Dielectric properties

Dielectric properties

1. DIELECTRIC PROPERTIES
Unit IV
PRESENTATION
BY
Dr.L.JOTHI
DEPARTMENT OF PHYSICS
N.K.R.GOVT. ARTS COLLEGE FOR WOMEN
NAMAKKAL

2. Overview of the presentation
•Dielectrics-Introduction
•Basic Definitions in dielectrics
•Types of Electric Polarization
•Dielectric Loss
•Clausius-MosottiRelation
•Determination of Dielectric Constant
•Dielectric breakdown
•PropertiesofdifferenttypesofInsulatingMaterials
•Uses of Dielectrics

3. Dielectrics

4. Dielectric Materials

5. Properties of Dielectric Materials

6. BASIC DEFINITIONS
Electric dipole
Anelectricdipoleisapairofequaland
oppositepointcharges-qandq,separatedby
adistance2a.Thedirectionfromqto-qis
saidtobethedirectionofthedipole.

7. Electric dipole moment
➢Theelectricdipolemomentisameasure
oftheseparationofpositiveand
negativeelectricalchargeswithina
system,thatis,ameasureofthe
system'soverallpolarity.
➢TheSIunitsforelectricdipolemoment
arecoulomb-meter(C⋅m);however,a
commonlyusedunitinatomicphysics
andchemistryisthedebye(D).

8. Electric dipole moment

9. Permittivity
Permittivity,isameasureoftheelectricpolarizabilityof
adielectric.Amaterialwithhighpermittivitypolarizes
moreinresponsetoanappliedelectricfieldthana
materialwithlowpermittivity,therebystoringmore
energyinthematerial.Inelectrostatics,thepermittivity
playsanimportantroleindeterminingthecapacitance
ofacapacitor.

10. Relative Permittivity
•Relative permittivity: Relative permittivity is defined as the
permittivity of a given material relative to that of the
permittivity of a vacuum. It is normally symbolisedby: εr.
•Dielectric constant: The dielectric constant is defined as the
relative permittivity for a substance or material.
•The permittivity εof a medium is given by
ε= D / E
Where:
ε= permittivity of the substance in Farad per metre
D = electric flux density
E = electric field strength

11. Dielectric Constant

12. Polarization
•When a dielectric material is placed in external electrical field,
it's molecule gain electric dipole moment and dielectric said to
be polarised.
•The induced moment per unit volume of dielectric material is
called the electric polarization of dielectric.

13. •Inmostneutralatomsormolecules,the
centerofpositivechargecoincideswiththe
centerofnegativecharge.
•Inthepresenceofachargedobject,these
centersmayseparateslightly.Thisresultsin
morepositivechargeononesideofthe
moleculethantheotherside.
•Thisrealignmentofchargeonthesurfaceof
aninsulatorisknownaspolarization.

14. Polarizability
•Polarizability,whichisrepresentedbythe
Greekletteralpha,α,isexperimentally
measuredastheratioofinduceddipole
momentptotheelectricfieldEthatinduces
it.
α=p/E
•Polarizabilityhas the SI units of C·m²V⁻¹.

15. Electric susceptibility
•Electricsusceptibility,isaquantitativemeasureoftheextentto
whichanelectricfieldappliedtoadielectricmaterialcauses
polarization,theslightdisplacementofpositiveandnegativecharge
withinthematerial.
•Formostlineardielectricmaterials,thepolarizationPisdirectly
proportionaltotheaverageelectricfieldstrengthEsothattheratio
ofthetwo,P/E,isaconstantthatexpressesanintrinsicpropertyof
thematerial.
•Theelectricsusceptibility,χe,inthecentimetre-gram-second(cgs)
system,isdefinedbythisratio;thatis,
χe=P/E.
•In the metre-kilogram-second (mks) system, electric susceptibility is
defined slightly differently by including the constant permittivity of
a vacuum, ε0, in the expression; that is, χe= P/(ε0E).

16. Types of dielectric polarization
There are four types of dielectric polarization
1. Electronic polarization
2. Ionic polarization
3. Orientational polarization
4. Space charge polarization

17. Electronic Polarization

18. Ionic polarization

19. Orientationalpolarization
OrientationalPolarizability, αOis determined from
µO= αOE

21. Space charge polarization

22. Total Polarization
The total amount of dielectric polarization in a
material is the sum of the electronic, ionic and
orientational polarizabilities.
α=αe + αi + α0
Since the space charge polarization is very small
and it is negligible.

23. Dielectric Loss

25. Clausius Mosotti Equation

27. Determination of Dielectric Constant
Schering BridgeSchering Bridge
The Schering bridge use for measuring the capacitance of the capacitor,
dissipation factor, properties of an insulator, capacitor bushing, insulating oil
and other insulating materials. It is one of the most commonly used AC
bridge. The Schering bridge works on the principle of balancing the load on
its arm.

