectionthatelectronsflowarounda
circuitandrecallthatthisisoppositetothedirectionof
conventionalcurrent.
18.1
*
Understandwhatacurrentisandrecallandusethe
equationthatrelatescurrent,chargeandtime.
18.1
345)
CONTINUED
6Astudentwantstomeasuretheresistanceoffilamentlamp.Sheusedan
ammeter,abattery,alampandavoltmeter.
aDrawthecircuitshebuilt. [2]
bThestudentobtainsthegraphbelowforthefilamentlamp.
Voltagein/V
iUsethegraphtofindthecurrentthroughthefilamentlampwhen
thevoltageis6.0V [1]
iiStatetherelationshipbetweenvoltage,currentandresistance. [1]
iiiCalculatetheresistanceofthelampwhenthevoltageis6.0V.
Givetheunit. [3]
ivExplaintheshapeoftheI-Vcharacteristic. [2]
[Total:9]
give:producean
answerfromagiven
sourceorrecallI
memory
explain:setout
purposesor
reasonsImake
therelationships
betweenthings
evidentIprovide
whyandIorhowand
supportwithrelevant
evidence

359. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
1can
See
Topic...
Needs
morework
Almost
there
Confident
tomoveon
Recallthenamegiventothevoltageacrossacell
orbattery.
18.2
Statetheunitsofelectromotiveforce(e.m.f.)and
potentialdifference(p.d.).
18.2
Showanunderstandingofwhate.m.f.is(notwhatit
standsfor).
18.2
Recallthenameoftheunitthatresistanceismeasuredin.18.3
Recalltheequationthatrelatesresistance,currentand
voltage.
18.3
Understandhowchangesinresistanceandvoltage
affectcurrent.
18.3
)
Describeanexperimenttodetermineresistanceusinga
voltmeterandammeter.
18.3
Relatetheresistanceofawiretoitslengthanddiameter
(withoutdoinganycalculations).
18.3
Recallandusetherelationshipsthatrelatetheresistance
ofawiretoitslengthandcross-sectionalarea.
18.3
Understandhowthegradientofacurrent-voltage
characteristicrelatestoresistance.
18.4
Sketchandexplainthecurrent-voltage(Z-k)
characteristicsofanohmicresistorandafilamentlamp.
18.4
Understandthatelectriccircuitstransferenergyfrom
thebatteryorpowersourcetothecircuitcomponents
andthenthesurroundings.
18.5
Recallandusetheequationsforelectricpowerand
energy.
18.5
CalculateelectricalenergyinkWhandworkoutitscost.18.5
346
>

360. drawandinterpretcircuitdiagrams
describehowcurrentandresistancevaryindifferentcircuits
highlightthehazardsofusingelectricityanddescribeandexplainelectricalsafetymeasures,including
fuses,circuit-breakers,andearthwires.
learnhowtocalculateresistances,currentsandvoltagesincircuits
describetheactionofdiodesandpotentialdividercircuits
>Chapter19
Electrics
circuits
INTHISCHAPTERYOUWILL

361. 1
>
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
GETTINGSTARTED
Spendtwominutesthinkingaboutthesequestions
beforecomparingnoteswiththepersonsitting
nexttoyou.Addto,orcorrect,yourownwork.Be
preparedtoshareyourthoughtswiththeclass.
Howcanyoutellaseriescircuitapartfromaparallel
circuit?
UsingwhatyourememberfromChapter18about
howtheresistanceofawiredependsonitslength
anddiameter,tryansweringthefollowingquestions:
Whydoeslesscurrentflowwhentherearetwo
lampsinseries'^
Whydoesmoretotalcurrentflowwhenthere
aretwolampsinparallel?
HOWCLOSEAREWETOCREATINGARTIFICIALINTELLIGENCE?
InChapter18wemetAlessandroVolta.In1800he
inventedthefirstbatteryandthefirstelectriccircuit.
Developmentofthefirstelectriclightbulbfollowed
almostimmediately,thoughitwasalmostacentury
beforeitwasgoodenoughtosell.Electriclighting
becamethefirstofmanyapplicationsofelectric
circuits.Forexample,electriccircuitscantransport
electricityfromwhereitisgeneratedtowhereitis
used(seeChapters7and21).Aswewillseelaterin
thischapter,circuitscanalsobeusedtoautomate
tasks,likeswitchingonalampwhenitgetsdark,and
transferringenergyinalightbulbintheprocess.
Thesemiconductortransistorisperhapsthemost
importantinventionofthelastcenturybecauseitis
muchsmallerandcheaperthanthedevice(calleda
valve)thatitreplaced.Withouttransistorswewould
nothavesmartphonesorpersonalcomputersas
thesearebasedonminiatureelectriccircuitsthat
usemillionsoftransistors.Ifsmartphoneswere
madeusingtheoldtechnology,theywouldfilla
room.Makingcomputerssmallermeanstheycan
beusedtocontrolmoreappliances.
Driverlesscarswillbecontrolledbycomputers,
whichwillmakedecisionsbasedonprograms
writtenbypeople.However,analienmightthink
thecarsareautomatons(makingdecisionsby
themselves).Robots(Figure19.1)alreadyexist
thatcouldbemistakenforartificialintelligence
(machinesthatcanthinkforthemselves).Some
scientistsworrythatweareclosetocreating
artificialintelligence,perhapsbyaccident,which
mightbeathreattoourfutureexistence.Science
fictionmovieslikeExMachinapaintableakfuture.
Figure19.1:Rico'scombatrobotfromthesciencefiction
film,JudgeDredd,1995.
Discussionquestions
1Drawatablewithtwocolumns.Inthe
leftcolumn,writedownthenamesoften
householdappliances(suchas'television')that
relyonelectricity,and,therefore,includean
electriccircuit.Intheright-handcolumn,write
downthenamesofappliancesthatdonot
needelectricity.Whatdoyounotice?
2Trydescribingourworldifelectricityhadnot
beendiscovered.
3Whatarethepotentialpositivesandnegatives
ofautomationandartificialintelligence?
348>

362. 19Electriccircuits
19.1Circuitcomponents
Figure19.2showsthecircuitsymbolsfortheelectricalcomponentsthatyouaremostlikelytomeetinthiscourse.
Youshouldtrytorememberthem.Acompletelistisgivenintheappendixattheendofthebook.Thesymbolsin
bluearesupplementarycontent.
junctionof
conductors
switch
resistor
cell battery
—
O O—
powersupply
variableresistor
fuse
potentialdivider
heater
magnetisingcoil generator transformer
lightemittingdiode d.c.powersupply a.c.powersupply
Figure19.2:Circuitsymbolsforelectricalcomponents.
Resistors
Aresistor(Figure19.3)canbeusedtocontrolthe
amountofcurrentflowingaroundacircuit.Aresistor
hastwoterminals,sothatthecurrentcanflowinone
endandouttheother.Theymaybemadefrommetal
wire(usuallyanalloy-amixtureoftwoormoremetals
withahighresistance)orfromcarbon.Carbon(likethe
graphiteinapencil)conductselectricity,butnotaswell
asmostmetals.Hencehigh-resistanceresistorstendto
bemadefromgraphite,particularlyasithasaveryhigh
meltingpoint.
349y

363. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Figure19.3:Aselectionofresistors.Somehavecolour-coded
stripestoindicatetheirvalue,andothersuseanumbercode.
Avariableresistorcanbeusedtoalterthecurrent
flowinginacircuit.Figure19.4ashowstheinsideofa
variableresistor-noticethatithasthreeterminals.
variableresistor
Figure19.4a:Avariableresistor.Theresistanceisprovided
byatrackofresistivewireorcarbon.Theresistanceinthe
circuitdependsonthepositionoftheslidingcontact,b:The
currentflowingaroundthiscircuitdependsontheposition
oftheslideronthevariableresistor.Imagineslidingthe
arrowtotheright.Thecurrentwillthenhavetoflowthrough
moreresistance,andsoitwilldecrease,c:Symbolsfora
variableresistor.
Asthecontrolisturned,thecbntactslidesoverthe
resistivetrack.Thecurrententersatoneendandflows
throughthetrackuntilitreachesthecontact,whereit
leavestheresistor.Theamountoftrackthatitflows
throughdependsonthepositionofthecontact.Variable
resistorslikethisareoftenusedforthevolumecontrol
ofaradioorstereosystem.(Youmayhavecomeacross
arheostat,whichisalaboratoryversionofavariable
resistor.)
Figure19.4bshowsanexampleofacircuitthatcontains
avariableresistor,andFigure19.4cshowstwodifferent
circuitsymbolsforavariableresistor.Notethatthe
uppersymbolhasthreeterminals(liketheresistoritself),
butthiscircuitonlymakesuseoftwoofthem.
KEYWORDS
variableresistor:aresistorwhoseresistancecan
bechanged,forexamplebyturningaknobor
movingaslider
I
light-dependentresistor(LDR):adevicewhose
resistancedecreaseswhenlightshinesonit
r
Light-dependentresistors
Alight-dependentresistor(LDR)isatypeofvariable
resistorwhoseresistancedependsofftheamountoflight
fallingonit(Figure19.5).AnLDRismadeofamaterial
thatdoesnotnormallyconductwell.Inthedark,an
LDRhasahighresistance,oftenover1MQ.However,
lightcanprovidetheenergyneededtoallowacurrentto
flow.ShinelightonanLDRanditsresistancedecreases.
Inbrightlight,itsresistancemaydecreaseto400fl.
Figure19.5a:Alight-dependentresistor.Theinterlocking
silverfingersarethetwoterminalsthroughwhichthecurrent
entersandleavestheresistor.Inbetween(black-coloured)
istheresistivematerial,b:Inthecircuitsymbol,thearrows
representlightshiningontheLDR.
350>

364. 19Electriccircuits
LDRsareusedincircuitstodetecttheleveloflight,for
exampleinsecuritylightsthatswitchonautomaticallyat
night.Somedigitalclockshaveonefitted.Whentheroom
isbrightlylit,thedisplayisautomaticallybrightenedso
thatitcanbeseenagainstitsbrightsurroundings.Ina
darkenedroom,thedisplayneedonlybedim.
Thermistors
Athermistor(Figure19.6)isanothertypeofresistor
whoseresistancedependsonitsenvironment.Inthis
case,itsresistancedependsonitstemperature.The
resistancechangesbyalargeamountoveranarrow
rangeoftemperatures.
Figure19.6a:Athermistor,b:Theresistanceofathermistor
dependsonthetemperature.Inthiscase,inthemiddleofthe
curve,itsresistancedropsalotasthetemperatureincreases
byasmallamount,c:Inthecircuitsymbol,thelinethrough
theresistorindicatesthatitsresistanceisnotfixedbut
dependsonanexternalfactor(inthiscase,thetemperature).
ForNTCthermistors,theresistancedecreasesastheyare
heated,perhapsfrom2kQatroomtemperatureto20Qat
100°C.NTCstandsfornegativetemperaturecoefficient.
Thesethermistorsareusefulfortemperatureprobes-see
thediscussionofthermometersinSection9.4.
Questions
1aDrawthecircuitsymbolforaresistor.
bDrawthecircuitsymbolforavariableresistor.
2aWhatdoesLDRstandfor?
bDrawitscircuitsymbol.
cWhathappenstotheresistanceofanLDR
whenlightisshoneonit?
3aDrawthecircuitsymbolforathermistor,
bGiveoneuseforathermistor.
cExplainwhyathermistorissuitablefor
theuseyouchoseinb.
Relays
Arelayisatypeofswitchthatworksusingan
electromagnet.Figure19.7showsthat,whenarelayis
used,therearetwocircuits:
themagnetisingcoil(electromagnet)oftherelayis
inonecircuit
theswitchisintheothercircuit.
Whenacurrentflowsthroughtherelayormagnetisingcoil
inthefirstcircuit,itbecomesmagnetised(Figure19.7).
Itpullsontheswitchinthesecondcircuit,causingitto
close,andallowingacurrenttoflowinthesecondcircuit
toturnonamotor.Thiswillbeshowninmoredetailin
Chapter20.
Thesecondcircuitofteninvolvesalargevoltage,which
wouldbedangerousforanoperatortoswitch,orwhich
couldnotbeswitchedbyanormalelectroniccircuit
(becausetheseworkatlowvoltage).Remember,whena
relayisused,therearetwocompletecircuits.
Figure19.7:Arelaycircuit.
KEYWORDS
NTCthermistor:aresistorwhoseresistance
decreaseswithincreasingtemperature
relay:aswitchcontrolledbyanelectromagnet
351>

365. ’CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Sensingcircuits
Arelaycanbeusedinacircuitthatsenseschangesin
temperatureorlightlevel.Figure19.8showsacircuit
thatwillturnonalampwhenthetemperaturerises.This
wouldbeuseful,forexample,inanindustrialfreezer.If
thefreezerfails,alargequantityoffrozenfoodcouldbe
ruined.HereishowthecircuitinFigure19.8works:
Whenthetemperatureislow,thethermistorhasa
highresistance.Thecurrentintheleft-handcircuitis
small,sotherelayremainsopen.Thereisnocurrent
intheright-handcircuit.
Whenthetemperaturerises,theresistanceofthe
thermistordecreases.Thecurrentthroughtherelay
coilincreases,pullingtherelayswitchclosed.Now
acurrentflowsintheright-handcircuitandthis
makesthelamplight.
Thiscircuitcouldbeadaptedtodetectchangesinlight
level.Forexample,thelightsinamuseumareswitched
offatnight.Athiefmightuseatorchandthiscouldbe
detectedusingalight-dependentresistorinplaceofthe
thermistor.
Figure19.8:Analarmcircuitthatusesarelay.
Question
4aRedrawthecircuitshowninFigure19.8sothat
aheateronlycomesoninthedaytime.Include
alight-dependentresistorinplaceofthe
thermistorandaheaterinplaceofthelamp.
bExplainwhytheheaterwouldbecoldwhenthe
LDRisindarkness.
cExplainwhytheheaterwouldbehotwhenlight
shinesontheLDR.
Diodes
Adiodeisacomponentthatallowselectriccurrentto
flowinonedirectiononly.Itscircuitsymbol(Figure
19.9a)representsthisbyshowinganarrowtoindicate
thedirectioninwhichcurrentcanflow.Rememberthat
thearrowinthediodesymbolshowsthedirectionin
whichconventionalcurrentcanflowthroughthediode.
Thebarshowsthatcurrentisstoppedifittriestoflowin
theoppositedirection.Itcanhelptothinkofadiodeas
beinga‘waterfall’inthecircuit(Figure19.9b).Charge
canflowoverthewaterfall,butitcannotflowinthe
oppositedirection,whichwouldbeuphill.Somediodes
giveoutlightwhenacurrentflowsthroughthem(Figure
19.9c).Adiodethatdoesthisiscalledalight-emitting
diode(LED).Again,itcanhelptothinkofthewaterfall.
Asthechargeflowsoverthewaterfall,someofthe
energyitlosesisgivenoutaslight. ,
Light-emittingdiodesarefamiliarinmanypiecesof
electronicequipment.Forexample,theyareusedasthe
smallindicatorlightsthatshowwhethera.stereosystem
ortelevisionison.Modemtrafficlightsoftenusearrays
ofbright,energy-efficientLEDsinplace®ffilament
bulbs.TheseLEDarraysuseverylittlepower,sotheyare
muchcheapertorunthantraditionaltraffic
lights.Also,
theyrequirelittlemaintenance,because,ifoneLEDfails,
theremainderstillemitlight.
chargecanflow
thisway
Figure19.9a:Circuitsymbolforadiode.Adiodeallows
currenttoflowinonedirectiononly-inthedirectionof
thearrow,b:Adiodeislikeawaterfall.Chargecanflow
downhill,butispreventedfromflowingbackuphill,c:Circuit
symbolforalight-emittingdiode.Thearrowsrepresentthe
lightthatisemittedwhenacurrentflowsthroughit.
KEYWORDS
diode:anelectricalcomponentthatallows
electriccurrenttoflowinonedirectiononly
light-emittingdiode(LED)atypeofdiodethat
emitslightwhenacurrentflowsthroughit
352>

366. 19Electriccircuits
Questions
5Adiodewillallowanelectriccurrenttoflowinone
directiononly.Usingalamp,batteryanddiode,
drawacircuitdiagraminwhich:
athelamplights
bthelampdoesnotlight.
6AfriendwantstoproducetheI-Vcharacteristicof
adiode.Drawthecircuitdiagramthathewillneed
tobuild.
19.2Combinations
ofresistors
Ifyouhavetworesistors,therearetwowaystheycanbe
connectedtogetherinacircuit:inseriesandinparallel.
Thisisillustratedfortwo10QresistorsinFigure19.10.
Itisusefultobeabletoworkoutthetotalresistanceof
tworesistorslikethis.Whatistheircombinedresistance
oreffectiveresistance?
Forthetwo10QresistorsinseriesinFigure19.10a,the
currenthastoflowthroughtworesistorsinsteadofone.
Theresistanceinthecircuitisdoubled,sothecombined
resistanceis20Q.
Forthetwo10QresistorsinparallelinFigure19.10b,
therearetwopossiblepathsforthecurrenttoflowalong,
insteadofjustone.Theresistanceinthecircuitishalved,
sotheeffectiveresistanceis5Q.
a 10Q 10Q
20Q
I I—>-
effectiveresistance
10Q
5Q
effectiveresistance
Figure19.10:Twowaysofconnectingtworesistorsina
circuit,a:Inseries,b:Inparallel.
Wehavenotreallyprovedthesevaluesforthecombined
oreffectiveresistance,butyoushouldseethattheyare
reasonablevalues.
Torecognisewhentworesistorsareconnectedinseries,
tracethepathofthecurrentaroundthecircuit.Ifallthe
currentflowsthroughoneresistorandthenthroughthe
other(asinFigure19.10a),theresistorsareconnected
inseries.Theyareconnectedend-to-end.Forresistorsin
parallel,thecurrentflowsdifferently.
Itflowsaroundthecircuituntilitreachesapointwhere
thecircuitdivides(asatpointXinFigure19.10b).Then
someofthecurrentflowsthroughoneresistor,and
someflowsthroughtheother.Thenthetwocurrents
recombine(asatpointYinFigure19.10b)andreturnto
thecell.Resistorsinparallelareconnectedside-by-side.
Resistorsinseries
Ifseveralresistorsareconnectedinseries,thenthecurrent
mustflowthroughthemall,oneafteranother.The
combinedresistance,R,inthecircuitissimplythesum
ofalltheseparateresistances.Forthreeresistorsinseries
(Figure19.1la),theequationfortheircombinedresistanceis:
R=Rt+R2+R3
Figure19.11bshowsthesamecurrent,I,flowingthrough
threeresistors.Remember,currentcannotbeused
upbecausechargeisconserved.Wecancalculatethe
combinedresistanceforthiscircuit:
combinedresistance=10Q+20Q+20Q=50Q
Sothethreeresistorscouldbereplacedbyasingle50Q
resistorandthecurrentinthecircuitwouldbethesame.
So,forresistorsinseries:
thecombinedresistanceisequaltothesumofthe
resistances
thecurrentisthesameatallpointsaroundthecircuit
thebiggertheresistance,thebiggerthep.d.acrossit.
a R, R2 R3
b
10V
Figure19.11a:Threeresistorsconnectedinseries,b:Values
ofcurrentandp.d.inaseriescircuit.Thesamecurrent,/,
flowsthrougheachofthethreeresistors.
353y

367. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Whenthecurrentthroughanelectricalconductor
isconstant,thep.d.acrossitwillincreasewhenits
resistanceincreases.Forexample,aslongasthe
combinedresistanceofthethreeresistorsinFigure
19.1lbiskeptthesame,thecurrentateverypointinthe
seriescircuitwillremainthesame.However,thereisa
biggerp.d.acrossthebiggerresistors.Infact,thep.d.
isproportionaltoresistance.Thep.d.acrossthe20Q
resistorsisdoublethevoltageacrossthe10(1resistor.
Ifthethreeresistorswerereplacedbyone50Qresistor
thentherewouldbeap.d.of10Vacrossit(equaltothe
cellvoltage).Itmightseemoddtothinkofconductors
havingresistancebuteventhebestconductorshave
resistance(althoughsuperconductivity-conductionwith
zeroresistance-isanareaofactiveresearch).
Resistorsinparallel
Thelightsinaconventionalhouseareconnectedin
parallelwithoneanother.Thereasonforthisisthateach
onerequiresthefullvoltageofthemainssupplytowork
properly.Iftheywereconnectedinseries,thep.d.would
besharedbetweenthemandtheywouldbedim.In
parallel,eachonecanbeprovidedwithitsownswitch,so
thatitcanbeoperatedseparately.Ifonelampfails,the
othersremainlit.
Theeffectiveresistanceofseveralresistorsconnectedin
parallelislessthanthatofanyoftheindividualresistors.
Thisisbecauseitiseasierforthecurrenttoflow.Youcan
seethisfortworesistorsinparallelinFigure19.12a.The
currentflowingfromthesourcedividesupasitpasses
throughtheresistors.Figure19.12bshowsthecurrent
fromthepowersupplysplittingupandpassingthrough
tworesistorsinparallel.
Figure19.12a:Tworesistorsconnectedinparallel,b:Values
ofcurrentandp.d.inaparallelcircuit.Thecurrentflowing
fromthesupplyissharedbetweentheresistors.
So,fortworesistorsinparallel:
theeffectiveresistanceislessthantheresistanceof
eitherresistor
thecurrentfromthesourceisgreaterthanthe
currentthrougheitherresistor.
Questions
7Whataretheadvantagesofconnectinglampsin
parallelinalightingcircuit?
8Threeresistorsareconnectedinserieswithabattery,
asshowninFigure19.13.
ResistorAhasthegreatestresistanceofthethree.
ThecurrentthroughAis1.4A.Whatcanyousay
aboutthecurrentsthroughBandC?
9Whatisthecombinedresistanceofthree30Q
resistorsconnectedinseries?
Voltageinseriescircuits
Whenresistorsareconnectedinserieswitheachotherin
acircuitwithapowersupply,thereisap.d.acrosseach
resistor.Fromthenumerica)exampleshowninFigure
19.1lb,youcanseethataddingupthep.d.sacrossthe
threeseparateresistorsgivesthep.d.ofthepowersupply.
Inotherwords,thep.d.ofthesupplyissharedbetween
theresistors.Wecanwritethisasanequation:
V=Vl+V2+V3
Festivelights,suchasthoseusedtocelebratedifferent
festivals,areoftenwiredtogetherinseries.Thisis
becauseeachbulbworksonasmallvoltage.Ifasingle
bulbwasconnectedtothemainssupply,thep.d.across
itwouldbetoogreat.Byconnectingtheminseries,the
mainsvoltageissharedoutbetweenthem.Thereisa
disadvantage;ifonebulbfails(itsfilamentbreaks),they
allgoout.Thisisbecausethereisnolongeracomplete
circuitforthecurrenttoflowaround.
354>

368. 19Electriccircuits
WORKEDEXAMPLE19.1
Three5.0flresistorsareconnectedinserieswitha
12Vpowersupply.Calculatethecombinedresistance
ofthethreeresistors,thecurrentthatflowsinthe
circuit,andthep.d.acrosseachresistor.
Step1:Drawacircuitdiagramandmarkonitall
thequantitiesyouknow.Addarrowstoshow
howthecurrentflows(Figure19.14)
Figure19.14:Sketchdiagram.
Step2:Calculatethecombinedresistance.
R=Ry+R2+R2
7?=5.On+5.OQ+5.OQ
R-I5fl
Step3:Calculatethecurrentflowing.Ap.d.of12V
ispushingcurrentthrougharesistorof15fl
totalresistance.So:
y
currentI=—
_
12V
15fl
=0.8A
Step4:Calculatethep.d.acrossanindividual
5.0flresistorwhenacurrentof0.8Aflows
throughit.
p.d.F=/J?=0.8Ax5.0fi=4.0V
Notethatthe12Vofthesupplyissharedout
equallybetweentheresistors,sinceeachhasthesame
resistance.So,wecouldhaveworkedoutthep.d.
acrossanindividualresistorwithoutknowingthe
current(12V-i-3=
4V).
Answer
Thecombinedresistanceofthethreeresistorsis15Q,
thecurrentthatflowsinthecircuitis0.8A,andthe
p.d.acrosseachresistoris4.0V.
Question
10One4flresistorandone6flresistorareconnected
inaseriescircuitwitha6Vpowersupply.Calculate:
athecombinedresistanceofthetworesistors
bthecurrentthatflowsinthecircuit
cthep.d.acrosseachresistor.
Potentialdividercircuits
Often,apowersupplyorabatteryprovidesafixed
potentialdifference.Toobtainasmallerp.d.,ora
variablep.d.,thisfixedp.d.mustbesplitupusinga
circuitcalledapotentialdivider.Figure19.15showstwo
formsofpotentialdivider.
KEYWORDS
potentialdivider:partofacircuitconsisting
oftworesistorsconnectedinseriestoobtaina
smallervoltagethansupplied
InthecircuitshowninFigure19.15a,tworesistorsR^
andRbareconnectedinseriesacrossthe6Vpower
supply.Thep.d.acrossthepairisthus6V.(Ithelps
tothinkofthebottomlineasrepresenting0Vand
thetoplineas6V.)Thep.d.atpointX,betweenthe
tworesistors,willbepart-waybetween0Vand6V,
dependingonthevaluesoftheresistors.Iftheresistors
areequal,thep.d.atXwillbe3V.Thep.d.ofthesupply
willhavebeendividedinhalf,hencethename‘potential
divider’.
Toproduceavariableoutput,wereplacethe
tworesistorswithavariableresistor,asshownin
Figure19.15b.Byalteringtheresistanceofthevariable
resistor,thevoltageatXcanhaveanyvaluebetween0V
and6V.
355)

369. b 6V
V=6V
ov
Figure19.15a:Asimplepotential-dividercircuit.Theoutput
voltageisafractionoftheinputvoltage.Theinputvoltageis
dividedaccordingtotherelativevaluesofthetworesistors,
b:Avariableresistorisusedtocreateapotentialdivider
circuit,whichgivesanoutputvoltagethatcanbevaried.
Question
11aTworesistorsaregoingtobeconnectedto
formapotentialdividercircuit.Shouldthey
beconnectedinseriesorinparallelwith
eachother?
bStatebrieflythefunctionofapotential
dividercircuit.
Normally,whentheresistanceofanelectricalconductor
isincreased,thecurrentthroughitdecreases.However,
thecurrentthroughapairofresistorswillbeconstantif
theirtotalresistanceisconstant.Iftheresistanceofone
resistorisincreased,theresistanceoftheotherresistor
hastobedecreasedtomaintainthesametotalresistance
andthesameconstantcurrent.Thep.d.willbebigger
acrossthebiggerresistor.Thisiswhatishappeningin
Figure19.15b.Thecurrentthroughthevariableresistor
isconstantbuttheresistance(andthevoltage)aboveand
belowXchanges.
Moreaboutpotentialdividercircui
InthecircuitshowninFigure19.16a,thep.d.acrosstl
lampisequaltothee.m.f.acrossthecell(thatis,9V).
Figure19.16:Potentialdividercircuits.
ThecircuitinFigure19.16bhastwolampsinserieswitl
a9Vcell.Imaginethattheresistanceofbothlampsis
1fl.Wecancalculatethecurrentthroughthisseries
circuitusingtheequationR=
weusedtodefineresistance.
V
1'
whichistheequation
V_
9V
R2fl
=4.5A
Thisallowsustoworkoutthevoltageacrosseachlamp
usingV=IR:
^
=4.5Ax1Q=4.5V
F2=4.5Ax1fl=4.5V
Noticethatthesevoltagesadduptothee.m.f.ofthecel
^ceii=+F2=4.5V+4.5V=9V
356y

370. 19Electriccircuits
Step2:
Step3:
Step4:
Question
4V
Y,=8V
Figure19.18
Figure19.17:Sketchdiagram.
CONTINUED
KEYEQUATION
WORKEDEXAMPLE19.2
Thisseemstomakessense.Thebiggertheresistanceof
thelamp,thebiggertheworkdoneperunitcharge,or
voltage,requiredtopushthechargethroughit.
WorkoutV\andFjninFigure19.18a.
WorkoutK]andR}inFigure19.18b.
12a
b
Apotentialdividercircuitisrequiredtoproducean
outputvoltageof8Vacrossaresistor,R},of600Q.
Thesupplyvoltageis12V.Whatistherequiredvalue
oftheseriesresistor,R2?
Step1:Sketchacircuitdiagramandmarkonthe
informationfromthequestion(Figure19.17).
RemembertoaddinthevoltageacrossR2
(thesumofthevoltagesacrossresistorsina
seriescircuitmustadduptothecellvoltage;
so,12V=8V+4V).
R2=
?Q
Writedownthepotentialdividerequation
appliedtothisproblem.
*i=Zl
/?2r2
Answer
Theseriesresistor,R2,needsavalueof3000.
/?=
600
=300Q
2
supplyvoltage=12Y
R,=600Q
Resistancefortworesistorsusedasapotentialdivider:
r2r2
Nowimaginethatlamp1hasaresistanceofR}=60and
lamp2hasaresistanceofR2=30(asinFigure19.16c).
ThecombinedresistanceisR=R1+R2=60+30=90.
Thistimethecurrentthroughtheseriescircuitis:
^
=^=1.0A
R90
Thistimethevoltageacrosseachlampis:
F]=1.0Ax60=6.0V
K2=1.0Ax30=3.0V
Again,thep.d.acrosseachlampaddsuptothee.m.f.of
thecell.However,thistimethep.d.acrosslamp1istwice
ashighasthep.d.acrosslamp2,whichisexactlythe
sameratioastheirresistances:
*2V2
Substitutevaluesfromthequestion.
600Q_8V_2
R24V
RearrangeandsolveforR2.

371. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Currentandresistancein
parallelcircuits
Inthesamewaythatenergyisconserved(Chapter6),
electricchargeisalsoconserved.Electronsinanelectric
circuitcannotappearfromnowhereordisappear.
Becausechargeisconserved,thechargearrivingatany
junctioninacircuitmustequalthecharge(andtherefore
thecurrent)leavingthesamejunction.Thismeansthat
thetotalcurrent(I=Q/t)comingintoajunctionmust
equalthetotalcurrent(charge/time)leavingthesame
junction.Imaginecarsarrivingatajunctionintheroad:
thenumberofcarsleavingthejunctionmustequalthe
numberofcarsthatarrived.Ifthatwerenotthecase,
thecarsandtheirpassengerswouldneedtoappearfrom
nowhereordisappear,whichwouldbequitealarming.
Therefore,weknowthatcurrentdividesuptopass
throughthebranchesofaparallelcircuit.Addingup
thecurrentsthroughthetwoseparateresistorsgivesthe
currentflowingoutofthepowersupply.Thecurrent
fromthesupplyisthesumofthecurrentsflowing
throughtheresistors:
Becausetheresistorsareconnectedsidebyside,each
feelsthefullpushofthesupply.
TocalculatetheeffectiveresistanceRfortworesistorsin
parallel,weusethisequation:
l=±+±
RR,R2
Therearetwowaystocalculatethistypeofsum:either
useacalculator,oraddupthefractionsbyfindingtheir
lowestcommondenominator.WorkedExample19.3
showshowtousethisequation,andhowtoworkoutthe
sumbyfindingthelowestcommondenominator.
WORKEDEXAMPLE19.3
One10(2resistoranda30Qresistorareconnectedin Step3:
parallelwitha12Vpowersupply.Calculate:
atheeffectiveresistanceofthetworesistors
bthecurrentthrougheachresistor
cthecurrentflowingfromthepowersupply.
Step1:Sketchacircuitdiagramandmarkonitallthe
quantitiesyouknow(Figure19.19).Addarrows
toshowhowthecurrentflows,likethis:
+
12V-
oo
R,=1OQ
"
>EZZ
Figure19.19:Sketchcircuit.
Eachresistorhasap.d.of12^acrossit.Now
wecancalculatethecurrentsusingtheequation:
12V
currentIthrough10(2resistor=
10Q
\=1.2A
12V
currentIthrough30(2resistor=
30(2
=0.4A
Noticethat,asyoumightexpect,thesmaller
(10Q)resistorhasabiggercurrentflowing
throughitthanthelarger(30(2)resistor.
Thecurrent,I,flowingfromthesupplyis
thesumofthecurrentsflowingthroughthe
individualresistors.
1=1.2A+0.4A=1.6A
Step2:Calculatetheeffectiveresistance.
7? R2
1=
_L
+
_1_
Rion30(2
1_
4
R30(2
R=7.5(2
R7.5(2
Thisisausefulwaytocheckthatyouhave
calculatedtheeffectiveresistancecorrectly.
Currentdividesinaparallelcircuit,butthe
totalamountmustremainthesame-electrons
cannotjustdisappear.
Note:wecouldhavereachedthesameresult
usingtheeffectiveresistance(7.5(2)ofthe
circuitthatwefoundinStep2:
I=^=^=
L6A
358>

372. 19Electriccircuits
CONTINUED
Answers
aThetworesistorstogetherhaveaneffective
resistanceof7.50.
bThe100resistorhasacurrentof1.2Aflowing
throughit.The300resistorhasacurrentof
0.4Aflowingthroughit.
cThereisa1.6Achargeflowingfromthepowersupply.
Questions
13Usetheideaofresistorsinseriestoexplainwhya
longwirehasmoreresistancethanashortwire(of
thesamethicknessandmaterial).
14Usetheideaofresistorsinparalleltoexplainwhya
thickwirehaslessresistancethanathinwire(ofthe
samelengthandmaterial).
15A15.00resistorisconnectedinserieswitha30.00
resistoranda15.0Vpowersupply.
aCalculatethecurrentflowingaroundthecircuit,
bWhichresistorwillhavethelargershareofthe
p.d.acrossit?
16One60resistorandone40resistorareconnected
inparallelwitha6Vpowersupply.Calculate:
atheeffectiveresistanceofthetworesistors
bthecurrentthrougheachresistor
cthecurrentflowingfromthepowersupply.
Puttingitalltogether
Wenowhaveenoughknowledgetocalculatethe
currentsandvoltagesinmorecomplexcircuits.Electrical
engineersoftensketchequivalentcircuitsinorderto
solvethemandwewilldothesame.Foreachresistor
inFigure19.20a,wearegoingtoworkoutthecurrent
passingthroughitandthepotentialdifferenceacross
it.Mostofthestepscanbedoneinadifferentorder.
Itistheproblem-solvingstrategy(orapproach)that
isimportanthere.Youmayneedtoreadthroughthis
exampleseveraltimes.Onceyouthinkyoucanfollowthe
steps,coveruptheworkingandseeifyoucansolveit
yourself.Itmaytakeyouseveralattempts.
InFigure19.20a,thereisthesamee.m.f.of9Vacross
bothbranchesofthiscircuit(thatis,9VbetweenAand
Bandas9VbetweenCandE):
e.m.f.=UAB=UCE=9V
ThecurrentthroughRt:
r
_^AB_
9V_
AB
7?!60
1.5A
C
"
D
R5=
8Q
E
Figure19.20:Circuitdiagramexamples.
ThepairofseriesresistorsR2andR^betweenCandD
canbereplacedwithasingleresistoroftheircombined
resistanceasshowninFigure19.20b:
R34=R3+R^=40+40=80
Thereisnowapairofparallelresistors(R2andR34)
betweenCandD.Theycanbereplacedwithasingle
resistor(Rcd)asshowninFigure19.20c:
_L
=±+
_L
^CD^2^34
_L
=
_L
+JL
R^d8080
1_
16
Rcd640
=40
359>

373. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
ThecombinedresistanceoftheseriesresistorsbetweenC
andE(Figure19.20c)is:
Rq^—/^cd+^5=40+8Q—12Q
ThecurrentbetweenCandEis:
T-
^CE
-
9V_
mrA
ZCe =0.75AC
*CE
12Q
Thep.d.acrossjR5:
^5=W5=
0.75Ax8Q=6V
Thep.d.acrossR^-.
VCD=/CERCD=0.75Ax40=3V
Thesepotentialdifferencesadduptothee.m.f.ofthe
source,asexpected.
ThecurrententeringjunctionA(Figure19.20a)must
equalthecurrentleavingthejunction:
^source
~^AB+/ce=1-5A+0.75A=2.25A.
IfyoulookatFigure19.20b,youwillseethatthere
aretwopathsbetweenCandD.Bothpathshaveequal
resistanceandthereisthesamep.d.of3Vacrossthem;
theymusteachcarryhalfofthetotalcurrentbetweenC
andD.Thiscanbeconfirmedbyworkingoutthecurrent
ineitherbranchbetweenCandD:
T-^CD
-^cd_
3V
R3480
0.375A
AtD,the0.375Athathascomedowntheleft-hand
branchjoinswiththe0.375Athathascomedownthe
right-handbranchsothat0.75ApassesbetweenD
andE.
Nowthatwehavethecurrentthroughthepairofseries
resistorsbetweenCandD(R3andR4)wecanworkout
thep.d.droppedacrosseach.Theirresistancevaluesare
thesamesotheyshouldeachhaveahalfshareofthe
dropinp.d.betweenCandD(thatis,1.5Vacrosseach
ofthem).Wecanconfirmthis:
F=ZCD
xR3=Icd
xl?4=0.375Ax40=1.5V
Allthecurrentandvoltagevaluesaresummarisedin
Figure19.21andTable19.1.
A 0.75"A
c
0.375A
Figure19.21
Resistor Source 1 2
Voltage/V 9.0 9.0 3.0
Current/A 2.25 1.5 0.375
Resistor 3 4 5
Voltage/V 1.5 1.5
Current/A 0.375 0.375 0.75
Table19.1
\
Question
17Workoutthecurrentthrough,andthevoltage
across,eachlampinthecircuit(Figure19.22).
SummariseyourresultsinatablelikeTable19.1.
Figure19.22
360>

374. 19Electriccircuits
ACTIVITY19.1
Seriesandparallelcombinationsofresistors
Task1:Thinkingaboutresistorcombinations
Yourteacherwillgiveyousometimetowritedownanswerstothefollowingquestions:
Whatisusedtomeasurecurrentandhowisitincludedincircuit?Drawasketchifithelps.
Whatisusedtomeasurevoltageandhowisitincludedincircuit?Drawasketchifithelps.
Howcantheresistanceofanelectricalcomponent(suchasalamp)bemeasured?
Howcanwecalculatethetotalresistanceforresistorsinseries?
Howcanwecalculatetheeffectiveresistancefortworesistorsinparallel?
Howisitpossibletomakemorethanonecellintoabatteryanddoesitmatterwhichwayroundthe
cellsare?
Thetwocircuits,whicharedescribedbelow,usetwolamps,a3.0Vbattery,andaswitch.Assumethateach
lamphasaresistanceof5fl.Usethisinformationtoworkoutthemissingvalues.Showyourworkingandthen
writeyouranswersontoacopyofTables19.2and19.3.Leavetheshadedboxesblank.
Part1:Seriescircuit
Useapencilandarulertodrawaneatseriescircuitwithtwolampsinserieswitha3.0Vbatteryand
aswitch.
Table19.2
Calculatedcurrent/A Calculatedvoltage/V Resistance/fl
battery 3.0
lamp1
lamp2
circuit
Part2:Parallelcircuit
Useapencilandarulertodrawaneatcircuitusingthesamecomponentsyouusedfortheseriescircuit
withthelampsinparallel.Makeitclearwhereyouthinktheswitchshouldbeplaced.
Table19.3
Calculatedcurrent/A Calculatedvoltage/V Resistance/fl
battery 3.0
lamp1
lamp2
circuit
Yourteacherwilldecidehowyouwillcheckyouranswersandhelpresolveanymisunderstandings.
361y

375. CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
Part3:Designyourown
1Designaslightlymorecomplicatedcircuit(forexample,twoparallellampsinserieswithathirdlamp).
2Predictthevoltagesandcurrentsforallthreelampsandthebattery.
3Writeaquestionbasedonyourcircuittotestanotherstudentorpairofstudents.
Task2:Buildingresistorcombinations(optional)
IfyouhavethetimeandequipmentyoumaygetthechancetobuildthecircuitsyouhavedrawninTask1.
Alternatively,youcouldbuildvirtualcircuitsinanonlinesimulation.Ifyouarebuildingphysicalcircuitsyou
willneed:
two2.5Vlampsinholders voltmeter
atleasttwo1,5VDcellsincellholders ammeter
atleastnineelectricalleads switch.
Ifyouarebuildingphysicalcircuitsyouneedtobeawareofthefollowingthings.
Theactualvalueofacomponentwillbedifferentfromthevalueprintedonitscase.Whenusingyourcircuits,
ensurethattheswitchisclosed(pressed)onlywhentakingmeasurements,otherwisethecellswilldrainquickly
andthee.m.f.willchange.Theresistanceofdifferentlampswillalsovarysothep.d.acrossthemwillnotbe
thesame.
The2.5Vlampsneedap.d.ofatleast2.0Vtolightup.However,yourcircuitsshouldstillwork(givingsensible
ammeterandvoltmeterreadings)whenthelampsdonotlightup.
Ifyouuseacopyofthetables(Tables19.4and19.5)torecordyourresults,leavetheshadedboxesblank.The
numberatthetop-lefthandcornerofeachcellreferstothestepnumber.Thisistomakeiteasierforyouto
findwhereyoushouldbewritinginyournumbers.Doyourworkinginyourexercisebook.
Ignore(donotrecord)anynegativesignsonthevoltmeterorammeter.Anegativesigntellsyouthatthemeteris
connectedthewrongwayround(thecurrentispassinginthewrongdirection)butthisdoesnotaffectthevalue.
Part1:Seriescircuit
1Useapencilandarulertodrawaneatseriescircuitwithtwo2.5Vlampsinserieswitha3.0Vbattery,a
switchandanammeter.
2Buildthecircuityouhavealreadydrawn.Labelthelampsas'lampTand'lamp2'andkeeptrackofthem.
3Bymovingtheammeter,measurethecurrentnexttolamp1,lamp2andthebattery.Recordyour
valuesinatablelikeTable19.4.
Table19.4
Current/A Voltage/V Resistance/Q
predictedmeasured predicted measured predictedcalculated
battery
1.3 1.4
lamp1
1.3 1.4 1.7
lamp2
1.3 1.5 1.6 1.7
circuit
1.8
4Measurethee.m.f.acrossthebatteryandthep.d.acrosslamp1andrecordyourvalues.
362>

376. 19Electriccircuits
8
Part2:Parallelcircuit
Table19.5
5
6
CONTINUED
5
6
2
3
8
9
10
11
Usingyourvaluesfromstep4,predictthep.d.acrosslamp2andrecordyourprediction.
Measurethep.d.acrosslamp2andrecordyourvalue.
Workouttheresistanceforeachlampbydividingthep.d.acrossitbythecurrentpassingthroughitby
usingR=
y
.Recordyourvalues.
Workoutthetotalresistanceofthetworesistorsinseries.Usethetotalresistanceoftheseriescircuit
andthebatterye.m.f.toworkoutwhatthecurrentshouldbeandcomparethistothevalueyou
actuallymeasured(inpart1,step3).
Useapencilandarulertodrawaneatparallelcircuitusingthesamecomponentsyouhavealready
usedwiththeseriescircuit.
Buildthecircuityouhavejustdrawn.
Usethee.m.f.ofthebatterytopredictthevoltageacrossthebatteryandeachlamp.Recordyour
predictionsinatablelikeTable19.5.
Part3:Designyourown
Nowthatyouhavesomeexperience,lookbackatthecircuityoudesignedyourself(Task1,part3).Usethe
valuesyouhavefoundforthebatterye.m.f.andtheresistancevaluesoflampstoworkoutthecurrentand
voltagevaluesthroughoutyourcircuit.Nowbuildthecircuitandcheckifyouwerecorrect.
Measureandrecordthee.m.f.acrossthebatteryandthep.d.acrosseachlamp.
Usetheresistancevaluesyouhavealreadycalculated(frompart1,step7)topredictthecurrent
througheachlamp(I=^).
Measureandrecordthecurrentthrougheachlamp.
Usethemeasuredcurrentsthroughthetwolampstopredictthecurrentthroughthebattery.
Measurethecurrentthroughthebatteryandcomparethistothevalueyoupredictedinstep7.
Usethevoltageandcurrentdatayouhavecollectedfortheparallelcircuittocalculatetheresistanceof
eachlampandrecordyourvalues.
Calculatetheeffectiveresistanceofthecircuitandrecordyourvalue.
Usethebatterye.m.f.andtheeffectiveresistanceoftheparallelcircuittopredictthecurrentthrough
thebattery.Compareittoyourmeasurement(inpart2,step8).
Current/A Voltage/V Resistance/0
predictedmeasured predictedmeasured predictedcalculated
battery
2.7 2.8 2.3 2.4
lamp1
2.5 2.6 2.3 2.4 2.9
lamp2
2.5 2.6 2.3 2.4 2.9
circuit
2.10

377. }CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
Didthecurrentchangeasitflowedaroundtheseriescircuit?
Whatistherelationshipbetweenthep.d.acrossthelampsandthee.m.fofthebatteryintheseriescircuit?
Whatistherelationshipbetweenthep.d.acrossthelampsandthee.m.f.ofthebatteryinaparallelcircuit?
Whatistherelationshipbetweenthecurrentthroughthelampsandthecurrentthroughthebatteryina
parallelcircuit?
Aretheresistancevaluesforthelampsinthetwocircuitsdifferent?Iftheyare,canyousuggestwhy?
Showalgebraicallythatfortworesistorsinparallel,theeffectiveresistanceistheproductoftheirvalues
dividedbythesumoftheirvalues:R=
Ri+R2
PEERASSESSMENT
Lookatthecircuitdiagramsdrawnbyyour
neighbourandlookathowtheybuiltthecircuits
basedontheirdiagrams.Giveyourneighbour
feedbackonhowtheydrewtheircircuits.The
circuitsshouldhavebeendrawnclearlyusingthe
correctsymbols,withapencilandaruler.Ifyour
neighbourhaddifficultydrawingorbuildingthe
circuitscanyougivethemsomesuggestionsthat
mightmakeiteasierforthem?
REFLECTION
Thisactivityinvolveddrawingcircuitsfroma
descriptionandthenbuildingthecircuit.
Ratehoweasyyoufoundboththesestepsona
scalefrom1(veryeasy)to5(veryhard).
Ifyoufoundeitherorbothstepsdifficult,what
wouldmakethemeasier?
ACTIVITY19.2
HowmuchdoIknowaboutcircuits?
Spendacoupleofminuteslabellingacopyof
thecircuitdiagramsinFigure19.23withasmuch
informationasyouknow,includingarrowstoshow
thedirectionofthecurrent.Assumeeachlamphas
adifferentresistance.Explainwhatishappening
atdifferentpointsofthecircuitandincludethe
followingwords:current,potentialdifference,
junction.
MakeatablelikeTable19.6andfillitinto
summarisetherulesforcurrentandvoltagein
seriesandparallelcircuits.
Table19.6
Current Voltage
series
parallel
CONTINUED
364>

378. 19Electriccircuits
19.3Electricalsafety
Mainselectricityishazardous,becauseofthelarge
voltagesinvolved.Ifyoucomeintocontactwithabare
wireat230V,youcouldgetafatalelectricshock.Here,
wewilllookatsomeaspectsofthedesignofelectrical
systemsandseehowtheycanbeusedsafely.
Electricalcables
Thecablesthatcarryelectriccurrentaroundahouseare
carefullychosen.Figure19.24showssomeexamples.For
each,thereisamaximumcurrentthatitisdesignedto
carry.A5Acable(Figure19.24a)isrelativelythin.This
mightbeusedforalightingcircuit,sincelightsdonot
requiremuchpower,sothecurrentflowingisrelatively
small.Thewiresina30Acable(Figure19.24c)aremuch
thicker.Thismightbeusedforanelectriccooker,which
requiresmuchbiggercurrentsthanalightingcircuit.
Thewiresineachcableareinsulatedfromoneanother,
andthewholecablehasmoreprotectiveinsulation
aroundtheoutside.Ifthisinsulationisdamaged,thereis
achancethattheuserwilltouchthebarewireandgetan
electricshock.
Figure19.24:Cablesofdifferentthicknessesarechosen
accordingtothemaximumcurrentthattheyarelikelyto
haveflowingthroughthem,a:5A.b:15A.c:30A.Each
cablehaslive,neutralandearthwires,whicharecolour
coded.Inthesecables,theearthwiredoesnothaveits
owninsulation.
Anotherhazardcanariseifanexcessivecurrentflows
inthewires.Theywilloverheatandtheinsulationmay
melt,causingittoemitpoisonousfumesorevencatch
fire.Thusitisvitaltoavoidusingappliancesthatdraw
toomuchcurrentfromthesupply.Fuseshelptoprevent
thisfromhappening.
Whenusingelectricity,itisimportanttoavoiddamp
orwetconditions.Recallthatwaterisanelectrical
conductor(seeSection18.1).So,forexample,ifyour
handsarewetwhenyoutouchanelectricalappliance,
thewatermayprovideaconductivepathforcurrentto
flowfromalivewirethroughyoutoearth.Thiscould
provefatal.
Multi-plugadapters
Thenumberofelectricalappliancespeopleuseintheir
homesisincreasing.Thesafestoptionistohavemore
wallsocketsfitted.Thealternativeistousemulti-plug
adaptorsthatallowustoplugmorethanonedeviceinto
thesamewallsocket.Multi-plugadaptorscomeintwo
mainvarieties.Thesaferoptionistouseamulti-waybar
extension,whichcomesasabankofsocketsontheendof
anextensionleadthatcanbepluggedintoawallsocket,
asshowninFigure19.25.Thesearegenerallyfittedwitha
fusetomatchthewallsocket(13AintheUK).
Figure19.25:Amulti-waybarextensionwithfouravailable
socketsandwhiteelectricalcable(ontheright).Thereisa
blockadapterwiththreesocketspluggedintotheleft-hand
endoftheextension.Pluggingmulti-plugadapterstogether
shouldbeavoided.
Bewaryofblockadaptors,whicharepluggeddirectly
intoawallsocket.Theyusuallyhaveacubicshape
andtwoormoresockets(seetheleft-handsideof
Figure19.25).Thesearelesslikelytocontainafuseso
thereisanincreaseddangerthatacurrentwillexceedthe
ratingofthewallsocket.Overloadingthesocketinthis
wayincreasesthechancethatit,andtheplug,willheat
upandcatchfire.
Itisimportantnottoexceedthecurrentratingofany
partofthesystem.Imaginetherearefourdevices,each
drawingacurrentof3A.Thiswouldnotoverloadawall
socketsotheycouldbepluggedintothesamewallsocket
viaafusedmulti-plugadaptor.However,ifanextension
isratedat10A,nomorethanthreeofitssocketsshould
beused,evenifmoresocketsareavailable.Avoidusinga
blockadapterthatdoesnothaveafuse.Neverjoinmulti¬
plugadapterstogether.Forexample,donotplugablock
adapterintoanextensionasshowninFigure19.25.
365y

379. CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Questions
18Namethetwomaintypesofmulti-plugadapters.
19Whatisoftenmissingfromablockadapterthat
makesitmoredangeroustousethanamulti-plug
adapter?
20Whyisitimportantnottooverloadawallsocketor
anymulti-plugadapterpluggedintoit?
Fuses
Fusesareincludedincircuitstostopexcessivecurrents
fromflowing.Thisprotectsthecircuitandthecablingfor
adomesticappliance.Ifthecurrentgetstoohigh,cables
canburnoutandfirescanstart.Afusecontainsathin
sectionofwire,designedtomeltandbreakifthecurrent
getsaboveacertainvalue.Usually,fusesarecontainedin
cartridges,whichmakesiteasytoreplacethem,butsome
fusesusefusewire,asshowninFigure19.26.Thethicker
thewireusedforthefuse,thehigherthecurrentthatis
neededtomakeitmelt(orblow).Afuserepresentsa
weaklinkintheelectricitysupplychain.Replacingafuse
ispreferabletohavingtorewireawholehouse.
KEYWORD
fuse:adevicethatbreaksthecircuitifthecurrent
exceedsacertainvalue;itisapieceofmetalwire
thatmeltswhentoomuchcurrentflowsthroughit
Itisimportanttochooseafuseofthecorrectvaluein
ordertoprotectanappliance.Thecurrentratingofthe
fuseshouldbejustabovethevalueofthecurrentthat
flowswhentheapplianceisoperatingnormally(see
WorkedExample19.4).
WORKEDEXAMPLE19.4
A2kWheaterworksona230Vmainssupply.The
currentflowingthroughitinnormaluseis8.7A.
Whatcurrentratingwouldasuitablefusehave?
3A
13A
30A.
Step1:The3Afusehasacurrentratingthatistoo
low,anditwouldmeltassoonastheheater
wasswitchedon.
Step2:The30Afusewouldnotmelt,butitis
unsuitablebecauseitwouldallowan
excessivecurrent(say,20A)to(Bow,which
couldcausetheheatertooverheat.
Step3:The13Afuseisthecorrectchoice,because
ithasthelowestratingabovethenormal
operatingcurrent.
Answer
13A
Figure19.26:Cartridgefusesandfusewire.Thethickerthe
wire,thehigherthecurrentthatcausesittomelt.
Tripswitch
Atripswitchcanreplaceafuse.Theswitchtripsand
breaksthecircuitwhenthecurrentflowingthroughthe
tripswitchexceedsacertainvalue.Somemodernhouse
wiringsystemsusetripswitchesinsteadoffusesinthe
fusebox(Figure19.27).Youhaveprobablycomeacross
tripswitchesonlaboratorypowersupplies.Iftoomuch
currentstartstoflow,thesupplyitselfmightoverheat
andbedamaged.Thetripswitchjumpsout,andyou
mayhavetowaitashortwhilebeforeyoucanresetit.
KEYWORDS
tripswitch:safetydevicethatincludesaswitch
thatopens(trips)whenacurrentexceedsa
certainvalue
366>

380. v
19Electriccircuit*
Figure19.27:Thisiswherethemainselectricitysupply
entersahouse.Ontheleftisthemeter.Thewhitebox
containsatripswitchforeachcircuitinthehouse,together
witharesidual-currentdevice,whichprotectstheusersof
anycircuit.
Questions
21Innormaluse,acurrentof3.5Aflowsthrough
ahairdryer.Chooseasuitablefusefromthe
following:3A,5A,13A,30A.Explainyourchoice.
22aWhyarefusesfittedinthefuseboxofa
domesticelectricitysupply?
bWhatdevicecouldbeusedinplaceoffuses?
23Whathazardscanarisewhenthecurrentflowingin
anelectricalwireistoohigh?
Usinganearthwireor
doubleinsulation
Wehavealreadyseenwhytheinsulationonwiresis
important-topreventfiresoranelectricshock.Thisis
becausethereisachancethatcurrentwillflowbetween
twobarewires(ashortcircuit),orfromonebarewire
andanypieceofmetalitcomesintocontactwith.
Thisiswhythemetalcaseofanelectricalapplianceis
earthedbyconnectingittotheearthwire.Theearthwire
providesalowresistanceelectricalpathtogroundand
reducesthechancesofafatalelectricshock.
Amainscircuitconsistsofalive(orline)wire,aneutral
wireandanearthwire.Thesearecarriedaround
buildingsaspartoftheringmainsandwecanplug
intoitviawallsockets.Ifyouhaveeverseeninside
anelectricalplugoranelectricalcable,youwillhave
noticeddifferentcolouredwires.Actually,thisisthe
plasticinsulation,anditiscolourcodedsothatpeople
canwireupplugsandappliancessafely.Thelivewire
(colouredbrownintheUK)carriestheelectricalcurrent
fromthewallsockettotheelectricalappliance(suchasa
television)andtheneutralwire(blueintheUK)carries
thecurrentbacktothesocket.Thereisusuallyalsoan
earthwire(yellowandgreenstripes).
Figure19.28ashowsthewiringwhenthereisnofault.
Noticethattheearthwireisconnectedbetweenthecasing
oftheapplianceandtheearthpinoftheplug.Thisisa
schematicdiagram(toshowyouwhatishappening).The
colouredwiresareallinsidetheelectricalcable(seeFigure
19.24)andtheconnectiontothemetalcaseisactually
madefrominsidetheappliance.
Imaginethatthereisnowafault(Figure19.28b)andthe
livewiretouchesthemetalcase.Thecasewillnowbeat
themainsvoltage(230VintheUK).Anyonetouching
thecasingcouldreceiveafatalshock.However,theearth
wireprovidesalowresistancepathtoground.Youcan
tracethepathofthecurrentwithyourfinger.Itpasses
throughthelivepinatthebottomrightoftheplug,
alongthelivewire(throughthefuse)tothemetalcasing
andthenalongtheearthwire.Becausetheresistanceof
thispathislowandthevoltageishigh,ahighcurrent
willpassthroughthefuse,whichwillalmostinstantly
melt,stoppinganycurrentflowingtotheappliance.
Aswitchontheelectricalappliancemustbeconnected
tothelivewire.Ifitwasconnectedtoeithertheneutral
wireortheearthwire,acurrentcouldstillpassintothe
applianceeveniftheswitchwasopen(off)andthiscould
leadtoafireortosomebodybeingelectrocutedifthey
touchedafaultyappliance.Imaginethattherewasa
faultandalivewiretouchedthemetalcasing.Nothing
wouldhappenandnocurrentwouldflowaslongasthe
circuitwasincomplete(anopencircuit).Ifthecasing
touchedsomethingthatcouldconductthecurrentto
ground,thenafiremightstart.Thecurrentcouldpass
throughsomeonetouchingthecasing,givingtheman
electricshock.Thinkaboutwhatwouldhappenifthe
fusewasconnectedtotheneutralorearthwire.
KEYWORD
earthed:whenthecaseofanelectricalappliance
isconnectedtotheearthwireofathree-pin
plug;theearthwireiselectricallyconnectedto
thegroundtopreventcurrentpassingthrough
anyonetouchingafaultyappliance
367)

381. CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
wiretouches
metalcase
wiringin
appliance
danger:
currentflows
earthwire
connectedtocase
throughmetal
Figure19.28a:Howthecurrentflowsfromthelivepinofthe
plugalongthelivewire(brown)totheapplianceandbackto
theplugandsocketalongtheneutralwire(blue),b:When
thereisafaultandabarewireintheappliancetouchesthe
metalcasing,thecurrentflowsthroughtheearthwire(green
andyellowstripes)insteadoftheneutralwire,andthefuse
meltsbecausealargecurrentflowsalongthislowresistance
path.
Electricalappliancesthataredoubleinsulateddonot
needanearthconnection(onlytwowiresarewiredinto
thethree-pinplug).Eveniftherearemetalpartsonthe
outercasing,theelectricalcircuitfortheapplianceis
insideacaseunderneath,whichismadefromelectrically
insulatingmaterial(forexample,plastic)sothatan
electriccurrentcannotpassthroughit.Thereisnoway
thataliveconductorcouldtouchtheoutercase.This
meansthatthereisnowaythatsomebodytouching
theoutercasecouldbeelectrocuted.Whenthesymbol
showninFigure19.29isstucktothecasingofan
electricalappliance,thenitisdoubleinsulated.
Figure19.29:Thesymbolfordoubleinsulation.Ifyousee
thisstucktothecasingofelectricalappliancethenitwillnot
needanearthwire.
KEYWORDS
doubleinsulated:whentheelectriccircuitforan
electricalapplianceisplacedinsideacasemade
fromanelectricalinsulatorsothatitisimpossible
foralivewiretotouchtheoutercasing
Questions
24Explainhowanearthwiremakesanelectrical
devicesafe.
25Whymustaswitchbeconnectedtothelivewireand
nottheearthwireorneutralwire?
26Whyshouldafusebeconnectedtothelivewireand
nottheearthwireorneutralwire?
I
27Explainhowafuseworks.
28Whatisdoubleinsulation?
ACTIVITY19.2
Challengingmisconceptionsusingrefutation
texts
Ifwhatyouthinkaboutanideaisincorrectthen
youhaveamisconception.Arefutationtextis
whenyoustatethemisconceptionandthenwrite
downthecorrectidea.
Mostpeopletalkaboutthecellorbatteryin
mobilephonesrunningoutofchargebelieving
that,whenwechargeourphones,weare
transferringcharge(electrons)intoit.However,
thiscannotbetrueorwemightgetanelectric
shockwhenwepickupafullychargedphone!The
cellorbatteryismorelikeapump,pushingthe
chargesroundthecircuit.Hereisanexampleofa
refutationtextaboutchargingaphone:
'Somepeoplethinkthatelectronsarepassing
intothebatterywhenwechargeitup,butweare
transferringenergytoitsbattery.'
Completeeachstatementtorefutetheincorrect
idea:
1Somepeoplethinkthatcurrentgetsusedup
byalamp,but...
2Somepeoplethinkthatallthelampsinaseries
circuitwillbethesamebrightness,but...
3Somepeoplebelievethatthecurrent
throughbothlampsinaparallelcircuitwillbe
thesame,but...
368y

382. -
19Electriccircuits
PROJECT
Theshockingtruthaboutelectricalsafety
Readthefollowingpassage.
Summaryofgovernmentresearchintorecent
housefiresandhospitaladmissions
Thefrequencyofhousefiresandpeopleexperiencing
electricshocksinthehome(somefatal)has
increased.Ourresearchsuggeststhatpeopleare
confusedaboutelectricalsafety.Forexample,more
thantwo-thirdsofpeoplesurveyedbelievethatan
applianceissafenomatterwhereinthecircuitthe
fuseisplaced.Mostofthosequestionedthinkitis
safeforthefuseorswitchtobeontheneutralwire
(onthereturnsideofthecircuit).Morethan80%of
peoplethinkitissafetofillalltheavailablesockets
onamulti-plugadapterwithanapplianceaslong
aseachappliancehasafuse.Apubliceducation
campaignshouldbelaunchedasamatterofurgency.
Option1:Agovernmentleaflet
Designagovernmentinformationleafletabout
electricalsafety.Giveitvisualimpact.Wherepossible,
uselabelledillustrationsinsteadoftext.Donot
exceed100words.Highlightthedangersofusing:
householdelectricalappliancesindamp
conditionsorwithdamagedinsulation
multi-plugadapters(especiallythosethatdo
notincludeafuse)andofjoiningmulti-plug
adapterstogether.
Emphasisetheimportanceofusingthecorrect
fuseratingandofplacingfusesandswitchesinthe
correctplaceinacircuit.
Option2:Anarratedanimation
Anarratedanimationisamovingpicturewitha
voiceover.Developanarratedanimationtoexplain
whyfusesandswitchesshouldbeonthelivewire
(thesupplyside)ofanappliance.Showthepath
takenbythecurrentwhenthefuse(and/orswitch)is
inthewrongplaceandafaultoccurs(forexample,
thelivewiretouchesthemetalcase).Becarefulhow
youshowthecurrentpassingthroughsomebody
toavoidupsettingviewers(byusinghumour,for
example).Thenshowthepathwhenthefuse(and/
orswitch)isinthecorrectplace.Ifyoudonothave
accesstoanimationsoftwareyoucouldphotograph
aseriesofdrawingstogetthesameeffect.
Option3:Explainingbystorytelling
Usecreativewriting(ashortstory)toexplainthe
science.Forexample,pretendthatyouareanevil
electrononthehuntforunsafeelectricalcircuitsand
describehowyoucanspotthemfromtheinside.Or
youcouldbeanelectronwithasenseofadventure,
lookingtobreakfreeoflifemovingalongawire.
Useoneoftheseideasifyoucannotthinkofanother
one.Whatevercreativedeviceyouchoose,youneed
toavoidgettingcarriedawaywiththestory,though
itdoesneedaclearplot.Thestoryissimplythereto
engagethereader;yourmissionistogetacrossthe
veryimportantsafetymessage.
SUMMARY
Aresistorcancontroltheamountofcurrentflowingaroundacircuit.
Avariableresistorcanalterthecurrentflowinginacircuit.
Alight-dependentresistor(LDR)isadevicewhoseresistancedecreaseswhenlightshinesonit.
Athermistorisadevicewhoseresistancedecreaseswithincreasingtemperature.
Arelayisaswitchcontrolledbyanelectromagnet.
Adiodeisacomponentthatallowsconventionalcurrenttoflowonlyinthedirectionofthearrow.
Alight-emittingdiode(LED)isadiodethatgivesoutlightwhencurrentflowsthroughit.
369y

383. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
Resistorsinseriesareconnectedend-to-end.
Forresistorsinseries,thetotalresistanceistheequaltothesumoftheresistors.
Thecurrentisthesameatallpointsaroundaseriescircuit.
Thevoltageofthesourceissharedbetweenresistorsinaseriescircuit.
Apotentialdividercircuitusesapairofresistorstoobtainasmallerp.d.thanprovidedbythesource.
Forresistorsarrangedinparallel,theeffectiveresistanceislessthanthevalueofthesmallestresistor.
Tocalculatetheeffectiveresistance,R,fortworesistorsinparallel,weusetheequation4?=+vk
RRtR2
Thecurrentfromthesourcedividestopassthroughparallelresistors.
Thecurrentfromthesupplyisthesumofthecurrentsflowingthroughparallelresistors:1=+12+13.
Lightsinahousearearrangedinparallelsothateachhasthesupplyvoltageacrossitandcanbecontrolledby
itsownswitch.
Themetalcaseofanelectricalapplianceisearthedbyconnectingittotheearthwiretopreventcurrentpassing
throughanyonetouchingafaultyappliance.
Excessivecurrentthroughawirecanmeltinsulation,causingittoemitpoisonousfumesorcatch^re.
Usingmulti-plugadapters(multi-waybarextensionsandblockadapters)increasestheriskofoverloadingplugs
andsockets.
Afusecontainsathinsectionofwire,designedtomeltandbreakthecircuitifthecurrentgetsaboveacertain
value.
Acircuitbreakerisasafetydevicethatautomaticallyswitchesoffacircuitwhenthecurrentbecomestoohigh.
Atripswitchisasafetydevicethatincludesaswitchthatopens(trips)whenacurrentexceedsacertainvalue.
EXAM-STYLEQUESTIONS
1ThecircuitdiagramshowsthreemeterslabelledX,YandZ.Youcan
workoutwhethereachofthemisavoltmeterorammeterfromtheway
inwhichtheyareconnected.Theswitchisopen.
370
>

384. 19Electriccircuits
CONTINUED
Whentheswitchisclosed,thereadingsonthemeterschange.
Choosethecorrectcombinationfromtheoptionsavailable. [1]
X Y Z
Aincreases decreases increases
Bdecreases decreases increases
Cincreases increases decreases
Ddecreases increases decreases
2Thecircuitshowsthreelampsandtwoammetersinacircuit.
WhenlampXstopsworking(thefilamentbreaks),thereadingsonthe
meterschange.Choosethecorrectcombinationfromtheoptionsavailable.[1]
Readingon
ammeter1
Readingon
ammeter2
Totalresistance
ofcircuit
Adecreases increases decreases
Bdecreases increases increases
Cincreases decreases decreases
Dincreases increases increases
3TworesistorsR]andR2areconnectedinseries.ResistorR[hasdoublethe
resistanceofresistorR2.
Fourpossiblestatementsaboutthiscircuitare:
1Thevoltageacross7?]istwicethatacrossR2.
2ThevoltageacrossR2istwicethatacrossRb
3Thecurrentisthesameinbothresistors.
4ThecurrentinR]istwicethecurrentinR2.
Whichpairofstatementsiscorrect? [1]
A1and4 B2and3 Cland3 D2and4
371>

385. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
COMMAND WORD
calculate:workout
fromgivenfacts,
figuresorinformation
372
>

386. 19Electriccircuits
[2]
[3]
[1]
d
[1]
CONTINUED
COMMAND WORDS
[1]
[1]
determine:establish
ananswerusingthe
informationavailable
Thetablegivesthecurrentthroughthreeoftheammeters.Copyand
completethetabletoshowthecurrentthroughtheothertwoammeters.
state:expressin
clearterms
a
b
NamethecomponentlabelledRinthecircuitdiagram.
StatetheminimumcurrentthroughcomponentRwhentheLEDs
emittinglight.
CalculatethevoltageacrossthecomponentRwhentheLEDislit.
CalculatetheresistanceofRtoensurethatacurrentof14mA
passesthroughit.
TheresistanceofcomponentRdecreasesasthetemperatureincreases.
DeterminewhathappenstothesizeofthecurrentthroughtheLED
asthetemperatureincreases.
completethetableto
place.
[11
[Total:5]
[Total:5]
6Thecircuitdiagramshowspartofasensingcircuit.TheLEDistobeusedas
atemperaturewarningindicatorwhenthetemperatureexceedsacertainvalue.
TheLEDrequiresacurrentofatleast14mAinordertoswitchon(thatis,
emitlight).

387. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
7Manycircuitscontainfuses.Figure19.26showsacartridgefuse.Alampis
connectedtothemainssupplyandtakesacurrentof6.2A.
aStatetworeasonswhyisafuseisincludedinthecircuit. [1]
bWhichofthesefusesshouldbeusedwiththelamp? [1]
A3A B5A C7A D13A
cExplainhowafuseworks. [2]
dNameanotherdevicethatdoesthesamejobasafuse. [1]
eExplainwhyisitimportanttoavoidtouchingalamporotherelectrical
appliancewithwethands. [2]
fStatewhatcanbeattachedtothemetalcaseofanelectricalappliance
topreventanelectricshockiftheapplianceisfaulty. [1]
[Total:8]
COMMAND WORD
explain:setout
purposesor
reasons;make
therelationships
betweenthings
evident;provide
whyand/orhowand
supportwithrelevant
evidence
SELF-EVALUATIONCHECKLIST
Afterstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutosee
anygapsinyourknowledgeandhelpyoutolearnmoreeffectively.
1can
See
Topic...
Needs
morework
Almost
there
v1
Confident
tomoveon
RecallwhatLDRsandthermistorsare. 19.1
Describewhatarelayisanddescribehowitworksina
switchingcircuit.
19.1
DescribewhatdiodesandLEDsdoandhowtheywork. 19.1
Recallwhethercurrentvariesaroundaseriescircuit. 19.2
Calculatethecombinedresistanceoftwoormore
resistorsinseries.
19.2
Statetherelativesizeofcurrentthroughthesource(for
example,acellorbattery)andeachbranchofaparallel
circuit.
19.2
Statehowthecombinedresistanceoftworesistorsin
parallelcomparestotheresistanceofeitherresistorby
itself.
19.2
Statetheadvantagesofconnectinglampsinparallelin
alightingcircuit.
19.2
Recallandusetherelationshipbetweenthesumofthe
p.d.sacrossthecomponentsinaseriescircuitandthe
p.d.acrossthesource(forexample,acell).
19.2
374)

388. 19Electriccircuits
CONTINUED
11
See Needs Almost Confident
Topic... moreworkthere tomoveon
Recallandusetherelationshipbetweenthecurrent
fromthesourceandthesumofthecurrentsinthe
separatebranchesofaparallelcircuit.
19.2
Calculatetheeffectiveresistanceoftworesistorsin
parallel.
19.2
Describetheactionofapotentialdivider. 19.2
Statethehazardsofusingelectricalappliancesin
dampconditions,whencablesoverheatorwhen
insulationisdamaged.
19.3
Statethehazardsofusingmulti-plugadapters. 19.3
Statewhatafuseisusedforanddescribehowit
works.
19.3
Chooseappropriatefuseratingsandcircuitbreaker
settings.
19.3
Explainthebenefitsofearthingthemetalcaseofan
electricappliance.
19.3
Statewhatatripswitchisandhowitworks. 19.3

389. investigatethemagneticfieldsaroundcurrent-carryingconductors
describesomepracticalusesofelectromagnets
observetheforceonacurrent-carryingconductorinamagneticfield
describetheprincipleofanelectricmotorandlistwaysofincreasingitsstrength
ectromagneti
INTHISCHAPTERYOUWILL:
describethefactorsaffectingthestrengthanddirectionofelectromagneticfieldsandforces
describethestructureandoperationofanelectricmotor.

390. 20Electromagneticforces
GETTINGSTARTED
InChapter16youstudiedmagnetism.Chapters
17-19coveredelectricity.Asyoucantellfromthe
title,thischapterbringselectricityandmagnetism
together.Therearemanylinksbetweenthetwo,
andalso,importantdifferences.
Onalargesheetofpaper,copyandcomplete
theVenndiagramshowninFigure21.1.Inthe
overlappingsection,writeallthethingswhichare
similar.Intheothersections,writedownthings
whichonlyapplytoelectricityortomagnetism.Two
examplesaregivenforyou.
ElectromagnetismwasfirstdescribedbyaDanish
scientistnamedOerstedintheearly19thcentury.
Usetheideasyouhaverecordedtodiscusswhat
mighthaveleadOerstedtothinktherewasalink
betweenelectricityandmagnetism.
THEMAGICOFMOTORS
Figure20.2:Smallmotorsinprostheticlimbsallowa
greaterrangeofmovementforpeopleusingthem.
Somethingsinscienceseemmagical.Forexample,
thefactthataplantcanfeeditselfusingsunlight
isamazing.Theelectricmotorisanotherexample
ofthemagicofscience.Asyouwilldiscoverinthis
chapter,puttingtogetheranelectriccurrentanda
magneticfieldcreatesmovement.Thisisthebasisof
allelectricmotors.
Electricmotorshaverevolutionisedourlives.Itis
easytotakethemforgranted,butanythingyou
pluginwhichinvolvesmovement,containsamotor.
Washingmachines,hairdryers,electricvehicles,
recordplayersandmanymoreapplianceswhich
makeourliveseasierandbetterusemotors.Just
thinkforamomentabouthowdifferentyourlife
wouldbewithoutmotors.
Motorshavemanymedicalapplications.Theyare
widelyusedinprostheticlimbs,andcanpump
fluidssuchasbloodduringdialysis.Surgeonscan
remotelycontrolmotoriseddevicestomakesurgery
lessinvasive.
Figure20.2showsagirlwithaprostheticarmeating
herdinner.Manyprostheticlimbshavesmallmotors
insidethem,greatlyincreasingthecapabilities
oftheartificiallimb.Here,themotorallowsthe
fingerstogriptheFrenchfriesandapplytheright
amountofforcetokeepholdofthemforhertothen
eat.Motorscanbelifechangingandinthiscase,
replacesthemovementlosttoanamputee.
Discussionquestions
1Listalltheapplicationsofmotorsyou.can
thinkof.Discusshowthejobeachapplication
performswouldhavebeendonebeforemotors
wereinvented.
2Discussifyouthinkthereareanyotherscientific
inventionsordiscoverieswhichhavehadas
muchimpactastheelectricmotor.
377)

391. /
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
20.1Themagneticeffect
ofacurrent
InChapter16,wesawthatanelectromagnetcanbe
madebypassingacurrentthroughacoilofwire(a
solenoid).Theflowofcurrentresultsinamagnetic
fieldaroundthesolenoid.Thefieldissimilartothefield
aroundabarmagnet(seeFigure16.8).
Ifyouuncoilasolenoid,youwillhaveastraightwire.
Withacurrentflowingthroughit,itwillhaveamagnetic
fieldarounditasshowninFigure20.3.Thefieldlinesare
circlesaroundthecurrent.
Everyelectriccurrentcreatesamagneticfieldaroundit.
Anelectromagnetusesthiseffect.Windingthewireinto
acoilisawayofconcentratingthemagneticfield.
Theright-handgripruletellsyouthedirectionofthefield
lines.Imaginegrippingthewirewithyourrighthandso
thatyourthumbpointsinthedirectionofthecurrent.
Thecurveofyourfingersshowstheshapeanddirection
ofthefieldlines.
Werepresentmagneticfieldsbydrawingfieldlines.The
arrowsonthelinesshowthedirectionofthefieldatany
point.Afieldlineshowsthedirectionoftheforceona
northmagneticpoleplacedinthefield.
Asolenoidisalengthofwirewoundtoformacoil.It
hasafieldpatternlikethatofabarmagnet,withfield
linesemergingfromanorthpoleatoneendofthecoil
andenteringasouthpoleattheother(Figure16.8).
Figure20.3:Magneticfieldaroundawi|e.
KEYWORDS
right-handgriprule:arulewhichgivesthe
directionoffieldlinesaroundastraightwirewhen
acurrentflowsthroughit
EXPERIMENTAL SKILLS20.1
Investigatingthemagneticeffectofacurrent
Oersted'sfirstexperimentwithelectromagnetism
involvedjustasimplecircuitandaplotting
compass.Youwillseewhatheobservedandlookat
thefieldsarounddifferentshapesofwires.
Safety:Thewirewillbecomeveryhotwhenthe
currentflows.Switchthepowersupplyoffas
soonasyouhavemadeyourobservations.Avoid
touchingthewire.
Ironfilingscanscratchthecorneaofyoureye.
Weareyeprotectionwhileusingironfilings.Ifyou
getfilingsinyoureye,tellyourteacherimmediately.
Donotrubyoureyeasthiswillmakeitworse.
Gettingstarted
InChapter16youinvestigatedthefieldaroundabar
magnet.
Whatdotheironfilingstellyouabouta
magneticfield?
Whatisthepurposeoftheplottingcompasses?
Sketchthemagneticfieldaroundabarmagnet.
378>

392. 20Electromagneticforces
379

393. 1
)CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
Questions
1Doyourobservationsofthefieldarounda
straightwireagreewiththeright-handgrip
rule?
2InChapter16,wesawamethodfor
determiningthepolarityofthefieldaround
asolenoid.Thisdependsonthedirectionof
currentflowateachendofthecoil.
Doyourobservationsagreewiththismethod?
Comparingthestrengthand
directionofmagneticfields
Currentinawire
Furtherfromthewire,thecircularfieldlinesarefurther
apart,showingthatthefieldisweaker.Ifthecurrentis
greater,thefieldwillbestrongerandsothelineswillbe
closertogether.
Thedirectionofthefieldlinesisreversedwhenthe
currentisreversed.
Currentinasolenoid
Thefieldlinesareclosetogetheratthepolesofthe
electromagnet.Furtherfromthecoil,thelinesarefurther
apart(illustratingaweakerfield).Insidethecoilthefield
linesrunparalleltoeachothershowingthatthefieldis
uniform(itsstrengthisconstant).Again,increasingthe
currentgivesastrongerfield.
Thepolarityofthefieldisreducedwhenthecurrent
isreversed.
Questions
1Copyandcompletethesesentences.
Thereisamagneticfieldaroundaconductorwhen
itcarries .
Thefieldlinesaroundastraightwireare.
Thedirectionofthesefieldlinescanbefoundusing
the rule.
Thefieldaroundasolenoidisthesameasthat
arounda .
2Whatmustbeaddedtoasolenoidtomakeitintoan
electromagnet?
3Acurrentflowsdownwardsinawirethatpasses
verticallythroughasmallholeinatabletop.Will
themagneticfieldlinesarounditgoclockwiseor
anticlockwise,asseenfromabove?
1
Electromagnetisminaction-therelay
Arelayisaswitchoperatedbyanelectromagnet.In
Chapter19,wesawhowarelaycanbeusedinanelectric
circuit.OnetypeisshowninFigure20.8,togetherwith
thecircuitsymbol.
Figure20.8a:Arelay,capableofswitchingacircuitcarrying
hundredsofamps,b:Thecircuitsymbolforarelay.The
rectanglerepresentstheelectromagnetcoil.
WhenswitchAisclosed,asmallcurrentflowsaround
thecircuitthroughthecoiloftheelectromagnet.
Theelectromagnetattractstheironarmature.As
thearmaturetips,itpushesthetwocontactsatB
together,completingthesecondcircuit.
Noticethatthereisnoelectricalconnectionbetween
thetwocircuits.
KEYWORD
armature:themovingpartofanelectromagnetic
devicesuchasarelayorbell
380>

394. 20Electromagneticforces
Questions
Figure20.10:Relayusedtoswitchonamotor.
PEERASSESSMENT
REFLECTION
ACTIVITY20.1
Arelayisusedtomakeasmallcurrentswitchalarger
currentonandoff.Forexample,whenadriverturns
theignitionkeytostartacar,asmallcurrentflowsto
arelayintheenginecompartment.Thisclosesaswitch
tocompletethecircuit,whichbringsahighcurrent
tothestartermotorfromthebattery.Thismeansthat
thewiresinthedashboardcanbethinwirescarrying
asmallcurrent.Thisissafer,neaterandcheaperthan
runningthickwirescarryingthelargecurrentneeded
bythestartermotorallthewayuptothedashboard.
Figure20.9showsarelaybeingusedinthisway.
Whataspectofthisactivitydidyoufindmost
useful?Givinganexplanationorlisteningto
anotherstudent'sexplanation?Ifyouwereto
continuewiththisactivity,wouldyoufindthe
feedbackyoureceiveduseful?
Presentyourworktoanothergroupofstudents.
Theaudienceshouldgivesmiley,straightorsad
facefeedbackon:
howclearandwelllabelledthediagramor
modelis
howwellthevideo,seriesofpicturesor
movementofthemodelshowsthewaythe
primarycircuitcontrolsthesecondarycircuit
theclarityoftheexplanationinthevoiceover.
Feedbackmayalsoincludeanypartsofthe
presentationwhichwereparticularlyhelpful,or
suggestionsforimprovementstothepresentation.
Writeoutthesestepsinthecorrectordertoexplain
whathappenswhenthecontrolswitchisclosed.
Thecontactsclosetogether.
Currentflowsthroughtheelectromagnetic
coil.
Modellingarelay
Itcanbedifficulttounderstandthewayarelay
workswithoutseeingitinaction.
Designateachingaidtohelpyoudescribe
ittootherstudents.Thiscouldbeavideoor
storyboardshowingtheeffectofswitchingon
thesmallcurrentcircuit.Alternatively,youcould
createaworkingmodelbydrawingtherelayon
cardandhavingseparatemovingpartssuchasthe
secondarycircuitconnectionsandthearmature.
Whicheveroptionyouchoose,youshouldwrite
avoiceoverscripttodescribeandexplainwhatis
happening.
4aFigure20.10showsanelectromagneticrelay
usedtoswitchonamotor.

395. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Currentflowsinthemotorcircuit.
Theelectromagnetattractsthearmature.
Theelectromagnetismagnetised.
bWhyisthearmaturemadeofiron?
cWhymustsoftironbeused?
5Figure20.11showsanelectromagnetinuseina
signalforamodelrailwayset.
Figure20.11:Electromagnetusedasasignal.
Explainwhathappenswhentheswitchisclosed,and
thenwhenitisre-opened.
20.2Forceonacurrent¬
carryingconductor
Theideaofanelectricmotoristhis.Thereisamagnetic
fieldaroundanelectriccurrent.Thismagneticfieldcan
beattractedorrepelledbyanothermagneticfieldto
producemovement.Thisiscalledthemotoreffect.
KEYWORDS
motoreffect:whencurrentflowsinawireina
magneticfieldwhichisnotparalleltothecurrent,
aforceisexertedonthewire
Anelectricmotorhasacoilwithacurrentflowing
aroundit(anelectromagnet)inamagneticfield.Itturns
becausethetwomagneticfieldsinteractwitheachother.
However,itisnotessentialtohaveacoiltoproduce
movement.Thebasicrequirementsare:
amagneticfield
acurrentflowingwhichcutsacrossthemagnetic
fieldlines.
Figure20.12showsawayiodemonstratethis.
Figure20.12:Themotoreffect.Thiscanbesummarisedas
current+magneticfield=movement.
Whenacurrentispassedthroughthealuminiumfoilit
hasamagneticfieldaroundit.Thisinteractswiththe
fieldofthestrong,permanentmagnetcreatingaforce.
Thefoilhasbeenpusheddownwardsbythisforce.
Thedirectionoftheforcecanbereversedby:
reversingthedirectionofthecurrent
reversingthedirectionofthefieldofthepermanen
magnetbyturningitround.
EXPERIMENTAL SKILLS20.2
Thecatapultfield
Safety:Thewireswillgethotwhencurrentflows.
Switchoffthepowersupplyassoonasyouhave
observedmovementandavoidtouchingthe
wires.
Youwillneed:
powersupply
2magnadurmagnetswithayoke
2steelrods
clamp
copperwire.
Gettingstarted
Thisexperimentworksbecausetwomagnetic
fieldsinteract.
Sketchthemagneticfieldbetweentwoopposite
polesandaroundawirewhichiscarryingcurrent.
Howcouldyoureversethedirectionofthefieldin
eachcase?
382>

396. 20Electromagneticforces
CONTINUED
Method
1Clamptwosteelrodshorizontally,parallelto
oneanother.
2Bendalengthofcopperwire(seeFigure20.13)
toforma'swing',whichcanhangbetweenthe
steelrods.
3Attachthetwomagnetstotheyoke,
ensuringthatoppositepolesarefacingeach
other.Placethemagnetsaroundtheswing,
asshowninFigure20.13.
Figure20.13:Set-upforinvestigation.
4Connectuptheendsofthesteelrodstoa
low-voltaged.c.powersupply.Thecurrent
shouldbeabletoflowalongonerod,through
theswingandbackthroughtheotherrod.
5Switchonthepowersupplyandobserve
whetheraforceactsontheswing.
6Investigatetheeffectsofreversingthe
currentandthemagneticfield(separately).
Question
1Listtwowaystoreversetheforceonacurrent¬
carryingconductorinamagneticfield.
Fleming'sleft-handrule
Figure20.14ausesarrowstorepresentthreeimportant
quantitiesinthedemonstrationinFigure20.12:
themagneticfieldofthepermanentmagnet
thecurrent
theforcewhichmovesthealuminiumstrip.
Themagneticfieldishorizontalbetweenthepoles.
Thecurrentisalsohorizontal,butatrightanglesto
thefield.Theforceisvertical,sothefoilmovesdown.
Thethreequantitiesareallatrightanglestoeachother
(Figure20.14a).Torememberhowtheyarearranged,
physicistsuseFleming’sleft-handrule(Figure20.14b).
Thisstatesthatifthethumbandfirsttwofingersofthe
lefthandareextendedatrightanglestoeachother,and
thefirstfingerpointsinthedirectionofthefield,the
secondfingerinthedirectionofcurrent,thenthethumb
willindicatethedirectionoftheforce.
KEYWORDS
Fleming'sleft-handrule:arulethatgivesthe
relationshipbetweenthedirectionsofforce,
fieldandcurrentwhenacurrentflowsacrossa
magneticfield
Itisworthpractisingholdingyourthumbandfirsttwo
fingersatrightangleslikethis.Ittakestimetogetthis
right.Thenlearnwhateachfingerrepresents:
theFirstfingerisField
theseCondfingerisCurrent
thethuMbisforceorMotion.
WeuseFleming’sleft-handruletopredictthedirection
oftheforceonacurrent-carryingconductorina
magneticfield.Bykeepingyourthumbandfingersat
rightanglestoeachother,youcanshowthatreversing
thedirectionofthecurrentorfieldreversesthedirection
oftheforce.(Donottrychangingthedirectionof
individualfingers.Youhavetotwistyourwholehand
aroundatthewrist.)
Figure20.14a:Force,fieldandcurrentareatrightanglesto
eachother,b:Fleming'sleft-handrule.
Themotoreffectinaction-
theloudspeaker
Figure20.15showsaloudspeaker.Thiscreatesasound
waveintheairduetothevibratingpapercone.Thecone
isattachedtoacoilofcopperwirewhichfitslooselyover
383)

397. i
>
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
thecentreofacyclindricalmagnet.Thecoilis,therefore,a
conductorwhichisfreetomoveinamagneticfield.When
acurrentflowsinthecoil,thecoilandconewillmove.
Thecurrentinputvarieswiththesoundbeingproduced
-avaryingsignalisreceivedfromamicrophone,MP3
playerorsimilar.Thechangingcurrentcausesthecoiland
conetomovetoandfroandproduceasoundwave.
Figure20.15a:Diagramofaloudspeaker,b:This
loudspeakerhasbeendismantledsoyoucanseethe
permanentmagnetandthecoil.
20.3Electricmotors
Figure20.16a:Thetypeofmotorusedinschool
laboratories,b:Themotortakenapart.Noticethecopper
coilsandthecurvedmagnetsinsidethecasingandthecoil.
Themovementcreatedinthdmotoreffectexperiments
isnotveryuseful.Theconductormovesoutofthefield
andtheeffectisover.Amotorisdesignedtousethe
motoreffecttocreateaturningmovement.
Figure20.17:Thismodelisusedtoshowtheprinciplesof
operationofanelectricmotor.
Theturningmovementhappensbecausethewireis
coiled.LookatthecoilinFigure20.17.Thecurrent
flowsuptheright-handsideofthecoil^fid
downthe
left.Thismeansthattheforcesonthetwowiresarein
oppositedirections.Theright-handwiremovesupand
theleft-handwiremovesdown.Thismakesthecoilspin.
Thebrushesandcommutatormakesurethatthecurrent
alwaysflowsthesamewayroundthecoil,inthiscase,
anticlockwise.
KEYWORD
commutator:adeviceusedtoallowcurrent
toflowtoandfromthecoilofad.c.motoror
generator
Forad.c.motorlikethis(Figure20.17)tobeofany
use,itsaxlemustbeconnectedtosomethingthatistobe
turned-awheel,apulleyorapump,forexample.This
modelmotorisnotverypowerful.Theturningeffectcan
beincreasedby:
increasingthenumberofturnsofwireinthecoil
increasingthecurrent
increasingthestrengthofthemagneticfield.
Questions
6Describetheenergytransfersthathappenin:
aanelectricmotor
baloudspeaker.
384>

398. c Electroma9neticforces
7Figure20.18showsatypeofammeter.Copyand
completethesentencestoexplainhowitworks.
When passesthroughthecoil,itexperiences
a becauseitisinamagnetic .This
causesthecoiltorotate.
Whenthecurrentincreases,theforcewillbe
andsothepointerwillmove onthescale.
8Astudentdoesanexperimenttoshowthemotor
effectusingtheapparatusshowninFigure20.19.
Thewiremovesupwards.
Figure20.19
aDescribetwowaysinwhichthestudentcould
reversetheeffecttomakethewiremove
downwards.
bThestudentmovesthewiresothatitpasses
fromthenorthtothesouthpoleofthemagnet
asshowninFigure20.20.Explainwhythewire
doesnotmove.
Electricmotorsexplained
WecanapplyFleming’sleft-handruletoanelectric
motor.Figure20.21ashowsasimpleelectricmotorwith
itscoilhorizontalinahorizontalmagneticfield.Thecoil
isrectangular.Whatforcesactoneachofitsfoursides?
Figure20.21a:Asimpleelectricmotor.Onlythetwolonger
sidesexperienceaforce,sincetheircurrentscutacrossthe
magneticfield,b:Thetwoforcesprovidetheturningeffect
neededtomakethecoilrotate,c:Whenthecoilisinthe
verticalposition,theforceshavenoturningeffect.
385>

399. >
CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
SideAB:thecurrentflowsfromAtoB,acrossthe
magneticfield.Fleming’sleft-handruleshowsthata
forceactsonit,verticallyupwards.
SideCD:thecurrentisflowingintheopposite
directiontothecurrentinAB,sotheforceonCDis
intheoppositedirection,downwards.
SidesBCandDA:thecurrenthereisparalleltothe
field.Sinceitdoesnotcrossthefield,thereisno
forceonthesesides.
Figure20.21bshowsasimplifiedviewofthecoil.The
twoforcesactingonitareshown.Theycausethecoil
toturnanticlockwise.Thetwoforcesprovideaturning
effect(ortorque),whichcausesthemotortospin.From
Figure20.21c,youcanseethattheforceswillnotturn
thecoilwhenitisvertical.Thisiswherewehavetorely
onthecoil’smomentumtocarryitfurtherround.
Keepingthemotorturning
Asthemomentumofthecoilcarriesitround,thewires
ABandCDswappositions.Thecommutatorspinswith
thecoil.Thebrushesdonotmove.Thismeansthat,in
themotorinFigure20.21,currentalwaysflowsinonthe
right,andanticlockwiseroundthecoil.Thismeansthat
themotorkeepsturninginthesamedirection.
Figure20.22:Aspinningelectricmotor.Everyhalfturnthe
commutatorreversestheconnectionstothepowersupply,
sothatthemotorkeepsturninginthesamedirection.
Thediagramsshowthecoilasifitwereasingleturnof
wire.Inpractice,thecoilmighthavehundredsofturns
ofwire,resultinginforceshundredsoftimesasgreat.A
coilcausesthecurrenttoflowacrossthemagneticfield
manytimes,andeachtimeitfeelsaforce.Acoilissimply
awayofmultiplyingtheeffectthatwouldbeexperienced
usingasinglelengthofwire.
Questions
9ForFleming’sleft-handrule,writedownthethree
quantitiesthatareat90°toeachother,and,nextto
eachone,writedownthefingerthatrepresentsit.
10InFigure20.23,crossesrepresentamagneticfield
intothepageanddotsrepresentafieldcomingout
ofthepage.Ineachcase,useFleming’sleft-hand
ruletodeterminethedirectionoftheforceonthe
wire.
Figure20.23
11Listtwowaystoincreasetheforceonacurrent¬
carryingconductorinamagneticfi^ld.
12Describethemotionthatwouldbeseenifthecoilin
amotorwasattacheddirectlytoad.c.powersupply
withoutacommutator.
20.4Beamsofcharged
particlesandmagnetic
fields
Amagneticfieldcanalsobeusedtodeflectabeam
ofelectrons,oranyelectricallychargedparticles.This
canbedemonstratedinthelaboratoryusingavacuum
tube(Figure20.24).Thereisaforceontheelectronsin
thesamewayaswesawearlierforacurrent-carrying
conductorinamagneticfield.Thedirectionoftheforce
isgivenbyFleming’sleft-handrule,butrememberthat
theconventionalcurrentisintheoppositedirectionto
theelectronflow.InFigure20.24,anelectronbeamis
travellingfromlefttorightinavacuumtube.Thisis
equivalenttoaconventionalcurrentflowingfromright
toleft.InFigure20.24athenorthpoleofamagnetis
heldbehindthetube,givingafieldwhichcomesoutof
thepage.Fleming’sleft-handruleshowsthattheforceon
thebeamisupwards.Figure20.24ashowstheeffectof
thisforce.Figure20.24bshowstheeffectofreversingthe
directionofthefield.
386y

400. 20Electromagneticforces
Figure20.24:Anelectronbeaminavacuumtube,being
deflectedbyamagneticfield.Aswithacurrentinawire,
reversingthefieldreversestheforceonthebeamof
electrons.
Thiseffectisusedinparticleacceleratorssuchasthose
atCERNinSwitzerland,tofocusandtodivertbeamsof
chargedparticles.Theparticlestravelatenormouslyhigh
speedsandsohavealotofkineticenergy.Hugefieldsare
neededtodivertthem.
Figure20.25:Thisgiantmagnet,whichhasamassof1920
tonnes,wasinstalled100metresbelowgroundina27km
tunnelatCERNtoprovideamagneticfieldforagiant
particledetector.
PROJECT
Inthisprojectyouwillresearcheithertheusesof
motorsorthemotoreffectindifferentsituations.
Option1
Collectpicturesofthingswhichusemotors.These
canbephotos,drawingsorpicturesfromtheInternet
orcatalogues.Trytofindpicturescoveringallthe
followingusesofmotors:
kitchenappliancesmedicine
hairandbeauty industry
appliances
rr
music
transportation.
Identifytheenergytransfersthathappenineach
appliance,includinganywasteenergy.
Useyourpicturestocreateaposteraboutmotors.
Includediagramsandtexttoexplainhowmotors
work,andhowsomemotorsareconstructedto
makethemstrongerthanothers.Includethewords:
magneticfield,current,electromagnet,forceandcoil.
Thiskitchencontainsatleastthreeelectricmotors.
Identifywheretheyare.
Motorseverywhere
387

401. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
Option2
Themotoreffectisusedinotherdevicesaswell
asmotors.Researchhowtheeffectisusedin
differentdevicessuchasdifferenttypesofmotors,
loudspeakers,movingcoilmilliammetersorparticle
accelerators.Prepareashortpresentation(which
couldbefilmed)orapostertoshowwhatyoufind
out.Youshouldinclude:
adescriptionofwhatthemotoreffectis(itcan
beshortenedtothreewordsandtwosymbols!)
detaileddiagramsshowingatleastthreeuses
ofthemotoreffect
anexplanationofhoweachoftheapplications
works
followupquestions(withanswers)tocheck
youraudienceunderstandyourpresentation
(includetheuseofFleming'sleft-handrulein
yourquestions).
SUMMARY
Thereisamagneticfieldaroundacurrent-carryingconductor.
Themagneticeffectofacurrenthasmanypracticalusesincludingtheelectricbellandrelay.
Thefieldaroundawireisstrongestnearthewire.
Thefieldinasolenoidisstrongestinsidethecoil.
Thestrengthanddirectionofthefielddependsonthesizeanddirectionofthecurrent.
Whenacurrentflowsthroughaconductorinamagneticfield,thereisaforceontheconductor.Thisiscalled
themotoreffect.
Thedirectionoftheforcecanbereversedbyreversingthefieldorthecurrent.
Acoiledconductorinamagneticfieldexperiencesaturningforce.Thisisthebasisoftheelectricmotor.
Thestrengthofamotorcanbeincreasedbyincreasingthefield,thecurrentorthenumberofturnsonthecoil.
Fleming’sleft-handruleallowsyoutofindthedirectionoftheforceonacurrent-carryingconductor.
Fleming’sleft-handrulecanalsobeusedtofindthedirectionoftheforceonabeamofchargedparticlesina
magneticfield.
Anelectricmotorusesasplit-ringcommutatortoproducecontinuousrotation.
388>

402. 20Electromagneticforces
EXAM-STYLEQUESTIONS
1Currentflowsupawireasshowninthediagram.Whatwillthemagnetic
fieldlooklike? [1]
Astraightlinesfromlefttoright
Bstraightlinesfromrighttoleft
Cclockwisecircles
Danticlockwisecircles
2Whichlinegivesthecorrectmaterialforthearmatureofarelay,andthe
propertywhichmakesitsuitable? [1]
Material Property
Acopper goodconductor
Bsoftiron bendy
Ccopper easilymagnetised
Dsoftiron easilydemagnetised
3Whichofthefollowingpairsofchangeswillbothincreasethestrength
ofanelectricmotor? [1]
AReversingthemagneticfieldandincreasingthenumberofturnson
thecoil
BIncreasingthenumberofturnsonthecoilandreversingthedirection
ofthecurrent
CReversingthedirectionofthecurrentandreversingthemagneticfield
DUsingamorepowerfulmagnetandincreasingthenumberofturns
onthecoil
389)

403. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
I
COMMAND WORDS
explain:setout
purposesor
reasons;make
therelationships
betweenthings
evident;provide
whyand/orhowand
supportwithrelevant
evidence
suggest:apply
knowledgeand
understanding
tosituations
wherethereare
arangeofvalid
responsesinorder
tomakeproposals/
putforward
considerations
describe:statethe
pointsofatopic/give
characteristicsand
mainfeatures
390
>

