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Topic:- Dielectric Materials
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2.  Lecture 3
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3.  Dielectric Materials
 Dielectric materials are also called as insulators.
 In dielectric materials, all the electrons are tightly bound to
their parent molecules and there are no free charges. In addition, the
forbidden energy band gap (e.g.) for dielectric materials is more than
3eV.
 Not possible for the electrons in the valence band to excite to
the conduction band, by crossing the energy gap, even with normal
voltage or thermal energy.
 Dielectrics are non-metallic materials of high specific
resistance and negative temperature coefficient of resistance

4.  Active and Passive Dielectrics
 The dielectric materials can be classified into active
and passive dielectric materials.
i. Active dielectrics
 When a dielectric material is kept in an external
electric field, if it actively accepts the electricity, then it is
known as active dielectric material. Thus, active dielectrics
are the dielectrics, which can easily adapt themselves to
store the electrical energy in it.
 ii. Passive dielectrics
 Passive dielectrics are the dielectrics, which
restrict the flow of electrical energy in them. So, these
dielectrics act as insulators.
 Examples: All insulating materials such as glass,
mica, rubber etc.,


5. 
Basic Definitions in Dielectrics
Electric Field
The region around the charge within which its effect is
felt or experienced is known as electric field.
The electric field is assumed to consist of imaginary electric
lines of force. These lines of force originate from the positive
charges and terminate to the negative charges .
Electric field strength or electric field intensity (E)
Electric field strength at any point is defined as the force
experienced by an unit positive charge placed at the point. It is
denoted by ‘E’.
‘q’ - magnitude of the charge in coulombs
‘f’ - force experienced by that charge in Newton,
electric field strength (E)
. Its unit is Newton / Coulomb (or) volt / metre.

6. Electric flux
It is defined as the total number of electric lines of force passing
through a given area in the electric field. (Emanated from the positive
charge). Unit: Coulomb
Electric flux density or electric displacement
vector (D)
It is defined as the number of electric lines of force passing
normally through an unit area of cross section in the field.
Its unit is Coulomb / m2

7. Permittivity
Permittivity is defined as the ratio of electric displacement vector (D)
in a dielectric medium to the applied electric field strength (E).
Mathematically the permittivity is, .Its unit is Farad /metre
The permittivity indicates the degree to which the medium can resist
the flow of electric charge and is always greater than unity.
Dielectric Constant
The dielectric constant or relative permittivity of a material
determines its dielectric characteristics. It is the ratio of the
permittivity of the medium and the permittivity of free space
0ε
ε
ε =r

8. Electric Polarization
Consider an atom. We know that it is electrically neutral.
Furthermore, the centre of the negative charge of the electrons
coincides with the positive nuclear charge, which means that
the atom has no net dipole moment.
However, when this atom is placed in an external electric
field, the centre of the positive charge is displaced along the
field direction while the centre of the negative charge is
displaced in the opposite direction.
When a dielectric material is placed inside an electric field,
such dipoles are created in all the atoms inside.

9. Polarizability (α)
When the electric field strength ‘E’ is increased, the strength of the
induced dipole is also increased. Thus, the induced dipole moment is
proportional to the intensity of the electric field.
Polarization vector
The dipole moment per unit volume of the dielectric material is
called polarization vector.
‘µ ’ - average dipole moment per molecule and
‘N’ - number of molecules per unit volume
polarization vector is given by,
Unit: Coulomb / m2

10. Relation between P,ε0 , ε r and E
The polarization ‘P’ is related to the electric flux density D as,
D = ε0E + P
Since D = ε0εr E, the above relation becomes,
ε0 εr E = ε0E + P
(or) P = ε0εr E− ε0E
i.e. P = ε0 (ε r − 1 )E
Electric susceptibility
The polarization P is proportional to the applied electric field
intensity E and it is in the same direction of ‘E’ It can be written as,

11. P ∝ E (or) P = ε0 χe E
Various Polarization mechanisms in
Dielectrics
Dielectric polarization is the displacement of charged
particles under the action of the external electric field. Several
microscopic mechanisms are responsible for electric polarization.
Four types of microscopic polarization mechanisms.
Electronic polarization
Ionic polarization
Orientation polarization and
Space-charge polarization.

12. i. Electronic Polarization
 Electronic Polarization occurs due to the displacement of positively
charged nucleus and negatively charged electrons in opposite directions,
when an external electric field is applied, and thereby a dipole moment is
created in the dielectric.
∴The induced dipole moment µ = αeE
where αe = electronic polarizability.
 Monoatomic gases exhibit this kind of polarization, Electronic
polarizability is proportional to the volume of the atoms and is independent
of temperature.
 The electronic polarizability = αe = 4πe0R3 ( Farad.m2) where R
is the radius of the atom.

13. Nucleus
Nucleus
Sphere of
electronic
charge
+Ze −Ze
Original Position
Field direction
Displaced
Equilibrium
position
x
Fig. (a) Position of +ve and –ve charges in an atom without
field (b) Position of +ve and –ve charges in an atom with
field

14. ii. Ionic Polarization
Ionic polarization arises due to the displacement of -ve
ions and + ve ions in opposite directions and it occurs in
ionic solids, in the presence of electric field. The
displacement is independent of temperature. Example : NaCl
crystal
- +
Cl Na
-
+
x2
x1
Fig. (a) Without field (b) With field

15. iii. Orientation Polarization
The orientation polarization arises due to the presence of polar
molecule in the dielectric medium.
Fig. (a) Without field (b) With field

16. Explanation:
 In the case of a CH3Cl molecule, the positive and negative
charges do not coincide. The Cl-
has more electro negativity than
hydrogen. Therefore, the chlorine atoms pull the bonded electrons
towards them more strongly than hydrogen atoms. Therefore, even in
the absence of field, there exists a net dipole moment.
 Now, when the field is applied, positive portion align along the
direction of field and negative portion align in the opposite direction of the
field. This kind of polarization is called as orientation polarization.
 This depends on temperature; when temperature is increased,
the thermal energy tends to randomize the alignment

17. Space-Charge Polarization
The space-charge polarization occurs due to the diffusion of
ions, along the field direction, thereby giving rise to redistribution of
charges in the dielectrics
Fig. (a) Without field (b) With field

18. Explanation
 Without the application of external field, the ions are orderly
arranged as shown in the
Fig.
 Now, when the field is applied, the ions diffuse with respect to
the direction of applied field. Thus the polarization occurs, known as
space charge polarization.
 Normally, this type of polarization occurs in ferrites and
semiconductors and will be very small.

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