Physics Thermodynamics Notes Grade 11

This presentation is prepared for the students of grades 11 and 12 concluding the chapter thermodynamics. Proper notes with diagrams, facts, and figures are presented. Numericals are solved too.

2. Unit 2: Heat And Thermodynamics
Heat and Temperature
Thermal Expansion
Quantity of Heat : i) Calorimetry
ii) Change of States
Thermal Properties of Matter : i) Gas Laws
ii) Kinetic Theory of Gases
Hygrometry
Transfer of Heat
First Law of Thermodynamics
Second Law of Thermodynamics

3. Chapter 1 Heat and Temperature
Heat
Heatisaformofenergywhichgivesusthesensationofwarmth.Theenergywhichistransferred
fromonebodytoanotherwithoutanymechanicalworkinvolvedisknownasHEAT.Heatenergyis
duetothemolecularvibrationinthebody.Onmicroscopiclevelheatenergypossessedbyabodyis
thesumofkineticenergyofallmoleculesthatmakesupthebody.
Everytypeofmotion(translational,vibrationalandrotational)provideskineticenergytothe
moleculesofthebody.Theenergyassociatedwithconfigurationandrandommotionofthe
moleculesofthebodyiscalledinternalenergy.Heatenergyisapartofinternalenergywhichis
transferredformonebodytoanotheronaccountoftemperaturedifferencebetweenthetwo
bodies.Thus,theheatenergyistheinternalenergyofabodyintransit.
Heatisaformofenergywhichistransferredfromonebodyathighertemperaturetoanotherbody
atlowertemperaturewhentheyareplacedincontactwitheachother.Itisnotnecessarilyaflow
fromabodywithmoreheatenergytoanotherwithlessenergy.Asheatenergyisatotalkinetic
energyofallmolecules,heatcontainedinabodydependonnumberofmoleculesi.emassofthe
body.Themeasurementofheatenergygainedorlostbyabodyiscarriedoutinadevicecalled
Calorimeter.
Its S.I unit is Joule and CGS unit is Calorie. 1 cal= 4.2 J. It is also measured in British thermal unit
(B.T.U). 1 B.T.U = 252 calorie.

4. .
Temperature
Temperatureisthemeasurementofdegreeofhotnessorcoldnessofabodyi.eitrepresentsthe
amountofheatcontainedinabody.Temperatureofabodyisthemeasureofaveragekinetic
energyofthemoleculescontainedinthebody.Asthetemperatureincrease,molecularmotion
increaseandthereforekineticenergyofmoleculesincrease.
Temperaturedeterminesthedirectionofflowofheatwhentwobodieswithdifferentthermalstate
arekeptincontact.Thebodieshavingdifferenttemperatureshavethemoleculeswithdifferent
kineticenergy.Sotheyhavedifferenttendencytoprovideheatenergytootherbodyortoreceive
heatenergyfromotherbody.Infact,temperatureisthemeasureofabilityofabodytotransfer
heattoanotherbody.Thehigherthetemperatureofabody,thegreateristhetendencyofthat
bodytotransferheat.
It is measured with a device called thermometer. Its S.I unit is Kelvin(K). For convenient
measurement, Celsius, Fahrenheit unit are used.

5. Differences between Heat and Temperature
Heat Temperature
1.Heat is a form of energy which gives the sensation of
warmth.
1.Temperature is the degree of hotness or coldness of
a body.
2.It is a measure of total kinetic energy of all
molecules of a body.
2.It is a measure of average kinetic energy of all
molecules of a body.
3.Twobodiesinthermalequilibriummayormaynot
havethesameamountofheat.
3.Two bodies in thermal equilibrium must have same
temperature.
4.Heat is exchangeable. It can flow from one body to
another body
4.Temperature is not exchangeable. Only heat can be
exchanged.
5.Heat flow between two bodies is independent on
the amount of heat contained in the bodies
5.Heat flow between two bodies depends on their
temperature.
6.Heat is not the fundamental property of matter.6.Temperature is the fundamental property of
matter.
7.Heat transfer is the reason behind the temperature
change
7.Temperature variation can be result of gain or loss
of heat.
8.Heat exchange is measured by calorimeter. It is
measured in Joule in S.I and calorie in CGS system
8.Temperature is measured by thermometer. It is
measured in Kelvin in S.I and ℃in CGS system.