404. 20Electromagneticforce*
CONTINUED
7Apermanentmagnetisplacedonatoppanbalance.Awireissuspended
betweenthepolesofthemagnetandattachedtoapowersupply.Thewire
isclampedsoitcannotmove.
wireconnectedto
d.c.powersupply
Whenthepowersupplyisswitchedon,acurrentflowsthroughthewireand
[4]thereadingonthetoppanbalancedecreases.Explainwhy.
8Thediagramshowsthemainpartofasimpledirectcurrentelectricmotor.
aNameandstatetherulewhichisneededtoworkoutthedirectionthe
motorwillmove. [2]
bAnimportantpartofthemotorismissingfromthediagram.Nameit
andstateitsfunction. [2]
cSuggesttwowaystoincreasethestrengthofthemotor. [2]
[Total:6]

405. yCAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
CONTINUED
9aDeterminethedirectionoftheforceonabeamofelectronspassingfrom
righttoleftinamagneticfieldintothepage. [1]
X X X X
X X X X
electron
beam
X X X X
X X X X
fieldintopage
bWhatwouldhappeniftheelectronbeamwasreplacedbyabeamof
positivelychargedions? [2]
cExplainwhythepathofthebeamcurvesasitpassesthroughthefield.[1]
COMMAND WORD
determine:establish
ananswerusingthe
informationavailable
SELF-EVALUATIONCHECKLIST
Afterstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutosee
anygapsinyourknowledgeandhelpyoutolearnmoreeffectively.
1can
See
Topic...
Needs
morework
Almost
there
Confident
tomoveon
Describeanexperimenttoshowthefieldarounda
straightwireandasolenoidwhentheycarrycurrent.
20.1
Drawthepatternanddirectionofthesefields. 20.1
Describethefactorsthataffectthedirectionand
strengthofthesefields.
20.1
Describetheactionofanelectricrelayandaloudspeaker.20.1
Interpretdiagramsofotherdevicesusing
electromagnets.
20.1
Statethatacurrent-carryingconductorinamagnetic
fieldexperiencesaforce.
20.2
UseFleming’sleft-handruletofindthedirection
ofthisforce.
20.2
Listthreewaysofincreasingthestrengthofanelectric
motor.
20.3
Describehowanelectricmotorproducesaturning
effect,includingthefunctionofthebrushesand
split-ringcommutator.
20.3
Recallthatabeamofchargedparticlesinamagnetic
fieldexperiencesaforceanddeterminethedirectionof
thisforce.
20.4
392y

406. >Chapter21
Electromagnetic
induction
yexplainhowatransformerworks.
explainhowana.cgeneratorworks
INTHISCHAPTERYOUWILL:

407. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
GETTINGSTARTED
InChapter18youlearntabouttheelectrical
quantitieswecanmeasureorcalculate.These
include:current,potentialdifference,resistance,
charge,energy,powerandelectro-motiveforce.
Foreachquantity,writeadefinition,theletter
usedtorepresentthequantityinanequation(for
example,Iforlength),anditsunit.
Writedownalltheequationsyouknowwhich
involvethesequantities.
Inasmallgroup,playagametocheckyourlearning:
Createasetofcards.Thereshouldbeonecardfor
thenameofeachquantity,onecardfortheunitof
eachquantity,onecardfortime,andonecardfor
seconds.
Placethecardsfacedowninapack.Thefirstplayer
turnstwocardsover.Iftheycanconnectthecards
inasentenceorequation,theyscoreapoint.
Returnthecardstothepack,mixthemup,andthen
thenextplayertakestheirturn.
Forexample,ifaplayerpicks'ohm'and'current',
theycouldsay:'whentheresistanceinohms
increases,thecurrentdrops'.Ifastudentmakesan
incorrectlink,otherplayerscanscoreapointby
spottingthemistakeandgivingacorrectanswer.
WHYMUSTELECTRICGUITARSHAVEMETALSTRINGS?
Figure21.1:Themetalcircles(onthewhitebarbelow
thestringsofthiselectricguitar)pickupsoundvibrations.
Noticethatthestringsdonottouchthepickup.
WesawinChapter20thatacurrentflowingina
magneticfieldexperiencesaforcewhichcanmakeit
move.Inthischapterwewillinvestigatethereverse
process.Whenaconductorandmagneticfield
moverelativetoeachother,acurrentflowsinthe
conductor.Thisistheprinciplebehindtheoperation
ofpowerstations.
Thiseffectisalsousedtopickupsoundvibrations
inanelectricguitar.Theguitarstringsare
magnetisedbyapermanentmagnetinthepick-up
belowthestrings.Thisiswhytheymustbemade
ofamagneticmetalsuchasnickelorsteel.The
pick-updevice-thewhitebarbelowthestrings
inFigure21.1-alsocontainscoilsofcopperwire
whicharewrappedaroundthepermanentmagnets.
Thestringsvibratewhentheguitarisplayed.
Theamplitudeandfrequencyofthesevibrations
dependonthenotebeingplayed.Wenowhave
relativemovementbetweenaconductor(thecoils)
andafield(aroundthemagnetisedstrings)andso
acurrentflowsinthecoils.Thiscurrentisamplified
andfedtoaloudspeaker.Thesizeandfrequencyof
thecurrentdependsonthevibratior^ofthestrings.
Thisallowsmusictobeplayed.
Figure21.2:Howthepi«kupinaguitarworks.
Figure21.2showsthefieldlinesfromthepermanent
magnetwhichmagnetisesthemetalstrings.The
changingcurrentgeneratedinthecoilispassed
tothespeakerwhichcontainsamagnet.Asthe
changingcurrentflows,thespeakerconemoves,
producingsound-thisisthemotoreffect.
Discussionquestions
1Theguitarpickupdoesnottouchthestrings.
Explainwhythisisanadvantage.
2Theelectromagneticpickupshownin
Figure21.2wouldnotworkforaguitarwith
nylonstrings.Explainwhy.
3Describetheenergychangesinvolvedfrom
theguitarbeingstrummedtothesoundbeing
heardfromthespeaker.
394>

408. 21Electromagneticinduction
21.1Generating
electricity
Amotorisadevicethattransfersenergybyanelectrical
currentintoamechanical(kinetic)energystore.An
electricalgeneratordoestheopposite-ittransfers
energyfromamechanicalenergystorebyanelectrical
current.Anelectricmotorcanbeusedinreverseto
generateelectricity.Ifyouconnectanelectricmotortoa
lampandspinitsaxle,thelampwilllight,showingthat
youhavegeneratedavoltagewhichcausesacurrentto
flowthroughthelamp(Figure21.3).
Figure21.3:Amotorcanactasagenerator.Spinthemotor
andthelamplights,showingthataninducedcurrentflows
aroundthecircuit.
Insidethemotor,thecoilisspinningaroundinthe
magneticfieldprovidedbythepermanentmagnets.The
resultisthatacurrentflowsinthecoil,andthisisshown
bythelamp.Wesaythatthecurrenthasbeeninduced,
andthemotorisactingasagenerator.
Therearemanydifferentdesignsofgenerator,just
astherearemanydifferentdesignsofelectricmotor.
Generatorsinpowerstationssupplyelectricityto
communities.Ifyouhaveabicycle,youmayhavea
generatorofadifferentsort-adynamo-forpowering
thelights.
Figure21.4:Thedynamorubsagainstthewheelcausingit
toturn.Thisgeneratescurrenttolightthebicyclelamp.
ThepowerstationgeneratorsshowninFigure21.10
generatealternatingcurrentatavoltageofabout25kV.
Theturbinesaremadetospinbythehigh-pressuresteam
fromtheboiler.Thegeneratorisonthesameaxleasthe
turbine,soitspinstoo.Acoilinsidethegeneratorspins
aroundinsidesomefixedelectromagnets,whichprovide
themagneticfield.Alargecurrentistheninducedin
therotatingcoil,andthisisthecurrentthatthepower
stationsuppliestoconsumers.
Allofthesegeneratorshavethreethingsincommon:
amagneticfield(providedbymagnetsor
electromagnets)
acoilofwire(fixedormoving)
movement(thecoilandmagneticfieldmoverelative
tooneanother).
Whenthecoilandthemagneticfieldmoverelativeto
eachother,acurrentflowsinthecoilifitispartofa
completecircuit.Thisisknownasaninducedcurrent.
Ifthegeneratorisnotconnecteduptoacircuit,there
willbeaninducede.m.f.(orinducedvoltage)acrossits
ends,readytomakeacurrentflowaroundacircuit.
KEYWORDS
inducede.m.f.:(orinducedvoltage)thee.m.f.
createdinaconductorwhenitcutsthrough
magneticfieldlines
395>

409. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Theprinciplesof
electromagneticinduction
Theprocessofgeneratingelectricityfrommotion
iscalledelectromagneticinduction.Thescienceof
electromagnetismwaslargelydevelopedbyMichael
Faraday.Heinventedtheideaofthemagneticfieldand
drewfieldlinestorepresentit.Healsoinventedthefirst
electricmotor.Thenheextendedhisstudiestoshow
howthemotoreffectcouldworkinreversetogenerate
electricity.Inthissection,wewilllookattheprinciples
ofelectromagneticinductionthatFaradaydiscovered.
Aswehaveseen,acoilofwireandamagnetmoving
relativetoeachotherareneededtoinduceavoltage
acrosstheendsofawire.Ifthecoilispartofacomplete
circuit,theinducede.m.f.willmakeaninducedcurrent
flowaroundthecircuit.
Infact,youdonotneedtouseacoil-asinglewireis
enoughtoinduceane.m.f.,asshowninFigure21,5a.
Thewireisconnectedtoasensitivemetertoshowwhen
acurrentisflowing.
Movethewiredownbetweenthepolesofthe
magnetandacurrentflows.
Movethewirebackupwardsandacurrentflowsin
theoppositedirection.
Alternatively,thewirecanbekeptstationaryand
themagnetmovedupanddown.Again,acurrent
willflow.
Youcanseesimilareffectsusingamagnetandacoil
(Figure21.5b).Pushingthemagnetintoandoutofthe
coilinducesacurrent,whichflowsbackandforthinthe
coil.Herearetwofurtherobservations:
Reversethemagnettousetheoppositepoleandthe
currentflowsintheoppositedirection.
Holdthemagnetstationarynexttothewireorcoil
andnocurrentflows.Theymustmoverelativeto
eachother,ornothingwillhappen.
Intheseexperimentsithelpstouseacentre-zerometer.
Then,whentheneedlemovestotheleft,itshowsthatthe
currentisflowingoneway;whenitmovestotheright,
thecurrentisflowingtheotherway.
KEYWORDS
electromagneticinduction:theproductionofan
e.m.facrossanelectricalconductorwhenthereis
relativemovementbetweentheconductoranda
magneticfield
Figure21.5a:Moveawireupanddownbetweenthepoles
ofastationarymagnetandaninducedcurrentwillflow,
b:Similarly,moveamagnetintoandoutofacoilofwireand
aninducedcurrentwillagainflow.
Increasingtheinducede.m.f.
Therearethreewaystoincreasethee.m.f.inducedina
coilorwire:
useastrongermagnet
movethewireorcoilmorequicklyrelativetothe
magnet
useacoilwithmoreturnsofwire.Eachturnofwire
willhaveane.m.f.inducedinit,andthesealladd
togethertogiveabiggere.m.f.
396
>

410. 21Electromagneticinduction
EXPERIMENTAL SKILLS21.1
Inducingelectricity
Inthisexperimentyouwillinduceane.m.f.and
investigatethefactorswhichaffecttheinduced
e.m.f.,includingthefactorsaffectingthesizeand
directionofthee.m.f.inducedwhenaconductor
movesrelativetoamagneticfield.
Safety:Somesensitivemetershavealocking
mechanismwhichpreventsthembeingdamaged
duringmovement.Checkyourmeterbeforeyou
begin.
Gettingstarted
Lookcarefullyatthemeteryouareusing.Explain:
whyzeroisatthecentreofthescale
howyoucantellitisaverysensitivemeter.
Youwillneed:
thininsulatedwire
strongbarmagnet
2magnadurmagnetsandayoke
sensitive,centre-zeroammeterorvoltmeter.
Method
1Coil2.0metresofthininsulatedwirewithbare
endstomakeasolenoidapproximately5cm
indiameter.Thecoilcanbeflatasshownin
Figure21.6ratherthanlong.
2Connecttheendsofthecoiltotheterminalsof
asensitivevoltmeterorammeter.
3Bringonepoleofabarmagnettowardsand
intothecentreofthecoil.Observethereading
onthemeter.
4Nowinvestigatehowthereadingonthemeter
changesindifferentcircumstances:
Movethebarmagnetatdifferentspeeds
intothecoil.
Usetheoppositepoleofthebarmagnet.
Movethebarmagnetoutofthecoil.
Holdthebarmagnetstationaryatdifferent
distancesfromthecoil.
5Straightenoutthewire.Keeptheends
connectedtothemeter.
6Mounttwomagnadurmagnetsonayoke.
Ensurethatoppositepolesarefacingeach
othersothatthereisastrongmagneticfield
betweenthemagnets.
7Holdasectionofthewire,approximately10cm
inlength,betweenyourtwohands.Movethe
wiredownwardsthroughthefield(Figure21.7).
Observethereadingonthemeter.
Figure21.7:Experimentalset-up(part2)
8Nowinvestigatehowthereadingonthemeter
changesindifferentcircumstances.
Movethewireatdifferentspeedsthrough
themagneticfield.
Reversethedirectionofthemagneticfield.
Movethewireoutofthemagneticfield.
Holdthewirestationaryinthemagnetic
field.
Figure21.6:Experimentalset-up.
397y

411. yCAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
CONTINUED
Question
1Copyandcompletethesesentencestorecord
yourobservations:
Ane.m.f.isinducedinaconductorinamagnetic
fieldifeitherthe orthe move
acrosstheother.
Thesizeofthee.m.f.inducedinacoilcanbe
increasedbyincreasing , or .
Thedirectionofthee.m.f.canbe by
reversingthefieldorthedirectionofmovement.
Questions
1Whichofthefollowingwouldnotinduceacurrent:
Amovingawirebetweenthepolesofamagnet
Btakingabarmagnetoutofacoilorwire
Choldingawirebetweenthepolesofamagnet
Dspinningacoilorwireinamagneticfield.
2Marcusholdsapieceofcopperwirewhichis
connectedtoanammeterbetweenthepolesof
amagnet,buthedoesnotgetareadingonthe
ammeter.Whatshouldhedotomakeacurrentflow?
3Astudentmovesabarmagnetintoacoilofwireand
acurrentflows.Describetwochangeswhichwould
makethecurrentflowintheoppositedirection.
4Ana.cgeneratorandacellcanbothbeusedtolight
abulb.Describehowthecurrentflowingthrough
thebulbisdifferentineachcase.
Inductionandfieldlines
Wecanunderstandelectromagneticinductionbythinking
aboutmagneticfieldlines.Figure21.8showsthefield
linesbetweenthepolesofahorseshoemagnet.Asthe
wireismoveddownbetweenthepolesofthemagnet,it
cutsthefieldlinesofthemagnet.Cuttingthefieldlines
inducesthecurrent.
Figure21.8:Asthewirecutsthroughthefieldlines,an
e.m.f.isinducedinit.Ifthewireispartofacompletecircuit,
acurrentwillflow.
Thisideahelpsustounderstandthefactorsthataffect
themagnitudeanddirectionoftheinducede.m.f.
Whenthemagnetisstationary,thereisnocuttingof
fieldlinesandsonoe.m.f.isinduced.
Whenthemagnetisfurtherfromthewire,thefield
linesarefurtherapartandsofewerare!cut,givinga
smallere.m.f.
Whenthemagnetismovedquickly,thelinesarecut
morequicklyandabiggere.m.f.isinduced.
Acoilgivesabiggereffectthanasinglewire,
becauseeachturnofwirecutsthemagneticfield
lines.Thismeansthateachcontributes
tothe
inducede.m.f.
Fleming'sright-handrule
Wehaveseenthat,whenawireismovedsothatitcuts
acrossamagneticfield,acurrentisinducedinthewire.
Howcanweworkoutthedirectionofthecurrent?
InChapter20,wesawtheoppositeeffect.Whena
currentflowsinamagneticfield,thereisaforceon
itsothatitmoves.Thedirectionsofforce,fieldand
currentweregivenbyFleming’sleft-handrule.Itisnot
surprisingtofindthat,inthecaseofelectromagnetic
induction,thedirectionsaregivenbyFleming’sright¬
handrule.AsshowninFigure21.9,thethumbandfirst
twofingersshowthedirectionsofthesamequantities.
Takecaretousethecorrecthand.
KEYWORDS
Fleming'sright-handrule:arulethatgivesthe
relationshipbetweenthedirectionsofforce,
fieldandcurrentwhenacurrentflowsacrossa
magneticfield
398y

412. 21Electromagneticinduction
Figure21.9a:Whenacurrentisinducedinawire,
motion,fieldandcurrentareatrightanglestoeachother,
b:Fleming'sright-handruleisusedtoworkoutthedirection
oftheinducedcurrent.
Ana.c.generator
Faraday’sdiscoveryofelectromagneticinductionledto
thedevelopmentoftheelectricitysupplyindustry.In
particular,itallowedengineerstodesigngeneratorsthat
couldsupplyelectricity.Atfirst,thiswasonlydoneon
asmallscale,butgraduallygeneratorsgotbiggerand
bigger,until,liketheonesshowninFigure21.10,they
werecapableofsupplyingtheelectricitydemandsof
thousandsofhomes.
Figure21.10:Theturbineandgeneratorinthegenerating
hallofanuclearpowerstation.Theturbinesarefedbyhigh-
pressuresteaminpipes.
Ageneratorofthistypeproducesalternatingcurrent
(a.c.).Alternatingcurrentflowsfirstonewaythenthe
otherasthecoilsturninthemagneticfield.
Figure21.11showsasimplea.c.generator,which
producesalternatingcurrent.Inprinciple,ana.c
generatorislikead.c.motor,workinginreverse.The
axleismadetoturnsothatthecoilspinsaroundin
themagneticfield,andacurrentisinduced.Theother
differenceisinthewaythecoilisconnectedtothecircuit
beyond.Ad.c.motorusesasplit-ringcommutator,
whereasana.c.generatorusessliprings.Thesliprings
rotatewiththecoil.Thebrushesrubagainsttheslip
ringsandsohavethesamee.m.f.asthesidesofthecoil.
KEYWORDS
a.c.generator:adevicesuchasadynamoused
togeneratealternatingcurrent
sliprings:adeviceusedtoallowcurrenttoflow
toandfromthecoilofana.c.generator
Figure21.11;Asimplea.c.generatorworkslikeamotorin
reverse.Theslipringsandbrushesareusedtoconnectthe
alternatingcurrenttotheexternalcircuit.
Whydoesthisgeneratorproducealternatingcurrent?
Asthecoilrotates,eachsideofthecoilpassesfirstthe
magneticnorthpoleandthenthesouthpole.
Figure21.12showsagraphofthis.Whenthecoilis
horizontal,itcutsthroughthefieldlinesinducinga
voltage.Asitturnstovertical,itcutsfewerfieldlines
sothevoltagedecreasestozero.Asitcontinuesbackto
horizontalitcutsthroughthefieldlinesintheopposite
direction,givingapeakvoltageintheoppositedirection.
Thismeansthattheinducedcurrentflowsfirstoneway,
andthentheother.Inotherwords,thecurrentinthecoil
isalternating.
399>

413. CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Thecurrentflowsoutthroughthesliprings.Eachring
isconnectedtooneendofthecoil,sothealternating
currentflowsoutthroughthebrushes,whichpress
againsttherings.
Therearefourwaysofincreasingthevoltagegenerated
byana.c.generatorliketheoneshowninFigure21.11:
turnthecoilmorerapidly
useacoilwithmoreturnsofwire
useacoilwithabiggerarea
usestrongermagnets.
Eachofthesechangesincreasestherateatwhich
magneticfieldfinesarecut,andsotheinducede.m.f.is
greater.Forthea.c.generatorshowninFigure21.11,
eachrevolutionofthecoilgeneratesonecycleof
alternatingcurrent.Spinthecoil50timeseachsecond
andthea.c.generatedhasafrequencyof50Hz.
Figure21.12:Agraphtorepresentanalternatingvoltage.
Asthecoilturns,thevoltagereachesapeakinone
direction,decreasestozero,thenreverses.
Figure21.12showshowthee.m.f.producedbyana.c.
generatorvariesasthecoilturns.Ifwethinkabout
howthecoilcutsthroughmagneticfieldlines,wecan
understandwhythea.c.graphvariesbetweenpositive
andnegativevalues.Withthecoilinthehorizontal
position,asshowninpositiona,itstwolongsidesare
cuttingrapidlythroughthemagneticfieldlines.This
givesalargeinducede.m.f.,correspondingtoapeak
inthea.c.graph.Whenthecoilisvertical(positionb),
itslongsidesaremovingalongthefieldlines,sothey
arenotcuttingthem.Thisgivesnoinducede.m.f.,a
zeropointonthea.c.graph.Whenthecoilhasturned
through180°topositionc,itwillbecuttingfieldlines
quicklyagain,butintheopftositedirection,sothe
inducede.m.f.willagainbelarge,butthistimeitwillbe
negative-thiscorrespondstoatroughonthegraph.
Directionoftheinducede.m.f.
Howisthedirectionofaninducedcurrentdetermined?
Theansweristhatthecurrent(likeallcurrents)hasa
magneticfieldaroundit.Thisfieldalwayspushesback
againstthefieldthatisinducingthecurrent.So,forthe
coilshowninFigure21.13,whenthemagnet’snorthpole
ispushedtowardsthecoil,thecurrentflowstoproduce
anorthpoleattheendofthecoilnearestthemagnet.
Thesetwonorthpolesrepeleachother.Thismeansthat
youhavetopushthemagnettowardsthecoilandthat
youhavetodowork.Theenergyyouuseinpushing
themagnetistransferredtothecurrent.Thatiswhere
theenergycarriedbythecurrentcomesfrom.Itcomes
fromtheworkdoneinmakingaconductorcutthrough
magneticfieldlines.
Aninducedcurrentalwaysflowsinsuch^a
waythatits
magneticfieldopposesthechangethatcauses
it.Thisis
knownasLenz’slaw.
KEYWORDS
Lenz'slaw:thedirectionofaninducedcurrent
alwaysopposesthechangeinthecircuitorthe
magneticfieldthatproducesit
inducedNpole
induced
current
induced
e.m.f.
Figure21.13:Considerabarmagnetbeingmovedintoa
coilnorthpolefirst.Lenz'slawtellsusthatthee.m.f.induced
willopposethis.Therefore,theendthemagnetapproaches
mustbecomeanorthpole,torepeltheapproachingnorth
pole.Thismeansthatthecurrentmustflowanticlockwiseat
thatendofthecoil.
400)

414. 21Electromagneticinduction
Questions
5Astudentmovesthenorthpoleofamagnettowards
acoilofwire(asshowninFigure21.13),sothatan
inducedcurrentflows.Statetwowaysinwhichthe
studentcouldcauseabiggerinducedcurrenttoflow.
6DrawadiagramsimilartoFigure21.13toshow
whathappenswhenthemagnetismovedawayfrom
thecoil.(Hint:remember,theinducedcurrentwill
opposethischange.)
ACTIVITY21.1
Generatingfitness
Figure21.14:Usinganexercisebiketochargea
mobilephone.
ThewomaninFigure21.14ischarginghermobile
phone.Asshepedalstheexercisebike,sheturns
ageneratortoinduceacurrentwhichisusedto
chargeherphone.
Produceapostertoattractcustomerstousea
bikechargerinapublicplace.Youshouldinclude
informationabouthowthesystemhelpsto:
savemoney
improvetheuser'shealth
protecttheenvironment.
Youshouldalsoincludeamoredetailedsectionfor
theusertoreadastheypedal.Thisshouldexplain
howthesystemworksandadvisethecustomer
howtochargetheirphoneasquicklyaspossible.
21.2Powerlinesand
transformers
Figure21.15:Electricityisusuallygeneratedatadistance
fromwhereitisused.Ifyoulookonamap,youmaybeable
totracethepowerlinesthatbringelectricalpowertoyour
neighbourhood.
Powerstationsmaybe100kmormorefromtheplaces
wheretheelectricitytheygenerateisused.Thiselectricity
mustbedistributedaroundthecountry.
High-voltageelectricityleavesthepowerstation.Its
voltagemaybeashighas1millionvolts.Toavoiddanger
topeople,itisusuallycarriedincablescalledpowerlines
slunghighabovethegroundbetweentallpylons.Linesof
pylonscarrywiresacrossthecountryside,headingforthe
urbanandindustrialareasthatneedthepower(Figure
21.16).Thisisacountry’snationalgrid.
KEYWORDS
powerlines:cablesusedtocarryelectricityfrom
powerstationstoconsumers
nationalgrid:thesystemofpowerlines,pylons
andtransformersusedtocarryelectricityaround
acountry
Whenthepowerlinesapproachtheareawherethe
poweristobeused,theyenteralocaldistributioncentre.
Herethevoltageisreducedtoalesshazardouslevel,
andthepowerissentthroughmorecables(overheador
underground)tolocalsubstations.Inthesubstation,the
voltageisreducedtothelocalsupplyvoltage,typically
230V.Whereveryoulive,thereislikelytobeasubstation
401>

415. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Figure21.16:Inanationalgrid,voltagesareincreasedtoreduceenergylosses,thenreducedtoarelativelysafelevel.
intheneighbourhood.Itmaybeinasecurelylocked
building,ortheelectricalequipmentmaybesurrounded
byfencing,whichcarriesnoticeswarningofthehazard
(Figure21.17).
DANGEROFDEATH
Figure21.17:Anelectricitysubstationhaswarningsigns
likethistoindicatetheextremehazardofenteringthe
substation.
Fromthesubstation,electricityisdistributedtohouses,
shops,etc.Insomecountries,thepoweriscarriedin
cablesburiedunderground.Othercountriesusetallpoles
andoverheadwires.
Whyusehighvoltages?
Thehighvoltagesusedtotransmitelectricalpower
aroundacountryaredangerous.Thatiswhythecables
thatcarrythepoweraresupportedhighabovepeople,
trafficandbuildingsontallpylons.Sometimesthecables
areburiedunderground,butthisismuchmoreexpensive,
andthecablesmustbesafelyinsulated.Thereasonfor
usinghighvoltagesistoreducethelossofenergydueto
thecablesheatingup.Thisheatinghappensmuchmore
whenthesamepoweristransmittedatalowervoltage.
Transformers
Atransformerisadeviceusedtoincreaseordecrease
thevoltageofanelectricitysupply.Theyaredesigned
402
>

416. 21Electromagneticinduction
tobeasefficientaspossible(upto99.9%efficient).
Thisisbecausetheelectricityweusemayhavepassed
throughasmanyastentransformersbeforeitreachesus
fromthepowerstation.Alossof1%ofenergyineach
transformerwouldrepresentatotalwasteof10%ofthe
energyleavingthepowerstation.
Powerstationstypicallygenerateelectricityat25kV.This
hastobeconvertedtothegridvoltage(typically400kV)
usingtransformers.Thisisknownassteppingupthe
voltage.
Figure21.18ashowstheconstructionofasuitable
transformer.Everytransformerhasthreeparts:
Aprimarycoil:theincomingvoltageVpisconnected
acrossthiscoil.
Asecondarycoil:thisprovidesthevoltageKstothe
externalcircuit.
Anironcore:thislinksthetwocoils.
transformer
Figure21.18a:Thestructureofatransformer.Thisisa
step-uptransformerbecausetherearemoreturnsonthe
secondarycoilthanontheprimary.Iftheconnectionsto
itwerereversed,itwouldbeastep-downtransformer,
b:Thecircuitsymbolforatransformershowsthetwocoils
withthecorebetweenthem.
Noticethatthereisnoelectricalconnectionbetween
thetwocoils.Theyarelinkedtogetheronlybythe
softironcore.Thewiresareinsulatedsonocurrent
flowsfromonecoiltotheother.Noticealsothatthe
voltagesarebothalternatingvoltages.Transformers
onlyworkwitha.c.Alltheydoischangethesizeof
ana.c.voltage.
Thepowerstationtransformerdescribedearliersteps
upthevoltagefrom25kVto400kV-itisincreasedby
afactorof16.Tostepupthevoltagebyafactorof16,
theremustbe16timesasmanyturnsonthesecondary
coilasontheprimarycoil.Bycomparingthenumbers
ofturnsonthetwocoilswecantellhowthevoltagewill
bechanged.
Astep-uptransformerincreasesthevoltage.There
aremoreturnsonthesecondarycoilthanonthe
primarycoil.
Astep-downtransformerreducesthevoltage.There
arefewerturnsonthesecondarycoilthanonthe
primarycoil.
KEYWORDS
transformer:adeviceusedtochangevoltageof
ana.c.electricitysupply
primarycoil:theinputcoilofatransformer
secondarycoil:theoutputcoilofatransformer
step-uptransformer:atransformerwhichincreases
thevoltageofana.c.supply
step-downtransformer:atransformerwhich
decreasesthevoltageofana.c.supply
Notethat,whenthevoltageissteppedup,thecurrentis
steppeddown,andwhenthevoltageissteppeddown,the
currentissteppedup.
Theratioofthenumberofturnstellsusthefactorby
whichthevoltagewillbechanged.Wecanwritean
equation,knownasthetransformerequation,relating
thetwovoltages,VpandKs,tothenumbersofturnson
eachcoil,ALandN,:
KEYEQUATION
voltageacrossprimarycoil_
numberofturnsonprimary
voltageacrosssecondarycoil.numberofturnsonsecondary
VN
p_*p
rsM

417. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
WORKEDEXAMPLE21.1
Thereareverylargetransformersbetweenpower
stationstothetransmissioncables.
Oneofthesetransformershas800turnsonits
primarycoiland16000turnsonitssecondarycoil.
Thevoltageacrossitsprimarycoilis25kV
aStatewhattypeoftransformerthisis.
bCalculatethevoltageacrossitssecondarycoil.
Step1:Writedownwhatyouknow:Thetransformer
hasmoreturnsonthesecondarycoilthanthe
primarysoitisastep-uptransformer.This
meansthevoltageshouldincrease.Usethisto
checkyouranswertothenextpart.
Step2:Drawasimpletransformerasshownin
Figure21.19andmarkonittheinformation
giveninthequestion.
V=25OOOV <>
V=?
IU- >
~r~
N=600 N=16000
f
S
Figure21.19:Transformersymbolwiththequantities
giveninthequestion.
Step3:Writedownthetransformerequation.
VsNs
Step4:Substituteinthevaluesfromthequestion.
25000V_
800
Fs
16000
Step5:RearrangetheequationtofindVs.
v
_
25000Vx16000
s
800
=500000V
Checkthatthisisgreaterthantheprimaryvoltageas
expected.
Answer
astep-uptransformer
bFs=500000V
Itisoftenusefultodoaquickmentalcalculationfirst.
Forexample,ifatransformerdecreasesvoltagefrom
230Vto60V,ithasreduceditbyaboutaquarter.This
meansthesecondarycoilmusthaveroughlyaquarterof
thenumberofturnsthanthoseontheprimary.Thisis
anapproximatecalculation.Ithelpsyoucheckwhether
yourfinalanswerissensible.
Questions
7aCopyandcompletethesesentences.
Electricityisdistributedbythenational
.Thisconsistsofsubstations,cables
and .
Theelectricityistransmittedatveryhigh
to energybeinglostasheat
inthecables.
Thevoltagecanbeincreasedordecreasedby
bFigure21.20showsatransformer.
Figure21.20:Atransformer.
NamethepartslabelledA,BandC.
cExplainwhetherthisisastep-uporstep-down
transformer.
8Atransformerhastenturnsontheprimarycoil
andfiveturnsonthesecondary.Avoltageof12Vis
appliedacrosstheprimarycoil.
aIsthisastep-uporstep-downtransformer?
bUsethetransformerequationtocalculatethe
outputvoltageofthetransformer.
9CopyandcompleteTable21.1.Calculatethe
missinginformationusingthetransformerequation.
404>

418. 21Electromagneticinduction
Table21.1
10
11
alternatingmagneticfieldline
Drawadiagramofthesituation.
primarycoil
21.3Howtransformers
secondarycoil
work
REFLECTION
Eveninawell-designedtransformer,someenergyislost
becauseoftheresistanceofthewires,andbecausethe
coreresiststheflowofthechangingmagneticfield.
Thinkaboutwhetheryoucanapplyanyofthese
ideaswhenyoudoothercalculations.
Transformersonlyworkwithalternatingcurrent(a.c.).
Tounderstandwhythisis,weneedtolookathowa
transformerworks(Figure21.21).Itmakesuseof
electromagneticinduction.
Figure21.21:Thea.c.intheprimarycoilproducesavarying
magneticfieldinthecore.Thisinducesavaryingcurrentin
thesecondarycoil.
Aportableradiohasabuilt-intransformersothatit
canworkfromthemainsinsteadofbatteries.Isthis
astep-uporstep-downtransformer?
Atransformerincreasesthe1100Vfromapower
stationto132000Vfortransmission.Calculate
thenumberofturnsontheprimarycoilwhenthe
secondarycoilhas6000turns.
Theprimarycoilhasalternatingcurrentflowingthrough
it.Itis,therefore,anelectromagnet,andproducesan
alternatingmagneticfield.Thecoretransportsthis
alternatingfieldaroundtothesecondarycoil.Nowthe
secondarycoilisaconductorinachangingmagnetic
field.Acurrentisinducedinthecoil.Thisisanother
exampleofelectromagneticinductionatwork.
Whenthesecondarycoilhasonlyafewturns,thee.m.f.
inducedacrossitissmall.Whenithasalotofturns,the
e.m.f.willbelarge.Hence,toincreasethevoltageout,
weneedasecondarycoilwithmanymoreturnsthanthe
primarycoil.
Whendirectcurrentisconnectedtoatransformer,there
isnooutputvoltage.Thisisbecausethemagneticfield
producedbytheprimarycoildoesnotchange.Withan
unchangingfieldpassingthroughthesecondarycoil,no
voltageisinducedinit.
NoticefromFigure21.21thatthemagneticfieldlinks
theprimaryandsecondarycoils.Theenergybrought
bythecurrentintheprimarycoilistransferredtothe
secondarycoilbythemagneticfield.Thismeansthatthe
coremustbeverygoodattransferringmagneticenergy.
Asoftmagneticmaterialmustbeused-usuallyanalloy
ofironwithasmallamountofsilicon.(Recallthatsoft
magneticmaterialsareonesthatcanbemagnetisedand
demagnetisedeasily.)
Doaquickapproximatecalculationfirst.
Decidewhetherthetransformerisstep-upor
step-down,andwhatthistellsyouaboutyour
answer.
Inthesecalculationsitisusefultothinkabout
whethertheanswerissensible.Whichofthe
followingsuggestionsarehelpfulforyou?What
doesthistellyouabouthowyouwork?
Drawthetransformerandwritethenumbers
onit.
WP Ns V.
Step-upor
step-down
transformer?
10 20 12step-up
10 1.2 12
50 240 6
10000 20115000

419. 1
)CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Questions
12aWhatisthefunctionofthecoreofatransformer?
bWhymustthecorebemadeofasoftmagnetic
material?
13Explainwhyatransformerwillnotworkwithdirect
current.
Calculatingcurrent
Totransmitacertainpower,P,wecanuseasmallcurrent,
I,ifwetransmitthepowerathighvoltage,V.Thisfollows
fromtheequationforelectricalpower(seeChapter18):
electricalpowerP=IV
WorkedExample21.2showshowthisworks.
Energysaving
Thereisagoodreasonforusinghighvoltages.Itmeans
thatthecurrentflowinginthecablesisrelativelylow,and
thiswasteslessenergy.Thiscanbeexplainedasfollows.
Aconsumerneedsacertainamountofpower.This
powercanbedeliveredas:
highvoltage,lowcurrentor
lowvoltage,highcurrent.
Calculatingpowerlosses
Whenacurrentflowsinawireorcable,someof
theenergyitiscarryingislostbecauseofthecable’s
resistance-thecablesgetwarm.Asmallcurrentwastes
lessenergythanahighcurrent.
Wecancalculatetherateofenergylossinthecables,or
thepowerloss,byputtingtogethertwoequationswemet
inChapter18:
power=currentxpotentialdifferenceorP=IV
and
potentialdifference=currentxresistanceorV=IR
P=IVbecomesP=KIR)orP=PR.
KEYEQUATION
powerloss=squareofcurrentinthecablexresistance
P=PR
Electricalengineersdoeverythingtheycantoreducethe
energylossesinthecables.Iftheycanreducethecurrent
tohalfitsvalue(bydoublingthevoltage),thelosseswill
beone-quarteroftheirpreviousvalue.Thisisbecause
powerlossesincablesareproportionaltothesquareof
thecurrentflowinginthecables:
doublethecurrentgivesfourtimesthelosses
threetimesthecurrentgivesninetimesthelosses.
WORKEDEXAMPLE21.2
A20kWgeneratorgivesanoutputof5kV.Thisis
transmittedtoaworkshopbycableswitharesistance
of20Q.Calculate:
athepowerlossinthecables
btheeffectofusingatransformertoincreasethe
outputvoltageto20kVassumingthatthepower
outputremainsthesame.
Whenansweringthisquestion,itisimportantto
realisetherearetwodifferentpowersinvolved-the
powerofthegeneratorandthepowerlossinthe
cables.
Step1:Calculatethecurrentinthewiresat5kV
usingtheequation:
generatorpower,P—
IV
Rearrangetheequationandsubstitutevalues:
I=P- 20000W-
5000V=4A
Step2:Calculatethepowerloss:
powerloss,P=PR=(4A)2x20Q=320W
(thisistheanswertoparta).
Step3:Calculatethecurrentinthewiresat20kV:
P=IV
I—
P
~K=20000W20000V=1A
Step4:Calculatethepowerloss:
P=PR=(1A)2x20Q=20W
Answer
a320W
bUsingthetransformertodecreasethevoltagegreatly
reducesthepowerloss,from320Wto20W.
FromtheresultsofWorkedExample21.2,youcansee
whyelectricityistransmittedathighvoltages.Thehighej
thevoltage,thesmallerthecurrentinthecables,andso
thesmallertheenergylosses.Increasingthevoltageby
afactoroffourreducesthecurrentbyafactoroffour.
Thismeansthatthepowerlostinthecablesisgreatly
reduced(infact,itisreducedbyafactorof42,whichis
16),andsothinnercablescansafelybeused.
Thecurrentflowinginthecablesisaflowofcoulombs
ofcharge.Athighvoltage,wehavefewercoulombs
flowing,buteachcoulombcarriesmoreenergywithit.
406>

420. 21Electromagneticinduction
Thinkingaboutpower
Ifatransformeris100%efficient,nopowerislostinits
coilsorcore.Thisisareasonableapproximation,because
well-designedtransformerswasteonlyabout0.1%ofthe
powertransferredthroughthem.Thisallowsustowrite
anequationlinkingtheprimaryandsecondaryvoltage,
FpandFs,totheprimaryandsecondarycurrents,Ipand
/s,usingP=IV.
KEYEQUATION
powerintoprimarycoil-poweroutofsecondarycoil
ZpXrp=zsxKs
Itisimportanttorememberthatthisequationassumes
thatnopowerislostinthetransformer.WorkedExample
21.3showshowtousethisequation.
WORKEDEXAMPLE21.3
Aschoolpowerpackhasanoutputvoltageof9V.Itis
pluggedintothe230Vmainssupply.Thepowerpack
containsatransformer.Theoutputcurrentofthe
powerpackis3A.Calculatethecurrentsuppliedto
theprimarycoilofthetransformerinthepowerpack.
Assumetherearenoenergylossesinthetransformer.
Step1:Drawatransformersymbolandmarkonthe
informationfromthequestion:
Figure21.22:Transformersymbolwiththe
knownquantities.
Step2:Writedownthetransformerpowerequation.
ZpxEp=ZsxFs
Step3:Substitutevaluesfromthequestion.
3Ax9V=Zs
x230V
Step4:RearrangeandsolveforZ8.
j
_
3Ax9V
’230V
=0.12A
CONTINUED
Answer
So,thecurrentsuppliedtotheprimarycoilis0.12A.
Insteppingdownthevoltage,thetransformerhas
steppedupthecurrent.Ifbothhadbeensteppedup,
wewouldbegettingsomethingfornothing-whichin
physics,isimpossible!
Questions
14Apowerstationgenerates230000W.
aCalculatethecurrentinthewireswhenthe
poweristransmitted:
iat230V
iiat23000V.
bExplainwhyitisbettertotransmitthe
electricityatahighervoltage.
15Thepowersupplytoafactoryis100kW.Thewires
havearesistanceof0.2Q.
aCalculatethepowerlossinthecableswhenthe
voltageacrossthewiresis250V.
bCalculatethepowerlosswhenthevoltageis
steppedupto12500V.
16Atransformerreducesthemainsvoltagefrom240V
to12Vforuseinaschoollaboratorypowerpack.
Thecurrentsuppliedbythepowerpackis2A.
Whatcurrentflowsintothepowerpack?
Whatassumptionmustyoumakeinthiscalculation?
REFLECTION
Lookbackthroughyourworkonthemotoreffect
andelectromagneticinduction.Arethereanyareas
youneedtoworkon?Askyourself:
Areyouclearaboutwhateacheffectis?
Canyourecallandunderstandthe
equations?
Canyourecallandusetherelevantrulesfor
findingdirectionsoffieldsorcurrents?
Makerevisioncardswiththekeythingsyouneed
toremember.
407)

421. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
PROJECT
Powertothepeople
Figure21.23:AtowninSweden.
InthisSwedishtowncoalisminedthenburnedto
generateelectricity.Theelectricityisdistributedto
homesviathenationalgrid.
Electricalpowergenerationanddistributionvaries
dependingonwhereyoulive.
Yourtaskistoresearchhowelectricityreaches
you.Workinginasmallgroupyouwillresearch
howelectricityisgeneratedanddistributedinyour
country.Youwillworktogethertoproducealarge
posterillustratingandexplainingwhatyoufind.
Yourposterwillhavealotofinformation,soplan
carefullyhowthedifferentpartswillcometogether.
Forexample,youmaydecidetostructureyour
posteraroundadiagramofthenationalgrid,with
informationboxesforeachstage.
Youmustinclude:
informationaboutthetypeofpowerstations
andrenewableresourceswhichareinuse
adiagramofageneratorandinformation
abouthowitworksandhowitisusedinpower
generation
adiagramsimilartoFigure21.16showinghow
electricityisdistributed
informationabouttheroleoftransformersin
thesystemandthedifferencebetweenstep-up
andstep-downtransformers
reasonswhythevoltageofthesupplyhastobe
changed.
Youcouldadd:
apiechartshowingthedifferentenergy
resourcesusedinyourcountry
adetailedexplanationofhowana.c.generator
works,includingwhythecurrentgeneratedis
a.c.(notd.c.)
detailsofthevoltagesatdifferentpartsofthe
grid,withcalculationsshowingthetransformers
neededateachstage
photographsfromyourlocalareashowing
pylons,substations,overheadcablesandsafety
notices.
.
SUMMARY
Whenthereisrelativemovementbetweenaconductorandamagneticfield,ane.m.f.isinducedacrosstheconductor.
Theinducede.m.f.maycauseaninducedcurrenttoflow.
IThee.m.f.inducedinacoilofwirecanbeincreasedby:
increasingthemagneticfieldstrength
increasingthenumberofturnsonthecoil
increasingthespeedofmovement.
Thedirectionoftheinducede.m.f.opposesthemovementwhichcausesit.
408)

422. 21Electromagneticinduction
CONTINUED
Thedirectionoftheinducede.m.fcanbefoundusingFleming’sright-handrule.
Directcurrentflowsinonedirection.Alternatingcurrentreversesdirectionrepeatedly.
Theoutputofana.c.generatordependsonthepositionofthecoilinrelationtothemagneticfield.
Atransformer,consistingofaprimarycoil,asecondarycoilandasoftironcorecanbeusedtochange
alternatingvoltages.
Astep-uptransformerincreasesvoltage,astep-downtransformerdecreasesvoltage.
Thenumberofturnsontransformercoilsandthevoltagesacrossthemcanbecalculatedusingtheequation.
VN
FsM
Transformersincreasevoltagefortransmissionbythenationalgridthenreducethevoltagetoasaferlevelfor
consumers.
a.c.intheprimarycoreofatransformercreatesachangingmagneticfieldwhich,inturn,inducesa.c.inthe
secondarycoil.
Whenatransformerisassumedtobe100%efficient,theprimaryandsecondarycurrentsandvoltagesare
relatedbytheequationZpxFp=IsxKs.
ThepowerlossesincablescanbecalculatedusingtheequationP=PR.
EXAM-STYLEQUESTIONS
1Whichoneoftheseisnotneededforelectromagneticinduction? [1]
Aaconductor
Bmovement
Cacoilofwire
Damagneticfield
2Whichstatementistrue? {1]
AAstep-downtransformerdecreasescurrent.
BAstep-uptransformerincreasesvoltage.
CAstep-uptransformerisusedinaschoollaboratorypowersupply.
DAstep-downtransformerhasmoreturnsonthesecondarycoilthan
ontheprimarycoil.
3Astudentinvestigatestheeffectofmovingabarmagnetintoacoil.
Shemeasuresthecurrentwhichflowsinthecoilwhenshemovesthemagnet.
Whichchangewouldincreasethecurrent? [1]
Ausingastrongerbarmagnet
Bmovingthemagnetintheoppositedirection
Cmovingthecoilinsteadofthemagnet
Dmovingthecoilfromsidetosideaswellasinandoutofthemagnet
409y

423. 1
yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
4Thediagramshowsacoilandamagnet.Whenthemagnetismovedawayfrom
thecoil,currentflows.
IW
Whichoneofthefollowingdescribeswhathappens? [1]
ACurrentflowsclockwiseinthecoil,creatinganorthpolewhichattractsthe
southpoleofthebarmagnet.
BCurrentflowsanticlockwise,creatinganorthpolewhichrepelsthesouth
poleofthebarmagnet.
CCurrentflowsanticlockwise,creatingasouthpolewhichrepelsthesouth
poleofthebarmagnet.
DCurrentflowsanticlockwise,creatinganorthpolewhichattractsthesouth
poleofthebarmagnet.
5aWhichstatementdescribeselectromagneticinduction? [1]
Atheproductionofane.m.facrossanelectricalconductorwhenthere
isrelativemovementbetweentheconductorandamagneticfield
Btheproductionofane.m.facrossanelectricalconductorwhenthere
isnomovementbetweentheconductorandamagneticfield
Ctheproductionofane.m.facrossanelectricalconductorwhenthere
isrelativemovementbetweentheconductorandaninducedcurrent
Dtheproductionofane.m.facrossanelectricalconductorwhenthere
isnomovementbetweentheconductorandaninducedcurrent
bDescribeanexperimenttodemonstrateelectromagneticinductionusing
ahorseshoemagnet,apieceofcopperwireandasensitiveammeter.
Youmayincludeadiagraminyouranswer. [3]
cStatetwofactorswhichaffectthesizeofthecurrentinducedinthis
experiment. [2]
[Total:6]
6aDrawadiagramofastep-uptransformerandlabelthethreeessential
partsofthetransformer. [4]
bAstudentconnectstheinputofastep-uptransformertoabattery.
Heconnectstheoutputtoabulb,butitdoesnotlight.
Explainwhy. [1]
[Total:5]
COMMAND WORDS
describe:statethe
pointsofatopic;give
characteristicsand
mainfeatures
state:expressin
clearterms
explain:setout
purposesor
reasons;make
therelationships
betweenthings
evident;provide
whyand/orhowand
supportwithrelevant
evidence
410>

424. 21Electromagneticinduction
CONTINUED
7aTransformersareusedinthenationalgridtochangethevoltageof
thesupply.FortransformersAandB,statewhetherthetransformeris
step-uporstep-down,andexplainwhythevoltagechangeisnecessary.[4]
bAtransformerinthenationalgridhas800turnsontheprimary
coiland16000onthesecondary.Theprimaryvoltageis25kV.
Calculatethesecondaryvoltage. [2]
[Total:6]
8Ateacherdemonstrateselectromagneticinductionusingacoil,abar
magnetandasensitive,centre-zeroammeter.
aTheteacherslowlymovesthemagnettowardsthecoil.Stateand
explainwhathappens. [2]
bTheteacherchangesdifferentfactorsintheexperiment.Statetheeffect
ofeachoftheseactionsontheammeterreading:
iTheteacherholdsthecoilstill.
iiTheteachermovesthecoilquicklytowardsthecoil.
iiiTheteacherholdsthemagnetstillandslowlymovesthecoiltowards
themagnet.
ivTheteacherquicklymovesthemagnetawayfromthecoil. [4]
[Total:6]
9Thediagramshowsana.c.generator.
COMMAND WORDS
calculate:workout
fromgivenfacts,
figuresorinformation
411>

425. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
*CONTINUED
Hl
[1]
c
[1]
d
COMMAND WORDS
[1]
give:producean
answerfromagiven
sourceorrecallI
memory
a
b
a
b
10Spotweldingisamethodofjoiningtwopiecesofmetaltogetherbypassinga
veryhighcurrentthroughthem.Thediagramshowsatransformerbeingused
tospotweldtwoironnails.
Explainwhythenailsbecomehotwhenthepowerisswitchedon.
Usethedatafromthediagramtocalculatethecurrentneeded
fromthepowersupply.Assumethetransformeris100%efficient.
Giveyouranswertotwodecimalplaces. [4]
[Total:5]
NamethepartlabelledA.
Nametherulewhichisusedtofindthedirectionofthecurrentina
conductormovinginamagneticfield.
Usethislawtofindthedirectionoftheforceonthesectionofwire
labelledBC.
Thebrightnessofthelampvariesasthecoilisturned.Explainwhythe
positionofthecoilaffectsthebrightnessofthebulb. [3]
[Total:6]

