Physics (BPH-151)

Dr. Alok Singh

School of Basic & Applied Sciences

Department of Physics
Harcourt Butler Technical University, Kanpur
B.Tech. First Semester Course
 Module 4:
Materials of Technological Importance:
Dielectric Materials
 Dielectric Materials
❑The non-conducting materials such as paper, wood,
glass, ceramics, polymers etc. do not have free charge
carriers, i.e., electrons or holes. Therefore, they prevent
the flow of electrical current through them.
❑"When the main function of non-conducting materials is
to provide electrical isolation then they are called
insulators".
❑"When the main function of non-conducting materials
is for charge storage then it is called dielectric".
❑The dielectrics are polarized under the influence of
external electric field.
 Dielectric Constant:
• Let us consider two parallel plates separated by a
distance 'd' connected with a dc supply of voltage
'V' as shown in figure 1. Now the circuit is
disconnected and dielectric is inserted between the
plates. Then, the voltage across the capacitor is
reduced V to V'. The change in voltage across the
plates can be related by a factor as ,
The capacitance without dielectric is given as,

and with dielectric is,

Now, from equation(1),

If 'A' is the area of the plates then,

[Where, ε is the permittivity of the material],

From equation (2),

Thus, the dielectric constant is the ratio of

permittivity of the material to the permittivity of free
space.
 Polar and Non Polar Molecules:
• When center of gravity of positive and negative
charges coincides in a molecule then it is known as
non-polar molecules. It does not have any
permanent dipoles. Ex. H2, N2, O2 etc.
When the center of gravity of positive charge and
negative charge do not coincides then the molecule
is known as polar molecules. It has permanent
dipoles with electric dipole moment. Ex. H2O, HCl
etc.
 Dielectric polarization:
• "Dielectric polarization is the displacement of
charge particles under the action of an electric field".
• Let a dielectric is made up of polar molecules, and it
is placed in a uniform electric field parallel to the
plate capacitor. The dipoles of the material are
randomly oriented in the absence of the electric
field. When an external electric field 'E' is applied , a
torque is exerted on the dipoles causing them to
align with the field. The alignment of the molecules
with the electric field depends upon the temperature
and on the magnitude of the applied electric field
In general, alignment increases with decrease in
dielectric temperature and with increase in external
electric field. The polarization of the material 'P' is
directly proportional to the electric field (ε) and can
be written as,
Where, is the proportionality constant known as
dielectric susceptibility of the material. describes
how easily a dielectric can be influenced
(polarized) by an external electric field. It is a
dimensionless quantity.
To describe the combined effect of the applied field
E and electric polarization (P), we define an
additional vector D, the electric displacement
vector.
For vacuum P=0,

For a dielectric medium,

 Relation between D, E and P:
The effective electric field across capacitor is
given by,
E’
The dielectric displacement vector is given by,

The polarization is given by, so,

 Relation between dielectric constant and
electrical susceptibility:
We know that,
 Polarizability:
Let us consider a dielectric slab placed in a uniform
external electric field, then the charges are displaced
to produce electric dipoles known as induced
electric dipoles. The electric dipole moment is
directly proportional to the applied electric field.
 The constant of proportionality α is called
polarizability.
The dielectric polarization is defined as dipole
moment per unit volume. So the polarization of the
material P is given by,

Where, N is the number of dipoles (atoms) per unit

volume.
 Types of polarization ( Polarizability):
There are four important types of polarization;

1) Electronic polarization
2) Ionic polarization
3) Orientation polarization
4) Space charge polarization
 Electronic polarization
Electronic polarization occurs in non- polar
dielectrics. In this type of polarization, the atom is
initially unpolarized in absence of electric field. As
the external electric field E is applied, there is a
displacement of an electron cloud of atom with
respect to heavy fixed nuclei to a distance that is less
than the dimension of atom or molecule. Electronic
polarization sets in a very short period of time ( 10-15
- 10-14 sec.).
Electronic polarization is independent of temperature
and described by,

where, is the electronic polarizability and N is the

number of atoms per unit volume.
Since we know that,
• Hence dielectric constant is dependent on the
electric polarizability. In case of monoatomic gas,

where R is the radius of an atom.

 Ionic Polarization (Polarizability):
Ionic polarization occurs in ionic crystals. Let us
consider NaCl molecule which is bound through
ionic bond. The inter-ionic distance of these ions
varies with the application of electric field. When
the applied electric is in the direction of ionic
bonds then the inter-ionic distance decreases,
while, an electric field opposite to it causes an
increase in inter-ionic distance.
The ionic polarization of the material is given by,

Where, is the ionic polarisability.

 Orientation Polarization:
Orientation polarization occurs in polar dielectrics in
which molecules have permanent dipole moment.
In absence of any external electric field, the
permanent dipoles are randomly oriented so that
the dielectric slab is electrically neutral. When an
external field is applied then a torque is exerted on
these permanent dipoles to align in the direction
of applied electric field E. Such type of polarization
is called orientation polarization.
It is dependent on temperature. The built up time for
orientation polarization is of the order of 10-10 sec.
The orientation polarization is given by,

Where, is the orientation polarizability, p is the

electric dipole moment, k is the Boltzmann's
constant and T is an absolute temperature.
 Space charge Polarization (Polarizability):
When an external electric field is applied on a
dielectric material the accumulation of the charges
at the electrodes occurs. Therefore, there is
tendency of redistribution of charges in the
dielectric medium in the presence of applied
electric field is known as space charge polarization.
 Total Polarization:
• When the material experiences all the three types of
polarization, then the total sum of polarisability is
the sum of electronic, ionic and orientation
polarizabilities, i.e.,

• The total polarization P is given by,

 Frequency Dependence of Dielectric Constant:

• When a non-polar dielectric is subjected to an

alternating electric field then, the polarization of
the material follows field reversal. The total
polarization of the material depends on the
frequency of an alternating field applied on the
dielectric. The average time taken by the dipole to
reorient themselves in the direction of the applied
electric field (f) is known as relaxation time and
the reciprocal of relaxation time is known as
relaxation frequency. When , 𝜏 < 𝑓 then, there
will be no polarization in the material. When , 𝜏 >
𝑓 then, there is polarization in the material.
• In audio frequency region, all types of
polarization occurs in dielectric as shown in
figure. In radio wave or micro wave frequency
regions , the permanent dipoles are unable to
follow the field reversal. Therefore, the
orientation polarization is vanished from the
dielectric material(Po=0). In infra-red region, ionic
polarization will be zero inside the material
(Pi=0). At optical frequency region, only
electronic polarization will occur in the dielectric
material. At ultra violet region, all polarization
become zero and dielectric constant becomes
unity.
 Dielectric Loss:
• Dielectric loss may be defined as the loss of energy in the
form of heat by a dielectric medium due to internal
friction developed in switching of dipoles to their normal
state under the action of an alternating field.
• Let us consider a dielectric material which is placed
between the plates of the capacitor under the action of
alternating electric field as shown in figure1. The action is
equivalent to the presence of a resistance accompanying
the capacitor as shown in figure2.
• The angle between I and Ic is denoted by δ and
is called as dielectric loss angle. The tangent of
this angle δ i.e., tan δ is called the loss tangent.

[Power loss due to resistance]

From equation (1), we have,
This expression shows that power loss or
dielectric loss depends on , provided factors
voltage, frequency and capacitance are constants.
 Applications of Dielectrics:
1. Glass
2. Mica
3. Ceramics
4. Asbestos
5. Resin
6. Rubber
7. Liquid dielectrics
8. Optical Fiber