6. Thermal Equilibrium:
Whentwobodiesatdifferenttemperaturearebroughtinthermalcontact,heatflowfrombodyat
highertemperaturetobodyatlowertemperatureandtheflowofheatcontinuestillthe
temperatureoftwobodiesbecomesequal.Thisconditionofequaltemperatureachievedbytwo
bodieskeptincontactiscalledthermalequilibrium.Nettransferofheatiszerowhentwobodiesin
contactareinthermalequilibrium.
Theprincipleofthermalequilibriumisappliedinthemeasurementoftemperature.Forinstance,by
placingathermometerincontactwithabodyandwaitinguntilthecolumnofliquidinthe
thermometerstopsrisingorfalling,wecanfindthetemperatureofthebody.Thereasonisthatthe
thermometerisinthermalequilibriumwiththebodywhentheycomesincontactwitheachother.

7. Zeroth Law of Thermodynamics
Statement:Zeroth law states that “If two systems are in thermal equilibrium with a third system,
then they are also in thermal equilibrium with each other”.
Now,iftheinsulatingwallbetweensystemsAandBisreplacedbyconductingwall,therewillbeno
furtherchangeintemperaturetakesplaceinsystemsAandBindicatingthattheyarealsointhermal
equilibriumwitheachother.IfT
A,T
B,andT
CbethetemperatureofsystemsA,BandCrespectively,
then,T
A=T
CandT
B=T
C
Hence,T
A= T
BThis experiment shows that if two systems (A and B) are in thermal equilibrium with
the third system C then they are also in thermal equilibrium with each other.
ToexplainZerothLawofthermodynamics,letusconsidertwo
systemsAandBinitiallyseparatedfromeachotherbyan
insulatingwallbuteachsystemisincontactwithathirdsystemC
throughaconductingwallasshowninfig.Thewholesystemis
surroundedbytheinsulatingwall.Iftheyareleftassuchfor
sometime,thesystemAwillbeinthermalequilibriumwith
systemCandsimilarlysystemBwillbeinthermalequilibrium
withsystemCastheyhaveconductingwall.

8. Concept of Temperature
TheconceptofthermalequilibriumorZerothlawgivesustheconceptoftemperature.Allbodies
whichareinthermalequilibriumhaveacommonpropertywhosevalueisthesameforallthe
bodies.Wecallthispropertyastemperature.Thus,temperatureofasystemcanbedefinedasthe
propertythatdetermineswhetherthesystemisinthermalequilibriumwiththeneighbouring
systemsornot.So,ifanumberofsystemsareinthermalequilibrium,thenacommonpropertyof
thesystemcanberepresentedbyasinglenumericalvaluecalledTemperature.Itmeansiftwo
systemsarenotinthermalequilibriumthentheyareatdifferenttemperatures.
AbsoluteZerotemperature
Thelowestpossibletemperatureofabodyatwhichtheentiremolecularmotionceaseiscalled
absolutezerotemperature.Atthistemperature,kineticenergyofmoleculesbecomeszerobutthe
intermolecularpotentialenergyisnotzero.Sincethetotalenergyofabodyissumofkineticenergy
andpotentialenergyofitsmolecules,absolutezeroisnotazeroenergytemperature.Itis
representedas0kelvinor-273.15℃.
Agraphmaybeplottedtoshowhowthepressureofafixedmassofgasvariesasthetemperatureis
changed.Itisfoundthatwhengraphwereextrapolated,pressurecomesouttobezeroat−273.15
℃andthisisthelowesttemperature.Lordkelvinsuggestedthat−273.15℃shouldberegardedas
thezeroofthetemperaturescale.

9. .
Such scale of temperature is absolute scale of
temperature and−273.15℃as absolute zero of this
new scale and is denoted as0Kelvin.
Thermometry:The branch of physics which deals with the measurement of temperature is
called thermometry. Instrument designed to measure temperature is called thermometer.
Construction of thermometers generally requires a measurable property of a substance which is
sensitive to temperature variation. For example in mercury thermometer, the mercury expand with
increasing temperature.
A thermometer is usually a glass tube essentially consisting of a bulb and capillary tube extending
out from the bulb. The bulb is filled with the thermometric substance which expand over the
capillary tube with the rise in temperature. The rate of expansion being proportional to the rise in
temperature. To measure the temperature, thermometer should be marked with appropriate scale
known as calibration of thermometer.