426. 21Electromagneticinduction
SELF-EVALUATIONCHECKLIST
Afterstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutosee
anygapsinyourknowledgeandhelpyoutolearnmoreeffectively.
Ican
See
Topic...
Needs
morework
Almost
there
Confident
tomoveon
Statethatane.m.f.isinducedwhenthereisrelative
movementbetweenaconductorandamagneticfield,
anddescribeanexperimenttodemonstratethis.
21.1
Listthewaysinwhichtheinducede.m.f.canbe
increased.
21.1
Statethattheinducede.m.f.opposesthechangewhich
causesit.
21.1
UseFleming’sright-handruletofindthedirectionof
theinducedcurrent.
21.1
Describethestructureandoperationofana.c.
generator.
21.1
Sketchagraphofvoltageagainsttimeforana.c.
generator.Statewhatpositionthecoilmustbeinforthe
peaks,troughsandzerosonthegraph.
21.1
Listthethreeessentialpartsofatransformer. 21.2
Describethedifferencebetweenastep-upanda
step-downtransformer,bycomparingtheirprimaryand
secondarycoils.
21.2
KNp
Recallandusetheequation—=—
. 21.2
Describehowandwhytransformersareusedinhigh
voltagetransmission.
21.2
Explainhowatransformerworks. 21.3
RecallandusetheequationIpxVp-IsxVs. 21.3
RecallandusetheequationP=I2R. 21.3

427. describethestructureoftheatom
atom
INTHISCHAPTERYOUWILL:
describethealphascatteringexperimentwhichprovidesevidenceforthenuclearatom
>
describefissionandfusionreactions.
nameandstatethemassandchargeoftheparticlesinthenucleus
representnucleiintheform/X
explainwhatisotopesare
>Chapter22
Thenuclear

428. 22Thenuclearatom
GETTINGSTARTED
Withapartner,spendtwominutesmakingnoteson
whatyouknowaboutatoms.Includeasketchofan
atom.
Nowdiscussthesestatementsanddecidewhichare
trueandwhicharefalse:
Atomsarethesmallestparticlesthereare.
Thereare92differenttypesofatom.
Mostofthemassofanatomisinthenucleus.
Atomshavenocharge.
DANGEROUS DISCOVERIES
Figure22.1a:Newunderstandingofthestructureof
theatomleadtothedevelopmentofthenuclearbomb
b:LiseMeitnerwasthefirstpersontodescribenuclear
fissionbutdidnotshareintheNobelprizeawardedfor
thisdiscovery.
Theearly20thcenturywasanexcitingtimetobea
physicist.Theideaofatomshadbeenaroundsince
DemocritusinancientGreece,butnowhugestrides
werebeingmadeinunderstandinghowatomswere
madeup.ErnestRutherfordhaddescribedanew
modeloftheatom,containingatiny,densenucleus
andtheparticlesinthenucleuswereidentified.
Thesediscoverieswerebeingmadealongsidethe
riseoffascisminEuropewhichleadtotheSecond
WorldWar.GermanphysicistOttoHahnsplita
uraniumnucleusandfoundbariuminthedebrisof
hisexperiment.LiseMeitner,aJewishcolleague
ofHahn,workinginexileinSweden,explainedthe
processandcalleditnuclearfission.Hahnwaslater
awardedtheNobelprizeforthisdiscovery.Nuclear
fissionreleasesahugeamountofenergyandthe
Hungarianscientist,LeoSzilard,alsoworkinginexile,
realisedthatthiscouldbeusedtomakeabomb.The
fearwasthatNaziscientistswouldcreatethisbomb,
allowingthemtowinthewar.
SzilardenlistedthehelpofEinsteininwritingtothe
USpresidenttopersuadehimofthenecessityof
developingthebombbeforetheNazisdid.After
thebombingofPearlHarbortheUSAjoinedthe
war.WithinayeartheManhattanProjectsuccessfully
producedthechainreactionneededforthebomb.
In1945,bombsweredroppedcausingthousandsof
deathsinHiroshimaandNagasakiinJapan.
J.RobertOppenheimer,oneoftheManhattan
Projectteamwhodevelopedthebomb,described
itusingwordsfromtheBhagavadGita,'NowI
ambecomeDeath,thedestroyerofworlds'.After
theendoftheSecondWorldWar,Oppenheimer
campaignedforinternationalcooperationtolimitthe
proliferationofnucleararms.
Therearenowabout27000nuclearbombsinthe
world.
Discussionquestions
1Towhatextentarescientistsresponsibleforthe
waysinwhichtheirdiscoveriesareused?
2Shouldthenuclearphysicistshavedestroyed
theirresearchwhentheyrealisedhow
destructiveitcouldbe?
22.1Atomicstructure
Atonetime,physicstextbookswouldhavesaidthat
atomsareverytiny,tootinyevertobeseen.Certainly,a
singleatomistoosmalltobeseenusingaconventional
lightmicroscope.Buttechnologynowallowsusto
seeintotheatom.Thereismorethanonekindof
microscopethatcanbeusedtoshowindividualatoms.
Figure22.2showsaphotographmadeusingascanning
tunnellingmicroscope.Thepictureshowssiliconatoms
onthesurfaceofacrystalofsilicon(thematerialthat
transistorsandcomputerchipsaremadefrom).The
diamondshapeshowsagroupof12atoms.Thewhole
crystalismadeupofvastnumbersofgroupsofatoms
likethis.
415

429. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Figure22.2:Individualsiliconatoms(brightspots,artificially
colouredbyacomputer)onthesurfaceofasiliconcrystal,
observedusingascanningtunnellingmicroscope.The
diamondshape(whichhasbeendrawnontheimage)
indicatesthebasicrepeatingpatternthatmakesupthecrystal
structureofsilicon.Inthisphotograph,thesiliconatoms
aremagnified100milliontimes.(Agoodlightmicroscope
canonlymagnifybyabout1000times.)Roughlyspeaking,
4000000000atomswouldfitintoalengthof1metre.
In1909,ErnestRutherfordandhiscolleaguesdiscovered
thateveryatomhasatinycentralnucleus.Thisgave
risetothe‘solarsystem’modeloftheatomshown
inFigure22.3.Inthismodel,thenegativelycharged
electronsorbitthepositivelychargednucleus.The
electronsareattractedtothenucleus(becauseofits
oppositecharge),buttheirspeedpreventsthemfrom
fallingintoit.
Figure22.3:Thenuclearmodeloftheatom.Sixelectrons
areillustratedorbitinganucleusmadeupofsixprotonsand
sixneutrons.
Formingions
Anatomhasequalamountsofpositiveandnegative
chargesoisneutraloverall.Electronscanbegainedor
lostbyatomsrelativelyeasily,forexamplebyrubbing
aninsulatoraswesawinChapter17.Thisleadstothe
formationofionsinaprocesscalledionisation.Anatom
whichgainsanelectronhasmorenegativethanpositive
chargeandsobecomesanegativeion.Anatomwhich
losesanelectronisleftwithmorepositivethannegative
chargeandsobecomesapositiveion.
KEYWORD
ionisationwhenaparticle(atomormolecule)
becomeselectricallychargedbylosingorgaining
electrons
Discoveringthenucleus
Electronswerediscoveredin1896bytheEnglishphysicist,
J. J.Thomson.HerealisedthatelectronsHeremuchsmaller
andlighterthanatoms.(Wenowknowthatthemassofan
electronisaboutJofthemassofahydrogenatom.)
1836
Heguessed,correctly,thatelectronsWerepartofatoms.
Otherscientistsarguedthat,sinceelectronshadnegative
charge,theremustbeotherparticlesinanatomwithan
equalamountofpositivecharge,sothatanatomhas
nooverallcharge-(itisneutral).Sinceelectronshave
verylittlemass,thepositivechargemustalsoaccountfor
mostofthemassoftheatom.Figure22.4showsamodel
thatillustratesthis.Theatomisformedfromasphereof
positivelychargedmatterwithtiny,negativelycharged
electronsembeddedinit.Thisiscalledtheplumpudding
model.Inthismodel,theelectronsarethenegatively
chargedplumsinapositivelychargedpudding.Youcan
seethatthisisadifferentmodelfromthesolarsystem
modelwedescribedearlier(Figure22.3).
Figure22.4a:Aplumpuddingisacakewithfruitdotted
throughit.b:InThomson'smodel,thenegativelycharged
electronsaretheplumsstuckinapositivelychargedpudding.
416>

430. 22Thenuclearatom
KEYWORDS
plumpuddingmodel-adisprovedmodelofthe
atomwhichimaginedittoconsistofapositive
'pudding'withelectronsdottedthroughit
alphapartide(a-partide):aparticlemadeupof
twoprotonsandtwoneutrons;itisemittedbyan
atomicnucleusduringradioactivedecay
Sowhydowenolongerthinkthatatomsarelikeplum
puddings?Theanswercomesfromanexperimentcarried
outbytheNewZealander,ErnestRutherford,andhis
colleagues,HansGeigerandErnestMarsden,aboutten
yearsafterThomson’sdiscoveryoftheelectron.
Theyfiredtinyparticlescalledalphaparticlesatavery
thinpieceofgoldfoil.Alphaparticlesaretiny,butan
alphaparticlehasalmost8000timesasmuchmassasan
electron,andtheyweremovingfast(youwilllearnmore
aboutalphaparticlesinchapter23).Theyusedgoldas
itiseasytogetgoldfoilwhichisonlyafewatomsthick.
Theircalculationssuggestedthiswasabitlikefiring
bulletsatplumpuddings.Theypredictedthatthealpha
particleswouldpassstraightthroughthegold.
GeigerandMarsdenfoundthatmostofthealpha
particlespassedstraightthroughthegoldfoil,scarcely
deflected.However,afewbouncedbacktowardsthe
sourceoftheradiation.Itwasasiftherewassomething
veryhardinthegoldfoil,likeaball-bearingburied
insidetheplumpudding.Whatwasgoingon?
Rutherfordrealisedthattheanswerwastodowith
electriccharge.Alphaparticlesarepositivelycharged.
Iftheyarerepelledbackfromthegoldfoil,itmustbe
byanotherpositivecharge.Ifonlyafewwererepelled,
itwasbecausethepositivechargeofthegoldatoms
wasconcentratedinatinyspacewithineachatom.
Mostalphaparticlespassedstraightthroughbecause
theyneverwentnearthisconcentrationofcharge(see
Figure22.6).Thistinycoreofconcentratedpositive
charge,attheheartofeveryatom,iswhatwenowcall
theatom’snucleus.
Rutherfordconcludedthat:
mostofthemassofanatomisconcentratedinthe
centralnucleus
thenucleusispositivelycharged
thenucleusistinycomparedtotheatom;anatomis
mainlyemptyspace.
scattered
particles
mostparticles
areundeflected
sourceof
particles
afewparticles
circularfluorescent
arescatteredback
screen
towardsthesource
Figure22.5:Theexperimenttoshowalphaparticle
scatteringbygoldfoil.Alphaparticlesfromthesourcestrike
thegoldfoil.Mostpassstraightthrough,andsomeare
scatteredabit.Afew,aboutonein8000,arescatteredback
towardsthesource.Theexperimentisperformedinavacuum
chamberasairwouldabsorbthealphaparticles.
Figure22.6:Mostalphaparticlespassstraightthrough
thegoldfoilbecausetheydonotpassclosetotheatomic
nucleus.Theclosertheyaretothenucleus,themorethey
aredeflectedorscattered.Onlythosewhichheadstraightto
thenucleusarereflectedstraightback.
Inlateryears,Rutherfordoftenspokeofthesurprising
resultsofthealphascatteringexperiment.Hesaid:
Itwasquitethemostincredibleeventthateverhappened
tomeinmylife.Itwasasifyoufiredafifteen-inch
artilleryshellatapieceoftissuepaperanditcameback
andhityou.
KEYWORD
nucleus:small,dense,positivelychargedregion
atthecentreofanatom

431. >
CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
Asenseofscale
RutherfordwasabletoanalysetheresultsfromGeiger
andMarsden’sexperimenttoworkoutjusthowbigthe
nucleusofagoldatomwas.Anatomissmall(about
1010metresacross)butitsnucleusisverymuchsmaller
(about10-15metresindiameter).Theelectronstravel
aroundthenucleus.Theyareeventinierthanthenucleus.
Andtherestoftheatomissimplyemptyspace.
Itishardtoimaginetheserelativesizes.Trypicturinga
glassmarbleabout1cmindiameter,placedatthecentre
ofafootballpitch,torepresentthenucleusofanatom.
Thentheelectronsareliketinygrainsofdust,orbiting
thenucleusatdifferentdistances,rightouttotheedgeof
thefootballground.
Itisevenhardertoimagine,whenyoustubyourtoeon
arock,thattheatomsoftherock(andyourtoe)are
almostentirelyemptyspace!
Asuccessfulmodel
Rutherford’smodeloftheatomwassoonacceptedby
otherscientists.Itgaveaclearexplanationofthealpha
particlescatteringexperiment,andfurthertestswith
othermetalsconfirmedRutherford’sideas.Itclearly
showedthattheplumpuddingmodelwaswrong.
Rutherford’smodelalsoallowedscientiststothinkabout
otherquestions.Chemistswantedtoknowhowatoms
bondedtogether.Physicistswantedtounderstandwhy
someatomsareunstableandemitradiation,andhow
X-raysareproduced.Theseareallquestionstowhichwe
nowhavegoodanswers,andRutherford’sdiscoveryof
theatomicnucleusdidalottohelpanswerthem.
Today,scientistshaveratherdifferentideasaboutatoms.
Theywanttocalculatemanydifferentquantities,and
somodelsoftheatomaremuchmoremathematical.
Quantumtheory,developednotlongafterRutherford’s
work,madetheatomseemlikeamuchfuzzierthing,
notacollectionoflittlespheresorbitingeachother.
However,theimportantthingaboutamodelisthatit
shouldhelpustounderstandthingsbetter,andhelp
ustomakenewpredictions.Rutherford’smodelofthe
nuclearatomhascertainlydonethat.
ACTIVITY22.1
Nuclearnews
Imagineyouareasciencereporter.Youhavebeen
senttointerviewRutherfordfollowingthealpha
particlescatteringexperiment.Thinkabouthowyou
CONTINUED
couldreportthisinawaythatconveysRutherford's
amazementatwhathisteamdiscovered.Prepare
eitheranewspaperreportoravideoedinterview
(clearlythiswouldn'thavebeenanoptionin1909
whentheexperimenttookplace!).
Rememberthattheaudiencearenotscientists,so
youwillneedtointroducetheideaofatomsand
explainwhatscientistsbeforeRutherfordthought.
Questions
1Intheplumpuddingmodeloftheatom:
awhataretheplums?
bwhatisthepudding?
2Inthealphaparticlescatteringexperiment,explain
whathappenedtoalphaparticles:
aheadingdirectlytowardsagoldnucleus
bpassingclosetoagoldnucleus
cpassingthroughtheemptyspacebetween
nuclei.
3HowdidthisexperimentprovethatIhenucleus
mustbeverysmall?
4Inthesolarsystemmodeloftheatom,whatforce
holdstheelectronsintheirorbitjiaroundthe
nucleus?
22.2Protons,neutrons
andelectrons
Wenowknowthattheatomicnucleusismadeupoftwo
typesofparticle,protonsandneutrons.Theprotonscarry
thepositivechargeofthenucleus,whiletheneutronsare
neutral.Negativelychargedelectronsorbitthepositively
chargednucleus.Protonsandneutronshavesimilar
masses,andtheyaccountformostofthemassofthe
atombecauseelectronsaresolight.Together,protons
andneutronsareknownasnucleons.
Table22.1summarisesinformationaboutthemassesand
chargesofthethreesub-atomicparticles.Thecolumns
headedRelativechargeandRelativemassgivethecharge
andmassofeachparticlecomparedtothatofaproton.
Itismucheasiertorememberthesevalues,ratherthan
theactualvaluesincoulombs(C)andkilograms(kg).
418y

432. 22Thenuclearatom
KEYWORDS
proton:apositivelychargedparticlefoundinthe
atomicnucleus
neutron,anunchargedparticlefoundinthe
atomicnucleus
nucleonaparticlefoundintheatomicnucleus;a
protonoraneutron
relativecharge;thechargeofaparticlerelative
tothechargeofaproton
relativemass:themassofaparticlerelativeto
themassofaproton
Atomsandelements
Oncetheparticlesthatmakeupatomswereidentified,it
wasmucheasiertounderstandthePeriodicTableofthe
elements(Figure22.7).Thisshowstheelementsinorder,
startingwiththelightest(hydrogen,thenhelium)and
workinguptotheheaviest.Infact,itisnotthemassesof
theatomsthatdeterminetheorderinwhichtheyappear,
butthenumberofprotonsinthenucleusofeachatom.
Everyatomofhydrogenhasoneprotoninitsnucleus,so
hydrogeniselementnumber1.Everyheliumatomhas
twoprotons,soheliumiselementnumber2,andsoon.
Table22.1Chargesandmassesofthethreesub-atomicparticles.
Particle Position Charge/C
Relative
charge
Mass/kg Relativemass
proton innucleus +1.6x10~19 +1 1.67x10~27 1
neutron innucleus 0 0 1.67x1(r27 1
electron
orbiting
nucleus
-UxKF1’ -1 9.11x1Q-31
1
1836
(practical|yzero>
H
1
He
2
Li
3
Be
4
B
5
c
6
N
78^
F
9
Ne
10
Na
11
Mg Al
13
Si
14
P
15
s
16
Cl
17
Ar
18
K
19
Ca
20
Sc
21
Ti
22
V
23
Cr
24
Mn
25
Fe
26
Co
27
Ni
28
Cu
29
Zn
30
Ga
31
Ge
32
As
33
Se
34
Br
35
Kr
36
Rb
37
Sr
3839^
Zr
40
Nb
41
Mo
42
Tc
43
Ru
44
Rh
45
Rd
46
AgCd
48
In
49
Sn
50
Sb
51
Te
52
I
53
Xe
54
Cs
55
Ba
56
Lato
Lu
Hf
72
Ta
73
w
74
Re
75
Os
76
Ir
77
Pt
78
Au
79
Hg
80
Tl
81
Pb
82
Bi
83
Po
84
At
85
Rn
86
Fr
87
Ra
88
Acto
Lr
La
57
Ce
58
Pr
59
Nd
60
Pm
61
Sm
62
Eu
63
Gd
64
Tb
65
Dy
66
7
Ho
67
Er
68
Tm
69
Yb
70
Lu
71
Ac
89
Th
90
Pa
91
u
92
Np
93
1Pu
94
Am
95
Cm
96
Bk
97
Cf
98
Es
99
Fm
100
Md
101
No
102
Lr
103
Figure22.7:ThePeriodicTableoftheelementsisawayoforganisingwhatweknowaboutthedifferentelements,basedon
theiratomicstructures.

433. >
CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
Eachelementhasitsownsymbol,consistingofoneor
twoletters,suchasHforhydrogen,andHeforhelium.
Sometimes,thesymbolforanatommaybewrittenwith
twonumbersinfrontofit,oneabovetheother,suchas:
2
He
Thisrepresentsanatomofhelium.Thebottomnumber
tellsusthattherearetwoprotonsinthenucleusofan
atomofhelium,andthetopnumbertellsusthatthere
isatotaloffournucleonsinthenucleusofanatomof
helium.Fromthis,itissimpletoworkoutthatthere
mustbetwoneutronsinthenucleus.
Wecanwritethegeneralsymbolforanelement(X)with
itsprotonnumber(Z),whichisthenumberofprotonsin
thenucleus,andnucleonnumber(^),whichisthenumber
ofnucleons(protonsandneutron)inthenucleus,as
follows:
zX
Zisprotonnumber(alsoknownastheatomicnumber)
andAisnucleonnumber(alsoknownasthemass
number).Thisisknownasnuclidenotation.
AneutralatomofelementXwillalsohaveZelectrons
orbitingthenucleus.
KEYWORDS
protonnumber(Z):(oratomicnumber)the
numberofprotonsinanatomicnucleus
nucleonnumber(A):(ormassnumber)the
numberofnucleons(protonsandneutrons)inan
atomicnucleus
REFLECTION
Whatstrategieshaveyoufoundusefulin
rememberingthepropertiesofeachofthe
subatomicparticlesandthedefinitionsofnucleon
andprotonnumbers?Shareyourstrategieswith
apartner.
Questions
5Copyandcompletethesesentences:
Anatomhasatiny,dense .Thiscontains
positivelycharged andneutral.
Thesetwoparticleshaveapproximatelythesame
Anucleuscanbedescribedbytwonumbers:the
protonnumber,andthenucleonnumberwhich
isthetotalnumberof and inthe
nucleus.
6CopyandcompleteTable22.2.
Subatomic
particle
Position
Relative
charge
Relative
mass
proton 1
electron -1
inthe
nucleus
Table22.2
7Aparticularneutralatomofboronisrepresented
by
aWhatisitsprotonnumber?
bWhatisitsnucleonnumber?
cWritedownthenumbersofprotons,neutrons
andelectronsitcontains.
8Acobaltatomcontains33neutrons.
aUsetheperiodictabletofinditsproton
number.
bWritedownanexpressionintheform^X
to
representthecobaltnucleus.
9Howmanytimesgreateristhemassofaproton
thanthemassofanelectron?
Elementsandisotopes
Itistheprotonnumber,thattellsuswhichelementan
atombelongsto.Forexample,asmallatomwithjusttwo
protonsinitsnucleus(Z=2)isaheliumatom.Amuch
biggeratomwith92protonsinitsnucleusisauranium
atom,becauseuraniumiselement92.
FromZandAyoucanworkoutathirdnumber,the
neutronnumber(TV),whichisthenumberofneutronsin
thenucleus.
protonnumber+neutronnumber=nucleonnumber
Z+N=A
KEYWORDS
neutronnumber(N):numberofneutronsinthe
nucleusofanatom
420>

434. 22Thenuclearatom
WORKED EXAMPLE22.1
Whatcanyoudeduceabouttheatomwhosenucleus
canberepresentedby
14
C?
Step1:Lookattheatomicsymbol.
ThesymbolCtellsusitisacarbonnucleus.
Step2:Identifythenumberofprotons:
Z=6
Step3:Identifythenumberofelectrons.
Itisaneutralatom,soithasthesamenumber
ofprotonsaselectrons=6.
Step4:Calculatethenumberofneutrons:
N=A-Z=14~6=8
Answer
I4Cisacarbonnucleuswithsixprotons,sixelectrons
andeightneutrons.
Theatomsofallelementsexistinmorethanoneform.
Forexample,Figure22.8showsthreetypesofhydrogen
atom.Eachhasjustoneprotoninitsnucleus,butthey
havedifferentnumbersofneutrons(0,1and2).This
meansthattheyaredescribedasdifferentisotopes
ofhydrogen.
Figure22.8:Hydrogenexistsinthreedifferentformsknown
asisotopes.Allthreehavethesameprotonnumber.
Thedifferentisotopesofanelementallhavethe
samechemicalproperties,butthosewithagreater
numberofneutronsareheavier.
Thedifferentisotopesofanelementallhavethe
samenumberofprotonsbutdifferentnumbersof
neutronsintheirnuclei.
KEYWORD
isotope:isotopesofanelementhavethesame
protonnumberbutdifferentnucleonnumbers
Table22.3showsatomsoftwoisotopesofhelium,
4
He
(themostcommonisotope)and
2
He(alighterandmuch
rarerisotope).Each
3
Hehastwoprotonsinthenucleus
andtwoelectronsorbitingit,butthelighterisotope
hasonlyoneneutron.Theseisotopesarereferredtoas
helium-4andhelium-3.
Symbolfor
isotope
Proton
numberZ
Neutron
number2V
Nucleon
numberA
jHe 2. 2 4
2
He 2 1 3
Table22.3:Isotopesofhelium.
Table22.4showstwoisotopesofuranium.Uranium-238
isthemostcommonisotope.Uranium-238hasthree
moreneutronsthanuranium-235,butthesamenumber
ofprotons.Uranium-235isusedinnuclearpower
stationsasitsnucleicanbesplittoreleaseahugeamount
ofenergy.
Symbolfor
isotope
Proton
numberZ
Neutron
numberN
Nucleon
numberA
235
u
92
U 92 143 235
238
u
92
u 92 146 238
Table22.4:Isotopesofuranium.
Isotopesatwork
Allelementshaveisotopes,someasmanyas36.For
mostchemicalelements,atleastoneisotopeisstable.
Otherisotopesareoftenunstable.Thismeansthatthe
nucleusislikelytogiveoutradioactivityinorderto
becomestable.Youwilllearnaboutradioactivedecayin
Chapter23.
Questions
10Carbonexistsintwoformsg2Candg4C.
aWritedownthenumberofprotons,neutrons
andelectronsineach.
bCopyandcompletethesesentences:
g2Candg4Caretwo ofcarbon.
Theyhavethesame number,but
different numbers.
Bothhavethesame properties.
421

435. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
11Table22.5liststheprotonandnucleonnumbersof
sixdifferentnuclei.
aCopyandcompletethetablebyfillinginthe
emptyspaces.
bWhichthreenucleiareisotopesofoneelement?
cWhichtwonucleiareisotopesofanotherelement?
dUsethePeriodicTable(Figure22.7)toidentify
thethreeelementsinTable22.5.
Table22.5Protonandnucleonnumbersofnuclei.
Nucleus
Proton
numberZ
Neutron
numberA
Nucleon
numberA
Nu-1 6 6
Nu-2 6 13
Nu-3 7 14
Nu-4 8 14
Nu-5 6 11
Nu-6 7 13
Chargeandmassofnuclei
Thechargeonanucleusisequaltothenumberof
protonsaseachprotonhasarelativechargeof+1.
Soallthreeisotopesofhydrogenhaveachargeof+1
asallcontainjustoneproton.
Themassofanucleusisequaltothemassofthe
nucleonsasbothprotonsandneutronshavearelative
massofone.Eachofthehydrogenisotopeshasa
differentmass.Protiumhasamassofone,deuterium
hasamassoftwoandtritiumhasamassofthree.
Question
12Writedownthemassandchargeofthenucleiofthe
heliumanduraniumisotopesshowninTables22.3
and22.4.
Nuclearfissionandfusion
InChapter7youlearntabouttheuseofnuclearfuels.
Theseproduceenergybysplittingnucleiwhichisa
processknownasnuclearfission.Figure22.9showsa
fissionreactioninwhichauranium-235nucleusishitby
aneutron.Theextraneutroncausesittobecomeunstable
anditsplits,makingtwonewnucleiandthreeextra
neutrons.Alargeamountofenergyisalsoreleased.These
neutronscanthensplit,leadingtoachainreaction.
Figure22.9:Auranium-235nucleusbeingsplittomake
bariumandkryptonnuclei,threemoreneutronsandalot
ofenergy.
Thisreactioncanberepresentedbyabalancednuclear
equation:
2^U+in^Ba+>^Kr+3in+energy
Boththeprotonnumbersandthenucleonnumbersmust
beequaloneachsideoftheequation:
Protonnumber:92+0=36+56+3(0)
92=92Z
Nucleonnumber:235+1=92+141+3(1)
236=236/
Nuclearfusionproducesevenmoreenergythannuclear
fission.Fusioniswhentwonucleijointogethertoform
alargernucleus.Thiscanonlyhappenatextremely
hightemperatures.Fusionistheprocessbywhichstars
produceheatandlight.Theequation,^elowshowsoneof
thefusionreactionswhichoccursintheSun.Youcansee
thattheprotonandnucleonnumbersbalance:
?H+?H^lHe+in
Somethingfornothing?
Asyouknow,energycannotbecreatedordestroyed.So,
wheredoesthehugeamountofenergyreleasedinfission
andfusioncomefrom?Toanswerthisweneedoneofthe
mostfamousequationsinphysics:
E=me2
Thetotalmassoftheparticlesbeforeafissionorfusion
reactionisfoundtobeslightlymorethanthetotalmass
afterthereaction.Themasswhichislostisconvertedto
energy,andEinstein’sequationallowsustocalculatethe
amountofenergyreleased:
energyreleased(£)=masslost(m)xthespeedoflight
squared(c2)
Thespeedoflightsquaredisahugenumber,soeven
averysmalllossofmasswillproducealargeamount
ofenergy.
422y

436. 22Thenuclearatom
PROJECT
Abriefhistoryofparticlephysics
Throughouthistorymankindhastriedtoexplainthe
worldaroundusandwhatitismadeof.Thishasled
toanunderstandingofatoms,thentheirconstituent
protons,neutronsandelectrons.Inthischapterwe
havecoveredpartofthisstory,buttherehavebeen
manyscientistsmakingdiscoveriesandoften
receivingNobelprizesfortheirwork.Andthe
storyisnotcomplete.In2012,scientistsatCERN
detectedtheHiggsboson.Itisaparticlewhichhad
beenpredictedtheoreticallyandwhichhelpsinour
understandingofgravity.
Figure22.10a:ErnestRutherford(shownhereinhislaboratoryatManchesterUniversity),b:TheLargeHadronCollider
(LHC)atCERNinSwitzerland.TheLHCissobigthatscientistscyclefromonepartofanexperimenttoanother.
Produceatimelineorpresentationshowingthemain
developmentsinparticlephysicsfromancientworld
thoughttopresentdayinvestigationsusingtheLarge
HadronCollider.
Youcouldinvestigatethecontributionsof:
theJainsinAncientIndia
ancientGreekssuchasDemocritusandLeucippus
JohnDalton
J.J.Thomson
ErnestRutherford,HansGeigerandErnest
Marsden
JamesChadwick
MurrayGell-Mann
FrancoisEnglertandPeterHiggs.
Youshouldincludediagramstoexplainexperiments
andmodelsoftheatom.Trytoincludeasmanyofthe
keywordsfromthischapterasyoucan.
Yourpresentationshouldfocusonthewayinwhich
ideasfollowonfromeachother.Itshouldinspire
peoplewiththeknowledgethatthereisstillalotto
bediscoveredbythephysicistsoftomorrow.
PEERASSESSMENT
Givefeedbacktoanothergroup.Writecommentsonthesepoints:
Doesthepresentationmakeitclearwhateachscientistcontributedtothestory?
Aretheexperimentsdescribedclearly?
Doyoufeelitgivesthereader/viewerasenseofscientistsbuildingontheworkofthosewhowent
beforethem?
423y

437. >
CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
SUMMARY
Theatomconsistsofatinypositivenucleussurroundedbymainlyemptyspacewithnegativeelectronsorbiting.
Positiveornegativeionsareformedwhenanatomlosesorgainselectrons.
Thenucleuscontainstwotypesofnucleons:positivelychargedprotonsandneutralneutrons.
Rutherford’salphaparticlescatteringexperimentprovidedevidencetosupportthenuclearmodeloftheatom.
Protonsandneutronshaveapproximatelythesamemass.Thisisnearly2000timesthemassofanelectron.
Protonsandelectronshaveequalbutoppositecharge.
AnucleuscanbedescribedusingthenotationX,whereXisthechemicalsymbol,Zthenumberofprotons
andAthenumberofnucleons.
Isotopesarenucleiofthesameelementwhichhavethesameprotonnumberbutdifferentnucleonnumbers.
Nuclearfissionisthesplittingofalargenucleus,whichreleasesalotofenergy.
Nuclearfusionisjoiningtogetherofsmallnuclei,whichreleasesevenmoreenergythanfission.
Nuclearfissionandfusioncanbedescribedusingnuclearequations.
EXAM-STYLEQUESTIONS
1Whichrowcorrectlygivestherelativechargeofaproton,neutron
andelectron?
Proton Neutron Electron
1 0 1
1 1 -1
-1 0 1
1 0 -1
Hl
2Anucleusisrepresentedbythenotationi^Pb.Whatparticlesdoesitcontain?[1]
A82protonsand193neutrons
B82protons,82electronsand111neutrons
C82neutronsand111protons
D82protonsand111neutrons
3Anelementhastwoisotopes.Whatisdifferentforthetwoisotopes? [1]
Athenumberofprotons
Bthechemicalsymbol
Cthenumberofneutrons
Dthechemicalpropertiesoftheisotope
4aNamethetwotypesofparticlesfoundinanatom’snucleus. [2]
bStatethenamegiventothetotalnumberofparticlesinanucleus. [1]
cDescribewhathappenstothechargeonanatomwhenitgains
anelectron. [2]
[Total:5]
COMMAND WORDS
state:expressinclear
terms
describe:statethe
pointsofatopic;give
characteristicsand
mainfeatures
424y

438. aHowmanyelectronsdoestheatomhave? [1]
bHowmanyprotonsdoesithave? [1]
cWhatisitsnucleonnumber? [1]
dHowmanyneutronsdoesithave? [1]
eCopyandcompletethisnuclearnotationtorepresentthe
berylliumnucleus:
gBe [1]
fAradioactiveisotopeofberylliumisknownasberyllium-11.Howmany
protons,neutronsandelectronsdoesanatomofthisisotopecontain?[3]
gWritedownthenuclearnotationforberyllium-11. [2]
[Total:10]
6The‘plumpudding’modelwasusedbyscientistsintheearly20thcenturyto
describethestructureoftheatom.
aScientistsknewthatatomscontainednegativeelectronsandthatatoms
wereneutraloverall.Whatdidtheyassumeaboutthe‘pudding’? [1]
bGeigerandMarsdencarriedoutanexperimentwhichshowedtheplum
puddingmodelwaswrong.Theyfiredalphaparticlesatgoldfoilwhich
wasonlyafewatomsthick.Thediagrambelowshowsthreealphaparticles
approachingthegoldnuclei.Copyandcompletethediagramtoshowwhat
happenedtothethreeparticles. [3]

439. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
SELF-EVALUATIONCHECKLIST
Afterstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutosee
anygapsinyourknowledgeandhelpyoutolearnmoreeffectively.
1can
See
Topic...
Needs
morework
Almost
there
Confident
tomoveon
Describethestructureoftheatom. 22.1
Describehowionsareformed. 22.1
DescribeRutherford’salphascatteringexperimentand
itsconclusions.
22.1
Nametheparticlesinthenucleusandstatetheirmass
andcharge.
22.2
DefineprotonnumberZ. 22.2
DefinenucleonnumberA. 22.2
Describenucleiusingthenotation^X.
22.2
Describethesimilaritiesanddifferencesbetweentwo
isotopesofanelement.
22.2
Describetheprocessesofnuclearfissionandfusion. 22.2
Statewhattheprotonandnucleonnumberstellus
aboutthechargeandmassofanucleus.
22.2
426>

441. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
GETTINGSTARTED
Inthischapterwewilllookattheradiationemitted
fromthenucleiofunstableisotopes,whichcan
causeionisation.Thiswillincludehighfrequency
electromagneticradiation.
Thesentencesaboveincludealotofkeywords
youhavemetelsewhereinthisbook.Writedown
asmanyasyoucaninatable.Trytodefineeach
wordfrommemorybeforereferringtoyournotes
ortotheglossaryatthebackofthebook.Research
showsthattryingtorecallinformationlikethisisa
greatwayofgettingittostickinyourmind.
Keyword Definitionfrommemory Definitionfromnotesorglossary
REFLECTION
Didyourememberthekeypartsofthedefinitions?Considermakingrevisioncardsandregularly
Howcanyouhelpyourselftolearnimportant checkingwhatyouknow.Findoutaboutmobile
definitions? phoneappswhichhelpyoudothis.
THERADIUMGIRLS
beginningtoshowthatradiumwasadangerous
substanceandsolimitedtheirownexposuretoit.
Someworkersquestionedwhether^heradiumwas
safeandwerereassuredthattheywerenotatrisk.
Figure23.1:ThenameTho-Radiareferstotheradioactive
isotopesofthoriumandradiumusedinthisfacecream.
Theradiumwaspaintedontothesewatchesby
youngwomeninfactories.Thesejobswerepopular
asradiumwasaglamorous,expensiveproductand
theworkwasmuchcleanerthanotherfactorywork.
Thewomenusedcamelhairbrushestoapplythe
radioactivepaint.Theyweretoldtosqueezethese
brushesbetweentheirlipsastheyworkedtokeep
afinepointonthebrush.Theseinstructionscame
fromfactoryownerswhoknewthatresearchwas
Radiumwasusedinpaintwhichconvertedthe
radiationitemittedintolight.Theprocessiscalled
luminescence.Thiswasusedtomakewatchand
clockfacesvisibleinthedark.
Radioactivitywasfirstdiscoveredin1896.Soon
afterwards,in1898,MarieCuriediscoveredradium
whichwasfoundtobeusefulintreatingcancer.
Itwasassumedthattheseemissionswhichcould
curecancer,mustbehealthy.Radiumwasusedto
treatanyconditionswherethepatientseemedto
needmoreenergy,fromanaemiatoimpotence.
Anindustrydevelopedsellingproductssuchas
radioactivewater,toothpasteandfacecreamsto
giveyoua'healthy'glow.Figure23.1showsan
advertforsuchacream.
UmbeaumaifUii&Lfe
doitibiemecede
THORWDIA
DawaquiHanjInfgBj
428>

442. 23Radioactivity
CONTINUED
Figure23.2:Theradiumgirlspaintedluminous
radioactivepaintonclockandwatchfaces.
Radiumischemicallysimilartocalciumandthe
radiumreplacedcalciuminthebonesofthe
women,weakeninganddamagingtheirbones.They
experiencedpainandmany-probablythousands-
developedboneandothercancerswhichwereoften
fatal.Whenthewomenbegantodie,somefactory
ownerstriedtoblametheirdeathsonavirus,oron
syphilis.
Eventuallythewomenwonacourtcasein1928
andthesurvivorsreceivedcompensation,though
thecompaniesappealedanditwasonlyin1939
thatthecasewasfinallywon.Luminouswatches
werestillsolduntilthe1960s,thoughthepainting
techniqueswerechanged.
Discussionquestions
1Whydidthefactoryownersnotinformthe
workersaboutthedangersofworkingwith
radium?
2Doyouthinkthissituationcouldhappen
today?Considerwaysinwhichlawsandaccess
toinformationmayhavemadethislesslikely.
23.1Radioactivityall
aroundus
Weneedtodistinguishbetweentwothings:radioactive
substancesandtheradiationthattheygiveout.Many
naturallyoccurringsubstancesareradioactive.Usually
theradiationinthesesubstancesisnotveryconcentrated,
sotheydonotcauseaproblem.Therearetwowaysin
whichradioactivesubstancescancauseusproblems:
Ifaradioactivesubstancegetsinsideus,its
radiationcanharmus.Wesaythatwehavebeen
contaminated.
Iftheradiationaradioactivesubstanceproduces
hitsourbodies,wereceiveadoseofradiation.
Wehavebeenirradiated.
Infact,weareexposedtolowlevelsofradiationallthetime
-thisisknownasbackgroundradiation.Inaddition,we
maybeexposedtoradiationfromartificialsources,suchas
theradiationwereceiveifwehaveamedicalX-ray.
Figure23.3showsthedifferentsourcesthatcontributeto
theaveragedoseofradiationreceivedbypeopleacross
theworld.Itisdividedintonaturalbackgroundradiation
(about85%)andradiationfromartificialsources(about
15%).Wewilllookatthesedifferentsourcesinturn.
14%medicine
(X-rays,forexample)
1%nuclearindustry
42%radon
(radioactivegasesintheair)
14%cosmicrays
11%foodanddrink
18%buildingsandsoil
n85%naturalradiation
15%artificialradiation
Figure23.3:Thispiechartshowsthedifferentsourcesof
radiationandhowtheycontributetotheaveragedoseof
radiationreceivedeachyearbyanindividual.Onlyabout
15%comesfromnon-naturalsources.
429>

443. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Sourcesofnatural
backgroundradiation
Theairisradioactive.Itcontainsaradioactivegas
calledradon,whichseepsuptotheEarth’ssurfacefrom
radioactiveuraniumrocksunderground.Becausewe
breatheinairallthetime,weareexposedtoradiationfrom
thissubstance.Thiscontributesabouthalfofourannual
exposure.(Thisvarieswidelyfromcountrytocountry,and
fromonepartofacountrytoanother,dependingonhow
muchuraniumthereisintheunderlyingrocks.)
Thegroundcontainsradioactivesubstances.Weuse
materialsfromthegroundtobuildourhouses,soweare
exposedtoradiationfromthese.
Ourfoodanddrinkisalsoslightlyradioactive.Living
thingsgrowbytakinginmaterialsfromtheairand
theground.Someofthesematerialsareradioactiveso
theplantswhichfeedintoourfoodchainswillalsobe
slightlyradioactive.
Finally,radiationreachesusfromspaceintheform
ofcosmicrays.Someofthisradiationcomesfromthe
Sun,somefromfurtheroutinspace.Mostcosmicrays
arestoppedbytheEarth’satmosphere.Ifyouliveupa
mountain,youwillbeexposedtomoreradiationfrom
thissource.
Becausenaturalbackgroundradiationisaroundusall
thetime,wehavetotakeaccountofitinexperiments.It
maybenecessarytomeasurethebackgroundleveland
thentosubtractitfromexperimentalmeasurements.
Sourcesofartificial
backgroundradiation
Mostradiationfromartificialsourcescomesfrom
medicalsources.ThisincludestheuseofX-raysand
gammaraysforseeinginsidethebody,andtheuseof
radiationfordestroyingcancercells.Thereisalways
adangerthatexposuretosuchradiationmaytrigger
cancer.Medicalphysicistsarealwaysworkingtoreduce
thelevelsofradiationusedinmedicalprocedures.
Overall,manymorelivesaresavedthanlostthroughthis
beneficialuseofradiation.
Today,mostnuclearweaponstestingisdone
underground.Inthepast,bombsweredetonatedon
landorintheair,andthiscontributedmuchmoretothe
radiationdosereceivedbypeoplearoundtheworld.
Whenyouflyinanaircraft,youarehighinthe
atmosphere.Youareexposedtomorecosmicrays.Thisis
notaseriousproblemfortheoccasionalflier,butaircraft
crewshavetokeepacheckontheirexposure.
Manypeople,suchasmedicalradiographersandstaffin
anuclearpowerstation,workwithradiation.Overall,a
powerstationdoesnotaddmuchtothenationalaverage
dose,butforindividualswhoworkthereitcanincrease
theirdosebyupto10%.
Finally,smallamountsofradioactivesubstancesescape
fromthenuclearindustry,whichprocessesuraniumfor
useasthefuelinnuclearpowerstationsandhandlesthe
highlyradioactivespentfuelafterithasbeenused.
Detectingradiation
RadiationcanbemeasuredusingaGeigercounter.
ThisconsistsofadetectorcalledaGeiger-Muller
tubewhichdetectsradiation,connectedtoacounter.
Thecounterrecordstherateatwhichradiationis
detected.Thisisknownasthecountrateanditis
measuredincountspersecond(count/s)orcduntsper
minute(count/min).
Figure23.4:UsingaGeigercountertomonitorradiation
levelsincrops.
KEYWORDS
radioactivesubstance:asubstancethatdecays
byemittingradiationfromitsatomicnuclei
radiation:energyspreadingoutfromasource
carriedbyparticlesorwaves
contaminated:whenanobjecthasacquired
someunwantedradioactivesubstance
irradiated:whenanobjecthasbeenexposedto
radiation
backgroundradiation:theradiationfromthe
environmenttowhichweareexposedallthetime
countrate:thenumberofdecayingradioactive
atomsdetectedeachsecond(orminute,orhour)
430y

444. 23Radioactivity
Questions
1Describewhatismeantbytheterm‘background
radiation’.
2Namethreesourcesofnaturalbackground
radiationandthreesourcesofartificialbackground
radiation.
3Whichtypeofnaturalbackgroundradiationare
airlinecrewsexposedtomorethanmostpeople?
23.2Radioactivedecay
Notallnucleigiveoutradiation.Somenucleiare
unstableandgiveoutradiationinordertobecomemore
stable.Thisprocessisknownasradioactivedecay.
IfyoulistentotheclicksorbeepsofaGeigercounter,
youmaynoticethatitisimpossibletopredictwhenthe
nextsoundwillcome.Thisisbecauseradioactivedecay
isarandomprocess.Radioactivesubstancescontain
unstablenucleiwhichwilldecayspontaneously.We
cannotpredictwhichnucleuswilldecaynextorwhenthis
willhappen.Thedirectioninwhichtheradiationwillbe
emittedisalsorandom.Radioactivedecayisnotaffected
byexternalfactorssuchastemperature.
Whyaresomenucleiunstable?
Radiationisemittedbythenucleusofanatomwhich
isunstable.Manyelementshaveisotopeswhichare
radioactivebecausetheirnucleiareunstable.Forsome
isotopesthisisbecausethenucleusistooheavy.Other
isotopesareunstablebecausetheyhavetoomany
neutrons.Anunstablenucleusemitsradiationinorderto
becomemorestable.
Fortunately,mostoftheatomsaroundushavestable
nuclei.WhentheEarthformed,about4500millionyears
ago,thereweremanymoreradioactiveatomsaround.
Thismeansthatthelevelofbackgroundradiation
usedtobemuchhigherthanitistoday.However,most
radioactiveatomshavedecayedtobecomestable.
Threetypesofradiation
Therearethreetypesofradiationemittedbyradioactive
substances(Table23.1).Thesearenamedafterthefirst
threelettersoftheGreekalphabet,alpha(a),beta(P)
andgamma(y).Alphaandbetaareparticles;gammaisa
formofelectromagneticradiation(seeChapter15).
Name SymbolMadeof
Charge
alpha a
2protons+
2neutrons
positive
beta P anelectron
negative
gamma Y
electromagnetic
radiation
neutral
Table23.1:Threetypesofradiationproducedbynaturally
occurringradioactivesubstances.
Analphaparticle(a-particle)ismadeupoftwo
protonsandtwoneutrons.(Thisisthesameasthe
nucleusofaheliumatom.)Becauseitcontains
protons,itispositivelycharged.
Abetaparticle(p-particle)isanelectron.Itisnot
oneoftheelectronsthatorbitthenucleus-itcomes
frominsidethenucleus.Itisnegativelycharged,and
itsmassismuchlessthanthatofanalphaparticle.
Agammaray(y-ray)isaformofelectromagnetic
radiationwithaveryshortwavelengthandhigh
frequency.ItissimilartoanX-ray,buthasmore
energy.
Figure23.5a:Alphaemission,b:Betaemission,c:Gamma
emission.
431>

445. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
KEYWORDS
radioactivedecaytheemissionofalpha,betaor
gammaradiationfromanunstablenucleus
randomprocess:aprocessthathappensata
randomrateandinrandomdirections;thetiming
anddirectionofthenextemissioncannotbe
predicted
betaparticle(0-particle):ahighspeedelectron
thatisemittedbyanatomicnucleusduring
radioactivedecay
gammaray(y-ray):electromagneticradiation
emittedbyanatomicnucleusduringradioactive
decay
Anatomofaradioactivesubstanceemitseitheran
a-particleora0-particle.Inaddition,itmayemitsome
energyintheformofay-ray.They-rayisusuallyemitted
atthesametimeasthea-particleor0-particle,butitmay
beemittedsometimelater.
Whenanatomofaradioactivesubstancedecaysby
a-or0-decay,itbecomesanatomofanotherelement.
Thisisbecausethenumberofprotonsinthenucleus
changes.
a-particleshaveamuchgreatermassthanbetaparticles,
sotheytravelmoreslowly,y-rays,likeallelectromagnetic
waves,travelatthespeedoflight.
Questions
4Copyandcompletethefollowingsentences.
Radioactivedecayhappenswhenanucleusis
.Thismaybebecauseitistoomassiveor
becauseitcontainstoomany .
Ana-particleconsistsof and
.A0-particleisan .y-radiationisa
formofhighfrequency radiation.
5A0-particleisidenticaltoanelectron.Howisit
differenttomostelectrons?
6Whichtypeofradiationtravelsatthespeedoflight?
Penetratingpower
Whenphysicistsweretryingtounderstandthenature
ofradioactivity,theynoticedthatradiationcanpass
throughsolidmaterials.Differenttypesofradiationcan
penetratedifferentthicknessesofmaterials.
a-particlesareabsorbedmosteasily.Theycantravel
about5cminairbeforetheyareabsorbed.Theyare
absorbedbyathinsheetofpaper,a-particlescannot
penetrateskin.
0-particlescantravelfairlyeasilythroughairor
paper.Buttheyareabsorbedbyafewmillimetresof
metalsuchasaluminium.
y-radiationisthemostpenetrating.Ittakesseveral
centimetresofadensemetallikelead,orseveral
metresofconcrete,toabsorbmostofthegamma
radiation.
Figure23.6showsthepenetratingpowerofeachtype
ofradiation.
Figure23.6:Thepenetratingpowerofy-rdyiationisthe
greatest,aradiationhastheleastamountofpenetrating
power.Thisisrelatedtotheirabilitytoionisethematerials
theyarepassingthrough.
II
Ionisation
Whenradiationpassesthroughair,itmayknock
electronsoutofatoms.Thismeansionsareformed.
Thisprocessiscalledionisation.
a-particlesarethemostionising.
y-radiationistheleastionising.
Astheradiationemittedbythenucleiofradioactive
substancescausestheionisationofthematerialsthat
absorbit,itisoftenknownasionisingnuclearradiation.
X-raysalsocauseionisationinthematerialsthey
passthrough,andsotheyarealsoclassedasionising
radiation.X-raysareverysimilartoy-rays.However,
X-raysusuallyhavelessenergy(longerwavelength)than
y-rays,andtheyareproducedbyX-raymachines,stars
andsoon,ratherthanbyradioactivesubstances.
Whensomethinghasbeenexposedtoradiation,ithasbeen
irradiated.Althoughitabsorbstheradiation,itdoesnot
itselfbecomeradioactive.Thingsonlybecomeradioactive
iftheyabsorbaradioactivesubstance.Soyoudonot
becomeradioactiveifyouabsorbcosmicrays(whichyou
432>

446. 23Radioactivity
doallthetime).Butyoudobecomeradioactiveifyou
consumearadioactivesubstance.Coffee,forexample,
containstinybutmeasurableamountsofradioactive
potassium.Youbecomecontaminatedbythecoffee.
Howionisationhappens
Toexplaintheionisingeffectofeachtypeofradiation
weneedtoconsidertheirkineticenergyandtheircharge.
Table23.2:Propertiesofionisingradiation.
Name Mass Speed/m/sCharge
a
approx,(massof
proton)x4
~3x107 +2
p
approx,(massof
proton)4-1840
~2.9x108-1
Y 0 3x10« 0
Considerana-particlepassingthroughtheair.
Ana-particleistheslowestmovingofallthethree
radiationsandhasthelargestcharge.Asthea-particle
collideswithanairmolecule,itmayknockanelectron
fromtheairmolecule,sothatitbecomescharged.
Thea-particlelosesalittleofitsenergy.Itmustionise
thousandsofmoleculesbeforeitlosesallofitsenergy
andcomestoahalt,a-radiationisthemoststrongly
ionisingradiation.
A0-particlecanalsoioniseairmolecules.However,itis
lessionisingfortworeasons:itschargeismuchlessthan
thatofana-particle,anditmovesfaster.Thismeansthat
itismorelikelytotravelstraightpastanairmolecule
withoutinteractingwithit.Thisiswhy0radiationcan
travelfurtherthroughairwithoutbeingabsorbed,
y-radiationisunchargedanditmovesfastestofall.
Thismeansthatitistheleastreadilyabsorbedinair,
andthereforeistheleastionising.Leadisagood
absorberbecauseitisdense(itsatomsarepackedclosely
together),anditsnucleiarerelativelylarge,sothey
presentaneasytargetforthey-rays.
Youshouldbeabletoseethepatternlinkingionising
powerandabsorption:
a-radiationisthemoststronglyionising,soitisthe
mosteasilyabsorbedandtheleastpenetrating.
y-radiationistheleaststronglyionising,soitisthe
leasteasilyabsorbedandthemostpenetrating.
Using,movingandstoring
radioactivematerialssafely
Anyelementcomesinseveralformsorisotopes(see
Section22.2).Somemaybestable,butothersare
unstable,thatis,theyareradioactive.Forexample,carbon
hastwostableisotopes(carbon-12andcarbon-13),but
carbon-14isanunstableisotope.Unstable(radioactive)
isotopesareknownasradioisotopes.
KEYWORDS
ionisingnuclearradiation:radiation,emittedby
thenucleuswhichcancauseionisation;alphaor
betaparticles,orgammarays
radioisotope:aradioactiveisotopeofanelement
Effectsofradioisotopesoncells
Touseradioisotopessafely,weneedtounderstandhow
theyaffectcells.Therearethreewaysinwhichradiation
candamagelivingcells.
Anintensedoseofradiationcausesalotof
ionisationinacell,whichcankillthecell.This
iswhathappenswhensomeonesuffersradiation
bums.Thecellsaffectedsimplydie,asiftheyhad
beenburned.Ifthesuffererisluckyandreceives
suitabletreatment,thetissuemayregrow.
IftheDNAinthecellnucleusisdamaged,the
mechanismsthatcontrolthecellmaybreakdown.
Thecellmaydivideuncontrollablyandatumour
forms.Thisishowradiationcancausecancer.
Iftheaffectedcellisagamete(aspermoreggcell),
thedamagedDNAofitsgenesmaybepassedon
tofuturegenerations.Thisishowradiationcan
producegeneticmutations.Occasionally,amutation
canbebeneficialtotheoffspring,butmoreoftenitis
harmful.Afertilisedeggcellmaynotdevelopatall,
orthebabymayhavesomeformofgeneticdisorder.
Weareleastlikelytobeharmedbyaradiationcoming
fromasourceoutsideourbodies.Thisisbecausethe
radiationisentirelyabsorbedbythelayerofdeadskin
cellsontheoutsideofourbodies(andbyourclothes).
However,ifanasourcegetsinsideus,itcanbevery
damaging,becauseitsradiationishighlyionising.That
iswhyradonandthorongasesaresodangerous.We
breathethemintoourlungs,wheretheyirradiateusfrom
theinside.Theresultmaybelungcancer.

447. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Today,weknowmuchmoreaboutradiationandthe
safehandlingofradioactivesubstances.Knowinghow
toreducethehazardsofradiationmeansthatwecan
learntolivesafelywithitandputittomanyworthwhile
purposes.
Knowingabouttheradiationproducedbyradioactive
materialshelpsusknowhowtohandlethemassafely
aspossible.Anyoneworkingwith,orbeingexposedto,
ionisingradiationmusttakesafetyprecautions,suchas
shielding,orlimitingtheirexposuretime.Figure23.7
showssomeoftheseprecautions.
Figure23.7a:Radiationsuitsarewornincontaminatedareas,b:Radiographersoperateequipmentfromaseparateroom,
c:Schoollaboratorysourcesarestoredinlead-linedwoodenboxesandlockedawayinalabelledmetalcabinetwhennotin
use.Thetweezersallowtheteachertohandlethesourcesatasafedistance,d:Radioactivematerialmustbeclearlymarked
whenitisbeingtransported.
Safetyprecautions
Table23.3showscommonsafetyprecautionswhendealingwithradioactivematerial.
Table23.3:Safetyprecautionsfordealingwithradioactivematerial.
Safetyprecaution Explanation
workersincontaminatedareaswear
protectivesuits
Thesuitwillabsorbradiation.Differentmaterialscanbeused
dependingonthetypeofradiation.Fory-rays,lead-linedsuitscanbe
used.
radioactivehazardlabels
Thesewarnpeopleofthedangersotheycanstayatasafedistance
andreducethetimetheyarenearthesourcefor.
photographicfilmdosimeterbadges
Thesemonitortheamountofexposureapersonhashad.Oncethe
safelimitisreached,workersmaybetransferredtootherareas.
recordkeeping
Schoolsarerequiredtorecordhowlongradioactivesourceswere
usedfor,andbywhom.Thisallowsthemtoensureno-oneisexposed
fortoolong.
remoteoperatingofscanners
Theoperatorusuallycontrolsthescannerfromaseparatearea.This
increasestheirdistancefromthesource.Theymayalsobebehinda
screenwhichwillabsorbsomeoftheradiation.
storageboxesforsources
Radioactivesourcesmustbestoredsecurely,usuallysurroundedby
leadtoabsorbmostoftheradiation.
434y

448. 23Radioactivity
Radioactivedecayequations
Radioactivedecaycanbedescribedusingbalanced
equationswiththenuclearnotationweusedin
Chapter22.Towritetheseequationsweneedtoconsider
theeffectofeachemissiononthenucleusemittingit:
a-decay:twoprotonsandtwoneutronsareemitted.
Theprotonnumberdecreasesby2andthenucleon
numberdecreasesby4.
p-decay:aneutronsplitsintoaprotonandan
electron;theelectronisemitted.Theprotonnumber
increasesby1andthenucleonnumberremains
thesame.
y-decay:thisistheemissionofenergyfromthe
nucleusanditdoesnotchangetheparticlesin
thenucleus.
Thesechangesallleadtothenucleusbecomingmore
stable.
Hereisanexampleofanequationforalphadecay:
2
Q4Am-►2,27U+
2
He+energy
Thisrepresentsthedecayofamericium-241,the
isotopeusedinsmokedetectors.Itemitsana-particle
(representedasaheliumnucleus)andbecomesan
isotopeofuranium.Ana-particlecanberepresented
as
2
Heor
2a.
Noticethatthenumbersinthisequationmustbalance
becausewecannotlosemassorcharge.So:
nucleonnumbers: 241 >237+4
protonnumbers: 94 »92+2
Hereisanexampleofanequationforbetadecay:
1
gC->N+_Je+energy
Thisisthedecaythatisusedinradiocarbondating.
Acarbon-14nucleusdecaystobecomeanitrogen-14
nucleus.Thep-particle,anelectron,isrepresentedby
_Jeor_]p.Ifwecouldseeinsidethenucleus,wewould
seethatasingleneutronhassplitintoaprotonandan
electron.So:
}p+>
Foreachofthesetwop-decayequations,youshouldbe
abletoseethatthenucleonnumbersandprotonnumbers
arebalanced.Wesaythat,inradioactivedecay,nucleon
numberandprotonnumberareconserved.
KEYWORDS
alphadecay:thedecayofaradioactivenucleus
bytheemissionofana-particle
betadecay:thedecayofaradioactivenucleusby
theemissionofap-particle
WORKEDEXAMPLE23.1
Aradioisotopeofthorium(^Th)decaysbyemitting
ana-particle.Theresultingnucleusisalsounstable
andemitsabetaparticle.Writeequationsforthe
twoemissions.
Step1:Ina-emission(2a)theprotonnumber
decreasesby2(inthiscase,from88to89).
FromthePeriodicTable,wecanseethat
Radium(Ra)hastheprotonnumber88.
Thenucleonnumberdecreasesby4(inthis
case,from229to225).
Step2:Inp-emission
1_”p)theprotonnumber
increasesby1(inthiscasefrom88to89).
FromthePeriodicTable,wecanseethat
Actinium(Ac)hastheprotonnumber89.
Thenucleonnumberremainsthesame.
Answer
a-emission:2^Th->^Ra
+
2a
p-emission:^Ra-*™Ac+?p
• 00 07
Deflectingradiation
Howcanwetellthedifferencebetweenthesethreetypes
ofradiation?Onemethodistoseehowtheybehavein
electricandmagneticfields.
Becausetheyhaveoppositecharges,aandp-particlesare
deflectedinoppositedirectionswhentheypassthroughan
electricfield(Figure23.8a).Positivelychargeda-particles
areattractedtowardsanegativelychargedplate,while
negativelychargedp-particlesareattractedtowardsa
positivelychargedplate,p-particlesaredeflectedmore
thana-particlesasthearelighter.'y-raysarenotdeflected
becausetheyareuncharged.
a-andp-particlesarecharged,so,whentheymove,they
constituteanelectriccurrent.Becauseoftheiropposite
signs,theforcesontheminamagneticfieldarein
oppositedirections(Figure23.8b).Thisisanexampleof
themotoreffect(Chapter20).Thedirectioninwhichthe
435y

449. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
particlesaredeflectedcanbepredictedusingFleming’s
left-handrule.Asinanelectricfield,y-raysarenot
deflectedbecausetheyareuncharged.
b fl
Figure23.8:a-andfl-radiations
aredeflectedinopposite
directions,a:Inanelectricfield,b:Inamagneticfield.
Questions
7Explainwhyemissionofa-orfl-particleschanges
thenucleustooneofadifferentelement.
8Theequationrepresentsthedecayofapolonium
nucleustoformaleadnucleus.Ana-particle
isemitted.
2$Po->2^Pb+qQ+energy
aCopyandcompletetheequation.
bShowthattheprotonnumbersareequalon
eachsideoftheequation.
cShowthatthenucleonnumbersareequalon
eachsideoftheequation.
9Writeabalancednuclearequationtoshowwhat
happenstothepoloniumisotope2yPowhenit
emitsafl-particle.
10y-raysandX-raysarebothformsofionising
radiation.
aStateonewayinwhichtheyaresimilar.
bStateonewayinwhichtheyaredifferent.
11Twobeamsofionisingradiationarepassedbetween
chargedmetalplates.Theyaredeflected,asshown
inFigure23.9.
Figure23.9
1
aNamethetypeofradiationforeachbeam,
bStatethepolarityofeachoftheplates.
cNamethetypeofradiationwhiclywould
notbe
deflectedbytheplates.
12Figure23.10showsaradiationdetectionbadge.
filmbadge
dosemeter jacket
Figure23.10:Aradiationdetectionbadge.
aExplainwhatyouwouldseeifthefilminside
thebadgewasdevelopedifthewearerhadbeen
exposedto:
ifl-radiation
iiy-radiation.
bSuggestareasonwhythebadgeisnotsuitable
fordetectingexposuretoalpharadiation.
23.3Activityandhalf-life
Theactivityofaradioactivesourceistherateatwhich
itsnucleidecay.ThiscanbemonitoredusingaGeiger
counterwhichmeasuresthecountrate,thenumber
ofemissionsdetectedeachsecond(orminute).
436y

450. 23Radioactivity
TheGeigercounterwillnotdetecteveryemission,so
activityandcountratearenotequal,butcountrateis
usedtomonitoractivity.
Theactivityofasourcedecreaseswithtime.Asnuclei
decayandbecomestable,therearefewerunstablenuclei,
sotherearefewerdecayseachsecond.Thecountrate
andactivitybothdecreasefollowingthesamepatternas
thenumberofundecayedatoms.
Allradioactivesubstancesdecaywiththesamepattern,
asshowninFigure23.1la.Thegraphshowsthatthe
amountofaradioactivesubstancedecreasesrapidlyat
first,andthenmoreandmoreslowly.Infact,because
thegraphtailsoffmoreandmoreslowly,wecannotsay
whenthelastatomswilldecay.Differentradioactive
substancesdecayatdifferentrates,somemuchfaster
thanothers,asshowninFigure23.1lb.
Wecannotsaywhenthesubstancewillhaveentirely
decayed.Wehavetothinkofanotherwayof
describingtherateofdecay.Asshownonthegraphin
Figure23.1la,weidentifythehalf-lifeofthesubstance.
Thehalf-lifeofaradioactiveisotopeistheaveragetime
takenforhalfoftheatomsinasampletodecay,orthe
timeforitsactivityorcountratetohalve.
Half-livescanvaryfromafractionofasecondto
thousandsofyears.Uraniumdecaysslowlybecauseit
hasaverylonghalf-life.Theradioactivesamplesused
inschoolsusuallyhavehalf-livesofafewyears,sothat
theyhavetobereplacedwhentheiractivityhasdropped
significantly.Someradioactivesubstanceshavehalf-lives
thatarelessthanamicrosecond.Nosoonerarethey
formedthantheydecayintosomethingelse.
KEYWORDS
activity:therateatwhichnucleidecayina
sampleofaradioactivesubstance
half-life:theaveragetimetakenforhalftheatoms
inasampleofaradioactivematerialtodecay
Explaininghalf-life
Afteronehalf-life,halfoftheatomsinaradioactive
samplehavedecayed.However,thisdoesnotmeanthat
alloftheatomswillhavedecayedaftertwohalf-lives.
FromthegraphofFigure23.1la,youcanseethatone-
quarterwillstillremainaftertwohalf-lives.Whyisthis?
Figure23.12showsonewayofthinkingaboutwhatis
goingon.Imaginethatwestartwithasampleof100
undecayedatomsofaradioactivesubstance(white
circlesinFigure23.12a).Theydecayrandomly(black
circlesinFigure23.12b-d)-eachundecayedatomhasa
50%chanceofdecayinginthecourseofonehalf-life.So,
lookingatthepanelsinFigure23.12,wecandescribethe
decaylikethis:
Atthestart,inFigure23.12a,thereare100
undecayedatoms.
Afteronehalf-life,inFigure23.12b,arandom
selectionof50atomshasdecayed.
Duringthenexthalf-life,inFigure23.12c,arandom
selectionofhalfoftheremaining50atomsdecays,
leaving25undecayed.
Duringthethirdhalf-life,inFigure23.12d,halfof
theremainingatomsdecay,leaving12or13.
(Ofcourse,youcannothavehalfanatom.)
half-lifehalf-life
Time Time
Figure23.11a:Adecaygraphforaradioactivesubstance.Acurveofthisshapeisknownasanexponentialdecaygraph,
b:Asteepergraphshowsthatasubstancehasashorterhalf-life.
437>

451. )CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
Sothenumberofundecayedatomsgoes100,50,25,12,...
andsoon.Itisbecauseradioactiveatomsdecayina
randomfashionthatwegetthispatternofdecay.Notice
that,justbecauseoneatomhasnotdecayedinthefirst
half-lifedoesnotmeanthatitismorelikelytodecayin
thenexthalf-life.Ithasnowayofrememberingitspast.
a
ooooooo|oOIO
ooo0'0ooooO'
oooo'oo0'0oo
oooooooooo
oooooooooo
olooOIOooooo
oooOlOooooo
ooooooOOoo
oooooOOooo
oooOlO00'0oo
b c d

o
eoeeeeeeoe
eeee

o
oe
oeoeoeeeeeoee

o
eeeeeeeeeeeeee
Key:

o
oeoeeoeeeeoeeeeo

o
oeeeeoeeeeeeeeo

o
oeoee eeeeeeeoee
undecayed
o
oeeoeeoeeeoeeeee
atoms

o
oeoeeeoeeeeeoee

oeeeeeeeeeeeeeeedecayed
atoms
eoeeoeeeeeeoeeee
Figure23.12:Thepatternofradioactivedecaycomesaboutbecausethedecayofindividualatomsisrandom.Halfofthe
atomsdecayduringeachhalf-life,butwehavenowayofpredictingwhichindividualatomswilldecay.
i
WORKEDEXAMPLE23.2
Thecountratefromaradioactivesourceisrecordedfor
tendays.Figure23.13ashowstheresults.
Usethegraphtofindthehalf-lifeofthesource.
Figure23.13a
Step1:Calculatethecountrateafteronehalf-life.
Theinitialcountrateis80count/s,soafterone
half-lifeitwillhavedroppedto40count/s.
Step2:Usethegraphtofindthetimetakenforthe
countratetodropto40count/s.
Thehalf-lifeistwodays.
Figure23.13b
438

452. 23Radioactivity
CONTINUED
Step3:Checkthisbyfindingthetimetakenforthe
countratetohalveagain,from40count/sto
20counts/s.
Thecountratedropsfrom40count/sto20
count/sintwodays,confirmingthatthe
half-lifeistwodays.
Answer
Thehalf-lifeistwodays.
Time(days)
Figure23.13c
WORKEDEXAMPLE23.3
Strontium-90hasahalf-lifeof28years.Thecountrate
ofasampleis480count/s.Howlongwillittakeforthe
countratetodropto30count/s?
Step1:DrawatablelikeTable23.4toworkoutthe
numberofhalf-livesforthecountratetodrop
to30cps.
Numberofhalf-livesCountrate/count/s)
0 480
1 240
2 120
3 60
4 30
Table23.4
So,thecountratehasfallento30count/s
afterfourhalf-lives.
Step2:Calculatehowlongthisisinyears.
Onehalf-lifeis28years,sofourhalf-lives:
4x28=112years.
Answer
112years
439y

453. >
CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
ACTIVITY23.1
Modellinghalf-life
Radioactivedecayisarandomprocess.Inthisactivity
youwillmodelradioactivedecayusinganother
randomprocess-throwingdiceorsmallcubes.
Youwillneedalargenumberofdice-atleast
100-orsmallcubeswithonesidemarked(asin
Figure23.14).
Figure23.14:Usingsmallcubestomodel
radioactivedecay.
Placeallthediceinacontainer,shakethecontainer
andthrowthedice.
Anyspinnershowing6,orcubewiththemarkedside
facingupwards,have'decayed'.Counthowmany
havedecayed.
CreateatablelikeTable23.5andrecordyourresults.
Thrownumber
(time)
Number
decaying
(activity)
Number
remaining(N)
Table23.5
I
Repeatuntilallthedicehavedecayed.
Drawtwographs:
activityagainsttime
numberof'nuclei'remainingagainsttime.
Questions
1Whatdoyounoticeaboutthetwographs?
2Calculatethehalf-lifefromeachgraph.
Whatdoyounotice?
Correctedcountrate
ThecountraterecordedbyaGeigercounterwill
includebackgroundradioactivityaswellasthecount
ratefromtheradioactivesource.Oftenthebackground
countisnegligibleincomparisontotheactivityofthe
source.Ifthebackgroundcountisbeingtakeninto
account,itshouldbesubtractedfromtheGeigercounter
measurements.
correctedcountrate=
measuredcountrate-backgroundcountrate
WORKEDEXAMPLE23.4
AscientistmonitoredaradioactivesourceeverytenminutesusingaGeigercounter.Sherecordedthe
followingreadings.
Time/min 0 10 20 30 40 50 60
Countrate/count/min 330 230 165 120 92 70 56
Aftertheexperimentsherecordedabackgroundcountof30count/min.
Plotagraphofcorrectedcountrateagainsttimeanduseittofindthehalf-lifeofthesource.
440)

454. 23Radioactivity
CONTINUED
Step1:Calculatethecorrectedcountrates.
Time/min 0 10 20 30 40 50 60
Countrate/count/min 330230165120 92 70 56
Correctedcountrate/count/min300200135 90 62 40 26
Step2:Plottheseresultsonagraph.
Step3:Theinitialcountrateis300count/min.Drawalinefromhalfofthis(3002=150count/min)tofind
thehalf-life.
half-life
Time1min
is18mins
Answer
half-life=18minutes
Questions
13Asampleofradioactiveiodinecontains6400
undecayedatoms.
aHowmanywillremainundecayedafterthree
half-lives?
bThehalf-lifeofthisisotopeofiodineiseight
days.Howmanyatomsremainundecayedafter
40days?
14Thehalf-lifeofthorium-227is19days.Howlong
willittakefortheactivityofthesourcetodecrease
by75%?
15Figure23.15showsthecountrateforaradioactive
sourceatdifferenttimes.Whatisthehalf-lifeofthis
source?
Time(days)
Figure23.15
441>

455. >
CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
16Carbonhastwoisotopes,carbon-12andcarbon-14.
Carbon-14isradioactive.Theproportionofthe
twoisotopesinlivingthingsremainsconstantwhile
theyarealive,butwhentheydie,theproportionof
carbon-14dropsastheisotopedecays.Archaeologists
studyingabonefinditemits20count/s,whereasa
similarmodemboneemits80count/s.
Thehalf-lifeofcarbon-14is5700years.Usethisto
estimatetheageofthebonethatthearchaeologists
found.
23.4Usingradioisotopes
Radioisotopesatwork
Nowwewilllookatsomeofthemanyusesof
radioisotopes.Wewilllookattheseusesinfourseparate
groups,relatedto:
theirdifferentpenetratingpowers
thedamagetheirradiationcausestolivingcells
thefactthatwecandetecttinyquantitiesof
radioactivesubstances
radioactivedecayandhalf-life.
Usesrelatedtopenetrating
power
Smokedetectors
Theseareoftenfoundindomestickitchens,andinpublic
buildingssuchasofficesandhotels.Ifyouopenasmoke
detectortoreplacethebattery,youmayseeayellowand
blackradiationhazardwarningsign(Figure23.16a).The
radioactivematerialusedisamericium-241,asourceof
a-radiation.Figure23.16bshowshowthesmoke
detectorworks.
TheAmericiumsourceusedinsmokedetectorshasa
longhalf-life-about430years.Thismeansthatthe
countratefromthesourcewillnotdropsignificantly
overthetimethedetectorisinuse.
b
smoke
Figure23.16a:Theinsideofasmokedetector.Thesource
ofradiationisasmallamountofamericiumr^41.
b:Blockdiagramofasmokedetector.Thealarmsounds
whensmokeabsorbsthearadiation.
Radiationfromthesourcefallsonadetector.Since
a-radiationischarged,asmallcurrentflowsinthe
detector.Theoutputfromtheprocessingcircuitis
off,sothealarmissilent. V
Whensmokeentersthegapbetweenthesource
andthedetector,itabsorbsthea-radiation.Now
nocurrentflowsinthedetector,andtheprocessing
circuitswitcheson,soundingthealarm.
Inthisapplication,asourceofa-radiationischosen
becausea-radiationiseasilyabsorbedbythesmoke
particles.
Thicknessmeasurements
Inindustry,fl-radiationisoftenusedintomeasure
thickness.Manufacturersofpaperneedtobesurethat
theirproductisofauniformthickness.Todothis,
fl-radiationisdirectedthroughthepaperasitcomesoff
theproductionline.Adetectormeasurestheamountof
radiationgettingthrough.
Ifthepaperistoothick,theradiationlevelwillbelow
andanautomaticcontrolsystemadjuststhethickness.
Thesametechniqueisusedinthemanufactureofplastic
sheetingandaluminiumfoil,fl-radiationisusedinthis
applicationbecausea-radiationwouldbeabsorbed
entirelybythepaper,plasticoraluminium.
442>

456. Figure23.17:Ifthematerialgetstoothick,lessradiation
willbedetected.Thiswillcausetherollerstobemoved
closertogether,makingthesheetthinner.
y-radiationwouldhardlybeaffected,becauseitis
themostpenetrating.
Sheetsteelfactoriesuseasimilarsystembutwitha
ysource.
Faultdetection
Sometimesy-raysareusedtodetectfaultsin
manufacturedgoods.Figure23.18showsanexample,
whereengineersarelookingforanyfaultsinsome
pipework.Ifthereisafault,radiationwillescape
throughthefault.Aphotographicfilmisstrappedtothe
outsideofthepipeandtheradioactivesourceisplaced
ontheinside.Whenthefilmisdeveloped,itlookslikean
X-raypicture,andshowsanyfaultsinthewelding.
23Radioactivity
Usesrelatedtocelldamage-
radiationtherapy
Cancertreatment
ThepatientshowninFigure23.19isreceivingradiation
aspartofatreatmentforcancer.Asourceofy-rays
(orX-rays)isdirectedatthetumourthatneedstobe
destroyed.Thesourcemovesaroundthepatient,always
aimingatthetumour.Inthisway,othertissuesreceive
onlyasmalldoseofradiation.Radiationtherapyisoften
combinedwithchemotherapy(usingdrugstotargetand
killthecancerouscells).
Figure23.19:Radiationcancausecancer,butitcanalso
beusedinitscure.Thispatientisbeingexposedtoy-rays
fromaradioactivesource.
Foodirradiation
Thisisawayofpreservingfood.Foodoftendecays
becauseoftheactionofmicrobes.Thesecanbekilled
usingintensey-rays.Becausetheseorganismsare
single-celled,anycelldamagekillstheentireorganism.
Differentcountriespermitdifferentfoodstobe
irradiated.Theresultissterilefood,whichhasbeenused
onspacemissions(wherelong-lifeisimportant)and
forsomehospitalpatientswhoseresistancetoinfection
bymicrobesmaybelow.Figure23.20showsadisplay
fromtheNehruScienceCentreinMumbaiwhich
demonstratestheadvantagesofirradiatingfood.
Figure23.18:Checkingforfaultsinametalpipe.
They-raysourceisstoredintheblackboxintheforeground,
butcanbepushedthroughthepipetoreachthepartthat
needschecking.
443>

457. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Figure23.21:Thesealedpackageensuresthesyringe
remainssterileoncey-rayshavekilledallpathogens.
Figure23.20:Themicrobeswhichwouldcausedecayhave
beenkilledbyradiationinthetopsample.
canuseradiationtodetecttiny(Quantitiesofsubstances,
farsmallerthancanbedetectedbychemicalmeans.Such
techniquesareoftenknownasradioactivetracing.Thishas
usesinmedicineandengineering.
Figure23.22:Thetracercanbedetectedinbothkidneys
andthebladder.Morey-raysaredetectedfromthekidney
seenontheright,suggestingaproblemthere.
Medicine
Thediagnosisofsomediseasesmaybecarriedoutusing
asourceofy-radiation.Thepatientisinjectedwitha
radioactivechemicalandascannerisusedtotracethe
pathofthechemical.Figure23.22showsascanofa
patientwithakidneyblockage.Thetracertechnetium-99
isinjectedintothepatient’sblood.Thescanshowsthat
thetracerisnotpassingthroughthekidneyshownonthe
rightaswellasitisthroughtheotherkidney.Thisindicates
thatthereisablockage.Thetechnetiumisotopeusedhas
arelativelyshorthalf-life-aboutsixhours.Thisislong
enoughforittobeusedtotracetheblockage,butitdoes
notremainradioactiveverylonginsidethepatient'sbody.
Engineering
Engineersmaywanttotraceundergroundwaterflow,for
example.Theymaybeconstructinganewwastedump,
andtheyneedtobesurethatpoisonouswaterfromthe
dumpwillnotflowintothelocalwatersupply.Under
highpressure,theyinjectwatercontainingaradioactive
chemicalintoaholeintheground(Figure23.23).Then
theymonitorhowitmovesthroughundergroundcracks
usingy-detectorsatgroundlevel.
Sterilisation
Sterilisationofmedicalproductsworksinthesame
wayasfoodirradiation.Syringes,scalpelsandother
instrumentsaresealedinplasticbagsandthen
exposedtogammaradiation.Anymicrobespresent
arekilled,sothat,whenthepackagingisopened,the
itemisguaranteedtobesterile,y-radiationisusedas
itcanpenetratetheplasticandcanpassthroughthe
equipment,makingsureallpartsaresterilised.
Usesrelatedtodetectability-
radioactivetracing
EverytimeyouhearaGeigercounterclick,ithasdetected
theradioactivedecayofasingleatom.Thismeansthatwe

458. 23Radioactivity
Figure23.23:Detectingthemovementofunderground
water.Engineersareinvestigatinghowwatermoves
underground.Thiscanalsoaffectthestabilityofbuildings
onthesite.Watercontainingasourceofy-radiationis
pumpedundergroundanditspassagethroughcracksis
monitoredatgroundlevel.
Usesrelatedtoradioactive
decay-half-lifeand
radiocarbondating
Becauseradioactivesubstancesdecayataratethatwe
candetermine,wecanusethemtodiscoverhowold
objectsandmaterialsare.Thebest-knownexampleof
thisisradiocarbondating.
Alllivingthingscontaincarbon.Plantsgetthis
fromatmosphericcarbondioxide,whichtheyusein
photosynthesis.Plant-eatinganimalsgetitfromtheplants
theyeattobuildtheirbodies.Meat-eatinganimalsgetit
fromtheirprey.Mostcarboniscarbon-12,whichisnot
radioactive.Atinyamountisradioactive(carbon-14),
whichhasahalf-lifeof5700years.Itemitsp-radiation.
Whenalivingorganismdies,thecarbon-14initsbody
decays.Astimepasses,theamountremainingdecreases.
Wecanmeasuretheamountremaining,andthenwork
outwhentheorganismwasalive.
Therearetwowaystomeasuretheamountofcarbon-14
presentinanobject:
bymeasuringtheactivityofthesampleusinga
detectorsuchasaGeigercounter
bycountingthenumberofcarbon-14atomsusing
amassspectrometer(alargemachinethatuses
magneticfieldstoseparateatomsaccordingtotheir
massandcharge).
TheTurinShroudwasfamouslydatedin1988
usingamassspectrometer.Theshroudwasdatedto
1325±33CE,whichmatchesthedatesoftheearliest
historicalrecordsofitsexistence.
Problemscanarisewithradiocarbondating.Itmaybe
thattheamountofcarbon-14presentintheatmosphere
wasdifferentinthepast.Nuclearweaponstestingadded
extracarbon-14totheatmosphereduringthe1950sand
1960s.Thismeansthatlivingobjectsthatdiedthenhave
anexcessofcarbon-14,makingthemappearyounger
thantheyreallyare.
KEYWORDS
radioactivetracing:usingaradioisotopeto
investigateaproblem
radiocarbondating:atechniquethatusesthe
knownrateofdecayofradioactivecarbon-14to
findtheapproximateageofanobjectmadefrom
deadorganicmaterial
Otherradioactivedatingtechniques
Geologistsusearadioactivedatingtechniquetofindthe
ageofsomerocks.Manyrockscontainaradioactive
isotope,potassium-40,whichdecaysbypemissiontoa
stableisotopeofargon.Argonisagas,anditistrappedin
therockasthepotassiumdecays.
Therocksofinterestformfrommoltenmaterial,for
example,inavolcano.Thereisnoargoninthemolten
rockbecauseitcanbubbleout.Aftertherocksolidifies,
theamountoftrappedargongraduallyincreasesasthe
potassiumdecays.Geologiststakeasampleandmeasure
therelativeamountsofargonandpotassium.Thegreater
theproportionofargon,theoldertherockmustbe.
Questions
17Asmokedetectorusesana-source.
Explainwhy:
aap-ory-sourcewouldnotwork
bitissafetohavethistypeofsmokedetectorin
yourhome
cthesourceusedmusthaveahalf-lifeofyears
ratherthandays.
18Describewhatwouldhappenifthesheetshown
inFigure23.17becametoothin.
19Whenmedicalequipmentistobesterilised,itis
firstsealedinaplasticwrapper.Whydoesthisnot
absorbtheradiationused?
445

459. >
CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
PROJECT
Demystifyingradioactivity
Wehaveknownaboutradioactivityforjustover
acentury.Inthattimeithasattractedalotof
sensationalattentionasanamazingcure-all,an
invisiblekillerorforitsfictionalabilitytomutate
humansoranimalsintomonsters.
Figure23.24:Thisistheinternationalwarningsignfor
ionisingradiation.
Overthelastcenturywehavelearntthat,although
dangerous,ionisingradiationcanbeveryuseful,and
ifhandledcorrectly,itcanbesafe.
Yourtaskistoprepareatelevisionsegmentor
magazinearticleaboutradioactivityaimedat
teenagerstoexplainthescienceofradioactivityand
itsusesanddangers.
Youshould:
describeandillustratethethreetypesof
radioactiveemission
explainhowwecanuseknowledgeabouttheir
penetratingpowertouseandstoresources
safely
explainwhatismeantbyhalf-lifeandwhysome
radioactivewastemustbestoredsecurelyfor
manyyears
includeasmanypracticalexamplesoftheuses
ofradioisotopesasyoucan.
Youshouldpresentyourworkasavideooragroup
presentationtotheclass,orinawrittenarticle.
Muchofthefearwhichsurroundsradioactive
materialsismadeworsebythelanguage
surroundingit.Wordssuchas'mutation'and
'half-life'arenotineverydayuseandcanaddtothe
feelingthatradiationisaweirdscientificthreat.To
counterthis,yourpresentationshouldincludeand
explainthefollowingkeyterms:
radioactive
a-particle
p-particle
y-ray
ionisation
mutation
radioactiveisotope
nucleardecay
half-life
background
radiation.
SUMMARY
Wearesurroundedbyionisingbackgroundradiationfromnaturalandman-madesources.
Unstableisotopesdecayrandomly.
Radioactivedecayleadstothreetypesofemissions-a-particles,p-particlesandy-rays.
aandpemissionschangethenucleustothatofadifferentelement.
aandpemissionscanbedescribedusingbalancednuclearequations.
Thehalf-lifeofaradioactivesourceisthetimetakenforhalfitsradioactivenucleitodecay.
a,pandy-radiationcanallionisecells,leadingtomutationsandtumours.Safetyprecautionsmustbetaken
whenusingradioactivematerials.
Whenusedsafely,radioactivematerialshavemanyuses,particularlyinmedicineandengineering.
446)

460. 23Radioactivity
EXAM-STYLEQUESTIONS
1Whichofthefollowingisanaturalsourceofbackgroundradiation? [1]
AmedicalX-rays
Bnuclearpowerstations
Ccosmicrays
Dnuclearweaponstesting
2Nuclearradiationcancauseionisationbyknockingelectronsoutofatoms.
Whichtypeofradiationisleastionising? [1]
Aa-particles
B0-particles
Cy-rays
Dtheyallhavethesameionisingpower
3Asourcecontains240gofaradioactiveisotope.Thehalf-lifeofthe
sourceisthreehours.Howmuchoftheisotopewillbeleftafter12hours?[1]
Anone B30g C15g D20g
4aEachofthestatementsbelowdescribeatypeofradiation.
Type1:Absorbedbyathinsheetofpaper
Type2:Absorbedbyathinsheetofmetal
Whichtypesofradiationaredescribedinthestatements?
Type1 Type2
A0-radiation a-radiation
By-radiation 0-radiation
Ca-radiation 0-radiation
D0-radiation y-radiation
bWhichstatementiscorrect?
Ay-radiationisanelectronemittedfromthenucleus
Ba-radiationhasanegativecharge
Cy-radiationhasapositivecharge
D0-radiationisanelectronemittedfromthenucleus [3]
[Total:10]
5Aradioisotopeofradiumhasahalf-lifeof1600years.
aDefinewhatismeantbytheterm‘half-life’. [1]
bThepaintinaluminouswatchfacecontains20mgoftheradioisotope
ofradium.Calculatethemassthatremainsundecayedafter3200years.[1]
cRadiumemitsionisingradiation.Statehowthiscanaffectlivingcells.[1]
dDescribethesafetyprecautionswhichmustbetakenwhenradioactive
radiumisusedinaschoollaboratory. [2]
[Total:5]
COMMAND WORDS
define:giveprecise
meaning
calculate:workout
fromgivenfacts,
figuresorinformation
state:expressin
clearterms
describe:statethe
pointsofatopic;give
characteristicsand
mainfeatures
447>

461. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
6Goldhasdifferentisotopes.Gold-198isradioactiveanddecaysby
P-emission.
aNameaparticlewhichisidenticaltoap-particle. [1]
bNameamaterialwhichcouldbeusedtostopp-particles,butwhich
wouldnotstopy-rays. [1]
cThegraphshowshowthecountratefromasampleofgold-198changes
withtime.
Usethegraphtofindthehalf-lifeofgold-198. [1]
[Total:3]
7a-particlesareatypeofionizingnuclearradiation.
aDescribethestructureofana-particle. [1]
bCompletethefollowingnuclearequationtoshowhowemittingan
a-particlechangesauraniumisotope.
“u^grh+O
|3|
*
448y

462. 23Radioactivity
COMMAND WORDS
explain:setout
purposesor
reasons/make
therelationships
betweenthings
evidentIprovide
whyand/orhowand
supportwithrelevant
evidence
deduceconclude
fromavailable
information
449y

463. )CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
SELF-EVALUATIONCHECKLIST
Afterstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutosee
anygapsinyourknowledgeandhelpyoutolearnmoreeffectively.
1can
See
Topic...
Needs
morework
Almost
there
Confident
tomoveon
Recallthemainsourcesofbackgroundradiation. 23.1
Statetheunitsofcountrate. 23.1
Explainwhatitmeanstosayradioactiveemissionis
random.
23.2
Describethenature,ionisingeffectandpenetrationof
a,pandy.
23.2
Describehowionisingradiationcanbedetected. 23.2
Describehowa,pandyareaffectedbyelectricand
magneticfields.
23.2
J
Statewhysomeisotopesareradioactiveanddecayto
becomedifferentelements.
23.2
Describehowa-andp-emissionchangenuclei,using
nuclearequations.
23.2
Definethehalf-lifeofaradioactivesourceanduseitin
calculations.
23.3
Describetheeffectsofionisingradiationonlivingcells,
andthesafetyprecautionswhichshouldbetakenwhen
usingradioactivematerials.
23.3
A
Describeandexplainpracticalusesofradioactive
materials.
23.4
450)

464. describetheorbitalmotionsoftheEarthandMoonandrelatethesetoourmeasuresoftime
describetheeightplanetsinourSolarSystemintermsoftheirformation,movementandsatellites
calculatethetimelighttakestotravelfromtheSuntotheplanets
explainthemovementofbodiesintheSolarSystemintermsofgravitationalattraction
INTHISCHAPTERYOUWILL
>Chapter24
Earthandthe
SolarSystem

465. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
GETTINGSTARTED

466. 24EarthandtheSolarSystem
HOWMANYPLANETS?
Figure24.2:Thisartist'simpressionshowstheSunand
theeightplanetswenowknoworbittheSun.
Humanshaveknownaboutthefirstfiveplanets,
Mercury,Venus,Earth,MarsandJupiter,foralong
timeaswecanseethemwiththenakedeye.In1610,
thenewlyinventedtelescopeallowedGalileoGalilei
todiscoverSaturn.UranuswasaddedbyWilliam
Herschelin1781.Carefulobservationsoftheorbit
ofUranusshoweditdidnotfollowasmoothorbital
pathastheotherplanetsdid.Astronomerspredicted
thiswasduetotheeffectofthegravitationalpullof
anotherplanet.Theycalculatedwherethisshouldbe
andfoundNeptunein1846.Tiny,distantPlutowas
discoveredin1930bringingthetotaltonineplanets.
Andsoitremaineduntil2006.
Inthe1990s,severalobjectswithsimilarmassto
Plutowerediscovered.Thiscreatedaproblem.
Shouldthesebenamedasplanets,andifnot,could
Plutostillbeclassedasaplanet?TheInternational
AstronomicalUniondecidedanewdefinitionfor
thetitleplanetwasneeded.Itcameupwiththree
rules.Tobeaplanet,abodymust:
orbitthestar(ourSun)
haveenoughmassthatitsgravitypullsitintoa
sphericalshape
havealargeenoughgravitationalpulltoclear
awayanyotherobjectsofasimilarsizenearits
orbitaroundtheSun.
Plutofailedthethirdruleduetothediscoveryofan
objectnamedEris,aboutthesizeofPluto,orbiting
closetoPluto.PlutoandEriswerebothreclassified
asadwarfplanets,leavingatotalofeightplanets.
Discussionquestions
1Whyisitimportanttohaveinternational
agreementaboutthedefinitionofaplanet?
Arethereanyotherareaswherescientistshave
agreedstandarddefinitions?
2AlotofpeoplefeltthatPlutoshouldhave
keptitsstatusasaplanet.Whatconsequences
wouldthishavehad?
24.1Earth,SunandMoon
Dayandnight
I'hcmostobvioussignofmovementintheSolar
Systemisthecyclicaldailychangefromlighttodark.
11isnotsurprisingthatourancestorsthoughtthe
SuntravelledroundtheEarth.Eachdayweseethe
apparentmovementoftheSunfromrisingintheeast
tosettinginthewest.Wenowknowthatthiseffectis
causedbytheEarthspinningonitsaxis(theimaginary
linebetweenthepoles).ThesideoftheEarthfacing
theSunexperiencesdaylightwhilsttheothersideisin
darkness.AtsunriseataparticularspotonEarth,the
Sunisjustvisibleontheeasternhorizon.AstheEarth
turns,thespotmovesintothefullglareoftheSunso
theSunappearsdirectlyoverheadatmidday.Asthe
Earthcontinuestoturn,thespotmovesoutofthedirect
sunlightuntil,atsunset,theSunappearstoslipbelow
thewesternhorizon.