Let, C1 – capacitor whose capacitance is to be determined,
r1 – a series resistance, representing the loss of the capacitor C1.
C2 – a standard capacitor (The term standard capacitor means the capacitor is free from loss)
R3 – a non-inductive resistance
C4 – a variable capacitor
R4 – a variable non-inductive resistance parallel with variable capacitor C4.

28. Breakdown in dielectric Materials

29. Classification of Breakdown
Breakdowninsoliddielectricoccurs,ifsoliddielectric
strengthlessthanelectricstress.BreakdownMechanism
insoliddielectricdependonthetimeofapplicationof
voltage,andcanbeclassifiedasfollows:
i.Intrinsic breakdown
ii.Thermal Breakdown
iii.Electrochemical Breakdown
iv.Discharge Breakdown
v.Defect Breakdown

30. Intrinsic Breakdown
•Inapureandhomogeneousdielectricunder
controlledtemperatureandenvironmental
conditionswegetaveryhighdielectric
(breakdown)strength.Thisisknownasthe
intrinsic.
•Dielectricstrengthwhichdependsmainlyonthe
characteristicsandstructureofthematerial.The
dielectricstrengthobtainedundersuch
conditionsisaroundMV/cmwhichisgenerally
notobtainedinpracticalconditions.

31. Thermal Breakdown

34. Electrochemical Breakdown

36. Discharge breakdown
•Dischargebreakdownisclassifiedasexternalor
internal.
•Theexternalbreakdowniscausedbyacorona
discharge.Suchdischargeisobservedatsharpedgesof
electrodes.Deteriorationaccompaniedbythe
formationofcarbon.Damagedareasbecome
conductingleadingtopowerarcandcompletefailure
tothedielectric.
•Internalbreakdownoccurswhentheinsulatorcontains
occludedgasbubbles.Whenlargebubblesarepresent,
thisoccursevenatlowvoltages

37. Corona Discharge
•The field is non-uniform, an increase in
voltage will first cause a discharge in the gas to
appear at points with highest electric field
intensity, namely at sharp points or where the
electrodes are curved or on transmission lines.
This form of discharge is called a corona
discharge and can be observed as a bluish
luminescence.

38. Defect breakdown
•Ifthesurfaceofthematerialhasdefectssuch
ascracklesandporositythenimpuritiessuch
asdustatthesediscontinuitiesleadingtoa
breakdown.Alsoifthedefectintheformof
strain,thatregionwillalsobreakonthe
applicationoffields.

39. Insulating Materials
➢Insulatingmaterialsarenonmetallicmaterials
thatdoesnotallowelectriccurrenttopass
throughitatnormaltemperatures.
➢Therearenumberofinsulatingmaterialsusedin
Electricalapparatusandmachinery.Theymaybe
organicorinorganic,uniformorheterogeneousin
composition,naturalorsynthetic.
➢Therearemanypropertiessuchasresistivity,
breakdownvoltage,thermalstabilityetc.,that
determinethesuitabilityofamaterialtobeused
asinsulatingmaterial.

40. Properties of insulating materials
Anidealinsulatingmaterialmustshouldhavethe
followingproperties.
•Highdielectricstrengthatthespecified
temperature
•Highresistivity
•Lowdielectrichysteresis
•Goodthermalconductivity
•Goodmoisturewithstandingcapacity
•Itshouldbechemicallyinert
•Itshouldbeabletowithstandvibration,abrasion
andbending

41. Classification of Insulating materials
Insulating materials are classified in to three
categories:
1. Solid Insulating Materials
2. Liquid Insulating Materials
3. Gaseous Insulating Materials

42. Insulating Materials and Examples
State Materials Examples
Solid Insulating Materials Fibrous materials wood, paper and card board, insulating textiles Materials
Impregnated fibrous materials impregnated paper, varnished or impregnated textiles
Non resinous materials asphalts and bitumens, waxes
Ceramics porcelain, steatite, alumina, titanate, etc.
Glass fused quartz or silica glass, Pyrex, fiber glass
Natural and synthetic rubbers natural rubber, hard rubber, butyl rubber, neoprene, hypalon, silicon rubber
Mica and its products; Asbestos and its products
Liquid Insulating Materials Oils Refined hydrocarbon minerals oils, Linseed oil, silicon liquids, vegetable oils etc.
Varnishes synthetic varnishes and spirit
Gaseous Insulating Materials Carbondioxide (CO2), Dry air, argon, nitrogen, etc.

43. Uses of Dielectrics

44. Thank you…