10. .
For this, two fixed points; ice point and boiling point of water are marked near two end of the
thermometer and the distance between these two point is divided into equal interval.
Advantages of using mercury as Thermometric Substance
Mercuryisusuallyusedasthermometricsubstanceinglassthermometersbecauseofthefollowing
reasons:
i.Itsboilingpointis357°Candfreezingpointis–39°C.Soitcanmeasurewiderangeof
temperature.
ii.Itisgoodconductorofheatandattainsthetemperatureofthebodyquickly.
iii.Itdoesnotwetthewalloftheglasstube.Thisisanimportantpointfortheconstructionofa
thermometer.

11. .
iv.Mercury is shinny and silvery white in colour. Hence the reading can be easily seen.
v.It has a uniform coefficient of expansion and contraction over a wide range of temperature.
vi.It has a low specific heat capacity and hence more sensitive to heat transfer.
vii.Its specific gravity is high. Therefore, even a small bulb can contain sufficient quantity of
mercury.
viii.Due to its cohesive nature, it is generally found in pure form in nature.
Disadvantages of using mercury as Thermometric Substance:
i.Since, freezing point of mercury is only –39°C. So, it is not useful to measure the temperature
of very cold places like Arctic and Antarctic region. In such place, we use Alcohol Thermometer.
ii.Its expansion is less than alcohol.
Advantages of using Alcohol as Thermometric Substance:
i.Its freezing point is very low i.e–117°C. So it is suitable in very cold places.
ii.Its expansivity is very large. So it is more sensitive than mercury thermometer.
Disdvantagesof using Alcohol as Thermometric Substance:
i.It cannot be used to measure the temperature above 78°C as its b.pis 78°C .

12. .
ii.It is colourless, so difficult to observe the reading.
iii.It sticks to the wall of glass tube.
iv.Its expansion is not uniform.
v.The specific gravity of alcohol is smaller than that of mercury.
Fixed point of Thermometer
During calibration of thermometer, two fixed points are marked near two ends in the thermometer.
i)Lower fixed point : It is a temperature of melting point of pure ice at standard atmospheric
pressure.
ii)Upper fixed point : It is a temperature of steam produced from pure boiling water at standard
atmospheric pressure.

13. Temperature Scales
Todefinetemperaturescalecompletely,werequiretemperaturesoftwofixedpointscalledUpper
fixedpointandlowerfixedpoint.
1.Centigrade(orCelsius)scale:Inthisscalelowerfixedpointismarkedas0°Candupperfixed
pointmarkedat100°C.Theintervalbetweenthetwofixedpointsisdividedinto100equal
parts.Eachpartordivisionrepresents1
o
C.
2.Fahrenheitscale:Inthisscalelowerfixedpointismarkedas32°Fandupperfixedpoint
markedat212°F.Theintervalbetweenthetwofixedpointsisdividedinto180equalparts.
Eachpartordivisionrepresents1
o
F.
3.Reaumurscale:Inthisscalelowerfixedpointismarkedas0°Randupperfixedpointmarkedat
80°R.Theintervalbetweenthetwofixedpointsisdividedinto80equalparts.Eachpartor
divisionrepresents1
o
R.
4.Kelvinscale:Inthisscalelowerfixedpointismarkedas273Kandupperfixedpointmarkedat
373K.Theintervalbetweenthetwofixedpointsisdividedinto100equalparts.Eachpartor
divisionrepresents1K.
5.Rankinscale:Inthisscalelowerfixedpointismarkedas492°R
Aandupperfixedpointmarked
at672°R
A.Theintervalbetweenthetwofixedpointsisdividedinto180equalparts.Eachpart
ordivisionrepresents1
o
R
A.