467. )CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
positionofEarth month
December summer
March autumn(fall) spring
June winter summer
September autumn(fall)spring
Figure24.4:TheEarthorbitstheSunevery365.25days.ThetiltoftheEarthcausesseasons.
southern
hemisphere
northern
hemisphere
season
winter
Aswellasthedailychanges,earlycivilisationswere
awareofperiodicchangeswhichhappenedoveralonger
time-thedifferencebetweenseasons.TheEarthorbits
theSun.Ittakesjustover365daystocompleteone
orbit.TheseasonsoccurbecauseofthetiltoftheEarth’s
axis.Figure24.4showshowtheseasonschangeasthe
EarthorbitstheSun.
Consideracountryinthenorthernhemisphere(thehalf
oftheEarthnorthoftheEquator).InFigure24.4a,due
tothetiltoftheEarth,itistippedawayfromtheSunand
theenergyfromtheSun’sraysismorespreadout,making
itcolder.Thismeansthatareareceivesfewerhoursof
sunlight.Thesecountriesareexperiencingwinter.In
Figure24.4c,thenorthernhemisphereistippedtowards
theSun,soitreceiveslongerhoursofmoredirect
sunlight.Thesecountriesareexperiencingsummer.
Years
Figure24.3:Aslighttravelsinstraightlines,onlyhalfthe
Earthreceivessunlightatanyonetime.
Earth'saxis

468. 24EarthandtheSolarSystem
KEYWORDS
axis:theimaginarylinebetweentheEarth'sNorth
andSouthpoles
orbit:thepathofanobjectasitmovesarounda
largerobject
hemisphere:halfofasphere;theEarthcanbe
consideredtobemadeoftwohemispheres
dividedbytheEquator
(the)Equator:animaginarylinedrawnround
theEarthhalfwaybetweentheNorthPoleandthe
SouthPole
CountriesattheEquatordonotexperienceseasons
becausetheSun’sraysalwayshitthematthesameangle.
Theseasonaldifferencesaremoreapparentthefurther
fromtheEquatoryouare.Inthefarnorthorsouth,
seasonsaresoextremethat,inwinter,theSunishardly
seenand,insummer,itcanbesunnyatmidnight.Figure
24.5showshow,inAlaska,theSundipslowerinthesky
towardsmidnightbutthenstartstoriseagain.
Figure24.5:Thismultipleexposurephotographshowsthe
positionoftheSuninthehoursbeforeandaftermidnightin
Alaskainmidsummer.
Months
ThemostobviousobjectinourskyaftertheSunisthe
Moon.TheMoonfeaturesinmanyfolktales.Ithasoften
beenseenasamysticalobjectduetoitsfainterlightand
itschangingshape.Withthebenefitoftelescopesand
spacetravel,weknowtheMoonisarockyspherewhich
weonlyseewhenitreflectslightfromtheSun.TheMoon
orbitsEarthevery27.5days.ItspositionrelativetoEarth
changesthewayitappearstousasdifferentpartsof
itareilluminatedbytheSun.Thiscausesthechanges
calledthephasesoftheMoon.ThephasesoftheMoon
areshowninFigure24.6.
firstquarter
waningcrescent
thirdquarter
Figure24.6:ThephasesoftheMoon.AstheMoonorbitsthe
Earth,thehalfoftheMoonthatfacestheSunwillbelitupby
theSun.AstheMoonmoves,theshapeofthelightpart,which
canbeseenfromtheEarth,changes.TheoutercircleofMoon
diagramsshowshowtheMoonlookstoanobserveronEarth.
KEYWORDS
phasesoftheMoon:thedifferentwaystheMoon
lookswhenviewedfromEarthoveraperiodof
onemonth
ACTIVITY24.1
Modellingday,nightandseasons
UsealamptorepresenttheSunandaballwith
arodthroughittorepresenttheEarth.Mark
yourpositionontheEarthusingapenorapiece
ofmodellingclay.Inadarkenedarea,holdthe
'Earth'neartothe'Sun',andturntheEarthonits
axistomodeldayandnight.
TilttheEarthonitsaxisandinvestigatethe
seasonsbymovingtheEartharoundtheSun.
Investigatethedifferenceinseasonsbetweenthe
southernandnorthernhemispheres.
Figure24.7:ModeloftheEarthandSun.
455>

469. yCAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
Questions
1Copyandcompletethefollowingsentences.
Adayisthetimetakenforthe .
Amonthisthetimetakenforthe .
Ayearisthetimetakenforthe .
TheEarthistiltedonitsaxisandthiscauses
whichdonotoccurattheEquator.
2Explainwhyitissummerinthenorthern
hemispherewhenitiswinterinthesouthern
hemisphere.Includeadrawinginyouranswer.
3LookatthephotographoftheMoon(Figure24.8).
Figure24.8:TheMoon.
aWhichphaseoftheMoonisthis?
bHowmanydaysaretherefromonefullMoon
tothenext?
24.2TheSolarSystem
Figure24.9:Anartist'simpressionoftheSolarSystemwitha
visitingcomet.Thepictureisnottoscale.
TheSolarSystemconsistsoftheSunwhichisour
star,andalltheobjectswhichorbitit.Itincludesthe
following:
Thereareeightplanets:Mercury,Venus,Earth,
Mars,Jupiter,Saturn,UranusandNeptune.
Thereareminorplanets,suchasPlutoandEris.
In2014,theInternationalAstronomicalUnion
recognisedfivedwarfplanetsbutitisbelievedthere
aremorethan200inall.
Moonsthatorbitplanetsanddwarfplanets.
Millionsofasteroidsandmeteoroids:theseare
rockyobjectswhicharesmallerthanplanets.Most
asteroidsarefoundintheasteroidbeltbetweenthe
orbitsofMarsandJupiter.
Comets,whichareoftendescribedasgiantsnowballs,
orbittheSuninveryirregularorbits.Whentheyare
furthestfromtheSun,theyarefrozenballsofgas,
rockanddust.AstheygetnearertotheSuntheyheat
upandleaveatrailofdustandgasesbehindthem.
(Note:thistrailofdustisnotthetailofthecomet;
thetailalwayspointsawayfromtheSufi,socould
actuallybeat90°tothemotionofthecomet.)
Figure24.10a:Asteroidsandmeteoroidssometimes
entertheEarth'satmosphere.Smallermeteoroidsburnup
intheEarth'satmosphereandareseenasshootingstars,
b:Itisbelievedthatthedinosaursbecameextinctduetoa
largeasteroidhittingtheEarthcreatingahugecraterand
throwingupsomuchdustthattheSun'srayscouldnotreach
Earthformorethanayear,c:CometHale-Boppwasvisible
tothenakedeyeinthesummerof1995.Itisnotexpectedto
bevisibleagainsoonasittakes2533yearstoorbittheSun!
456
>

470. 24EarthandtheSolarSystem
TheSun'sgravitationalpull
Theorbitsoftheplanetsarealmostcircular.Tomoveina
circleanobjectneedsaforcepullingittowardsthecentre
ofthecircle.Imaginespinningaballontheendofapiece
ofstring.Theballwillspininacircleaslongasyouhold
on.Onceyouletgo,theballwillflyoutwards.Theforce
neededtokeeptheplanetsorbitingtheSuncomesfrom
thegravitationalattractionoftheSun.
Theformationoftheplanets
Evidencecollectedbyastronomerssuggeststhatthe
planetswereformedatthesametimeastheSun.TheSolar
Systembeganasanebula,whichisahugeswirlingball
ofdustandgas.Mostofthisgaswashydrogen,butthere
werealsootherelementsformedbyfusioninotherstars,
whichhadexplodedattheendoftheirlifecycle,sending
theircontentsoutintothecloudsofinterstellargas.
Asgravitypulledthismasstogether,thecentreformed
astar.YouwilllearnmoredetailaboutthisinChapter
25.Theplanetsformedfromthematerialsofthenebula
whichwerenotpulledintotheSun.Thespinningmotion
ofthedustandgasformedaflat,spinningringdisc
knownasanaccretiondisc.Gravitypulleddustandgas
togethersotheyjoinedtomakerockswhichthenjointo
makelargerrocks.Theprocessofthedustandgasbeing
pulledtogetherbygravityiscalledaccretionanditled
totheformationoftheinner,rockyplanets.Theintense
heatforcedsomeofthelightermaterialsfurtheraway
andtheseformedtheouterplanets-thegasgiants.
Thefourinnerplanets,Mercury,Venus,EarthandMars,
aresmallandrocky.AfterMarsthereistheasteroidbelt.
Thisismadeupofleft-overpiecesofrock.Theouter
fourplanets,Jupiter,Saturn,UranusandNeptune,are
hugeballsofgases.Theseplanetsaremuchbiggerthan
theinnerplanets.
Figure24.11:Thisartist'simpressionshowsastarforming.
Theuneven,swirlingmassofrockandgasarounditis
flattenedbyitsrapidrotationintoanaccretiondiscwhere
theplanetseventuallyform.
KEYWORDS
planet:alargesphericalobjectthatorbitsthe
Sunwithoutanothersimilarobjectclosetoit
minorplanet:anobjectwhichorbitstheSunbut
isnotlargeenoughorfarenoughfromanother
objecttobedefinedasaplanet
asteroidsandmeteoroids:lumpsofrockwhich
orbittheSun
comet:aballofice,dustandgaswhichorbitsthe
Suninahighlyellipticalorbit
accretiondisc:arotatingdiscofmatterformed
byaccretion
accretion:thecomingtogetherofmatterunder
theinfluenceofgravitytoformlargerbodies
Distancesandtimesinthe
SolarSystem
DistancesintheSolarSystemarealmostunimaginably
big.TheEarthisapproximately150million
kilometresfromtheSun.Thisissimilartocircling
theEarth4000times.Distancesareoftenexpressedin
termsofhowlongittakeslighttotravel;onelight-year
isthedistancetravelledbylightinayear.Thenext
neareststaraftertheSunisProximaCentauri,whichis
4.2light-yearsfromEarth.Youwilllearnmoreabout
light-yearsinChapter25.
WORKEDEXAMPLE24.1
CalculatethetimeforlightfromtheSuntotravel
the150000000kmtoEarth.Giveyouranswer
inminutes.
Step1:Writedownwhatyouknow:
speedoflight=300000000m/s
distancetravelled=150000000km
Step2:Convertdistancetometres,sounitsare
consistent.
150000000km=150000000000m
457y

471. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
Step3:Writetheequationdownandcalculatethe
timetaken:
Q-distancetravelled
timetaken
speed
_
150000000000m
300000000m/s
=500seconds
Step4:Converttominutes
500-9-60=8.3minutes
Answer
8.3minutes
Questions
4TheMoonisapproximately390000kmfromEarth.
Calculatethetimeittakesforlighttotravelfromthe
MoontotheEarth.
5HowlongwillittakeforlightfromtheSunto
reach:
aMercury,whichisapproximately60000 000km
fromtheSun.
bNeptune,whichisapproximately
4500000000kmfromtheSun.
6Ittakessunlight43minutestbreachJupiter.
CalculatethedistancefromJupitertotheSun.
7Calculatehowmanykilometresalight-yearis
equivalentto.
Moreabouttheplanets
Table24.1givesdataabouttheplanetsintheSolar
System.Itshowshowtheplanetsdifferfromeachother,
forexamplelookingupfromthesurfaceofJupiteryou
mightsee16moons.
Forces
TheSunisatthecentreoftheSolarSystem.Itisby
farthemostmassiveobjectintheSolarSystemand
makesupabout99.8%ofthemassoftheSolarSystem.
Asgravitationalattractiondependsonmass,the
gravitationalfieldstrengthoftheSunisfarlargerthan
thefieldofanyotherobjectintheSolarSystem.
Theplanets,minorplanets,asteroidsandmeteoroidsand
cometsallorbittheSun.Theyareheldinorbitbythe
gravitationalattractionoftheSun.
Likeothernon-contactforcessuchasmagnetismand
staticelectricity,gravitationalattractiondecreaseswith
distance.Thismeansthattheouterplanetsexperience
lessgravitationalforcefromtheSunthantheinner
planetsdo.
Table24.1
Planet
Average
orbital
distance/
millionkm
Orbital
duration/
years
Density/kg/
m’
Surface
temperature
/°C
Gravitational
fieldstrengthat
thesurfaceof
theplanet/
N/kg
Numberof
Moons
Mercury 58 0.2 5500 -18to460 4 0
Venus 108 0.6 5200 470 9 0
Earth 150 1 5500 -8to58 10 1
Mars 228 1.9 4000 -8to-5 4 2
Jupiter 778 12 1300 15to20 26 16
Saturn 1427 30 700 -140 11 20
Uranus 2870 84 1300 -200 11 15
Neptune 4497 165 1700 -220 12 8
458y

472. 24EarthandtheSolarSystem
AlthoughtheplanetsaresmallcomparedtotheSun,
theyareverymassiveobjects.Jupiterhasamassof
1.9x1027kg.Themoremassivetheplanet,thegreater
thegravitationalforceexperiencedbyobjectsatits
surface.OnEarthweexperienceaforceoflON/kg.
OnEartha60kgstudenthasaweightof600N.
OnMercury,wheregravityis4N/kg,thesamestudent
wouldweigh240N.Thegravitationalpullofplanetsis
enoughtocausemoonstoorbitthem.
Orbitsandenergy
Theorbitsoftheplanetsarenotcompletelycircular.
Theirshapeisthatofaslightlysquashedcircle,calledan
ellipse.Theorbitsaredescribedaselliptical.Theamount
theorbitissquashediscalleditseccentricity.Comets
haveveryeccentricorbits.CometstravelfarfromtheSun
andthenreturnclosetoit.
Figure24.12:TheorbitofHalley'scometismuchmore
eccentricthanthoseoftheplanetsandminorplanets.
Whyareorbitselliptical?Toexplainthis,weneedto
thinkabouttheearlyswirlingmassoftheSolarSystem.
ImagineanobjectmovingpasttheSunathighspeed,
carriedalongbyitsownmomentumfromtheexplosive
startoftheuniverse.AsitpassesneartheSunthe
gravitationalforceoftheSunstartstoactontheobject
andtopullittowardstheSun.Thisforcealsocausesitto
accelerate.Thismeansthemassspeedsupanditskinetic
energycarriesitslightlyfurtherouttothefurthestpoint
oftheorbit.Theobjectslowsdownandispulledin
againtowardstheSun.
TheSunisnotquiteatthecentreofaplanet’selliptical
orbit.Thereisapointclosetothecentreofanellipse
calledthefocus.TheSunisatthefocusoftheelliptical
pathofeachoftheplanets.Theplanetmovescloser
to,andfurtheraway,fromtheSunduringeachorbit.
TheSun’sgravitypullstheobjectin,speedsitupand
thenthespeedcarriesitontothefurthestpartofthe
orbit.
Theobject’sorbitalspeedisthereforegreatestwhenitis
nearesttotheSunandslowestwhenitisfurthestfrom
theSun.
Comets,whichhavethemostellipticalorbitsofany
bodyintheSolarSystemaccelerategreatlyasthey
approachtheSunandareslungbackathighspeedtothe
farreachesoftheirorbits.
Aplanetorbitinginspacedoesnotexperienceany
frictionorairresistance,soitsenergyremainsthesame
throughoutitsorbit.Ithastwotypesofenergy:
kineticenergy
gravitationalpotentialenergy.
WhenitisnearesttheSun,aplanethasitsminimum
gravitationalpotentialenergyandismovingatitsfastest
sohasitsmaximumkineticenergy.Whenitisatits
furthestfromtheSun,ithasmaximumgravitational
potentialandminimumkineticenergy.
Figure24.13:Mercuryhasthemostellipticalorbitofany
oftheplanetsintheSolarSystem.AtpointA,itis46million
kmfromtheSun,travellingatitsfastestspeedbutwithleast
potentialenergy.AtpointB,itis70millionkmfromthe
Sun,travellingatitsslowestspeedbutwithmostpotential
energy.
Speeds
Thespeedofaplanetinorbitroundastariscalled
itsorbitalspeed(v).Astheplanets’orbitsarealmost
circular,thedistancetheytravelcanbecalculatedifwe
knowtheaverageorbitalradius,whichistheaverage
distanceoftheplanetfromtheSun,ortheaverage
radiusoftheorbit.

473. )CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
Figure24.14:Tocalculatetheorbitalspeed,weassume
thattheorbitsarecircular.
Thedistancetravelledbytheplanetisthecircumference
ofitsorbit.Thecircumferenceofacircleisequalto2w.
IfwealsoknowthetimefortheplanettoorbittheSun
-knownasitsorbitalperiod(T)-wecancalculatethe
speed:
speed=
distance
time
So,theaverageorbitalspeedv,canbecalculatedfrom
itsorbitalperiod,T,anditsaverageorbitalradiusr,
usingtheequation:
KEYEQUATION
, ,2xnxorbitalradius
averageorbitalspeed=—
orbitalperiod
_
2nr
v=
KEYWORDS
ellipse:asquashedcircle
eccentricity:ameasureofhowellipticalanorbitis
orbitalradius:theaveragedistanceoftheplanet
fromtheSun
orbitalperiod:thetimetakenforaplanetto
completeonefullorbitoftheSun
WORKEDEXAMPLE24.2
CalculatetheorbitalspeedofEarth.
Step1:Writedownwhatyouknow:
r=150000 000km
T=1year
Step2:ConvertTtoseconds.
1year=1x365=365days
365days=365x24=8760hours
8760hours=8760x60x60=31536000
seconds
Step3:SubstitutevaluesforTandrintotheequation
andcalculatev.
_
2w
v=
T
=
2^x150000000km
31536000s
=30km/s
Answer
30km/s
Planetarypatterns
Muchofwhatastronomershavediscoveredhasbeen
throughobservingtheskies,gatheringhugeamountsof
dataandthenlookingforpatternsinthedata.Ancient
astronomersknewtheplanetsweredifferentfrom
thestarsbecauseofthewaytheirpositionsinthesky
changed.MercurywasnamedbyancientGreeksafter
themessengerofthegods,whichisafittingnamefor
theplanetwhichorbitstheSunfasterthananyother.
Sometimeswecanlearnasmuchfromobservationsthat
areexceptionstoapatternasfromthosethatfitour
predictions.
ThedatainTable24.1canbeusedtoinvestigatepattern*
inthepropertiesandbehavioursofplanets.Plotting
dataonascattergraphcangiveaclearindicationof
whetherthereisacorrelationbetweentwosetsofdata.
Forexample,agraphofdensityagainstdistancefrom
theSun(Figure24.15)showsthatthereisnotaclear
correlationbetweenthetwo.However,itisclearthatthe
fourinnerrockyplanetsaremoredensethantheouter
gasgiants.
460>

474. 24EarthandtheSolarSystem
Figure24.15:Thereisapatterninthisdatabutnotadirect
correlation.
Questions
8aNametheforcewhichcausesplanetstoorbitthe
Sun.
bWhatshapeareplanetaryorbits?
cHowistheorbitofacometdifferenttotheorbit
ofaplanet?
dDescribetheenergychangesinacometasit
orbitstheSun.
9Calculatetheweightofa30kgsheepon:
aEarth
bMars
cJupiter.
10UseinformationfromTable24.1tocalculatethe
orbitalspeedsinm/sof:
aVenus
bSaturn.
11UsingTable24.1,drawandcommentonscatter
graphstoinvestigatetherelationshipbetween:
aorbitaldistanceandaveragetemperature
bgravitationalfieldstrengthandthenumberof
moons.
PROJECT
SolarSystemquiz
Somegreatwaysoflearningare:
findinginformationfromavarietyofsources
summarisingtheinformation
writingquestionsandanswersonthe
informationyouhavegathered
answeringquestionswrittenbyyourpeers.
Thistaskasksyoutobringallthesetogethertohelp
youbecomeanexpertonourSolarSystem.
MakeupaquizabouttheSolarSystem.Thequiz
canbeonpaper,thecomputer,oronamobile
devicesuchasphoneortablet(therearelotsof
goodquizmakingappsavailable).Itshouldbe
aimedatstudentswhohavestudiedthischapter,
andwhohaveagoodgeneralknowledge.Spend
sometimerevisingandresearchingtofind
interestingfactstoinclude.Youmaywanttorate
questionsas'easy','medium'or'hard'andgive
morepointsforharderquestions.Youcaninclude
mathematicalquestionsandquestionswhich
requiredatainterpretation.Youshouldincludeat
least20questions.Thinkabouthowyouwillgroup
yourquestions.Youcouldinclude:
Apictureround:usepicturesfromtheInternet
ordrawyourown.
Definitions:youcouldgiveadefinition,such
as,'thisisthetimeittakesfortheEarthtoorbit
theSun'or,'thishasthemostellipticalorbitof
anyobjectintheSolarSystem',andaskwhatis
beingdefined.
461y

475. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
Factsabouttheplanets:thiscouldinvolve
somequestionsforwhichtheanswerscanbe
workedoutfromadatachartyousupply.
Historyofastronomicaldiscoveries:usethe
Internetandthisbooktohelpyou.
Whenyouhavewrittenyourquestions,testth»m।
onanothergroup.Areyourquestionsclearer>'mt|l>
Iftherearetwopossibleanswersyouneedtoid'i
thequestiontomakeitmoreclear.
PEERASSESSMENT
Whenyouexchangequizzeswithanothergroup,givethemfeedbackontheirquestions.Ratequestion!,
'green','amber'or'red':
green:greatquestion
amber:goodideabutneedstobeclearer
red:donotusethisquestionasitismisleadingorcontainswronginformation.
Forquestionsratedamberorred,youshouldalsogivewrittenfeedback.
Afterfeedbackandimprovementworkfollowingthefeedback,tryyourquizoutonsomeotherstudent).
REFLECTION
Thinkaboutwhatyoufoundmostusefulinthisproject.Wasitresearchingandsummarising?Maybeyou
enjoyedwritingthequestions,orthechallengeofansweringquestionssetbyothers.Whatdoesnistellyou
abouthowyouliketolearn?Howwillyouapplythisinyourfuturerevision?
SUMMARY
TheEarthspinsonitsaxisevery24hourscausingdayandnight.
TheEarthistiltedonitsaxis.ThiscausestheseasonsastheEarthorbitstheSunevery365days.
TheMoonorbitstheEarthevery27.5days,causingthephasesoftheMoon.
TheSunisorbitedbyfourrockyinnerplanets,fourgaseousouterplanetsandminorplanets,moonsandconic1
AllobjectsorbitingtheSunarekeptinorbitbyitsgravitationalattraction.
LightfromtheSuntakesapproximatelyeightminutestoreachtheEarth.Thedistancesforsunlighttoreach
otherplanetscanbecalculatedusingtheequationspeed=distance/time.
Thespeedofanobjectinorbitcanbecalculatedusingtheequationv= whereristheradiusoftheorbit
andTistheorbitalduration.
Theorbitsoftheplanetsareslightlyelliptical.TheSunisnotatthecentreoftheellipse.Cometshavehighly
ellipticalorbits.
TheSuncontainsalmostallofthemassoftheSolarSystemandsohasaverystronggravitationalfield.
AsdistancefromtheSunincreases,itsgravitationalfieldstrengthdecreasesandtheorbitalspeedofanyorbitnir1
objectdecreases.
462y

476. 24EarthandtheSolarSystem
Whichofthefollowingobjectsisaplanet?
AtheMoon BHale-Bopp CPluto
WhichstatementabouttheorbitsoftheEarthandMooniscorrect? [1]
Thediagramshowshowpeople1000yearsagothoughttheSolarSystemlooked.
[1]
b
[1]
[1]
d
[1]
ONTINUED
iXAM-STYLEQUESTIONS
COMMAND WORDS
A
B
C
D
calculate:workout
fromgivenfacts,
figuresorinformation
TheMoonrotatesonitsaxisin24hoursandorbitstheEarthin27.5days.
TheEarthrotatesonitsaxisin24hoursandorbitstheSunin365days.
TheMoonorbitstheSunin27.5days.
TheEarthrotatesonitsaxisin24hoursandorbitstheMoonin27.5days.
Stateonewayinwhichthismodelisdifferentfromwhatwenowknow
abouttheSolarSystem.
Stateonewayinwhichthismodelissimilartowhatwenowknowabout
theSolarSystem.
StateonewayinwhichtheplanetsMercury,Venus,EarthandMarsare
similar.
StateonewayinwhichJupiterandSaturnaredifferenttotheplanetsin
partc.
Marsis228millionkmfromtheSun.Calculatethetimeittakesforlight
totravelfromtheSuntoMars.Thespeedoflightis3x108m/s.
state:expressinclear
terms
WhatforcekeepstheplanetsinorbitroundtheSun?
Amomentum BairresistanceCtension
[3]
[Total:7]
[1]
DUranus
WhenanorbitingobjectisatitsclosesttotheSun,ithasitsmaximumkineticenergyandminimum
gravitationalpotentialenergy.
Planetarydataaboutorbitaldistance,orbitalduration,density,surfacetemperatureandgravitationalfield
strengthcanbeanalysedtoshowpatternsinthepropertiesandbehaviouroftheplanets.
[1]
Dgravity

477. 5LaurieisstandingatpointXontheEarth’ssurface.
N
Moon
Earth X
S
COMMAND W(H
'
6ThetableshowssomedataabouttheobjectsorbitingtheSun.
Usetheinformationinthetabletoanswerthefollowingquestions.
c
[2]
d
[2]
e
lightfrom
theSun
[1]
[11
m
m
[i]
tn
[2]
describe:statethe
pointsofatopic;
1
|ii
characteristicsand
mainfeatures
explain:setout
purposesor
reasons;make
therelationships
betweenthjigs
evident;provide
whyand/orhow.in<I
supportwthrelevant
evidence
a
b
a
b
d
e
f
HowcanyoutellitisnighttimeatpointX?
RedrawthediagramtoshowwherepointXwillbeafter12hours.
TheMoondoesnotemitlight.ExplainhowLaurieisabletoseethe
Moon.
NametheforcewhichkeepstheMooninorbitaroundtheEarth.
DescribethemovementoftheMoon.
AballdroppedonEarthwillfallfasterthananidenticalballdropped
ontheMoon.WhatdoesthistellyouabouttheMoon’sgravity?
Nametheobjectthatisnotaplanet.
WhichplanettakestheleasttimetogoroundtheSun?
Astudentwrites,‘thefurtherawayfromtheSunaplanetis,thelower
itsdensity’.Towhatextentdoyouagreewiththisstatement?
WhatdatafromthetablesuggeststhatPlutoandMarsarethetwoleast
massiveobjects?
CalculatetheaverageorbitalspeedofJupiter.Giveyouranswerinkm/s
totwosignificantfigures. [3]
[Total:9]
[1]
[Total:7]
Object
Distance
fromSun/
millionkm
Averagesurface
temperature/
°C
Density
/kg/m3
Surface
gravity
/N/kg
Timeof
orbit/
years
Venus 108 470 5200 9 0.6
|Earth 150 15 5500 10 1.0
Mars 228 -30 4000 5 1.9
|Jupiter 778 -150 1300 26 12
Saturn 1427 -180 700 11 30
|Pluto 5900 -230 500 4 248

478. 24EarthandtheSolarSystem
SELF-EVALUATIONCHECKLIST
Afterstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutosee
mygapsinyourknowledgeandhelpyoutolearnmoreeffectively.
1can
See
Topic...
Needs
morework
Almost
there
Confident
tomoveon
ExplainhowtheEarth’srotationonitsaxiscauses
dayandnightandtheapparentchangeinpositionof
theSun.
24.1
1ExplainhowtheEarth’stiltedaxisanditsrotation
aroundtheSuncauseseasons.
24.1
1ExplainhowtheMoonorbitingtheEarthleadsto
|thedifferentphasesoftheMoon.
24.1
StatehowlongittakesfortheEarthtorotate,forthe
EarthtoorbittheSimandfortheMoontoorbitthe
Earth.
24.1
NamethedifferentobjectswhichorbittheSun. 24.2
1Explainthedifferencebetweentheinnerandouter
iplanets.
24.2
|Calculatethetimeittakesforlighttotravelfromthe
Suntoagivenplanet.
24.2
Nametheforcewhichkeepstheplanetsinorbit. 24.2
Describetheshapeofplanetaryorbitsandstatethe
positionoftheSunwithinthisshape.
24.2
Calculateorbitalspeedusingtheequationv=
24.2
Describehowgravitationalpotentialenergyandkinetic
energyvaryasanobjectmovesinanellipticalorbit.
24.2
ExplainwhytheSunhasthelargestgravitational
fieldofanyobjectintheSolarSystem.
24.2
Interpretplanetarydataanddescribepatternsinthis
data.
24.2
465y

479. learnthattheredshiftoflightfromdistantgalaxiessupportstheBigBangtheory
describetheSunandgalaxies,includingtheMilkyWay
learnabouttherelativeseparationofplanets,starsandgalaxies
INTHISCHAPTERYOUWILL:
learnthatthisredshiftcanbedescribedbyHubble'slaw,whichcanbeusedtoworkouttheageof
theUniverse.
describehowstablestars(suchastheSun)arepoweredbythethermonuclearfusionofhydrogen

480. 25StarsandtheUniverse
GETTINGSTARTED
Spendtwominutesthinkingaboutthesequestions
beforecomparingnoteswithyourneighbourfora
furthertwominutes,addingtoorcorrectingyour
ownwork.Bepreparedtoshareyourthoughtswith
theclass.
Listthedifferencesbetweenplanetsandstars.
WheredoestheSungetitsenergy?
Whatcolourarestars?
Whatisagalaxyandwhatisthenameof
ourgalaxy?
ListwhatyouknowabouttheUniverse.
WHATMAKESTHESUNSHINE?
WeknowmanythingsabouttheSunbutalotofthat
knowledgehasbeengainedveryrecently.Working
outwhatmakestheSunshinewasaprocessof
eliminatingdifferenthypotheses(ideas)untilonewas
foundthatbestfitstheevidence.
TheGreekphilosopherAristotlebelievedtheSun
wasmadeofether,aperfectsubstancethatglows
forever.However,in1613,GalileoGalileiobserved
sunspotsontheSunandthese'imperfections'
showedthattheSuncouldnotbemadeofether.
Coalwasburnedinsteamenginestopowerthe
UK'sIndustrialRevolution.Thismadescientists
wonderwhethertheSunwasagiantlumpofcoal
butcalculationsshowedthataSunmadeofcoal
wouldshineforlessthan1500yearsandthisisa
shortertimethanrecordedhistory.However,efforts
tounderstandsteampowerledtotheprincipleof
conservationofenergy.Thisledscientiststolookfor
othersourcesofenergy(thatcouldbetransferred
bylight).
ScientistslikeHermannvonHelmholtzbelieved
thekineticenergyofmeteorites(lumpsofrock)
collidingwiththeSuncouldbethissourceof
energy.However,thetotalmassofmeteoriteswas
toosmallandtheywerenotmovingfastenoughto
providetherequiredenergy.
OtherscientistsimaginedthattheSunwasonce
muchbiggersothatitonlyjustfittedinsidethe
Earth'sorbit.Butthegravitationalenergyreleased
whenitcollapsedtoitspresentsizecouldonly
haveprovidedenoughenergyfor100millionyears,
whichwasnotenoughtimefortheevolutionof
differentspeciesonEarthtohavetakenplace.
Thenradioactivitywasdiscovered,andEinstein
showedthatmasscanbetransformedintoenergy.
ThisledscientiststoworkoutthattheSunis
poweredbythermonuclearfusion,thoughafully
formedtheorydidnotappearuntil1939.
Figure25.1:TheSunshining.
Discussionquestions
1Listatleastthreethingsthatmostpeopleused
tobelieveaboutwhatmakestheSunshine.For
eachone,writedownhowscientistsshowed
thatthebeliefwasincorrect.
467y

481. >
CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
25.1TheSun
TheSunisanaverageormediummassstarandismade
upofabout75%hydrogenandabout24%helium.The
rest(about1%)ismadeupofotherelements,suchas
oxygenandcarbon.
TheglowinghydrogenatthesurfaceoftheSunradiates
energy.About40%ofthisenergyisvisiblelight,about
50%isinfraredradiation,andtheremaining10%is
ultraviolet.Earth’satmosphereabsorbsalotofthis
ultravioletradiation.Theozonelayer,inparticular,
absorbsmostoftheharmful(moreionising)ultraviolet.
Starsarepoweredbynuclearreactionsthatreleaseenergy.
StablestarslikeourSunarepoweredbythenuclearfusion
(orthermonuclearfusion)ofhydrogenintohelium.This
makestheSunshine.ItissohotinsidetheSunthatmatter
existsasplasma(positiveionsandelectrons).Althoughthe
Sunproducesgammaraysbecauseofthenuclearfusion
process,collisionswiththeplasmameanthatittakes
about100000yearsforthatenergytoreachtheSun’s
surface.Inaddition,becausetheenergyisspreadovera
bigsurface(calledthephotosphere)thetemperatureis
lowerattheSun’ssurface(about5800Kcomparedtoa
coretemperatureofabout15000000K).
TheEarthorbitstheSunatadistanceofabout150
millionkilometres,whichiswithinthehabitablezone.
Thisisthezonewherewatercanexistinliquidform(an
essentialrequirementforlifeasweknowit).Ifitwas
hotter,thewatervapourwouldnevercondense;ifitwas
colder,icewouldnevermelt.
TheSunhasamassof2x1030kg.Thisisreferredtoas
thesolarmassasitprovidesasimplewayofcomparing
themassofotherstarstothemassofourSun.For
example,astarwitheightsolarmasseswouldhave
eighttimesthemassoftheSun.TheSuncontainsover
99.86%ofthemassoftheSolarSystemsoitexertsa
biggravitationalforceontheplanetsandcausesthemto
follownearlycircularorbits.
1
KEYWORDS
stablestar:astarthatisnotcollapsing<>i
becausetheinwardforceofgravityisbtil.intmiIIn
radiationpressure,whichpushesoutward-.
plasma:acompletelyionisedgasinwhuhlift
temperatureistoohighforneutralatom1I
soitconsistsofelectronsandpositively<limu«
atomicnuclei
solarmass:equaltothemassoftheSun
(2x1030kg)
Questions
1FromwhichtwoelementsistheSunnunlh
made?
2Giveanapproximatevaluefor:
athetemperatureintheSun’score
bthetemperatureoftheSun’ssi.face
cthesolarmass
dthepercentageofsunlightthatisinIhi
infrared,visibleandultraviol:tpart-ulth
electromagneticspectrum.
3Nametheprocessthatmakesstablestars,muIi
asourSun,shine.
4ImaginethatEarthorbitedastarthatgive-ill
mostofitsenergyintheultravioletregionulih-
spectrum.Discusswhetheroureyeswouldnilhci
evolvedtoseevisiblelight.
ACTIVITY25.1
WhatcolouristheSun?
Spendtwominuteswritingdownyourthoughtsand
answerstothequestions.Thenspendoneminute
discussingthemwithapartner.
Yourteachermaygiveyouadditionaltimeto
researchthesequestionsusingtheInternet,orask
supplementaryquestionstohelpyoureachth-
correctanswer.
1WhydomostpeoplethinktheSunisyellow?
2IsthisthecorrectcolouroftheSun?
Howdoweknow?
468

482. REFLECTION
25StarsandtheUniverse
Didyoualreadyknowthecorrectanswerto
Activity25.1?
Itisimportantinsciencetoavoidlookingfor
evidencethatsupportsanideathatyoualready
thinkiscorrect.Scientistsmustalsoavoidnot
lookingforevidenceatallandassumingthatthey
knowtheanswer.IfyouthoughttheSunis
yellow,didyouquestionthisidea?Ifyouguessed
thattheSunisnotyellow,didyouknowwhat
questionstoasktoworkoutitscorrectcolour?
Wereyouabletothinkobjectivelyandfind
evidencetosupportthecorrectanswer?Thisis
howscienceprogressesanditistheapproach
outlinedintheScienceincontextsectionWhat
makestheSunshine?thatledtocorrectly
understandinghowstarsshine.
25.2Starsandgalaxies
Whenyoulookintothenightsky,thelightthatyou
."cfromthestarshasbeentravellingformanyyears.
ANtionomersusethisideaasawayofmeasuringvast
distances.Alight-yearisameasureofdistance(not
Iline).Itisthedistancethatlighttravelsthroughspacein
lineyear.Lighttravelsataconstantspeedof3x108m/s
lliroughavacuum.Thismeansthatthetimeittakesto
Iravelsomewhereisdirectlyproportionaltodistance.
<)nclight-yearisthedistancethatlighttravelsinoneyear,
illatance=speedxtime
So,onelight-year=3x108m/sx365.25daysx24hours
3600seconds=9.5x10l5m.
KEYWORD
light-year:thedistancetravelledinspacebylight
Inoneyear(itisequivalenttoabout9.5x1015m)
Ihedistancebetweenstarsismuchbiggerthanthesize
ofeachsolarsystem.AftertheSun,ournextnearest
MarisProximaCentauri,whichisabout4.2light-years
ijway.WhenyouseeProximaCentaurithelightleftit
I2yearsago;sunlightonlytakeseightminutestoreach
lisbecausetheSunismuchclosertous.Plutohasan
oilipticalorbitbut,onaverage,itis40timesfurtherfrom
IlieSunthantheEarthis.Butthisisdwarfedbythe
distancebetweenProximaCentauriandtheSun,whchis
7000timesfurtherfromtheSunthanPlutois.
Questions
5TheSunisabouteightlight-minutesaway.Ittakes
sunlightabouteightminutestoreachEarthonits
journeyfromtheSun.
aGiventhatthespeedoflightis3x108m/s,how
farawayistheSuninkilometres?
bHowmanyyearswouldittakeacartogetto
theSuntravellingat120km/h?
6AfterourSun,ProximaCentauriisournextnearest
star.Itisabout4.2light-yearsaway.
aHowmanysecondsdoesittakelightfrom
ProximaCentauritoreachEarth?
bHowfarawayisProximaCentauriinkm?
cHeliosI&IIholdtherecordasthefastestever
spaceprobesat252738km/h(about70km/s).
Howmanyyearswouldittakethesespace
probestoreachProximaCentauri?
dHowlongwouldittakethemtoreachthe
nearestgalaxy25000light-yearsawayfromus?
Theforceofgravitypullsstarstogetheringroupscalled
galaxies.OurSunisoneofmanybillionsofstarsinour
galaxy,theMilkyWay.Theremightbe200billion(2x10’')
starsintheMilkyWay,about20starsforeveryperson
onEarth.TheMilkyWayisaspiralgalaxywithacentral
bulge(seeFigure25.2).Ithasadiameterof100000light-
yearsandthediscisabout2000light-yearsthick.OurSolar
Systemislocatedabout30000light-yearsfromthegalactic
centre,two-thirdsofthewayalongaspiralarm.TheMilky
WayisspinningandittakesourSolarSystemabout225
millionyearstotraveloncearoundthegalaxy.
TheMilkyWayisoneofmanybillionsofgalaxies,
thatmakeuptheUniverse.Mostpeopleconsiderthe
AndromedaGalaxy(Figure25.3)tobeourclosestgalactic
neighbouranditiscertainlyourclosestspiralgalaxy.
However,ournearestgalacticneighbouristheCanisMajor
469

483. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
DwarfGalaxy,whichis25000light-yearsawayfromus
and42000light-yearsfromthecentreoftheMilkyWay.
Figure25.3:AninfraredimageoftheAndromedaGalaxy,
ourclosestspiralgalaxy.
Questions
7aMaketwosketchestoshowtheMilkyWay
Galaxy;onesketchshouldshowitsspiral
structureandtheothershouldshowthegalaxy
edgeon.
bOnyoursketchesmarkthediameterofthe
MilkyWayinlight-years.
cMarkthepositionoftheSunintheMilkyWay.
dHowmanystarsarethereintheMilkyWay?
8TheSolarSystemhasexistedfor4.6billionyears.
HowmanytimeshastheSolarSystemtravelled
aroundtheMilkyWayinthattime?
9HowcantheCanisMajorDwarfGalaxybecloserto
usthanwearetothecentreofourowngalaxy?
10Assumingthattheaveragemassofastarisequalto
themassoftheSun(2x1030kg),whatisthemass
oftheMilkyWay?
11ImaginethattheMilkyWayisshrunkdownto
fitintothespacebetweentheEarthandtheSun.
Onthisscale,calculatehowfarawaythefollowing
bodieswouldbefromEarth:
aProximaCentauri(inkm)
bPluto(inkm)
ctheSun(inmetres)
dtheMoon(incm).
12Writeasentenceortwocomparingyouranswers
toquestion25.11withthelengthofapencil,
thelengthofacricketpitch(about20metres),a
400metreathleticstrack,andtheradiusofthe
Earth(6400km).
ACTIVITY25.2
Howdoastronomersmeasuredistancesto
farawayobjects?
Astronomershavemanytechniquestomeasur<'
distancesinspace.Fornearbystarswithinour
owngalaxy,theycanuseparallax.Thisiswhen
thestarappearstomoveacrosstheskywhen
viewedfromoppositesidesofourorbitaround
theSun,asshowninFigure25.4.
Youcanexperiencethisyourself.Stretchout
anarminfrontofyouandstickupyourthumb
Closeoneeyeandopentheotherandthen
swapoverwhichisclosedandopen.Yourthumb
shouldappeartomovefromsidetosideagaim-t
thebackground(whichshouldbeatleasttwo.urn
lengthsaway).
backgroundstars
^y^&
\/*
cVstar /
/i\,paralaxangle/
/\/
Earthinsummerjr"
Q
Earthinwlnb
'
A"- —
""B
Figure25.4:Parallaxinnearbystars.
Whenatelescopeispointedatanearbystarin
thesummeritappearstobeatlocationXagainiJ
thebackgroundstars.Whenthetelescopeis
pointedinthesamedirectionsixmonthslater
(shownbythedashedlinefromB),theastrononv
wouldneedtoswingthetelescopethroughtwlc«
theparallaxangleinordertogetthetelescope
backontothestar,whichappearstohavemoved
topositionYagainstthebackgroundstars.
1Ingroups,usethebiggestspaceavailableto
youtomarkoutthreepositionstorepresent
thelocationsoftheEarthinsummer(A),the
Earthinwinter(B),anddistantstar(C),locateI
roughlysouthofAandB.Ensurethatthe
distantstarisontheperpendicularbisector<>l
thelinejoiningAandB.Measurethedistant
betweentheSunandthestar.
470)

484. 25StarsandtheUniverse
CONTINUED
2MeasurethedistancebetweenAandB.
Compassappsarestandardonmobile
phones.StandatpositionAandusetheapp
tomeasuretheangletoC.ThenmovetoB
andmeasuretheangletoC.Subtractthetwo
anglesanddividebytwotogettheparallax
angle.Usetrigonometryorascaledrawing
tofindthedistancebetweentheSunand
thestar.Checkwhetheryouransweriswithin
10%ofthedistancemeasured.Ifyouranswer
isincorrect,identifythesourceoftheerror.
3Repeatstep2withthelayoutproducedby
othergroupsbutkeepthedistancetoyour
'star'asecretuntiltheend.Thewinning
groupistheonethatgetsconsistently
closesttotheactualvalue.
Aprotostar-howastarisborn
Aprotostaristhefirststepinstarformation.Starsform
frominterstellarcloudsofgasanddustthatcontain
hydrogencalledmolecularclouds,whicharebothcold
anddenseenoughforstarformation.TheOrionNebula
(Figure25.5)intheMilkyWayisabout1350light-years
awayanditistheclosestregionofstarformationtous.
Itisvisibletothenakedeyeinthenightskyjustsouthof
Orion’sBeltintheconstellationofOrion.
Figure25.5:TheOrionNebula,theclosestregionofstar
formationandvisibletothenakedeye.
I"hecollapseofaclumpofmolecularclouddueto
gravitationalattractionstartsaseriesofenergytransfers.
Astheforceofgravitypullsthehydrogengas
moleculesclosertogether,theirgravitationalpotential
energyistransferredtokineticenergy.Asthemolecules
collide,theirkineticenergyistransferredintothermal
energy.Theclumpcontractsintoaspinningsphere
ofsuper-hotgasknownasaprotostar.Aprotostar
continuestogrowbypullinginmorematerialfromthe
molecularcloud.Itsfinalmassdetermineswhathappens
toit.Aprotostarbecomesstablewhentheinwardforce
ofgravitationalattractionisbalancedbyanoutward
forceduetothehightemperatureofastarcausedby
nuclearfusion.
KEYWORDS
protostar:averyyoungstarthatisstillgathering
massfromitsparentmolecularcloud
interstellarcloud:acloudofgasanddustthat
occupiesthespacebetweenstars
molecularcloud:acloudofinterstellargasthat
consistsmostlyofmolecularhydrogenandiscold
anddenseenoughtocollapsetoformstars
Questions
13Whattwopropertiesdomolecularcloudshavethat
allowthemtocollapse?
14aExplainhowstarsareformed.
bWhatcausesthecentreofastartowarmup
whenitforms?
15aExplainwhatismeantbynuclearfusion.
bWhycannuclearfusiononlyoccurathigh
temperatures?
Stablestars
Hotbodiesradiateheatandthisradiationexertsa
forcecalledradiationpressure.Thehottertheobject
is,thehighertheradiationpressure.Theveryhigh
temperatureofastarleadstoaradiationpressurethat
actsoutwards,makingthestarexpand.Thisactsin
theoppositedirectiontotheforceofgravitypulling
thestarinwards,makingthestarcontract.Whenthese
forcesarebalanced,thestarisstableandstaysthesame
sizeasshowninFigure25.6.Anincreaseinthecore
temperatureofastarincreasestheradiationpressure
andthestarincreasesinsize.Astarshrinkswhenitscore
temperaturefalls.

485. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
KEYWORDS
radiationpressure:theoutwardforceduetothe
hightemperatureofthestar
Figure25.6:Astarisstablewhentheinwardpullofgravity
isbalancedbytheoutwardpushofradiationpressure,which
isactuallyaforce.
Thelifecycleofalessmassive
starlikeourSun
Likeallstars,itbeginslifeasaprotostarbeforea
enteringastableperiod.Oncethestarstartsrunningout
ofhydrogen,nuclearreactionsslowdown.Thisreduces
theradiationpressuresothestarcontracts.Thisturns
somegravitationalpotentialenergyintothermalenergy,
whichraisesthetemperatureofboththecoreofthe
starandtheoutershellofhydrogen.Thecorebecomes
hotenoughforthefusionofhelium.Heliumneedsa
highertemperaturetofusebecausethereisabigger
electrostaticrepulsionbetweentheheliumnuclei.Thisis
becauseeachheliumnucleushasachargeof+2(instead
of+1forhydrogen).Heatingtheoutershellcausesitto
expandandthencool(turningitred).Therefore,thestar
becomesaredgiant,whichisabiggerstarwithacooler
surface.
OurSunis4.6billionyearsoldandishalf-waythrough
itstimeasastablemainsequencestar.Itwillbecome
aredgiantinabout5billionyearsfromnowwhenit
willexpandbeyondEarth’sorbit.Eventually,thecore
collapsesintoawhitedwarfstatr.AwhitedwarfCHiumi
exceedamassofabout1.4solarmassesandtypiculh
hasaradiusof1000km.WhentheSunbecomesawhin
dwarf,itsradiuswillbeabout1%ofitspresentradiu
whichmeansitwillshrinktoaboutthesizeoftheInIh
Thoughithasawhitehotsurface(hencethecoioUi)il4
nothotenoughinsidetofuseheavierelements,soitwill
cooltobecomeablackdwarf.Radiationpressurebit>"*
awayitsoutershelltocreateaplanetarynebulalikeIIn
Cat’sEyeNebula(Figure25.7).
KEYWORDS
redgiant:astarthatbeganwithfewerthaneight
solarmassesandisburningheliuminitscore;11'
shellofhydrogenhasexpandedandcooled
mainsequence:astablestarthatisburning
hydrogeninitscore;onceithasusedup12%t>l
itshydrogenitgoesontoanotherstageofitslib
cycle
whitedwarf:thefinalstageofastarthatstartciI
withfewerthaneightsolarmassesafterallitsfuid
hasbeenusedup
planetarynebula:abubbleofgassurroundinga
whitedwarfstarthatusedtobetheoutershell<>l
aredgiantfromwhichitcollapsed
Figure25.7:TheplanetarynebulaNGC6543,knownastin
Cat'sEyeNebula,takenfromtheHubbleSpaceTelescop
Thewhitedotinthemiddleisawhitedwarfstar,whichih
whatourSunwillbecomeinabout5billionyears.
472y

486. carbonfusion
siliconfusion
ironfusion
magnesium
fusion
Figure25.8:Moremassivestarshaveshellsofdifferent
elements,withtheheavierelementsfallingtowardsthecore.
Onceallthefuelhasrunout,thestarcollapsesonefinal
timeandthenexplodesasasupernova.Thisprovides
theenergyrequiredtocreateelementsheavierthan
ironandpushthemintospaceasanebula,alongwith
lighterelements(includinghydrogen).Thenebula
providesthebuildingblocksforpossiblefuturestars
iindsolarsystems.Asupernovawillbrieflyoutshineits
galaxy.TheCrabNebula(Figure25.9)iswhatremains
ofasupernovaobservedbyChineseastronomersin
1054.Whathappensafterasupernovadependsonthe
inassofthecorethatremains.Ifthecoreislessthan
aboutthreesolarmasses,aneutronstarforms.Theforce
ofgravityissostrongthatelectronsandprotonsare
forcedtogethertocreateneutrons.Anevenmoremassive
corewillcontinuecollapsinguntilitbecomessodense
thatnotevenlightcanescapeandthestarbecomesa
blackhole.
oxygenfusion
neonfusion
non-burninghydrogen
hydrogenfusion
heliumfusion
exceedingeightsolarmasses
Likeallstars,itbeginslifeasaprotostarbeforea
enteringastableperiod.Thecoreofmoremassive
starsgetssohotthatthenucleiofheavierelementscan
IUse.Thestarishotenoughforthefusionoflighter
elementstocontinueinshellsfurtherfromthecore,as
ihowninFigure25.8.Theoutershellexpandsintoared
Mupergiant.However,itisnotpossibletomakeelements
heavierthanironbynuclearfusionsoastarwithatleast
eighttimesthemassoftheSunendsitscycleofnuclear
reactionswithironatitscoresurroundedbyshellsof
progressivelylighterelements.
25StarsandtheUniverse
Figure25.9:Asupernovain1054leftbehindtheCrab
Nebulaandaneutronstarsomewherewithin.
KEYWORDS
redsupergiant:similartoredgiants,theyform
whenstarswithatleasteighttimesthemassof
theSunrunoutofhydrogenfuelintheircorebut
fusionofhydrogencontinuesintheoutershells
supernova:anexplodingstarthatbeganlifewith
morethaneightsolarmassesandhasrunoutof
fuel
neutronstar:acollapsedstarcomposedalmost
entirelyofneutronswhichformswhenastarwith
morethaneightsolarmassesreachestheendof
itslife
blackhole:thefinalstageinthelifecycleof
astarthatstartedwithmorethaneightsolar
masses;ithasenoughmassleftoverafter
explodingasasupernovatocollapsetoapoint
wheregravityissostrongthatnotevenlightcan
escape
ThepossiblelifecyclesofstarsissummarisedinFigure
25.10.Allstarsbeginasaprotostarbutthefuturepath
ofastarisdeterminedbyitsmasswhenitmovesonto
themainsequence,astageinitslifewhenitisstableand
burninghydrogen.Starsthataremoremassivespend
lesstimeonthemainsequenceastheyhaveahighercore
temperatureanduseuptheirfuelmorequickly.
473>

487. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
dyingstar
redgiant
neutronstar
redsupergiant supernova
blackhole
planetary
nebula
starsstartingout
withmorethan
8solarmasses
starsstartingout
withlessthan
8solarmasses
molecular
cloud
stablemain
sequencestars
Figure25.10:Thelifecycleofastardependsonitsinitial(starting)mass.Allstarsbeginasprotostarsinmolecuarcloud*'
beforejoiningthemainsequence.StarswithastartingmassoffewerthaneightsolarmassesfollowthetoprowwhileImmlet
starsmovealongthebottomrowandexplodeassupernovae.Afterthesupernovastage,thelighterstarsbecomeneutron
starswhiletherestbecomeblackholes.
whitedwarf blackik
TheBigBangcreatedhydrogen,heliumandatraceof
lithium,sostarshaveproducedtherest.Thismeans
theSolarSystemcontainsstardust.TheSunisathird
generationstar,whichmeansthatitincludesmatterthat
hasbeenthroughtwopreviousstars,includingonethat
endedinasupernovaexplosion.Infact,wemayhave
atomsinourbodiesthatcomefrommanystarsbecause
supernovaexplosionsmixupmatterintheinterstellar
medium.
Questions
16Explainthefollowingtermsaboutthelifecycleof
astar:
aprotostar
bmainsequencestar
credgiant
dwhitedwarf
esupernova
fneutronstar
gblackhole.
17Constructaflowcharttoshowhowthestageslisted
inquestion25.16fittogetherinthelifecycleofstars.
25.3TheUniverse
TheUniversehasonlyrecentlybeendiscovered:onNew
Year’sDay,1925.ThisiswhenEdwinHubble’sscientiln
paperwaspresentedthatendedthe‘GreatDebate’and
provedthattheUniverseisbiggerthantheMilkyWay,
Astronomershadobservedwhatlookedlikewhirlpool*i>1
gasanddustinsideourgalaxy.ButwhenHubblefociiMI
thenewMountPalomartelescopeontothemherealised
theywereothergalaxiesbeyondourown.Alongwith>1
othergalaxiesanddwarfgalaxies,wearepartofthelotd
groupofgalaxies,whichispartoftheVirgoSupercluslti
Spectroscopy-learningabout
starsfromtheirstarlight
Itisremarkablewhatstarlightcantellusaboutastar.
Spectroscopy,orthescientificstudyofspectra,began
withIsaacNewtonin1666.Hediscoveredthataprism
(atriangularblockofglass)disperseswhitelightintotin
coloursofthevisiblespectrumascoveredinChapterII
In1814,JosephFraunhofernoticedthatmanydarklint
crossthespectrumofsunlight.Thesedarklinesarethe
wavelengthsoflightmissingfromthesunlightbecaust
474y

488. 25StarsandtheUniverse
thecoolgasintheSun’satmosphereabsorbsthem.
Aspectrumwiththeseabsorptionlinesisknownasan
absorptionspectrum(seeFigure25.11).Forreasonsthat
arebeyondthiscourse,eachelementhasauniquesetof
lines(sometimesknownasaspectralfingerprint)similar
toabarcode.Theselinesallowastronomerstoworkout
whatelementsareinsideastar.
Figure25.11a:Anabsorptionspectrumfoundinan
experimentonEarth,b:Theredshiftedspectrumobserved
fromadistantgalaxy.
Thespectrumforhydrogenandotherelementshas
beenfoundinexperimentsonEarth.However,when
astronomerslookedforthesamespectraindistant
galaxies,theydiscoveredthattheyareredshifted(shifted
towardstheredendofthespectrum).
Thisdoesnotmeanthatelectromagneticradiation
(includingvisiblelight)fromdistantgalaxiesturnsred.
Itmeanslightshiftstowardslongerwavelengthsbecause
thewaveisstretchedoutandthewavelengthisincreased.
Forexample,ascanbeseeninFigure25.11,absorption
linesthatarenormallyinthebluepartofthespectrum
canshiftintothegreenpartofthespectrum.
KEYWORDS
absorptionspectrum:darklinesinaspectrum
thatareproducedwhenlightpassingthrough
coolergasisabsorbed
redshift:anincreaseintheobservedwavelength
ofelectromagneticradiation(includingvisible
light)fromastarorgalaxybecauseitismoving
awayfromus
TheDopplereffect
YoumayalreadyhavenoticedtheDopplereffect.Asa
veryfastvehiclepassesyou,thevolumeofthesound
risesandfalls.However,thepitchalsoincreasesasthe
vehicleapproachesanddecreasesasitrecedes(moves
away).Thesoundwaveiscompressedinfrontofthe
vehicleasitapproaches.Thisisbecauseoncethecrest
(orcompression)ofasoundwaveleavesthecar,thecar
catchesupwithitbeforethenextcrestofthewaveleaves
thecar,asshowninFigure25.12.Thesoundwaveis
stretchedoutbehindthevehicleasitmovesaway.
Figure25.12:Thisdiagramshowscrestsofasoundwave
modifiedbytheDopplereffect,withthewavecompressed
aheadoftheambulance(sopersonAhearsahigherpitch)
andredshiftedbehind(sopersonBhearsalowerpitch).
Bythetimeitemitsthenextcrest,theambulancehas
movedforward,closingthegaponthepreviouscrest.
475

489. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
TheDopplereffectisapropertyofallwaves,including
light.Lightfromgalaxiesthataremovingawayfromus
isredshifted.Thisturnedouttobethefirstimportant
cluethattheUniverseisexpandingandsuggestedthatthe
galaxiesmusthavebeenclosertogetherinthepast.This
ledtothetheorythattheUniversehadabeginning:theBig
Bangtheory.ThisistheideathattheUniverse(space,time,
matter,energy)wascreatedatasinglepoint13.8billion
yearsagoandhasbeenexpandingandcoolingeversince.
KEYWORDS
BigBangtheory:theUniverse(space,time,
matter,energy)wascreatedatasinglepoint13.8
billionyearsagoandhasbeenexpandingand
coolingeversince
Hubble'slaw
AstronomersusetheDopplereffecttoworkouthow
fastgalaxiesaremovingawayfromus(ortowardsus).
Thespeedofthegalaxiesisdirectlyproportionaltothe
amountofredshift.In1921,EdwinHubbleplottedthe
recessionspeedofgalaxies(howfasttheyaremoving
away)againsttheirdistancefromus.Hegotagraphlike
Figure25.13.Itshowsthatthespeedatwhichgalaxies
aremovingawayfromusisdirectlyproportionaltotheir
distancefromus.
Distance/millionsoflight-years
Figure25.13:AtypicalHubbleplotthatshowsthatthe
velocityofgalaxiesandclustersisdirectlyproportionalto
theirdistancefromus.
HislineofbestfitisHubble’slaw:
v—H^d
Here,vistherecessionspeedofgalaxies(howIasiIh.»
aremovingawayfromus)anddistheirdistanceInanm
Thegraphconfirmsthatthefurtherawaythegainmvl»iI
thefasteritismovingawayfromus.
distanceofthegalaxyfromEarth
KEYEQUATION
TheHubbleconstant,isthegradientofthisgraphand
itistheratioofthespeedatwhichgalaxiesaremmmu
awayfromEarthtotheirdistancefromEarth:
I
So:
t,
Hubbleconstant=
speedofgalaxymovingawayfrontIuth
=
d_1
universevu
V
#0
EstimatefortheageoftheUniverse
v
H.
Therefore,theageoftheUniverseis:
^universe=V^=
” =4.5XIQ17S
22xJQ-18
=14.4x109years
TheUniversebeganatasinglepoint(calledasingulaiII\i
about14.5billionyearsago.
ThecurrentestimateforHois2.2x10*18persecond
Thereciprocal(inverse)oftheHubblecorlftantisknown
asHubbletimebecauseitcanbeusedtowftrkouttin.ly*
oftheUniverse.Fromtheequationthatlinksdistant>
speedandtime,weknowthat:
time=
distance
speed
KEYWORDS
Hubble'slaw:distantgalaxiesaremovingaway
fromEarthwithaspeed,v,thatisproportionalto
theirdistance,d,fromEarth;v=HodwhereHois
theHubbleconstant
Hubbleconstant:theslopeofagraphofgalaxy
speedagainstdistance
Hubbletime:theinverseoftheHubbleconstant,
whichgivesanestimatefortheageofthe
Universe
476y

490. 25StarsandtheUniverse
Questions
18
a
b
Distance/millionsoflight-years
a
Figure25.14:Recessionspeedofsuperclustersplotted
againstdistance.
ThisquestionisabouttheDopplereffect.Usea
compasstodrawacircleofradius5cm.Thenmove
thepointofthecompass0.5cmtotheleftanddraw
acircleofradius4cm.Repeattheprocess,moving
thecompasstotheleftandreducingtheradiusof
thecircleby1cm,untilyouhaveanestofcircles
thatareclosertogetherontheleft-handsidethan
theother.Theyrepresentcircularsoundwaves
producedbytheengineofaracingcar,atascaleof
1cm=1metre.
aheadoftheracingcar?
behindtheracingcar?
iftheracingcarisnotmoving?
ii
iii
Thediscoveryofthecosmic
microwavebackground
radiation(CMBR)
Mostcultureshaveacreationmythabouthowthe
Universebegan.Sciencehasonlybeenabletooffer
analternativeexplanationduringthelastcentury
andmanypuzzlesremain.Ourbestknowledgeatthe
momentisthattheUniversebeganasahotbigbang
fromatinypointsmallerthanapinhead(calleda
singularity)about13.8billionyearsago.TheUniverse
wasunimaginablyhotanddensebutithasbeen
expandingandcoolingeversince.TheearlyUniverse
wassohotthatneutralatomscouldnotform.They
wouldinstantlyionise.Lightwascontinuouslyscattered
offthechargedparticles(ionsandelectrons).Byanalogy,
lightisscatteredinfog,whichiswhyyoucannotsee
veryfarintoit.OncetheUniversewasabout379000
yearsold,andaboutthesizeoftheMilkyWay,the
temperaturedroppedto3000Kandneutralatoms
formed.LightwasnolongerscatteredandtheUniverse
becametransparent.Itwaslikethefogsuddenlylifted
(disappeared)andtheairbecameclear.However,the
continuedexpansionoftheUniversehascausedthe
wavelengthofthislighttoredshiftovertime.
Thewavelengthofthislight,inthemicrowaveregion
oftheelectromagneticspectrum,waspredictedin
1948.In1964,theUSscientistsAmoPenziasand
RobertWilson(Figure25.15)builtaradiotelescopebut
struggledtoeliminatenoise(unwantedsignal).Itdid
notmatterwhereintheskytheypointedtheirtelescope,
thenoisewasconstant,andsotheyassumedthatitwas
aproblemwiththeirequipment.However,itturned
outthattheyhadaccidentallydiscoveredmicrowave
radiation.Theywerelookingatlightthathadleftthe
surfaceoflastscatteringwhentheuniversewasonly
379000yearsold(almost14billionyearsago)andhad
beenredshiftedsothatitswavelengthwasnowmore
than1000timeslonger.Thismicrowaveradiation,called
cosmicmicrowavebackgroundradiation(CMBR),has
atemperatureof2.726K(Figure25.16).Penziasand
WilsonwereawardedtheNobelPrizeforphysicsin1978
fortheiraccidentaldiscovery.
Whatisthewavelengthofthesoundwhenthe
racingcarisnotmoving?
Whatisthewavelengthofthesound:
aheadoftheracingcar?
behindtheracingcar?
Soundtravelsat330m/s.Whatisthefrequency
ofthesoundheard:
thevalueoftheHubbleconstantyou
workedoutinpartaitoworkouthow
farawaythesuperclusterisfromusand
plotitspositiononasketchedcopyof
Figure25.14.
iiUseyouranswertopartaiitoworkout
theageoftheUniverse.
UseFigure25.14tocalculatetheHubble
constant,expressed:
inkm/spermillionlight-years
persecond.(Hint:itwillhelptorecallthat
1light-year=9.5x1012km.)
TheredshiftoftheSaraswatiSupercluster
suggeststhatitisreceding(movingaway)
fromusataspeedof84000km/s.Use
Explainwhatyouwouldhearastheracing
carpassesyou.Whydoesthecarhavetobe
travellingfast?
Figure25.14isaplotoftherecessionspeedof
superclustersagainsttheirdistancefromEarth.

491. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Figure25.15:RobertWilsonandArnoPenziasinfrontof
theradiotelescopethatdetectedthecosmicmicrowave
backgroundradiation.
Figure25.16:Fullskymapofcosmicmicrowave
backgroundradiation.Thisistheradiationemittedwhen
theUniversewas379000yearsoldbutredshiftedinto
themicrowaveregionoftheelectromagneticspectrum.
ThisshowsthattheUniversehasauniformtemperature
of2.726Kinalldirections,withonlyverytinyvariations,
indicatedbythefalsecolours.
Despitethename,theBigBangwasnotanexplosion.
Itistheexpansionofthespacebetweenthegalaxies.
Imagineyouruniverseisthetwo-dimensionalsurfaceof
aballoon.Everythinginsideoroutsideoftheballoon
doesnotexist.Asspaceexpands(thatis,theballoon
inflates),clustersofgalaxiesmovefurtherapartwith
theirrecessionspeedsincreasingwithdistanceasshown
inFigure25.17.Nomatterthedirectionwelook,
galaxiesappeartobemovingawayfromus,suggesting
weareatthecentreoftheUniverse.However,aliensona
distantgalaxywouldalsothinktheywereatthecentreof
theUniverse,withallothergalaxiesmovingawayfrom
them.Actually,theUniversedoesnothaveacentreor
anedge.Donotworryifyoufindthisideaimpossibleto
1
2
ACTIVITY25.3
imaginebecausenobodycan.ThebestwecandonI
presentmodels.
Figure25.17:TheUniverserepresentedbythesurfaC"ofi"
expandingballoon.
UsingHubble'slawtofindthecentreofthe
Universe
Workindividually,inpairs,oringroups(four
wouldbeideal)onthistask.Ifyouhavqastrip
ofelasticwithbuttonssewnintoit,youcando
thisasanexperimentundertheguidanceofyour
teacher.Youneedtostretchtheelasticbythe
sameamountforeach'timeinterval'sothatit
lookssomethinglikeFigure25.18.Ifbuttonsand
elasticarenotavailable,takemeasurementsfrom
thediagram.
Eachbuttonrepresentsaclusterqfgalaxies.
ClusterArepresentsthelocalgroup(wherethe
MilkyWayGalaxyislocated).Thescaleis3mm

10millionlight-years.
Measurethedistancesbetweenthecentres
oftheclustersandrecordthemonacopythe
Table25.1a.Compareyourmeasurements
withotherstudentsandreachaconsensus
(agreement).Checkthatthevalueinthefinal
columnforeachrowisequaltothesumof
thethreepreviouscolumns.
Ifyouareworkinginagrouptheneach
ofyoushouldtakeoneoftheclusters
andworkoutthedistancetoalltheother
clusters,usingthedistancesyouagreedin
thepreviousstep.Forexample,ifyouare
allocatedcluster(button)C,youwillbea
scientistlookingattheexpandinguniverseas
ifyouwerelivingonaplanetinclusterCand
workingoutthedistancestoclustersB,A
andD.Dothisforallthedifferenttimesand
completeyourowntable,whichshouldlook
somethingliketheoneinTable25.1b.
478>

492. 25StarsandtheUniverse
CONTINUED
t=1billionyears
t=2billionyears
C B
C B
Figure25.18:ModellingtheexpandingUniverse.
Table25.1a:Agreeddistancesbetweenclusters
Time
(billion
years)
C-B B-A A-D
Total
(C-D)
1
2
3
4
5
3Foryourallocatedcluster,plotagraphof
distance(inmillionsoflight-years)againsttime
(inbillionsofyears)totheotherthreeclusters.It
willsavetimeandmakeiteasiertocompareif
youplotallthreegraphsonthesameaxesbut
ensureyoulabeleachgraphwiththecorrect
letterforeachcluster.
4Useyourplotteddistance-timegraph(s)fromthe
previoussteptoworkoutthe(recession)speed
Table25.1b:Exampletable(forclusterC)
Time
(billion
years)
Distance
toB
Distance
toA
Distance
toD
1
2
3
4
5
foreachcluster(inmillionlight-yearsperbillion
years)andcommentonwhatyoufind.Sketcha
graphofspeedagainstoriginaldistance.How
doyouinterpretyourresults?
Comparewhatyoufindwithyourclassmates.
Checkthattherecessionspeedisthesamefor
pairsofgalaxies.Forexample,thespeedofBas
seenfromDshouldbethesameasthespeedof
DasseenfromB.

493. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
MeasuringdistanceswithPEERASSESSMENT
PROJECT
ArewealoneintheUniverse?
ThepossibilityoflifeelsewhereintheUniverse
fascinatesandfrightenspeopleinequalmeasure
andisthesubjectofmanysciencefictionstories,
bothinbooksandmovies.Sciencefiction(as
opposedtofantasy)oftenworksbetterifthe
underlyingscienceiscorrector,atleast,possible.
WhenlookingforlifebeyondEarth,itisimportant
toconsiderwhatconditionsarerequiredforlife
onEarth.Weneedliquidwater(asasolventfor
thechemicalreactionsneededforlife),asource
ofenergy(wedependonsunlight),andchemical
buildingblocks.Scientistshavediscoveredthat
moststarshavesolarsystemsoforbitingplanets,
increasingthechancesthattheconditionsforlife
onEarthexistonotherplanets,beyondtheSolar
System.
However,liquidwaterisnottheonlypossible
solventforchemicalreactions.Ammoniaandliquid
hydrocarbons(likemethane)arealternativesand
haveraisedhopesoffindinglifeonTitan,the
largestmoonorbitingSaturn.
Sunlightisnottheonlysourceofenergy.In1977,
hydrothermalvents(calledblacksmokers)were
discoveredatmid-oceanridgesseveralkilometres
belowtheoceansurface.Eventhoughtheyare
fardeeperthanlightcanreach,ecosystemshave
formedaroundthem.Theseecosystemsget
theirenergyfromthechemicalreactionbetween
oxygenandhydrogensulphide.Thishasincreased
hopesoffindinglifebeneaththeiceofEuropa,
oneofJupiter'smoons.Microbeshavealsobeen
discovereddeepundergroundintheEarth'scrust.
LifeonEarthisbasedoncarbon,whichcanbond
withtoupfourotheratomsandformsmore
compoundsthanallotherelementscombined.
However,lifecouldalsobebasedonsilicon,which
isinthesamegroupinthePeriodicTableandcan
alsoformahugevarietyofcompounds.
Lookattheworkoffellowstudentswhohave
completedthistask(yourteammatesifyouhave
beenworkinginagroupoffour).
Whichpieceofworkisthemostaccurate
andeasiesttofollow?Ifyouhadbeenabsent
andmissedthislessonwhoseworkwould
youwanttoreadtohelpyouunderstandthe
missedwork?
Identifythereasonswhythisworkstandsout
butalsohowitcouldbeimproved.
Inyourgroupmakealistofthingsthat
everyonecandotosetouttheirworkmore
clearly.
Yourteachermayaskyourgrouptodiscuss
thepieceofworkyouhavechosen.
Question
20Describeindetailtwopiecesofevidencethat
supporttheBigBangtheory.
supernovae
HavingfoundevidencethattheUniverseisexpandnir
scientistsarenowaskingwhatwillhappentothe
Universeinthefuture.Observationsin1998oftype
lasupemovaeinaverydistantgalaxysuggestthat
theexpansionoftheUniverseisaccelerating.TypeIi
supernovaeoccurwhenawhitedwarfstarinabinaiy
starsystem(whentwostarsorbiteachother)pulls
inmaterialfromitscompanionstaruntilitreachesa
certainmassandthenexplodes.Theimportantpointi*
thatthesesupernovaealwayshavethesameluminosih
(outputpower)thereforetheyactasa‘standardcandh
Byrecordinghowbrightthesupernovaappearstobi
andknowinghowbrightitreallyis,itispossibletowoiI
outhowfarawayitis.Scientistsarestillnotableto
explainwhytheexpansionoftheUniverseisspeeding
up,buttheyhavesuggestedthatsomethingcalled*dtn1
energy’isresponsible.Ifyoucontinuetostudyphysics
youwilllearnmoreaboutthis.

494. r
25StarsandtheUniverse
CONTINUED
Youhavetwotasks.
1UsetheInternettofindmoreinformationonone
ofthefollowing:
ThesearchforEarth-likeextrasolarplanets
(planetsinothersolarsystemsthat
matchtheconditionsrequiredforlifeon
Earth):youwillfindtherearemanyother
requirementsformorecomplicatedlife
forms(forexample,thespinoftheplanet
hastobestable).Youcanprobablythinkof
morebeforegoingonline.
ThesearchforlifewithinourSolarSystem:
youcouldlookat,forexample,Mars,
EuropaorTitan.
2Usetheinformationpresentedhereorthe
informationyouhavefoundontheInternet
tocompleteashortpieceofcreativescience
fictionwriting,basedonscientificfact.Aimfor
amaximumof800words(orwhatyourteacher
suggests).Useyourimagination,butmakesure
thatyourstoryisconciseandthescienceis
clearandcorrect.
SUMMARY
OurSunisanaveragestarandismademainlyofhydrogenandhelium.
Stablestarsshinebecauseofthethermonuclearfusionofhydrogen.
TheSunshinesintheinfraredandultravioletaswellasthevisiblelight.
Alight-yearisthedistancethatlighttravelsinoneyear.
Alight-yearisdefinedas9.5x10'
5
metres.
TheSunisofoneofmanybillionsofstarsinourMilkyWayGalaxy.
Thedistancebetweenstarsisthousandsoftimesbiggerthanthedistancebetweenastarandplanets
initssolarsystem,ifithasone.
OurMilkyWayGalaxyisoneofbillionsofothersintheUniverse.
Ournearestgalaxyismorethan25000light-yearsfromEarth.So,galaxiesaremuchfurtherapartthanstars,
andstarsaremuchfurtherapartthanplanets.
Allstarsbeginasprotostarsfromthecollapseofinterstellar(molecular)gascloudsandthenstartusing
hydrogenastheirfuel.
Astablestaruseshydrogenasitsfuelandtheoutwardforceofradiationpressureduetoitshightemperature
balancestheinwardforceofgravity.
Lowmassstars(lessthaneighttimesthemassoftheSun)swellintoredgiantswhentheyrunoutofhydrogen
fuelfortheirnuclearreactions.
Whenredgiantsrunoutofhelium,theyformaplanetarynebulaandshrinkintowhitedwarfs.
Highmassstars(morethaneighttimesthemassoftheSun)willexplodeassupernovaethatcreateelements
heavierthanironandsendthismaterialintotheinterstellarmediumasanebulaethatformtherawmaterialfor
newstarsandsolarsystems.
Astarthatexplodesasasupernovacollapsestobecomeaneutronstaror,ifithasabiggermass,ablackhole.
Redshiftistheincreaseinthewavelengthofelectromagneticradiation(includingvisiblelight).
481)

495. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
CONTINUED
Lightfromdistantstarsandgalaxiesisredshifted,whichsuggeststhattheyaremovingawayfromus.
ThefactthatgalaxiesaremovingawayfromussuggeststhattheUniverseisexpandingandsupportstheBig
Bangtheory.
Thespeedthatagalaxyappearstobemovingawayfromuscanbefoundfromtheredshiftinthestarlight.
TheHubbleequationdescribeshowthespeedofgalaxiesisproportionaltotheirdistancefromus.
ThereciprocaloftheHubbleconstanttellsustheageoftheUniverse.
Thecosmicmicrowavebackgroundradiation(CMBR)iselectromagneticradiationfromtheearlyUniversethal
hasbeenredshiftedanditswavelengthstretchedintothemicrowaveregionoftheelectromagneticspectrum.
Thedistancetoadistantgalaxycanbedeterminedbythebrightnessofatypelasupernova.
EXAM-STYLEQUESTIONS
1Whydostarsshine? [1]
ATheyareburning.
BNuclearfusionistakingplaceinsidethestar.
CTheyaremadeofhotgasesthatareheatedupwhentheycollapsed
fromgasclouds.
DTheyaremadeofether.
2Whatisalight-year? [1]
Athedistancelighttravelsinoneyear
B366days
Cthetimeittakeslighttotravelinoneyear
D1.44x1011metres
3Whichofthefollowingisnotastar? [1]
Aneutronstar
Bwhitedwarf
Csupernova
Dredgiant
4WhichofthefollowingisnotevidenceoftheBigBang? [1]
Acosmicmicrowavebackgroundradiationeverywhere
Bgalaxiesaremovingclosertogether
Cstarsthatareredshifted
DtheUniverseisexpanding
5aWhatdowecallalargegroupofstars? [1]
bWhydothesegroupsform? [1]
cWhatisthenameofthegroupwherewelive? [1]
dHowmanyothergroupsofstarsdowethinkexistintheUniverse? [1]
[Total:4]
482)

496. 25StarsandtheUniverse
Whatcausestheredshiftinthelightarrivingfromdistantgalaxies?[2]
[3]
[Total:9]
8WhatistheBigBangtheoryandwhatevidenceistheretosupportit? [3]
CONTINUED
[2]
COMMAND WORD
[5]
Findtheageofthisfictionaluniverse.
describe:statethe
pointsofatopic;give
characteristicsand
mainfeatures
Thetableshowsdataforaparalleluniverse.Plotagraphofrecessional
speed(inkm/s)versusdistance(millionlight-years)toeachgalaxyfor
thisparalleluniverse.
7DescribethelifecycleoftheSunfromhowitformeduntilthepointat
whichitrunsoutoffuel.
UsethegraphyouplottedtofindtheHubbleconstant,makingitclear
howyoudidit. [2]
Distance
(millionlight-years)
Velocity(km/s)
0.0 0
1.6 270
2.3 360
3.0 470
1.7 890
5.6 990
6.6 1050
7.3 1150

497. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
SELF-EVALUATIONCHECKLIST
Afterstudyingthischapter,thinkabouthowconfidentyouarewiththedifferenttopics.Thiswillhelpyoutosee
anygapsinyourknowledgeandhelpyoutolearnmoreeffectively.
1can
See
Topic...
Needs
morework
Almost
there
Confident
tomoveor
DescribewhattheSunismadeof. 25.1
Recallwhatpartsoftheelectromagneticspectrum
areemittedbytheSun.
25.1
Describewhatpowersastablestar. 25.1
Recalltherelativedistancesbetweenplanets,stars
andgalaxies.
25.2
Recallwhatalight-yearis. 25.2
Definealight-year. 25.2
Describehowaprotostarisformedandhowit
becomesastablestar.
25.2
DescribethelifecycleofastarliketheSun,
includingthenamesofthedifferentstages.
25.2
Describethelifecycleofstarsexceedingeightsolar
masses,includingthenamesofthedifferentstages.
25.2
Describetheroleofsupernovaeincreatingheavy
elementsandspreadingthem.
25.2
RecallthenumberofstarsintheMilkyWayandthe
numberofgalaxiesintheUniverse.
25.2
Recallwhatredshiftis. 25.3
Recallhowtheredshiftofelectromagneticradiation
fromdistantstarsandgalaxiessupportstheBig
Bangtheory.
25.3
Knowthattheredshiftoflightfromdistantgalaxies
canbeusedtoworkouttheirspeedofrecession
(howfasttheyaremovingawayfromus).
25.3
RecallHubble’sLawanduseittoworkouttheage
oftheUniverse.
25.3
RecallwheretofindtheCMBR(cosmicmicrowave
backgroundradiation)anddescribeitsorigin(howit
cameintoexistence).
25.3
Describehowthedistancetoagalaxycanbe
determinedusingatypelasupernova.
25.3
484y

498. Appendix
>Appendix
Electricalsymbols
You'llneedtoknowthefollowingelectricalsymbolsforthe‘Electricityandmagnetism’
sectionofthesyllabus.Thesymbolshighlightedinbluearesupplementcontent.
cell
batteryofcells
—
1
or
F--1
powersupply oo
d.c.powersupply +
oo
a.c.powersupply o~
o
fixedresistor —11 1—
variableresisitor
thermistor — 1/
1—
light-dependentresisitor
l
heater —m 11
potentialdivider
magnetisingcoil
_TO"YV.
transformer
switch
earthorground
junctionofconductors
lamp 0—
motor
generator G
ammeter —®
—
voltmeter —®
—
diode
light-emittingdiode
fuse 1 1
relaycoil EZ

499. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Symbols
You'llneedtoknowtheusualscientificsymbolsforanumberofphysicalquantities
and,whererelevant,theunitsthatthey'remeasuredin.Thetablesbelowshowyou
whatyouneedtoknowfortheCoreandSupplementoptionsofthesyllabus.
Core
Quantity UsualsymbolUsualunit
length 1,h,d,s,x km,m,cm,mm
area A m2,cm2
volume V m3,cm3,dm3
weight W N
mass m,M kg,g
time t h,min,s
density P g/cm3,kg/m3
speed u,V km/h,m/s,cm/s
acceleration a m/s2
accelerationof
freefall
9 m/s2
force F N
gravitational
fieldstrength
9 N/kg
momentof
force
Nm
workdone W J,kJ,MJ
energy E J,kJ,MJ,kWh
power P W,kW,MW
pressure P
N/m2,N/cm2
temperature 0,T °C,K
486

500. Appendix
Core
Quantity UsualsymbolUsualunit
frequency f Hz,kHz
wavelength A m,cm
focallength f m,cm
angleof
incidence
i degree(°)
angleof
reflection
r degree(°)
angleof
refraction
r degree(°)
Criticalangle c degree(°)
potential
difference/
voltage
V V,mV,kV
current 1 A,mA
rn.f. E V
idistance R Q
।harge Q C
।ountrate count/s,
counts/minute
halflife s,minutes,h,
days,weeks,
years
Supplement
Quantity UsualsymbolUsualunit
wavelength A nm
refractiveindexn
HubbleconstantHo S’1
487>

501. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
ossary
Commandwords
BelowaretheCambridgeInternationaldefinitionsforcommandwordswhichmaybeusedinexams.
TheinformationinthissectionistakenfromtheCambridgeAssessmentInternationalEducationsyllabus(0625/0*1')
forexaminationfrom2023.Youshouldalwaysrefertotheappropriatesyllabusdocumentfortheyearofyour
examinationtoconfirmthedetailsandformoreinformation.ThesyllabusdocumentisavailableontheCambridgt
AssessmentInternationalEducationwebsitewww.cambridgeinternational.org.
calculate:workoutfromgivenfacts,figuresor
information
comment:giveaninformedopinion
compare:identify/commentonsimilaritiesand/or
differences
deduce:concludefromavailableinformation
define:giveprecisemeaning
describe:statethepointsofatopic;givecharacteristics
andmainfeatures
determine:establishananswerusingtheinformation
available
explain:setoutpurposesorreasons;makethe
relationshipsbetweenthingsevident;providewhyand/or
howandsupportwithrelevantevidence
Keywords
a.c.generator:adevicesuchasadynamousedto
generatealternatingcurrent.
absolutezero:thetemperatureatwhichparticleshaveno
kineticenergy.
absorptionspectrum:darklinesinaspectrumthatare
producedwhenlightpassingthroughcoolergasisabsorbed.
accelerationduetogravity:theaccelerationofanobject
fallingfreelyundergravity.
accelerationoffreefall:theaccelerationofanobject
fallingfreelyundergravity.
acceleration:therateofchangeofanobject’svelocity.
accretion:thecomingtogetherofmatterunderthe
influenceofgravitytoformlargerbodies.
accretiondisc:arotatingdiscofmatterformedby
accretion.
activity:therateatwhichnucleidecayinasampleofa
radioactivesubstance.
give:(areason/example)produceananswerfromagiven
sourceorrecall/memory
sketch:makeasimplefreehanddrawingshowingthek
features,takingcareoverproportions
airresistance:frictionactingonanobjectmoving
throughair.
alphadecay:thedecayofaradioactivenucleusbythe
emissionofana-particle.
alphaparticle(a-particle):aparticlemadeupoftwo
protonsandtwoneutrons;itisemittedbyanatomic
nucleusduringradioactivedecay.
alternatingcurrent(a.c.):electriccurrentthat
(periodically)changesdirectioninacircuit.
ammeter:ameterformeasuringelectriccurrent.
ampere,amps(A):theSIunitofelectriccurrent.
amplitude:thegreatestheightordepthofawavefromil
undisturbedposition.
analoguesignal:asignalwhichvariescontinuouslyin
frequencyandamplitude.
analogue:adisplaythathashands(oraneedle)andis
oftennotveryprecise.
identify:name/select/recognise
justify:supportacasewithevidence/argument
predict:suggestwhatmayhappenbasedonavailable
information
state:expressinclearterms
suggest:applyknowledgeandunderstandingto
situationswheretherearearangeofvalidresponsesin
ordertomakeproposals/putforwardconsiderations
488>

502. Glossary
bonds:anothernamefortheforcesbetweenparticles.
Brownianmotion:themotionofsmallparticles
suspendedinaliquidorgas,causedbymolecular
bombardment.
calibrate:tomarkastandardscaleontoameasuring
instrument
calibrated:shouldagreecloselywithastandardor
agreeswhencorrectionapplied.
cell:adevicethatprovidesanelectromotiveforce(e.m.f.)
inacircuitbymeansofachemicalreaction.
centreofgravity:allthemassofanobjectcouldbe
locatedhereandtheobjectwouldbehavethesame
(whenignoringanyspin).
changesofstate:changingfromonestateofmatterto
another.
charge:carriedaroundacircuitbythecurrent;negative
chargeiscarriedbyelectrons.
chemicalenergy:energystoredinbondsbetweenatoms
thatcanbereleasedwhenchemicalreactionstakeplace.
clockwise:turninginthesamedirectionasthehandson
aclock.
collision:themeetingofparticlesorofbodiesinwhich
eachexertsaforceupontheother.
comet:aballofice,dustandgaswhichorbitstheSunin
ahighlyellipticalorbit.
commutator:adeviceusedtoallowcurrenttoflowto
andfromthecoilofad.c.motororgenerator.
compression:aregionofasoundwavewherethe
particlesarepushedtogether.
condensing:changingfromgastoliquid.
conductor:amaterialthatallowsanelectriccurrentto
flowthroughit.
contaminated:whenanobjecthasacquiredsome
unwantedradioactivesubstance.
convectioncurrent:thetransferofthermalenergybythe
motionofafluid.
convection:thetransferofthermalenergythrougha
materialbythemovementofthematerialitself.
conventionalcurrent:thedirectionpositivecharges
wouldflowinacompletecircuit,fromthepositive
tonegativeterminalsofacell,andoppositetothe
directionthatelectronsflow.
converginglens:alensthatcausesraysoflightparallel
totheaxistoconvergeattheprincipalfocus.
coulomb(C):theSIunitforelectriccharge.
countrate:thenumberofdecayingradioactiveatoms
detectedeachsecond(orminute,orhour).
angleofincidence:theanglebetweentheincidentray
andthenormaldrawnatthepointwheretherayhits
thesurface.
angleofreflection:theanglebetweenthereflectedray
andthenormaldrawnatthepointwheretherayhits
thesurface.
angleofrefraction:theanglebetweenarefractedrayand
thenormaltothesurfaceatthepointwhereitpasses
fromonemediumtoanother.
anticlockwise:turningintheoppositedirectionfromthe
handsonaclock.
armature:themovingpartofanelectromagneticdevice
suchasarelayorbell.
asteroidsandmeteoroids:lumpsofrockwhichorbit
theSun.
atom:thesmallestpartofanelementthatcanexist.
1attractiveforces:forcesbetweenparticleswhichhold
theparticlesinfixedpositionsinasolid.
averagespeed:thespeedcalculatedfromtotaldistance
travelleddividedbytotaltimetaken.
axis:theimaginarylinebetweentheEarth’sNorthand
Southpoles.
backgroundradiation:theradiationfromthe
environmenttowhichweareexposedallthetime,
barmagnet:arectangular-shapedpermanentmagnetwith
anorthpoleatoneendandasouthpoleattheother,
battery:twoormoreelectricalcellsconnectedtogether
inseries.
/betadecay:thedecayofaradioactivenucleusbythe
emissionofa0-partiele.
betaparticle(0-particle):ahighspeedelectronthatis
emittedbyanatomicnucleusduringradioactivedecay.
BigBangtheory:theUniverse(space,time,matter,
energy)wascreatedatasinglepoint13.8billionyears
agoandhasbeenexpandingandcoolingeversince,
biofuel:material,recentlyliving,usedasafuel,
blackhole:thefinalstageinthelifecycleofastarthat
startedwithmorethaneightsolarmasses;ithasenough
massleftoverafterexplodingasasupernovatocollapse
toapointwheregravityissostrongthatnotevenlight
canescape.
boer:devicewherethermalenergyistransferredto
watertoturnitintosteam.
boilingpoint:thetemperatureatwhichaliquidchanges
toagas(atconstantpressure).
boling:changingfromliquidtogasatafixed
temperaturecalledtheboilingpoint.

503. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
crest:(orpeak)thehighestpointofawave.
criticalangle:theminimumangleofincidenceatwhich
totalinternalreflectionoccurs.
current:therateatwhichelectricchargepassesapointin
acircuit.
current-voltagecharacteristic:agraphofcurrentonthe
verticalaxisandvoltageonthehorizontalaxis.
density:theratioofmasstovolumeforasubstance.
diffraction:whenawavespreadsoutasittravelsthrough
agaporpasttheedgeofanobject.
digitalsignal:asignalthatconsistsofaseriesofpulses
whichareeitheronoroff.
digital:adisplaythatshowsnumbersandisoftenprecise,
diminished-usedtodescribeanimagewhichissmaller
thantheobject.
diode:anelectricalcomponentthatallowselectric
currenttoflowinonedirectiononly.
directcurrent(d.c.):electriccurrentthatflowsinthesame
directionallthetime.
dispersion:theseparationofdifferentwavelengthsof
lightbecausetheyarerefractedthroughdifferentangles,
displace:movingsomethingtoanotherplacesowater
ismovedoutoftheway(upwards)whenanobjectis
loweredintoit.
dissipated:energythatisspreadoutbecomesnotuseful
orwanted.
diverginglens:alensthatcausesraysoflightparallelto
theaxistodivergefromtheprincipalfocus.
doingwork:transferringenergybymeansofaforce.
doubleinsulated:whentheelectriccircuitforanelectrical
applianceisplacedinsideacasemadefromanelectrical
insulatorsothatitisimpossibleforalivewiretotouch
theoutercasing.
drag:frictionthatactsonanobjectasitmovesthrougha
fluid(aliquidoragas).
earthed:whenthecaseofanelectricalapplianceis
connectedtotheearthwireofathree-pinplug;theearth
wireiselectricallyconnectedtothegroundtoprevent
currentpassingthroughanyonetouchingafaulty
appliance.
eccentricity:ameasureofhowellipticalanorbitis.
efficiency:thefraction(orpercentage)ofenergysupplied
thatisusefullytransferred
electricfield:aregionofspaceinwhichanelectriccharge
willexperienceaforce.
electricalconductor:asubstancethatallowstheflowof
electrons(electricalcurrent).
electricalinsulator:asubstancethatinhibitsthefl<
electrons(electricalcurrent).
electricalpower:power=currentxp.d(P=VI).
electromagnet:acoilofwirethatactsasamagnet
anelectriccurrentpassesthroughit.
electromagneticinduction:theproductionofane.i
acrossanelectricalconductorwhenthereisrelativ
movementbetweentheconductorandamagnetic
electromagneticradiation,energythatistransferre
electromagneticwaves.
electromagneticspectrum:thefamilyofradiations
tolight.
electromotiveforce(e.m.f.):theelectricalworkdon
asource(cell,batteryetc.)inmoving(aunit)char;
aroundacircuit;thevoltageacrosstheterminalsc
asource.
I
electron:anegativelychargedparticle,smallerthat
anatom.
electroncharge:theelectricchargeofasingle
electron=-1.6x10-'9C.
electrostaticcharge:apropertyofanobjectthatcz
ittoattractorrepelotherobjectswithcharge.
ellipse:asquashedcircle.
energy:quantitythatmustbechangedortransferr
makesomethinghappen.
enlarged:usedtodescribeanimagewhichisbiggei
theobject.
(the)Equator:animaginarylinedrawnroundtheI
halfwaybetweentheNorthPoleandtheSouthPol
equilibrium:whennonetforceandnonetmoment
onabody.
evaporation:changingfromaliquidtoagasatany
temperature.
event:somethingthathappensortakesplace,ofter
aspecifictimeandplace.
extension:theincreasedlengthofanobject(forex:
aspring)whenaload(forexample,weight)isattac
toit.
fixedpoints:knownvaluesusedtocalibrateameas
instrument
Fleming’sleft-handrule:arulethatgivestherelatic
betweenthedirectionsofforce,fieldandcurrentv
currentflowsacrossamagneticfield.
Fleming’sright-handrule:arulethatgivesthe
relationshipbetweenthedirectionsofforce,fielda
currentwhenacurrentflowsacrossamagneticfiel
fluid:asubstancewhichcanflow;liquidsandgase:
fluids.
490y

504. Glos*
focallength:thedistancefromthecentreofthelenstoits
principalfocus.
force:theactionofonebodyonasecondbody;
imbalancedforcescausechangesinspeed,shapeor
direction.
fossilfuels:material,formedfromlong-deadmaterial,
usedasafuel.
Irequency:thenumberofvibrationsorwavesperunitof
lune.
Ifiction:theforcethatactswhentwosurfacesrubover
oneanother.
fuse:adevicethatbreaksthecircuitifthecurrent
exceedsacertainvalue;itisapieceofmetalwirethat
meltswhentoomuchcurrentflowsthroughit.
HHlvanometer:ameterformeasuringtinyelectriccurrent,
iummaray(y-ray):electromagneticradiationemittedby
anatomicnucleusduringradioactivedecay.
generator:adevicewhichgenerateselectricityusing
electromagneticinduction.
geothermalenergy:energystoredinhotrocks
underground.
gravitationalfieldstrength:isthegravitationalforce
।ujrtedperunitmassplacedatthatpoint.
gravitationalpotentialenergy(g.p.e):theenergystoreof
miobjectraisedupagainsttheforceofgravity;more
pvncrally,itisthedistancebetweenparticlesorbodies,
gravity:theforcethatexistsbetweenanytwoobjectswith
HUIU.
hulllife:theaveragetimetakenforhalftheatomsina
impleofaradioactivematerialtodecay.
hard(material):amaterialthat,oncemagnetised,is
todemagnetise.
Immisphere:halfofasphere;theEarthcanbeconsidered
I
"
bemadeoftwohemispheresdividedbytheEquator.
In11;theunitoffrequency;1Hz=1wavepersecond.
1lookelaw:theextensionofanobjectisproportionalto
ifloadproducingit
Ilubhleconstant:theslopeofagraphofgalaxyspeed
iiynmstdistance.
Ilubhletime:theinverseoftheHubbleconstant,which
ri.mestimatefortheageoftheUniverse.
law:distantgalaxiesaremovingawayfrom
Inthwithaspeed,v,thatisproportionaltotheir
di11ih
।d,fromEarth;v=HodwhereHoistheHubble
*
।hi(.ml.
ku'whatweseewhenweviewanobjectbymeansof
।
'iiledrays.
immerse:tocoversomethinginafluid(usuallywater)!
thattheobjectissubmerged.
impulse:thechangeinanobject’smomentum,FAp,or
theforceactingonanobjectmultipliedbythetimefor
whichtheforceacts(fxt).
incidentray:arayoflightarrivingatasurface.
inducede.m.f.:(orinducedvoltage)thee.m.f.createdii
conductorwhenitcutsthroughmagneticfieldlines,
inducedmagnetism:whenamagneticmaterialisonly
magnetisedwhenplacedinamagneticfield(forexamp
whenbroughtclosetothepoleofapermanentmagne
infraredradiation:electromagneticradiationwhose
wavelengthisgreaterthanthatofvisiblelight;someth
knownasthermalradiation.
insulator:amaterialthatmakesitverydifficultforan
electricalcurrenttoflowthroughit.
internalenergy:theenergyofanobject;thetotalkinet
andpotentialenergiesofallofitsparticles.
internalreflection:whenarayoflightstrikestheinner
surfaceofamaterialandsomeofitreflectsback
insideit.
interruptcard:allowsthespeedofanobjectpassing
throughafightgatetobecalculated;atimerstartswhi
thecardbreaksthebeamandstopswhenthebeamisi
longerbroken.
interstellarcloud:acloudofgasanddustthatoccupiei
thespacebetweenstars.
inverselyproportional:twoquantitiesareinversely
proportionalwhenincreasingonequantitydecreasest
otherbythesamefactor;doublingonequantityhalves
theother.
inverted:usedtodescribeanimagewhichisupsidedov
comparetotheobject.
ionisation:whenaparticle(atomormolecule)become:
electricallychargedbylosingorgainingelectrons.
ionisingnuclearradiation:radiation,emittedbythe
nucleuswhichcancauseionisation;alphaorbeta
particles,orgammarays.
irradiated:whenanobjecthasbeenexposedtoradiatio
isotope:isotopesofanelementhavethesameproton
numberbutdifferentnucleonnumbers.
joule(J):theSIunitoftransferredenergy(orworkdon
workdoneistheforceofonenewton(1N)whenappliei
throughadistanceofonemetre(1m);1J=1Nm.
Kelvintemperaturescale:(ortheabsolutetemperature
scale)thetemperaturemeasuredfromabsolutezero.
Adifferenceintemperatureof1kelvinisthesameas£
differenceof1C.0Kisapproximately-273°C.
491

505. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
kineticenergy:theenergystoreofamovingobject.
kineticparticlemodelofmatter:amodelinwhichmatter
consistsofmovingparticles.
lamina:flattwo-dimensionalshape.
laser:adeviceforproducinganarrowbeamoflightofa
singlecolour(monochromatic)orwavelength.
laterallyinverted:animageinwhichleftandrighthave
beenreversed.
Lenz’slaw:thedirectionofaninducedcurrentalways
opposesthechangeinthecircuitorthemagneticfield
thatproducesit.
lightgates:allowthespeedofanobjectpassingbetween
themtobecalculatedelectronically.
light-dependentresistor(LDR):adevicewhoseresistance
decreaseswhenlightshinesonit.
light-emittingdiode(LED):atypeofdiodethatemits
lightwhenacurrentflowsthroughit.
light-year:thedistancetravelledinspacebylightinone
year(itisequivalenttoabout9.5x105m).
limitofproportionality:uptothislimit,Hooke’slawis
obeyed(soextensionisproportionaltoload).
load:theforce(usuallyweight)stretchesanobject(aspring),
longitudinalwave:awaveinwhichthevibrationisforward
andback,paralleltothedirectionofpropagationof
thewave.
lubrication:usuallyaliquid,itallowstwosurfacesto
slidepasteachothermoreeasily.
magneticfieldlines:representthedirectionthemagnetic
forcewouldhaveonthenorthpoleofamagnet.
magneticfield:aregionofspacearoundamagnetor
electriccurrentinwhichamagneticpoleexperiences
(feels)aforce.
magnetised:whenamagneticmaterialhasbeenmade
magnetic.
mainsequence:astablestarthatisburninghydrogenin
itscore;onceithasusedup12%ofitshydrogenitgoes
ontoanotherstageofitslifecycle.
mass:isthequantityofmatterabodyiscomposedof;
masscausestheobjecttoresistchangesinitsmotion
andcausesittohaveagravitationalattractionforother
objects.
meltingpoint:thetemperatureatwhichasolidmeltsto
becomealiquid.
melting:changingfromsolidtoliquid.
meniscus:curveduppersurfaceofaliquid.
minorplanet:anobjectwhichorbitstheSunbutisnot
largeenoughorfarenoughfromanotherobjecttobe
definedasaplanet.
model:awayofrepresentingasysteminorderto
understandhowitfunctions,usuallymathematical,
molecularcloud:acloudofinterstellargasthatconsilt
mostlyofmolecularhydrogenandiscoldanddense
enoughtocollapsetoformstars.
molecule:twoormoreatomsjoinedtogetherbycheniual
bonds.
moment:theturningeffectofaforceaboutapivot;giwu
byforcexperpendiculardistancefromthepivot...
momentum:thequantitymassxvelocity,/;=mv.
monochromatic:describesarayoflight(orother
electromagneticradiation)ofasinglewavelength.
motoreffect:whencurrentflowsinawireinamagnetii
fieldwhichisnotparalleltothecurrent,aforceisexcrt<d
onthewire.
nationalgrid:thesystemofpowerlines,pylonsand
transformersusedtocarryelectricityaroundacountry
negativecharge:thetypeofelectricchargecarriedby
electrons.
neutral:havingnooverallpositiveornegativeclarge,
neutronnumber(N):numberofneutronsinthenucleus
ofanatom.
neutronstar:acollapsedstarcomposedalmostentirely
ofneutronswhichformswhenastarwithmorethan
eightsolarmassesreachestheendofitslife.
neutron:anunchargedparticlefoundintheatomic
nucleus.
newton(TV):theforcerequiredtogiveamassof1kgan
accelerationof1m/s1
non-renewables:anenergyresourcethatisgoneforever
onceithasbeenused.
normal:thelinedrawnatrightanglestoasurfaceatthe
pointwherearayhitsthesurface.
NTCthermistor:aresistorwhoseresistancedecreases
withincreasingtemperature.
nuclearenergy:energystoredinthenucleusofanatom,
nuclearfission:theprocessbywhichenergyisreleasedby
thesplittingofalargeheavynucleusintotwoormore
smallernuclei.
nuclearfusion:theprocessbywhichenergyisreleased
whentwosmalllightnucleijointogethertoformanew
heaviernucleus.
nucleonnumber(A):(ormassnumber)thenumberof
nucleons(protonsandneutrons)inanatomicnucleus,
nucleon:aparticlefoundintheatomicnucleus;aproton
oraneutron.
492)

506. Glossary
nucleus:small,dense,positivelychargedregionatthe
centreofanatom.
observations:whatyouseehappeninginanexperiment,
ohm(JI):theSIunitofelectricalresistance;
IQ=1V/A.
ohmicresistor:hasaconstantresistance;itsI-V
characteristicisastraightline,sothatthecurrent
throughitisdirectlyproportionaltothevoltageacrossit.
orbit:thepathofanobjectasitmovesaroundalarger
object.
orbitalperiod:thetimetakenforaplanettocomplete
onefullorbitoftheSun.
orbitalradius:theaveragedistanceoftheplanetfrom
theSun.
oscillation:arepetitivemotionorvibration.
pascal:theSIunitofpressure,equivalenttoonenewton
persquaremetre;1Pa=1N/m2=1Pa.
I
jriod:thetimeforonecompleteoscillationorwave;the
limeittakesanobjecttoreturntoitsoriginalposition,
permanentmagnet:magnetisedmagneticmaterialthat
producesitsownmagneticfieldthatdoesnotgetweaker
withtime.
phasesoftheMoon:thedifferentwaystheMoonlooks
WhenviewedfromEarthoveraperiodofonemonth,
pivot:thefixedpointaboutwhichaleverturns;also
Inownasthefulcrum.
planemirror:(orflatmirror)amirrorwithaflat,
inflectivesurface.
planet:alargesphericalobjectthatorbitstheSun
withoutanothersimilarobjectclosetoit.
planetarynebula:abubbleofgassurroundingawhite
dwarfstarthatusedtobetheoutershellofaredgiant
Itomwhichitcollapsed.
pht<ma:acompletelyionisedgasinwhichthe
bmperatureistoohighforneutralatomstoexistso
Hconsistsofelectronsandpositively-chargedatomic
nuclei.
l>b(tingcompass:verysmallcompasswithaneedlethat
lineupwithmagneticfieldlines,allowingchangesin
helddirectiontobeobservedandplottedoveravery
hurtdistance.
plumpuddingmodel:adisprovedmodeloftheatom
Inchimaginedittoconsistofapositive‘pudding’with
।heIronsdottedthroughit.
plumbbob:amass(usuallylead)hangingfromastringto
drIIncaverticalline.
po
1dvecharge:thetypeofelectricchargecarriedinthe
nuelensofanatom.
potentialdifference(p.d.):theworkdoneby(aunit)
chargepassingthroughanelectricalcomponent;another
nameforthevoltagebetweentwopoints.
potentialdivider,partofacircuitconsistingoftwo
resistorsconnectedinseriestoobtainasmallervoltage
thansupplied.
powerlines:cablesusedtocarryelectricityfrompower
stationstoconsumers.
power:therateatwhichworkisdone,ortherateat
whichenergyistransferred.
precise:whenseveralreadingsareclosetogetherwhen
measuringthesamevalue.
pressure:theforceactingperunitareaatrightanglesto
asurface.
primarycoil:theinputcoilofatransformer.
principalaxis:thelinepassingthroughthecentreofa
lensperpendiculartoitssurface.
principalfocus:(orfocalpoint)thepointatwhichraysof
lightparalleltotheaxisconvergeafterpassingthrougha
converginglens.
principleofconservationofenergy:energycannotbe
createdordestroyed;itcanonlybestoredortransferred.
principleoftheconservationofmomentum:thetotal
momentumisconstantanddoesnotchangebecauseof
aninteractionbetweenbodies(suchascollisions).
principleofmoments:whenanobjectisinequilibrium,the
sumofanticlockwisemomentsaboutanypointequalsthe
sumofclockwisemomentsaboutthesamepoint.
process:aseriesofactionsorsteps,oftentakingplace
overalongperiodoftime.
proton:apositivelychargedparticlefoundintheatomic
nucleus.
protoncharge:theelectricchargeofasingle
proton=+1.6x1019C.
protonnumber(Z):(oratomicnumber)thenumberof
protonsinanatomicnucleus.
protostar:averyyoungstarthatisstillgatheringmass
fromitsparentmolecularcloud.
P-waves:fastmoving,longitudinalseismicwaves.
radiationpressure:theoutwardforceduetothehigh
temperatureofthestar.
radiation:energyspreadingoutfromasourcecarriedby
particlesorwaves.
radioactivedecay:theemissionofalpha,betaorgamma
radiationfromanunstablenucleus.
>

507. >
CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
radioactivesubstance:asubstancethatdecaysbyemitting
radiationfromitsatomicnuclei.
radioactivetracing:usingaradioisotopetoinvestigatea
problem.
radiocarbondating:atechniquethatusestheknown
rateofdecayofradioactivecarbon-14tofindthe
approximateageofanobjectmadefromdeadorganic
material.
radioisotope:aradioactiveisotopeofanelement.
randomprocess:aprocessthathappensatarandomrate
andinrandomdirections;thetiminganddirectionofthe
nextemissioncannotbepredicted.
rarefaction:aregionofasoundwavewheretheparticles
arefurtherapart.
raybox:apparatususedtoproducearayoflight.
raydiagram:adiagramshowingthepathofraysoflight.
ray.anarrowbeamoflight.
realimage:animagethatcanbeformedonascreen.
redgiant:astarthatbeganwithfewerthaneightsolar
massesandisburningheliuminitscore;itsshellof
hydrogenhasexpandedandcooled.
redsupergiant:similartoredgiants,theyformwhenstars
witheighttimesthemassoftheSunrunoutofhydrogen
fuelintheircorebutfusionofhydrogencontinuesinthe
outershells
redshift:anincreaseintheobservedwavelengthof
electromagneticradiation(includingvisiblelight)froma
starorgalaxybecauseitismovingawayfromus.
reflectedray:arayoflightwhichhasbeenreflectedfrom
asurface.
reflection:thechangeindirectionofarayorwavewhen
itstrikesasurfacewithoutpassingthroughit.
refraction:thebendingoflightwhenitpassesfromone
mediumtoanother.
refractiveindex:theratioofthespeedsofalightwavein
twodifferentmedia.
relativecharge:thechargeofaparticlerelativetothe
chargeofaproton.
relativemass:themassofaparticlerelativetothemass
ofaproton.
relay:aswitchoperatedbyanelectromagnet.
renewables:anenergyresourcethatwillbereplenished
(replaced)naturallywhenused.
resistance:ameasureofhowdifficultitisforanelectric
currenttoflowthroughadeviceoracomponentina
circuit;itisthep.d.acrossacomponentdividedbythe
currentthroughit.
resistor:acomponentinartelectriccircuitwhose
resistancedecreasesthecurrentflowing.
resultantforce:thesingleforcethathasthesameeffectoi
abodyastwoormoreforces.
right-handgriprule:arulewhichgivesthedirectionof
fieldlinesaroundastraightwirewhenacurrentflows
throughit.
rippletank:ashallowwatertankusedtodemonstrate
howwavesbehave.
ripple:asmalluniformwaveonthesurfaceofwater.
Sankeydiagram:aflowdiagramthatrepresentsthe
principleofconservationofenergy:thewidthofthe
arrowsisproportionaltoenergy.
scalarquantity:somethingthathasmagnitudebutno
direction.
scattergraph:awaydisplayingtwosetsofdatatoseeil
thereisacorrelation,orconnection.
secondarycoil:theoutputcoilofatransformer.
seismicwaves:wavescausedbyearthquakes.
sliprings:adeviceusedtoallowcurrenttoflowtoand
fromthecoilofana.c.generator.
soft(material):amaterialthat,oncemagnetised,iseasy
todemagnetise.
solarcell/photocell/photovoltaiccell:anelectricaldevice
thattransferstheenergyofsunlightdirectlytoelectricity
byproducingavoltagewhenlightfallsonit.
solarmass:equaltothemassoftheSun(2x1030kg).
solarpanel:usedtocollectenergythatistransferredby
lightfromtheSun.
solenoid:anelectromagnetmadebypassingacurrent
throughacoilofwire.
solidfriction:theresistancetomotioncausedwhentwo
surfacesareincontact.
solidifying:(orfreezing)changingfromliquidtosolid,
specificheatcapacity:theenergyrequiredperunitmass
perunittemperatureincrease.
spectrum:(plural'spectra')waves,orcolours,oflight,
separatedoutinorderaccordingtotheirwavelengths,
speedoflight:thespeedatwhichlighttravels(usuallyin
avacuum:3.0x108m/s).
speed:thedistancetravelledbyanobjectperunittime,
springconstant:istheconstantofproportionalityin
Hooke’slawandisameasureofthestiffnessofaspring,
stable:anobjectthatisunlikelytotoppleover,often
becauseithasalowcentreofgravityandawidebase.
494>

508. B«lnbl<star:astarthatisnotcollapsingorexpanding
1»i<iusetheinwardforceofgravityisbalancedby
।ulUitionpressure,whichpushesoutwards.
|«i«<idard:isanabsoluteorprimaryreferenceor
mt.inurement.
|*
inrofmatter:solid,liquidorgas.
electricity:electricchargeheldbyachargedinsulator.
1'downtransformer:atransformerwhichdecreasesthe
111.Igeofana.c.supply.
*i«puptransformer:atransformerwhichincreasesthe
illageofana.c.supply.
»l<.tnenergy:(orelasticenergy)energystoredinthe
।linlgedshapeofanobject.
bMi|»।nova:anexplodingstarthatbeganlifewithmore
limneightsolarmassesandhasrunoutoffuel.
N। slowmoving,transverseseismicwaves,
letupirature:ameasureofhowhotorcoldsomething
imeasureoftheaverageenergyoftheparticlesina
uilmtance.
IIlerminilvelocity:thegreatestspeedreachedbyanobject
ilivnmovingthroughafluid.
Illi
।malconduction:thetransferofthermalenergybythe
'Ibhitionofmolecules.
f
lln।nullconductor:asubstancethatconductsthermal
inigy.
Illwimalenergyenergytransferredfromahotterplace
।
"
iolderplacebecauseofthetemperaturedifference

Imienthem.
Blhrminlexpansion:theincreaseinvolumeofamaterial
.<lumitstemperaturerises.
Ill’hiilinsulator:asubstancethatconductsverylittle
iIkimalenergy.
lii’
।Internalreflection(TIR):whenarayoflightstrikes
liinnersurfaceofamaterialand100%ofthelight
i.Hi.r.backinsideit.
Ifhitormer:adeviceusedtochangevoltageofana.c.
11"11icitysupply.
It*niwave:awaveinwhichthevibrationisatright
Hi.h.tothedirectionofpropagationofthewave.
|Wt|»switch:safetydevicethatincludesaswitchthatopens
11111.)whenacurrentexceedsacertainvalue.
UiMHthelowestpointofawave.