14. Temperature Scales: UFP LFP
1. Centigrade Scale 100℃ 0℃
2. Fahrenheit Scale 212℉ 32℉
3. Reaumer Scale 80°R 0°R
4. Kelvin Scale 373K 273K
5. Rankin Scale 672°R
A 492°R
A

15. If C,F,R,K and R
Abe the temperature of a body in Celsius, Fahrenheit, Reaumer, Kelvin and Rankin
scale respectively, then we have conversion relation
Temperature−LFP
UFP−LFP
=
C−0
100−0
=
F−32
212−32
=
R−0
80−0
=
K−273
373−273
=
R
A
−492
672−492
∴
C
100
=
F−32
180
=
R
80
=
K−273
100
=
R
A−492
180
Relation between temperature scales
In general, the conversion of one scale into
another is given by the relation,
R
1–L
1
U
1–L
1
=
R
2–L
2
U
2–L
2
=�??????�����??????�������????????????���
R
1 = temp. reading of 1
st
temp. scale
L
1= lower fixed point of 1
st
temp. scale
U
1 = upper fixed point of 1
st
temp. scale
R
2= temp. reading of 2
nd
temp. scale
L
2 = lower fixed point of 2
nd
temp. scale
U
2 = upper fixed point of 2
nd
temp. scale

16. .
1.AtwhatpointofthermometricscaledoesKelvinreadingcoincidewith
Fahrenheitscalereading?
Solution:
Let,FandKbetheFahrenheitscalereadingandKelvinscalereading
respectively.Thenaccordingtothequestion,wehave,F=K=x(say)
wehave,
F−32
180
=
K−273
100
Or,
X−32
180
=
X−273
100
or, 18(x–273) =10(x–32)
or,x=574.25
Thus,KelvinscalereadingcoincideswithFahrenheitscalereadingat574.25K
or574.25F.

17. 2. At what temperature will the Kelvin scale reading double the Fahrenheit
reading?
Solution:
Let,theKelvinscalereadingbe2xandFahrenheitreadingbex.
i.e.F=x K=2x
Asweknow,
or,
K−273
100
=
F−32
180
or,
2x−273
100
=
x−32
180
or,9(2x–273)= 5(x–32)
or,18x–2457= 5x–160
or,13x=2297
x=176.7 and2x=353.4
Hencetherequiredtemperatureis353.4Kor176.7F.

18. 3. If a Fahrenheit thermometer reads 100
o
when standard centigrade scale thermometer reads 37
o
, what is the error in the Fahrenheit thermometer?
Solution:
CorrectreadinginCelsius(C)=37
o
FaultreadinginFahrenheit(F)=100
o
CorrectreadinginFahrenheit(F)=? error=?
Now,fromconversionrelation,
C
100
=
F−32
180
37
100
=
F−32
180
F=98.6
o
FaultreadinginFahrenheit(F)=100
o
&CorrectreadinginFahrenheit(F)=98.6
o
Error=Faultreading–Correctreading=F–F=100–98.6=+1.4
o
(Ans)
Similarly,Correctioninthermometer=error=1.4
0

19. .
Relation between Difference in Temperature of different Temperature
Scales
∆C
100
=
∆F
180
=
∆K
100
=
∆R
80
=
∆R
A
180
Where∆C,∆F,∆K,∆Rand∆R
Arepresents change in temperature in Celsius, Fahrenheit, Kelvin,
Reaumerand Rankine temperature scales, respectively.
1.Whatisthetemperaturechangeincentigradedegreewhenthetemperaturechange
inFahrenheitscaleis72°
Solution:Here,changeintemperatureinCentigradescale(C)=?
changeintemperatureinFahrenheit(F)=72
o
F
∆C
100
=
∆F
180
Or,
∆C
100
=
72
180
 C=40C

20. 2.A centigrade and a Fahrenheit thermometer are placed in hot water. The water is then cooled.
What fall of temperature will the Fahrenheit thermometer register, when the centigrade
thermometer records the fall in temperature as 45°.
change in temperature in Celsius (C) = 45°C
change in temperature in Fahrenheit (F) = ?
If C
1and F
1be the initial temperature readingrecorded by the centigrade and Fahrenheit thermometers
respectively then we can write
C
1
100
=
F
1
−32
180
Or,
C
1
5
=
F
1
−32
9
……………………………….(i)
Similarly, If C
2and F
2be the final temperature readings recorded by the centigrade and Fahrenheit
thermometers respectively then we can write
C
2
100
=
F
2
−32
180
Or,
C
2
5
=
F
2
−32
9
……………………………….(ii)
Subtracting equation(ii) from equation(i), we get
C
1
−C
2
5
=
F
1
−F
2
9
or,
??????
5
=
??????
9
∴F=
��
5
×9=81°FHenceFahrenheitthermometerregister81°Ffallintemperature.

21. .
Faulty thermometer:A thermometer having wrong calibration is known as faulty thermometer.
Numerical Problems
1.A faulty thermometer has its fixed points marked at –2 and 98. What is the correct temperature
on the Celsius scale when the thermometer reads 20C?
Solution: Forfaultythermometer Forcorrectthermometer
ReadingR
1=20C ReadingR
2=?
LowerfixedpointL
1=–2C LowerfixedpointL
2=0C
UpperfixedpointU
1=98C UpperFixedpointU
2=100C
Now,usingtherelation,
R
1–L
1
U
1–L
1
=
R
2–L
2
U
2–L
2
Faulty temperature scale Correct temperature scale
Lower fixed point = L
1 Lower fixed point = L
2
Upper fixed point = U
1 Upper fixed point = U
2
Temperature reading = R
1 Temperature reading = R
2
The conversion relation is
R
1–L
1
U
1–L
1
=
R
2–L
2
U
2–L
2

22. .
or,
20–(–2)
98–(–2)
=
R
2–0
100–0
Or,
22
100
=
R
2
100
∴ The correct temperature R
2= 22C (Ans)
2. A thermometer has wrong calibration. It reads the melting point of ice -12C. It reads 72C in
place of 60C. Calculate the temperature of boiling point of water on this scale.
Solution:
Forfaultythermometer Forcorrectthermometer
ReadingR
1=72C ReadingR
2=60C
Lowerfixedpoint(m.pofice)L
1=–12C Lowerfixedpoint(m.pofice)L
2=0C
Upperfixedpoint(b.pofwater)U
1=? UpperFixedpoint(b.pofwater)U
2=100C
Now,usingtherelation,
R
1–L
1
U
1–L
1
=
R
2–L
2
U
2–L
2
72–(–12)
U
1–(–12)
=
60–0
100–0
or,
84
U
1+12
=
60
100
∴U
1=128C
Thetemperatureofboilingpointofwaterinfaultythermometeris128C(Ans)

23. 3.Acentigradethermometerreads1Catmeltingpointoficeand99Cattheboilingpointof
wateratnormalpressure.Whatisthecorrecttemperaturewhenitreads25Candatwhat
temperatureisitsreadingexactlycorrect?
Solution:
Forfaultythermometer Forcorrectthermometer
ReadingR
1=25C ReadingR
2=?
LowerfixedpointL
1=1C LowerfixedpointL
2=0C
UpperfixedpointU
1=99C UpperFixedpointU
2=100C
Now,usingtherelation,
R
1–L
1
U
1–L
1
=
R
2–L
2
U
2–L
2
or,
25−1
99−1
=
R
2–0
100–0
or,
24
98
=
R
2
100
∴R
2=
24
98
×100=24.5C
LetθCbethetemperatureatwhichreadingiscorrect.Then,
ReadingR
1=θC ReadingR
2=θC
LowerfixedpointL
1=1C LowerfixedpointL
2=0C
UpperfixedpointU
1=99C UpperFixedpointU
2=100C
Now,usingtherelation,
R
1–L
1
U
1–L
1
=
R
2–L
2
U
2–L
2
or,
θ−1
99−1
=
θ–0
100–0

24. .
Or
θ−1
98
=
θ
100
or, 100θ–100 = 98θ
Or, 2θ= 100 ∴θ= 50C (Ans)
4. The distance between the upper and lower fixed point is 80cm. Find the temperature on the
Celsius scale if the mercury level rise to a height 10.4cm above the lower fixed point.
Solution: let θbe the required temperature on Celsius scale.
heightofHglevelatLFP,H
0=0cm TemperatureReadingθ=?
heightofHglevelatUFP,H
100=80cm LowerfixedpointL
2=0C
heightofHglevelaboveLFP,H
θ=10.4cm UpperFixedpointU
2=100C
Now using the relation,
H
θ–H
0
H
100–H
0
=
θ–L
2
U
2–L
2
or,
10.4−0
80−0
=
θ−0
100−0
Or,
10.4
80
=
θ
100
∴ θ= 13C (Ans)