«n
।;adevicethatismadetoturnbymovingair,
iiiiinorwater;oftenusedtogenerateelectricity.
P "effect:whenaforcecausesanobjecttorotateor
*i
11klmaketheobjectrotateoftherewerenoresistive
inn11
Glossary
ultrasound:anysoundwithafrequencyhigherthan
20000Hz.
ultravioletradiation:electromagneticradiationwitha
wavelengthshorterthanthatofvisiblelight.
unmagnetised:whenamagneticmaterialhasnotbeen
mademagnetic.
unstable:anobjectthatislikelytotoppleover,often
becauseithasahighcentreofgravityandanarrowbase,
upright:usedtodescribeanimagewhichisthesameway
upastheobject.
variableresistor:aresistorwhoseresistancecanbe
changed,forexamplebyturningaknobormovinga
slider.
vectorquantity:hasbothmagnitude(size)anddirection.
vectortriangleagraphicalrepresentationofvectors
intwodimensionssothattheresultantvectorcanbe
calculated.
velocity:thespeedofanobjectinastateddirection,
virtualimage:animagethatcannotbeformedona
screen.
voltage:theenergytransferredorworkdoneperunit
charge;itcanbeimaginedasthepushofabatteryor
powersupplyinacircuit.
voltmeter:ameterformeasuringthep.d.(voltage)
betweentwopoints.
volts(V):theSIunitofvoltage(p.d.ore.m.f.);1V=1J/C.
volume-thespaceoccupiedbyanobject.
watercycle:waterevaporatesfromthesurfaceofthe
Earth,risesintotheatmosphere,cools,condenses,and
fallsasrain.
watt(W):theunitofpowerwhen1Jofworkisdoneper
unittime;1W=1J/s.
wavespeed:thespeedatwhichawavetravels.
wavefront:alinejoiningadjacentpointsonawavethat
areallinstepwitheachother.
wavelength:thedistancebetweentwoadjacentcrests(or
troughs)ofawave.
weight:thedownwardforceofgravitythatactsonan
objectbecauseofitsmass.
whitedwarf:thefinalstageofastarthatstartedwith
fewerthaneightsolarmasseswhenallitsfuelhasbeen
usedup.
workdone:theamountofenergytransferredwhenone
bodyexertsaforceonanother;theenergytransferredby
aforcewhenitmoves;workdone=energytransferred.
495

509. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
Keyequations
, changeinvelocity
acceleration= —
timetaken
„. usefulenergyoutput
efficiency=
totalenergyinput
a=—
At energytransferred=currentxp.d.xtime
i ccr11//2a
gradient
accelerationoffreefall(m/s2)= —
E—
IVt
„workdoneonthecharge
e.m.f.=
charge
E-—
Q
accelerationoffreefall=g=
average:theaverageof12,15and16is14.3:
12+15+16_
143
bE^=-mv2
2
, j2xirxorbitalradius
averageorbitalspeed=
orbitalperiod
_
2^r
V
T
bEp=mgbh
_
4?r2
g
G
c_
bE
mbO
_
w
%
m
changeinpressure=densityxaccelerationdueto
gravityxdepth
gravitationalpotentialofthelead
=massofleadxgxheightoltut
bp=pgbh
impulse=forcextimeforwhichforceacts
conversionbetweenKelvintemperarureanddegrees
Celsius
T(K)=0(°C)+273
impulse=Fbt=b(mv)
imageheightu
magnification=——
;=-
objectheightd
criticalangle:
»=—
sinc
momentofaforce=forcexperpendiculardistant
fromthepivot
charge
current=
momentum,p-mv
time
. usefulenergyoutput,,„
percentageefficiency= x100"n
totalenergyinput
density=
-»_
volume
bE
power,p=—
496y

510. Keyequations
power=currentxp.d.
P=IV
powerintoprimarycoil=poweroutofsecondarycoil
4X= Vs
powerloss=squareofcurrentinthecablexresistance
P=PR.
force
pressure
died
F
P~A
,workdonebythecharge
p.d.= -
charge
refractiveindex:
_
sinz
n———
sinr
relationshipbetweenpressureandvolumeforgasata
constanttemperature:
pV=constant
p.d.
resistance=
current
resistancefortworesistorsusedasapotentialdivider:
R2V2
, _
changeinmomentum
resultantforce=
unittime
bt
, . energyrequired
specificheatcapacity=
massxchangeintemperature
.., . energyrequired
specificheatcapacity=
massxtemperatureincrease
speed=
distance
time
springconstant=—:
force
—
unitextension
voltageacrossprimarycoil_numberofturnsonprimary
voltageacrosssecondarycoilnumberofturnsonsecondary
VN
Vs“X
wavespeed=frequencyxwavelength
v-fX
weight=massxg
workdonebyaforce=forcexdistancemovedbythe
forceinthedirectionof
theforce
W=Fd=bE
497>

511. )CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
>Index
absolutetemperaturescale163
absolutezero157,163
absorbers201-3
absorptionspectra475
a.c.generators399400
acceleration24,25-31,45-6
calculation32
duetogravity48
equation32
forceandmassand524
offreefall48
fromspeed-timegraphs33-6,37-8
units33,52
accelerator46,52
accretion457
accretiondiscs457
activity43642
air,convectionin198
airpressure161
airresistance45,49
airbags55,56
aircraft60
Alaska455
alphadecay435
alphaparticles(a-particles)417,431-3,442
deflection435-6
alphascattering417-18
alternatingcurrent(a.c.)323
aluminiumfoil382,442
amber312,326
americium,isotopes435,442
ammeters324-5,330,349,385
ammonia480
ampere(amp)(A)324-5
amplitude(A)222,262,263
analogueclocks10
analoguemeters324
analoguesignals286-7
AndromedaGalaxy469-70
angleofincidence(i)232,237
angleofreflection(r)232
angleofrefraction(r)237
anticlockwisemoments70
argon,isotopes445
Aristotle467
armatures380
artificialintelligence348
asteroidbelt456
asteroids103,456,457
atmosphericpressure93, 94,95-6
atmosphericstopper95
atomicnumbersseeprotonnumbers
atomicstructure415-18
atoms156, 196,315-16,414-24
andelements419-20
size418
attraction,electrostatic310,315
attractiveforces159
averageorbitalradius460
averagespeed20,21
axisofEarth454,455
axisofsymmetry75
backgroundradiation429-30,440
balloondebates288-9
balloons196-7,310
barmagnets295,297-8,400
batteries105,108,323,338
symbol349
seealsocells
beams,balancing68-9,71-2
beamsofchargedparticles,andmagneticfields
386-7
bells352,380
bending86,90-1
betadecay435
betaparticles(0-particles)431-3,442-3,445
deflection435-6
BigBangtheory476
bimetallicstrips172-3
binarystarsystems480
biomassfuels(biofuels)127-8
blackdwarfstars472,474
blackholes473,474
blacksmokers480
blockadaptors365
bluetooth286
boilers130
boiling155-6,175,182
boilingpoints163,180,181
498

512. Index
bonds159
bonfires206
Boyle’slaw164-5
brakes46,52
brakingforce149
brightness341-2
Brownianmotion158,159
brushes386,399
bungeejumping86
Burnell,JocelynJoyce278
burns284
buses25,77-8
cabletelevision286
cakemonstermodel329-30
calibrations3,163
cameras250,282
cancer278, 283,284,428-9,430,433
treatment278,283,443
candlewax181
CanisMajorDwarfGalaxy469-70
carcoolingsystems206
carcrashes55-6
carignition381
carbon480
isotopes433,435,442,445
carpets316
cars,driverless348
catapultfield382-3
catapults106
Cat’sEyeNebula472
cavitywalls204,205
cells(battery)323,327-8,333,337-8
symbol349
cells(living)433,443
Celsiustemperaturescale163
centreofgravity71,72,73,75-7
finding76-7
CERN387,423
CFLs341-2
changesofstate155-6,179-85
charge(electric)(0324,326
conservationof358
equations326,358
units315
chargedparticles315-16
charging310-12
frictionand312-13,316
chemicalenergy105,106
chemotherapy443
circuitbreakers366
circuitdiagrams323
circuits(electrical)347-70
components349
model329-30
circularmotion50-2
circumference,measurement4
circumnavigation19
climate206-7
clocks10,11-12,92
clockwisemoments70
CMBR477-8
coal128,129
cobalt296
collisions56-9,103
comets103,456,457,459
communications285-9
commutators384,386
compasses296,299,302-3
plotting298,378-9
compressionofgases86,90-1,93
compressionofwaves218,264
condensation155,180
conduction191-6,197,204
conductionelectrons325
conductors
electricalseeelectricalconductors
thermal192-4,195
conservationofcharge358
conservationofenergy110-14,338,467
conservationofmomentum57-8
constantofproportionality91
contactforces45
contamination429,430
continentaldrift284
convection196-200,204
convectioncurrents197,198,199,207
conventionalcurrent302-3
converginglenses246-8,249,250-1,252
converters287
cooling180,182,183,184
copperrods313
cosmicmicrowavebackgroundradiation(CMBR)
477-8
coulomb(C)315,316
countrate430,436
corrected440-1
CrabNebula473
crests261,262,264
criticalangle(c)242-4
andrefractiveindex244
crowbars68
cubit2
Curie,Marie428
499)

513. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
current(electric)(7)323-7 dwarfplanets453,456 s
changing331 dynamos395
equation326,335
induced395-6 Earth
magneticeffect378-82 circumference2,13
measurement324-5 conditionsforlifeon480
inparallelcircuits354,358-64 gravitationalforce459
inpowertransmission406 gravity48,49,114
units324-5 magneticfield294-5,299
currentflow325-6 orbitofMoonround455
current-voltagecharacteristics335-7 orbitofSun454-5,468
orbitalspeed460
dams93,129 spin453
darkenergy480 tiltofaxis454
dayandnight453 earthwires367,368
deceleration24,25-7,32, 33,46 earthedappliances367
deformations86 earthsymbol349
density5-9,197 earthquakes263,264
measurement7 eccentricity459,460
andpressure95-6 echoes219,220
units6 effectiveresistance353,354,358-9,364
depth,andpressure95-6 efficiency111-12,118,144-5,147
deuterium421,422 calculation113
diaphragm161 Einstein,Albert260,467
diffraction270-2 Einstein’sequation422
diffusion158-9 elasticdeformation86
digitalclocks10 elasticenergy105,106
digitalmeters324 elasticlimit(limitofproportionality)88,90
digitalphonecalls287 electricchargeseecharge(electric)
digitalsignals287 electriccookers365
diminishedimages248 electriccurrent
dinosaurs456 seecurrent(electric)
diodes337,349,352-3 electriceels322
light-emitting(LEDs)349,352 electricfieldlines315
directcurrent(d.c.)323 electricfields314-16
discharging310-12 electricforce315
dispersionoflight253-5 electricguitars394
displacement4-5 electricmotorsseemotors
distance electricshocks316,324,365,367,369
fromspeed-timegraphs27-9,37 electricalcables365
measurementinspace470-1 electricalcircuitsseecircuits(electrical)
measurementwithsupernovae480 electricalcomponents349
units20 electricalconductors313-14,337
distance-timegraphs19,24-5,31-2,37 forcesoncurrent-carrying382-4
distortion286,287 junctionof349
diverginglenses246-7,251-2 electricalenergy106,337^42
DNA433 calculation338-9
Dopplereffect475-6 equation338
doubleinsulation368 units339-40
drag45 voltageand338
seealsofriction electricalinsulators313-14,337
draughts183 electricalplugs367-8

514. Index
electricalpower338
equation338
electricalquantities321-43
electricalresistanceseeresistance(electrical)
electricalsafety365-9
electricitycost340-1
electricitygeneration395^01
electricitymeters339,367
electricitysupplyindustry399
electromagneticforces37688
electromagnetichazards284-5
electromagneticinduction393-409
andfieldlines398
principles396
electromagneticradiation107,200
applications278
electromagneticspectrum277-89
electromagneticwaves200,279-84
communicatingusing285-9
speed280
uses281-4,285
electromagnets300-3,378
strength300,301-2
electromotiveforce(e.m.f.)327-8,337
combining328
equation328
electronbeams386-7
electroncharge315,316,418-19
electronflow325-6
electrons196,312-14,315,416,418-19
conduction325
free196,313
mass416
electrostaticcharge310
electrostaticenergy106
elements419-21
ellipses459,460
emergencystops149
e.m.f.seeelectromagneticforce
emitters201-3
endoscopes245
energy
calculation114-17
makingbetteruseof112-13
types104-5,106
energychanges102-19
energyconservation110-14,338,467
energydissipation113
energyefficiency111-12,118,144-5,147
calculation113
energyflowdiagrams109
energyresources123-36
comparing131-3
usingtogenerateelectricity130
energystores104—7,135
energytransfers102-19,141,147,261,338
equation340
engineering,radioactivetracingin444-5
enlargedimages248
environmentalimpact131
Equator454,455
equilibrium69,72
Eris453
Europa480
evaporation155-6,182-3
events107
exercisebikes401
expansion,thermal172-5
expansionjoints173
exponentialdecaygraphs437
extension
inbending90-1,93
ofspring87-8,89,90,91
extrasolarplanets481
eyes251,252,284
eyesightproblems251-2
falling49-50
Faraday,Michael396
faultdetection443
ferrite296
festivelights354
fibreoptics245
filamentlamps323,336,341-2
fireworks162
fixedpoints163
Fleming’sleft-handrule383,385-6,436
Fleming’sright-handrule398-9
fluids197
focallength247
focalpoint(principalfocus)247,249
fog477
foodirradiation443-4
forces42-62,66-79
actingonsolids86
addition59-60
alongsamestraightline46-7
arrowrepresentation43, 44,46,59-60
attractive159
balanced47
calculation52-4
doingnowork143
andkineticmodel159
andmassandacceleration52-4
501>

515. >
CAMBRIDGE IGCSE™PHYSICS:COURSEBOOK
andmatter84-97 Glennie,Evelyn215
momentsofseemoments globalwarming154,206-7
andmotion42-62 gold184,417
asrateofchangeofmomentum55 graphite349
resultant46-7,50,59-60 graphs
andturningeffects66-79 distance-time19,24-5,31-2,37
unbalanced45-6 load-extension87-8
units44,52 speed-time19,26-9,33-6,37-8
forensics283 gravitationalfield,uniform49
Fossett,Steve19 gravitationalfieldstrength48,458
fossilfuels125,128-9,131,133 gravitationalpotentialenergy(g.p.e.)104,106,114-15,4>9
fracking135 calculation115
Franklin,Benjamin309,312,326 equation143
Fraunhofer,Joseph474-5 gravitationalwaves260,273
freeelectrons196,313 gravity47-9,143,457
freezers352 greenhouseeffect206
frequency(/)222,223,262-3,266,280 Gupta,Sunil309
units222,262
friction45,61,111,310 habitablezone468
andcharging312-13,316 Hahn,Otto415
coefficientof45 Hale-Bopp456
solid45,61 half-life437^12
fundamentalforces315 modelling440
fuses349,366 Halley’scomet103,459
hammerthrow52
galaxies469-74 hardmaterials296
recessionspeed476 heaters,symbol349
GalileoGalilei453,467 height115
galvanometers324,325 helium181,472,474
Galvini,Luigi322 isotopes421
gammadecay435 Helmholtz,Hermannvon467
gammarays278,283,284,430,431^4,443-4 hemispheres
non-deflection435 compassesfor303
productionbySun468 ofEarth454-5
gas,asfuel125,128 Herschel,William279,453
gasgiants457 hertz(Hz)222,262
gaslaws164-6 Higgsboson423
gases155,158-9 highjump116
andconduction196 high-wireartistes75
density6 homeinsulation204-5
expansion173-4 Hooke,Robert88,92
andkineticmodel160-1 Hooke’slaw88,91
particlestructure157 horseshoemagnets398
Geiger,Hans417 housefires369
Geigercounters430,431,436-7,440 Hubble,Edwin474,476
Geiger-Mullertubes430 Hubbleconstant476
generators130,349,395-401 Hubbletime476
geneticmutations433 Hubble’slaw476,478
geothermalenergy129,133 hydrocarbons480
glaciers154 hydroelectric/waterpower127,133
glass90,173,205,324 hydrogen457,468,471,472,474
andlight237-9,240,241-5,253 isotopes421,422

516. Index
hydrothermalvents480
HyperloopOne43-4,61
ice171
density6,171
images233
immersion4
impulse54-5
equation54
incidentrays232-5
inducedcurrent395-6
inducede.m.f.(inducedvoltage)395-7
direction400
increasing396,400
inducedmagnetism296-7
infraredradiation200-3,279-80,282,284,286
insulation
double368
electrical365
home204-5
insulators
electrical313-14,337
thermal192,194-5
internalenergy105,106,175
temperatureand162,175
internalreflection241-2
internet286
interruptcards22
interstellarclouds471
inverselyproportionalquantities165
invertedimages248
ionisation416,432-3
ionisingnuclearradiation432-3
iron296,300,405
ironfilings297,298,378-9
irradiation429-30
isotopes421-2,431
joule(J)142, 147,339
Jupiter453,456,457,458,459
Kelvintemperaturescale163
kilowatthour(kWh)339
kineticenergy(k.e.)104,106,116-17,459
equation116
kineticparticlemodelofmatter156-67
evidencefor158
explanationsusing158-9
forcesand159
gasesand160-1
kites309
lamina76-7
lamps323,336,341-2
symbol349
LargeHadronCollider(LHC)423
lasers231,245,256
laterallyinvertedimages233
lattices336
lawofreflection232,241
lead432,433
LEDlamps341-2
LEDs349,352
length
measurement3-4
units4-5
lenses246-53
converging246-8,249,250-1,252
diverging246-7,251-2
andeyesightproblems251-2
raydiagramsfor248-50,251
Lenz’slaw400
life
beyondEarth480-1
conditionsfor480
lifters124
lifting68,141
light229-55
energytransferby106
speedof219,239,240,267,280
lightbulbs104,108,109,147
energy-saving341-2
light-dependentresistors(LDRs)349,350-1
light-emittingdiodes(LEDs)349,352
lightgates21-2
lightrays231-9
changingdirection238-9
lightwaves219,270,272
light-years457,469
lightingcircuits354,365
lightning309
LIGO260
limitofproportionality(elasticlimit)88,90
linesofforce315
liquids155,158-9
andconduction196
density6,8
expansion173,174
particlestructure157
lithium474
litre166
livewires367,368
load87
load-extensiongraphs87-8
503>

517. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
longsight252
longitude12
longitudinalwaves217,264-5
loudness161,216,222
loudspeakers106,263,383—4
lubrication111
lumen341-2
luminescence428
luminosity480
lungcancer433
MacArthur,Ellen19
maglev43,61
magma154
magneticattraction295,299-300
magneticfieldlines297-9,303,378,380
andinduction398
magneticfields297-303
andbeamsofchargedparticles386-7
comparingstrengthanddirectionof380-1
interacting299-300
magneticmaterials296
magneticrepulsion295,299-300
magnetisedmaterials296-7
magnetisingcoils,symbol349
magnetism293-304
induced296-7
magnets295-8,398,400
magnifyingglasses247,250-1
mainsequencestars472
mantisshrimp85
marineexploringvehicles93
Mars453,456,457,458
Marsden,Ernest417
mass5,48-9
forceandaccelerationand52-4
units48,52
andweight48-9
massnumbersseenucleonnumbers
massspectrometers445
materialtesting224
measurements1-14
mechanicalenergy106
medicaldiagnosis444
medicalscans224,444
Meitner,Louise415
melting155-6
meltingpoints163,180,181
meniscus4
Mercury(planet)453, 456,457,458, 459,460
mercury(element)181,341
merry-go-rounds51
metals
aselectricalconductors323-4,325
specificheatcapacity176,177
andthermalconduction195-6
meteorites467
meteoroids456,457
mgh143
microbes443^1,480
microscopes246,282
microwaves282,284,285,477-8
MilkyWay469-70
minorplanets456,457
mirrors232-6
mnemonics288
mobilephones(cellphones)282,284,286,296
charging368,401
models156,261,300
molecularclouds471,474
molecules156
moments68-9
balancing70-2,73,76
calculation69-74
equation69
principleof70
units69
momentum54-9
incollision56-9
conservationof57-8
equation55
monochromaticlight253—4
months454,455
Moon455-6
gravity48,49,114-15
orbitofEarth455
phases455
reflectorsonsurface230
Moonbase,energyfor124
moons456
mosquitosoundalarm223
mosquitos54
motion18-38,42-62
motoreffect382,383^1,385,388,394
motorisedshorts377
motors377,382-6,387-8
asgenerators395
keepingturning386
starter381
symbol349
multi-plugadaptors365-6
multi-waybarextensions365
muons309
musicalsounds217,225
504y

518. Index
Musk,Elon43
mutations433
nationalgrid401-2,408
naturalgas125,128
negativecharge310,326
negativeions416
Neptune453,456,457,458
neutralcharge312,418-19
neutralwires367,368
neutronnumbers(TV)420-2
neutronstars473,474
neutrons418-22
Newton,Isaac253,474
newton(N)44,52,142
nickel296
nightandday453
non-magneticmaterials296
non-metals,andthermalconduction195-6
non-renewables130
normallines232
NorthPole,geographical295,299,302
northpoles295,302
northernhemisphere454-5
N'l'Cthermistors351
nuclearbombs415
nuclearenergy105,106,129,133,421
nuclearequations422
nuclearfission129,415,422
nuclearfuels124,129,131,133
nuclearfusion133,134,422,468,473
nuclearpowerstations399,421,430
nuclearweaponstesting445
nucleonnumbers(X)420-2
nucleons418-19
nucleus416-19
charge417,422
discovering416-17
mass422
lize418
unstable431
ifovations158
ihcancurrents207
ihm(Q)330,331
ohmicresistors336-7
ml128
tppenheimer,J.Robert415
iplicalfibres245,286
mhitalperiod(T)460
oihitalradius459-60
speed459-60
iquation460
orbits454,455,457,459
OrionNebula471
oscillation10,11-12
oscilloscopes221-2,224
ozonelayer279
P-waves264—5
parachutes49-50
parallax470-1
parallelconnection327,330,358-64
particleaccelerators387
particlemovement,andtemperature157
pascal(Pa)94,166
p.d.seepotentialdifference
peaks(crests)261,262,264
pendulums10-12
penetratingpower432,442-3
Penzias,Arno477-8
perfume158
period(7)10,262-3
units262
PeriodicTable419
periscopes236
permanentmagnets295-7
photocells126
photosphere468
photosynthesis125,127-8,230,282
photovoltaiccells126
pivots68,75
planelamina76-7
planemirrors233-6
planetarynebulas472,474
planetarypatterns460-1
planets453,456-61
dataabout458
formation457
plasma468
plasticrods310-11,312,314
plasticsheeting442
platetectonics294
plottingcompasses298,378-9
pluckedinstruments225
plugs367-8
plumpuddingmodel416-17,418
plumbbob10
Pluto453,456,469
polevault114,115
poles,like/unlike295,300
polymers,aselectricalinsulators323-4,325
polypropylene316
polythenerods312-14
poppers85
505>

519. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
positivecharge310,326
positiveions416
potassium,isotopes445
potentialdifference(p.d.)327-8,338
equation335
units327
potentialdividercircuits349,355-7
power145-8
calculation146
electrical338
equation146,338,406
units147
water127,133
wave126-7
wind126
powerlines401-5
powerlosses406-7
equation406
powerratings338
powerstations394,395,401
powersupply,symbols349
precisionofmeasurements3
pressure(p)93-6,160-1
atmospheric93,94,95-6
calculation94-6
changein95
anddepthanddensity95-6
equation94
andtemperature164
units94,166
andvolume164-6
primarycoils403,405,407
principalaxis247
principalfocus(F)247,249
principleofconservationofenergy110-14,338,467
principleofconservationofmomentum57-8
principleofmoments70
prisms253
processes107
projectors250
protium421,422
protoncharge315,316,418-19
protonnumbers(Z)420-2
protons315-16,418-22
charge418
protostars471,472,473
ProximaCentauri457,469
pulsars278
pylons401-2
pyramids12-13,109,144
Pyrexglass173,174
quantumtheory418 *
radiation133,200-3,204,429-30
deflection435-6
detection430
types431-2
radiationdetectionbadges434,436
radiationenergy106
radiationpressure471-2
radiationsuits434
radiationtherapy443^4
radio,wind-up118
radiofrequencyidentification(RFID)281
radiowaves266,278,281,284,286
radioactivedecay431-6
equations435
explaining436-42
modelling440
usesrelatedto445
radioactivehazardlabels434
radioactivesubstances429-30,437
safetyissues433-6
radioactivetracing444-5
radioactivity427^16
radiocarbondating435,445
radiographers430,434
radioisotopethermoelectricgenerator(RTG)108,
118
radioisotopes433
K
effectsoncells433-4
using442-5
radium428-9
radon430,433
rainbows253
raindrops230
ramps144-5
randomprocesses431,432,440
rarefaction218,264
rayboxes231
raydiagrams234-6
forlenses248-50,251
rays231-9
changingdirection238-9
realimages233
forming248
recessionspeed476
redgiants472,474
redsupergiants473,474
redshift475,476
reflectedrays232-5
reflection
lawof232,241
506y

520. Index
oflight230,231-6
ofripples268,269
totalinternal(TIR)241-6,253
reflectors202,230
refractedrays237-9
refraction
oflight230,236^41
ofripples268-9
refractiveindex(n)240-1
andcriticalangle244
equation240
refrigerators126,183,199
regenerators287
reindeer190-1
relativecharge418-19
relativemass418-19
relayraces29-31
relays349,351-2,380-1
remotecontrols282
renewables130
repulsion,electrostatic310,315
resin312
resistance(electrical)(R)330-7
effective353,354,358-9,364
equations330,331,358
measurement332-5
andthickness331,332
units330
resistors327,330,332,335-6,349-51
combinationsof353-64
light-dependent(LDRs)349,350-1
ohmic336-7
inparallel353,354,358-64
inseries353-7,359-64
variable349,350
resultantforces46-7,50,59-60
retina251,252
Richer,Jean322
right-handgriprule378
ringmains367
rippletanks260-1,262,263,264,267-70
ripples260,261
diffraction271-2
reflection268,269
refraction268-9
Ritter,Johan279
rivets172
robots348
rockets59,60,109
rocks,dating445
rubber92
rulerformeasurement3
running104
Rutherford,Ernest415,416,417-18,423
S-waves264-5
Sankeydiagrams111
satellitetelevision282,285
satellites285
scalarquantities25-6,59-60
scattergraphs460
scrapyards301
scubadivers12,165
sea,waveson263,266,270
seasons454-5
seatbelts55
secondarycoils403,405, 407
securityalarms282
securitymarking283
see-saws68-9,70,71-2
seismicwaves264-5
seismometers263
semiconductors337
sensingcircuits352
seriesconnection328,330,353-7,359-64
shadoof74
ships12
shortcircuits367
shortsight251-2
signals,railway382
silicon415-16,480
sinewaves262
sliprings399-400
smokedetectors435,442
smokeparticles158,159
snorkels84
softmaterials296
solarcells126
solarmass468
solarpanels126,133
SolarSystem451-63
distancesin457-8
forces458-9
timesin457-8
solarsystemmodelofatom416
solarwind295
solenoids300,302-3,378,380
solidfriction45,61
solidification155-6,180
solids155,158-9
density6,8-9
expansion172,174
particlestructure157
sonar224
Sorensen,Kirk124
507>

521. )CAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
sound214-26
Dopplereffect475
energytransferby105,106
hearingsounds223
needformediumtotravelthrough218-19
seeingsounds221-2
speedof219-21,267
soundwaves217-19,221-2,263,264, 270,272
southpoles295,302
southernhemisphere454-5
spaceshuttle44
space-timecontinuum260
spacecraft22,44,143
specificheatcapacity(s.h.c.)175-9
equation175
spectroscopy474-5
spectrum/spectra253
visible253,280,282,286,288
speed20-4
calculation31-2
determinationinlaboratory21-2
equation20,22
measurement20
asscalarquantity25
andtime20-1
seealsovelocity;wavespeed
speedcameras20
speedoflight219,239,240,267,280
speedofsound219-21,267
speed-timegraphs19,26-9,33-6,37-8
spindryers51
springbalances73-4
springconstant91
springs
extension87-8,89,90,91
stretching86-90,91
uses92,264
sprinters148
stability74-8
stablestars468,471-2
standardsofmeasurement2,3
stardust474
starlight474-5
stars200,466-82
lifecycle472-4
startermotors381
statesofmatter154-6
changesofstate155-6,179-85
staticelectricity308-17
explaining312-14
steamengines140
stearicacid180-1
steel296,443 *
steel‘tyres’172
sterilisation283,444
stiffness91
stilettoheels94
stoppingdistance149
strainenergy105,106
straws,drinkingthrough94
stretching86-93
springs86-90,91
stringinstruments217,225
strontium,isotopes439
submarines12,93
substations401-2
Sun125,133,279,467-9
colour468-9
composition468
asenergysource125,126,133,263
fusionreactions422
gravitationalfieldstrength458
gravitationalpull457
mass458,468
orbitofEarthround454-5,468
radiationfrom200-1,430
temperature468
theoriesonshining467
sunlight126,133,279
sunshields201
sunspots467
superconductivity354
supemovae473,474
distancemeasurementwith480
Typela480
sweating183
swingball56-8
switches104,323,333,349
symmetry75-6
Szilard,Leo415
tailwind23
TASERs322
technetium,isotopes444
telecommunications245
telescopes246,282,470,474,477-8
temperature162—4
andinternalenergy162,175
particlemovementand157
andpressure164
temperaturescales163
terminalvelocity49
thermalconduction191-6,197,204
thermalconductors192-4,195
508>

522. Index
thermalenergy45,105,106,111
thermalenergytransfers106,189-209
consequences204-9
thermalexpansion172-5
thermalimaging200-1
tllermalinsulators192,194-5
thermalpropertiesofmatter170-85
thermistors349,351,352
thermometers162,163,173
thermonuclearfusion467,468
thermostats173
thicknessmeasurement3,442-3
1homson,J.J.416
thorium124,131
liotopes435
thoron433
minienergy129,133
lime
measurement9-12
andspeed20-1
units20
1ll.m480
Illnnic,RMS12,89
idiimak134
'tillinternalreflection(TIR)241-6,253
'iilliclights352
Hams25,43
11ailsformerequation403
Hainformers402-7
howwork405-7
itep-down403
ilep-up403
ymbol349,404
uiniistors348
uainversewaves264-5
tnhuchets67
HIptwitches366-7
iiIlium421,422
timlghs261,262,264
।niigiten181,336,341-2
tuningforks216,218
।iibines130,395,399
IminShroud445
mimugeffectsofforces66-79
><<alsomoments
muting86
charge315
current324-5
density6
distance20
electricalenergy339-40
force44,52
frequency222,262
length4-5
mass48,52
moment69
period262
potentialdifference327
power147
pressure94,166
resistance330
time20
volume4-5,166
weight48
workdone142
Universe474-81
age476
modelling478-80
unmagnetisedmaterials296-7
unstableisotopesseeradioisotopes
unstableobjects75
uprightimages248
uranium129,131,133,422,430
half-life437
isotopes421,435
Uranus453,456,457,458
vactrain43
vacuumflasks205-6
vacuumtubes386-7
variableresistors349,350
vectorquantities25-6,59-60
addition60
vectortriangles59,60
velocity25-6,32,51
addition60
terminal49
seealsospeed
Venus453,456,457,458
Verne,Jules19
vibrations216-19,221-2
VirgoSupercluster474
hIiiwound223,224
Iniviolet(UV)radiation278,279-80,
'83.284
milts
moderation33,52
virtualimages233,251
visiblelightspectrum253,280,282,286,288
volcanoes154
volt(V)327,328
Volta,Alessandro322,348
509

523. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
voltage327-30
andelectricalenergy338
equations328,354
inpowertransmission402,406,407
inseriescircuits354-5,360-4
steppingup403
voltmeters327-8,330,349
volume(V)
measurement4
andpressure164-6
units4-5,166
wavespeed263,266,267
equation266
wavefronts268-9
wavelength(k)262,266-7,271-2,280
weather207
Wegener,Alfred294
weighingmachines92
weight5,48-50
andmass48-9
units48
wheelbarrows68
whistles223
‘wallofdeath’51
wallsockets365,367
water
anomalousexpansion171
boilingpoint163,180
changesofstate179
convectionin197-8
density6,171
depth224
aselectricalconductor324,365
evaporation182
inhabitablezone468
meltingpoint163,180
specificheatcapacity175-6,178
andthermalconduction194
undergroundflowdetection444-5
watercycle133
waterpower/hydroelectricpower127,133
waterwaves260-4
Watt,James140
watt(W)140,147,338
wavepower126-7
waveproperties259-73
whitedwarfstars472,474,480
whitelight253
Wilson,Robert477-8
wind23,133,207
windinstruments217,225
windpower126
windows204-5
wires
cross-sectionalarea337
fieldlinesaround379
length332-5,337
resistance331,332-5,337
thickness331,332
wood127-8
workdone107,141-2,146
calculation142-5
equation142,143
units142
X-rays278,283,284,418,430
asionisingradiation432
years454-5
510

524. Acknowledgements
>Acknowledgements
Theauthorsandpublishersacknowledgethefollowingsourcesofcopyrightmaterialandaregratefulforthepermissions
granted.Whileeveryefforthasbeenmade,ithasnotalwaysbeenpossibletoidentifythesourcesofallthematerialused,
ortotraceallcopyrightholders.Ifanyomissionsarebroughttoournotice,wewillbehappytoincludetheappropriate
acknowledgementsonreprinting.
Thankstothefollowingforpermissiontoreproduceimages:
UniversalHistoryArchive/GI;EncyclopaediaBritannica/GI;ApcortizasJr/GI;UniversallmagesGroup/GI;Yuri_
Arcurs/GI;JohnWBanagan/GI;Hadynyah/GI;YevgenRomanenko/GI;MikePowell/GI;AlexanderShcherbak/
GI;MarkDadswell/GI;UniversallmagesGroup/GI;Pool/GI;PeterMacdiarmid/GI;RodgerBosch/GI;ThreeLions/
GI;JohnGurzinski/GI;BarcroftMedia/GI;EducationImages/GI;MichaelSteele/GI;AndreasWagner/GI;Bill
Pugliano/GI;RobynBeck/GI;MikeKemp/GI;UniversalHistoryArchive/GI;BostonGlobe/GI;Dea/Castiglioni/GI;
JohnMLundPhotographyInc/GI;SimonDack/Alamy;PaulStarosta/GI;TomohiroTakita/GI;548901005677/GI;
PaulSouders/GI;EarlGibsonIII/Wirelmage/GI;Duncan1890/GI;PhotoAlto/MilenaBoniek/GI;Eclipse_images/
GI;ImageSource/GI;Runstudio/GI;TetraImages/GI;ErinIlisco/GI;TayRees/GI;AlexisRosenfeld/GI;Universal
HistoryArchive/GI;Stockbyte/GI;Mgkaya/GI;RobertDaly/GI;PallavaBagla/GI;VeenaNair/GI;Photography
byJeremyVillasis.Philippines/GI;MontyRakusen/GI;RenaudVisage/GI;MarkusDaniel/GI;Vcg/GI;Christophe
Simon/GI;KeithLance/GI;Nastasic/GI;DonaldMiralle/GI;LionelBonaventure/GI;EricMeola/GI;Patcharin
Saenlakon/GI;Elementallmaging/GI;RoyMehta/GI;Imgorthand/GI;JoseLuisPelaezInc/GI;FajrulIslam/GI;
TopicImagesInc./GI;KristenPrahl/GI;ImageSource/GI;Portisheadl/GI;NarongNiemhom/GI;KlausVedfelt/GI;
Jgi/GI;PeterMuller/GI;ScienceHistoryImages/Alamy;Anilyanik/GI;Capelle.r/GI;Mixetto/GI;BurakKarademir/
GI;PawelOpaska/GI;Nudiblue/GI;MartynF.Chillmaid/SPL(x2);DavidPorter/GI;MattMeadows/SPL;Steve
Bronstein/GI;HarrisonShull/GI;CatherineFallsCommercial/GI;Blondiegirl/GI;MediaProduction/GI;Mint
Images/GI;AnusjaMenon/GI;MarkRalston/GI;MintImages/GI;Floortje/GI;AlexCao/GI;AndreasSolaro/GI;
Fbp/GI;UniversalHistoryArchive/GI;PeterDazeley/GI;Thankyouforchoosingthispicture!WibodeVries/GI;
Skaman306/GI;DrGarySettles/SPL;MartynF.Chillmaid/SPL;AtomicImagery/GI;Bsip/GI;JosephGiacomin/
GI;DemlO/GI;StefanoMontesi-Corbis/GI;SopaImages/GI;SirapholSiricharattakul/GI;PAThompson/GI;
OlexandrKyzyma/Alamy;Fotosearch/GI;TomGowanlock/GI;PascalGoetgheluck/SPL;Pbwpix/Alamy;Thomas
Barwick/GI;TimBrown/SPL;EdwardBerthelot/GI;NgSokLian/GI;VincentBesnault/GI;HiroyukiIto/GI;Andrew
Berezovsky/Alamy;Westend61/GI;JoseA.BernatBacete/GI;SalmonRaymond/GI;Pixologicstudio/SciencePhoto
Library/GI;JamesMarshall/GI;ShepardSherbell/GI;SopaImages/GI;SciencePhotoLibrary;SteveAllen/SPL;
EncyclopaediaBritannica/GI;GettyImages/GI;Science&SocietyPictureLibrary/GI;KlausTiedge/GI;Tristan
Savatier/GI;SandraKonzett/GI;AndrewLambertPhotography/SPL(x2);Dra_schwartz/GI;Andreygudkov/GI;
MartynF.Chillmaid/SPL;HHakim/GI;SunisaKanphian/GI;MattMeadows/GI;PierreVauthey/Sygma/Sygma/
GI;SamOgden/SPL;BSIP/UniversalImagesGroup/GI;Science&SocietyPictureLibrary/GI;UniversalHistory
Archive/GI;RussellKightley/SPL(x2);Spauln/GI;Fstopl23/GI;RadiusImages/GI;DonSmith/GI;Giphotostock/
SPL;GabrielVisintin/GI;MarkGarlick/SciencePhotoLibrary/GI;Claudiad/GI;Kickers/GI;GoceRisteski/GI;
MichaelRoberts/GI;Westend61/GI(x2);Bsip/UniversalImagesGroup/GI;DailyHeraldArchive/SSPL/GI;MirageC/
GI;SDIProductions/GI;WladimirBulgar/SciencePhotoLibrary/GI;DasrilRoszandi/AnadoluAgency/GI;Peter
Dazeley/GI;RogerRessmeyer/GI;BSIP/UniversalImagesGroup/GI;UniversalImagesGroup/GI;AndrewLambert
Photography/SPL;TimRobberts/GI;DerinThorpe/GI;DouglasSacha/GI;DianaRobinsonPhotography/GI;Heidi
OrcinoPhotography/GI;SamEdwards/GI;JurgenReisch/GI;Comstock/GI;Badins/GI;Prisma/UIG/GI;Wikimedia;
Landscapes,Seascapes,Jewellery&ActionPhotographer/GI;Simon2579/GI;PhilDegginger/SPL;Giphotostock/
SPL;JoseLuisPelaezInc/GI;HdhLucas2012/GI;Giphotostock/SPL;LesterLefkowitz/GI;ReinhardDirscherl/
ullsteinbild/GI;CraigF.Walker/TheDenverPost/GI;MichaelGottschalk/Photothek/GI;DamienMeyer/GI;Scott
Olson/GI;RichardBlanshard/GI;ImageSource/GI;DonnaLu;AndrewLambertPhotography/SPL;SciencePhoto
Library;AndrewLambertPhotography/SPL;Gecko753/GI;Giphotostock/SPL;TrevorCliffordPhotography/SPL.
CharlesD.Winters/ScienceSource/SPL(x2);Jean-PierreClatot/GI;MiodragIgnjatovic/GI;Halbergman/GI;Science

525. yCAMBRIDGEIGCSE™PHYSICS:COURSEBOOK
PhotoLibrary/GI;TrevorCliffordPhotography/SPL(x2);PhilippeHuguen/GI;AshleyCooper/GI;SimonMcGill/
GI;Beyondlmages/GI;StocktrekImages/GI;Augenstein/Ullsteinbild/GI;DigitalArt/GI;SteveLupton/GI;Dorling
Kindersley/GI(x2);Prof.PeterFowler/SPL;FabriceCoffrini/GI;Apic/GI;DailyHeraldArchive/SSPL/GI;Public
HealthEngland/SPL;Wellphoto/GI;Hudzilla/GI;AdamGault/GI;ImaginechinaLimited/Alamy;TrevorClifford
Photography/SPL;SciencePhotoLibrary/GI;MarkWilliamson/SPL;PhotoAlto/OdilonDimier/GI;CentreJean
Perrin,ISM/SPL;BabakTafreshi/GI;RogerRessmeyer/GI;Nasa/TheLifePictureCollection/GI;MattAust|Star-
trails.de/GI;Stocktrek/GI;JimSugar/GI;MarkGarlick/SciencePhotoLibrary/GI;SpaceFrontiers/HultonArchive/
GI;MarkGarlick/GI;TomWalker/GI;AmerGhazzal/GI;MarkGarlick/SciencePhotoLibrary/GI;NormanPosselt/
GI;A.V.Ley/GI;MarkGarlick/SciencePhotoLibrary/GI;DorlingKindersley/GI;ZhouNan/VisualChinaGroup/
GI;ManuelBrevaColmeiro/GI;Science&SocietyPictureLibrary/GI;YeAungThu/GI;Historical/GI;Nasa/GI;Ted
Thai/TheLifePictureCollection/GI;EncyclopaediaBritannica/GI.
Key:GI=GettyImages.SPL=SciencePhotoLibrary.
512

527. CambridgeIGCSE™Physics
Thisprintanddigitalcoursebookhasbeendevelopedfromextensiveresearch
throughlessonobservations,interviewsandworkwithourresearchcommunity
(theCambridgePanel)tomeetyourspecificneeds.Activitiesandquestions
developyouressentialscienceskills,withafocusonpracticalwork.Exam-style
questionsgiveyouvaluablepractice.Projectsprovideopportunitiesforassessment
forlearning,cross-curricularlearninganddevelopingskillsforlife.Thereare
multipleopportunitiestoengageinactivelearning,suchasscriptingapodcast,
todiscussionsanddebates.Activitiesbuildincomplexitytosupportyourlearning,
andworkedexampleshelpyouwheneveryouneedtouseanequation.
Theresourceiswritteninaccessiblelanguagewithfeaturestosupport
Englishasasecondlanguagelearners.
EndorsedbyCambridgeAssessmentInternationalEducationforfull
syllabuscoverage
Developsyourscientificenquiryskills,suchasmakingpredictions,
recordingobservations,handlingdata,interpretingdataand
evaluatingmethods
Theprojectfeatureattheendofeachchapterprovidesopportunitiesfor
assessmentforlearning,cross-curricularlearningandskillsforlifedevelopment
Answerstoallquestionsareaccessibletoteachersonlineat
www.cambridge.org/go
Formoreinformationonhowtoaccessanduseyourdigitalresource,
pleaseseeinsidethefrontcover
Thisresourceisendorsedby
CambridgeAssessmentInternationalEducation
SupportsthefullCambridgeIGCSEand
IGCSE(9-1)Physicssyllabuses(0625/0972)
forexaminationfrom2023
HaspassedCambridgeInternational's
rigorousquality-assuranceprocess
Developedbysubjectexperts
ForCambridgeschoolsworldwide
CambridgeCompletelyCambridge
CambridgeUniversityPressworkswithCambridge
AssessmentInternationalEducationandexperienced
authorstoproducehigh-qualityendorsedtextbooks
anddigitalresourcesthatsupportCambridgeTeachers
andencourageCambridgeLearnersworldwide.
Tofindoutmorevisit
cambridge.org/cambridge-international
BrighterThinking
BetterLearning
BuildingBrighterFuturesTogether
Cambridge
UNIVERSITYPRESS
ISBN978-1-108-88807-3