25. 5. The pressure of air in a constant volume gas thermometer is 80cm and 109.3cm of Hg at 0C and
100C respectively. Find the temperature of hot water bath when bulb is inserted in it and record
the pressure as 100 cm of Hg
Solution: let θbe the required temperature on Celsius scale.
PressurereadingatLFP(P
0)=80cm TemperatureReadingθ=?
PressurereadingatUFP(P
100)=109.3cm LowerfixedpointL
2=0C
Pressurereadingat(P
)=100cm UpperFixedpointU
2=100C
Now using the relation,
P
θ–P
0
P
100–P
0
=
θ–L
2
U
2–L
2
or,
100−80
109.3−80
=
θ−0
100−0
or,
20
29.3
=
θ
100
∴ θ= 68.25C (Ans)

26. Types of Thermometers
There are various thermometers which depends on some measurable property of a thermometric
substance which are highly sensitive with temperature variation. For example, liquid thermometer
uses the property of expansion on heating, gas thermometer is based on change in pressure with
temperature at constant volume, variation of electrical resistance with temperature is applied in gas
thermometer etc.
1.LiquidThermometer:Liquidthermometersarebasedontheprincipleofchangeinvolumeof
liquidwiththechangeintemperature.Thethermometricliquidusedinthesethermometers
mustbehighlysensitivetoheatsothatexpansioncanbeobservedinverysmallrisein
temperature.Thecommonlyusedthermometricliquidaremercury,alcohol,ethanol,kerosene
etc.Thesethermometerscanmeasurethetemperaturerangefrom-200
o
Cto600
o
C.For
example,Mercurythermometer,Alcoholthermometer,ethanolfilledthermometeretc.
2.GasThermometer:Gasthermometerarebasedontheprincipleofvariationofpressureor
volumewithchangeintemperature.Thesethermometersareverysensitiveasithaslarge
expansion.InsuchthermometersgaslikeH
2,N
2,andHeareused.Gasthermometerscanbe
usedforwiderangeoftemperaturemeasurementfrom-270
o
Cto1600
o
C.Forexample,Constant
volumeH
2thermometer,Callendar’sconstantpressurethermometeretc.

27. .
3.ResistanceThermometer:Resistancethermometerdependsontheprinciplethat,the
resistanceofasubstancechangeswithtemperaturewhencurrentisflowingthroughit.Insuch
thermometermateriallikePlatinumisusedwhoseresistancechangesbycurrentflowing
throughtheplatinumwireinside.Itcanmeasurethetemperaturerangingfrom-250
o
Cto
1200
o
C.Forexample,Platinumresistancethermometer.
4.ThermoelectricThermometer:Thermoelectricthermometerisbasedonprincipleof
thermoelectricityi.eproductionofthermoemfinathermocouplewhenitstwojunctionsare
keptatdifferenttemperature.Thisisverysensitivethermometerthatprovideaccurate
temperatureresultbetween-250
o
Cto1600
o
C.Forexample,copperconstantanthermometer.
5.RadiationThermometer:Radiationthermometerisbasedonthequantityofheatradiation
(Infrared)emittedbyabody.ThesearealsocalledPyrometersusedtomeasurehigh
temperatureorradiationfromsunbeyond1250Kto5000
o
C.Itisalsousedtomeasurevery
highorlowsurfacetemperaturewithoutanycontactwiththesurface.
6.VapourPressureThermometer:VapourPressurethermometerisbasedontheprincipleof
changeinvapourpressurewithchangeintemperature.Itisusedtomeasurelowtemperature
rangingfrom-272.5
o
Cto-150
o
C.Forexample,Heliumvaporpressurethermometer.

28. Short Answer Questions
1.What are the effects of heat?
Ans: when heat is supplied to a body , it can bring about the following effects due to change in
temperature.
i.Rise in Temperature
ii.Change in Volume
iii.Change of State of Matter
iv.Electric Effect
v.Change in Electric Properties
vi.Change in Physical and Chemical Properties
2.Why heat flows from the body at higher temperature to the body at lower temperature?
Ans:Theaveragekineticenergyofthemoleculesofthebodyathighertemperatureisgreaterthan
thatofthebodyatlowertemperature.Whenthisbodyisbroughtinthermalcontactwiththebody
atlowertemperature,thesemoleculesgiveupsomeoftheirKineticenergytolessenergetic
moleculesofthebodyatlowertemperature.Thisultimatelystartstoincreasethekineticenergy
andhencethetemperatureofthebodyatlowertemperature.Thisphenomenonstopswhentheir
averagekineticenergybecomesequali.e.whenboththebodyreachestofinalcommon
temperature.

29. .
3.Absolutezerotemperatureisnotthetemperatureofzeroenergy.Explain.
Ans:ThetotalenergyofabodyisequaltothesumoftotalK.EandP.Eofitsmolecules.Atabsolute
zerotemperature(T=0K=-273.15℃),onlytheK.Eofthemoleculesiszerobuttheintermolecular
P.Eofthebodyisnotzeroandthisremainingenergyiscalledzero-pointenergy.Thus,absolutezero
temperatureisnotthetemperatureofzeroenergy.
4.Howcanthesensitivityofathermometerbeincreased?
Ans:Thesensitivityofthermometercanbeincreasedbyusingfollowingsteps:
1.Thebulbofthermometerisconstructedtohavelargesurfacearea.
2.Thecapillarytubeismadeverythin.
3.Byusingaliquidofhighexpansivity.
5.Amongtwothermometers,onewithsphericalbulbandanotherwithcylindricalbulb,which
willrespondquicklytotemperaturechange?
Ans:Thegainorlossofheatbyabodyisdirectlyproportionaltoitssurfacearea.(i.e.Q∝A).As
thesurfaceareaofcylindricalbulbisgreaterthanthatofasphericalbulb,foragivenvolume,
cylindricalbulbwillrespondquicklytotemperaturechange.
6.whatisthetemperatureofavacuum?
Ans:Accordingtomoleculartheory,thetemperatureofabodyisthemeasureofaveragekinetic
energyofthemolecules.Asthevacuumconsistsofnomolecules,theaverageK.E.ofthemolecules
iszero.So,thetemperatureofvacuumiszero.However,nospaceintheuniverseiscompletelyfree
fromradiation.Fromthisviewvacuummusthavecertaintemperaturewhichdependsonthe
energyofradiation.

30. .
7.Athermometerislaidoutindirectsunlight.Doesthisthermometermeasurethetemperature
oftheair,ofsunorwhat?
Ans:Whenathermometerislaidoutindirectsunlight,itwillnotbeinthermalequilibrium
withtheairarounditandsun.Thus,itexchangesheatwithitssurroundingandtriestobein
thermalequilibriumwhichmakesthethermometricsubstancetochangethereadingwhich
dependsonitsowntemperature.Soitdoesnotmeasurethetemperatureofairandthesun;
insteaditmeasuresthetemperatureofthermometricsubstanceusedinthethermometer.
8.Astudentclaimedthatthermometersareuselessbecauseathermometeralwaysregistersits
owntemperature.Howwouldyourespond?
Ans:whenathermometerisusedtomeasuretemperatureofabody,therewillbeexchangeof
heatbetweenthebodyandmercuryinsidethebulbofthermometeruntiltheyattainacommon
temperature(i.e.thermalequilibrium).Itistruethatthethermometerregistersitsown
temperature.Butduringthemeasurement,thetemperaturerecordedbythermometerisalsothe
temperatureofthebodyinthermalequilibrium.Hencestudent’sclaimwaswrongthat
thermometerareuseless.
9.Whyaclinicalthermometershouldnotbesterilizedbyboiling?
Ans:Therangeofclinicalthermometerisusuallyfrom95℉to110℉andtheboilingpointofwater
is212℉(100℃).Soonsterilizingbyboiling,thecapillaryofthermometerwillburstduetothermal
expansionofmercuryinthecapillary.

31. .
10.Why water is not used as thermometric substance?
Ans:somereasonsfornotusingwaterasthermometricsubstanceare:
i.ithashighvalueofspecificheatcapacitysoitislesssensitivetoheat.
ii.itdoesnothaveuniformexpansionasitshowsthephenomenonofanomalousbehaviour
withintemperaturerangeof0to4
o
C.
iii.itsticksonglassanddoesnotgivecorrectreading.
iv.itistransparent&cannotbeseeneasily
v.itsfreezingpointis0
o
C&boilingpoint100
o
C.So,itcanmeasurethesmallrangeoftemperature.
11.Gasthermometerismoresensitivethanliquid(mercury)thermometer.Why?
Ans:thecoefficientofthermalexpansionofgasismuchlargerthanthatofcoefficientofthermal
expansivityofmercury.Sotherewillbemuchlargerchangeinvolumeofgasascomparedto
mercuryforsameriseintemperature.
12.Mercuryboilsat357
o
C.Howcanthenamercurythermometerbeusedtomeasuretemperature
upto500
o
C?
Ans:Theboilingpointofliquidrisewithincreaseinpressure.Ifthespaceabovethemercury
columninsidethethermometerisfilledwithnitrogengasathighpressure,theboilingpointof
mercuryriseabove500
o
C.Inthiswayitispossibletomeasurethetemperatureupto500
o
Cusing
mercurythermometer.

32. .
13.CanthetemperatureofabodybenegativeontheKelvinscale?
Ans:NO.Thisisbecausetheabsolutezeroonthekelvinscaleistheminimumpossibletemperature.
Ifthetemperaturefallsfromabsolutezero,volumeofgaswillbenegativewhichisimpossible.
14.ExplainthesignificanceofKelvintemperaturescale.
Ans:Kelvintemperatureistheabsolutetemperaturescalewhichisindependentonthe
thermometricpropertiesofanyparticularsubstance.Whiletheothertemperaturescalesdepends
onthespecificpropertyofthethermometricsubstanceused.
15.Thetriplepointofwaterisastandardfixedpointinmodernthermometry.Why?
Ans:Thetriplepointofwaterisuniquei.eitoccursatparticularpressureandtemperature(0.46cm
and273.15K).Butthemeltingpointoficeandboilingpointofwaterchangesduetopresenceof
impuritiesinwater.Hencetriplepointofwaterchosenasstandardfixedpointinmodern
thermometry.

33. Numerical Questions
1.Convert the following temperatures:
i.98.4℉into ℃
ii.-80℃into ℉
iii.5000°R
A into ℉
iv.-460℉into K
v.36°R into ℃
(Ans : i. 36.9℃, ii. -112℉, iii. 4540℉,iv. 0K, v. 45℃)
2.At what temperature Celsius and Fahrenheit scale coincide? (Ans : -40°)
3.Calculate the temperature on absolute scale whose value is twice the reading on centigrade
scale. (Ans: 546K)
4.At what temperature will the reading on Fahrenheit thermometer be double than that of the
centigrade one? (Ans: 160℃or 320℉)
5.A faulty Fahrenheit thermometer shows 206℉where as a centigrade thermometer register 96
℃. Find the correction for the faulty thermometer. (Ans: -1.2℉)

34. .
6.What was the temperature change in Celsius degrees if it is changed from 44.0
o
F to –56.0
o
F
Solution:
•Here,changeintemperatureinFahrenheit(F)=[(–56.0)–44.0]
o
F=-100
o
F
changeintemperatureinCentigradescale(C)=?
Wehave,
∆C
100
=
∆F
180
Or,
∆C
100
=
−100
180
 C=55.6
o
C(Ans)
7.TwobeakerofwaterAandBinitiallyareatsametemperature.Thetemperatureofthewaterin
beakerAisincreasedby10
o
FandthetemperatureofthewaterinbeakerBisincreasedby10K.
Afterthetemperaturechanges,whichbeakerofwaterhasthehighertemperature?(Ans
beakerB)
Hint: F = 10
o
Fand K = 10K
Convert K = 10K into
o
Fand the result is 18
o
F
So temperature of beaker A increased by 10
o
Fand that of B increased by 18
o
F.
Hence beaker B has higher temperature.

35. .
8.A faulty thermometer has its fixed points marked as 5℃and 95℃. What is the correct
temperature in Celsius when this thermometer reads 95℃? (Ans: 100℃)
9.A faulty Celsius thermometer reads -4℃when placed in melting ice and 98℃when placed in
contact with steam at normal pressure. What is the correct temperature on Celsius scale when
this thermometer reads 32°?(Ans: 35.3℃)
10.A centigrade thermometer reads 5℃at melting point of ice and 90℃at boiling point of water
at normal pressure. What is the correct temperature when it reads 20℃, and at what
temperature is its reading exactly correct? ( Ans: 17.65℃, 33.33℃)
11.The temperature of melting ice and that of steam above water boiling at normal pressure are
marked 40 and 200 respectively on a certain thermometer. What is i) the temperature in ℃
when the reading on this thermometer is 80 ii) the thermometer reading when the
temperature is 60℃?( Ans: i) 25℃ii)136 )
12.A constant volume gas thermometer shows pressure reading of 50cm and 90 cm of mercury at
0℃and 100℃respectively. Find the temperature reading when the pressure reading is 60cm
of mercury. (Ans: 25℃)