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UNIT 5 : DIELECTRICS AND TRANSDUCERSCONDENSED MATTER PHYSICS FOR ENGINEERS
Electrical& Electronics Engineering Stream- (EC, EE, EI and ET)
Introduction:
Materials such as glass, ceramics, polymers, and paper are non-
conducting materials. They prevent the flow of current through them.
Therefore, they can be used for insulation purposes. When the main
function of non-conducting materials is to provide electrical
insulation, they are called Insulators.
Distance
Figure 1: Band diagram of an insulator
When non-conducting materials are placed in an electric
field, they undergo appreciable changes because of which they act as
stores of electric charges. When charge storage is the main function,
the materials are called Dielectrics. For a material to be a good
Dielectric, it must be an insulator.
The forbidden energy gap (Eg) between the valence band and
conduction band is very large (fig.1) in dielectrics and excitation of
electrons from valence band to conduction band is not possible under
ordinary conditions. Therefore, conduction cannot occur in a
dielectric. Even if the dielectric contains impurities, extrinsic
conduction cannot occur as observed in case of extrinsic
semiconductors. The resistivity of an ideal dielectric is infinity, in
practise dielectrics conduct electric current to a negligible extent and
their resistivity range from 1010 to 1020Ωm.
, UNIT 5 : DIELECTRICS AND TRANSDUCERS
CONDENSED MATTER PHYSICS FOR ENGINEERS
Electrical& Electronics Engineering Stream- (EC, EE, EI and ET)
Polar and Non-polar dielectrics:
A dielectric material doesn‟t possess any free electrons. All the
electrons are bound very strongly to the respective nuclei of the
parent molecules. Each molecule consists of equal amount of
positive and negative charges. All the positive charges are
concentrated in the nuclei, which are surrounded by electron clouds
in which all the negative charges are distributed.
In the molecules of some dielectric materials, the effective
centre of the negative charge distribution coincides with the effective
centre of the positive charges. Such materials are called non-polar
dielectrics.eg Hydrogen, carbon dioxide etc.
In some dielectric materials, the effective centres of the
negative and positive charges in the molecules do not coincide with
each other even in the absence of any external field. Each molecule
behaves as though it consists of a pair of equal negative and positive
charges separated by a small distance. Such a pair is referred to as a
permanent dipole and the materials comprising of such dipoles are
called polar dielectrics. HCl, H2O etc.
Induced dipole:
If an atom is placed in an electric field of strength „E‟, the electron
cloud will be displaced in the direction opposite to „E‟ by a distance
„d‟ with respect to the nucleus. The centres of gravity of positive and
negative charges in the atom no longer coincide.
The atom is equivalent to the system of charges, q=Ze of equal
magnitude but opposite in sign separated by a distance„d‟. Such a
system is called an electric dipole. The dipole is induced in the atom
due to the action of external electric field. Though a dipole is
electrically neutral, the induced dipole sets up its own electric field
, UNIT 5 : DIELECTRICS AND TRANSDUCERS
CONDENSED MATTER PHYSICS FOR ENGINEERS
Electrical& Electronics Engineering Stream- (EC, EE, EI and ET)
which is opposite in direction to the external field.The dipole
moment µ is a vector, directed along the axis of the dipole from the
negative charge to the positive charge.
When an atom or molecule is placed in an electric field, the
field tends to displace the equilibrium position of the bound charges
because of which dipole moment is induced in the molecule. The
molecule is then said to be polarized.
When the molecule is polarized, restoring forces due to
coulomb attraction come into play which tends to pull the displaced
charges together. The charges separate until the restoring force
balances the force due to the electric field.
The induced dipole moment is proportional to the field strength. The
larger the field, greater the displacement of charges and hence larger
the induced dipole moment. The induced dipole moment is given by.
µ=αE α is the polarizability of the molecule.
It characterizes the capacity of electric charges in a molecule to
suffer displacement in an electric field. The unit of polarizability is
Fm2. The induced dipole moment vanishes as soon as the electric
field is switched off.
Permanent dipole
In some molecules known as polar molecules, the centres of
gravity of the charges of opposite sign are separated even in the
absence of external field. Such molecules are said to have intrinsic
dipole moment and carry permanent dipoles.
Polarization
Displacement of positive and negative charges in the molecules of a
dielectric under the action of applied electric field leading to the
development of dipole moment is known as dielectric polarization.
, UNIT 5 : DIELECTRICS AND TRANSDUCERS
CONDENSED MATTER PHYSICS FOR ENGINEERS
Electrical& Electronics Engineering Stream- (EC, EE, EI and ET)
Consider an electrically neutral slab inserted between the plates of a
parallel plate capacitor as shown in figure. Dielectric is imagined to
be divided into large number of identical cells of volume dv. Under
the action of external electric field, charges are induced in each cell
and each cell acquires a dipole moment dµ.
Then intensity of polarization “P” is defined as the total dipole
moment per unit volume of the material.
d
P dv = v
Dielectric constant
For isotropic materials the electric flux density E and the electric
induction (or electric displacement) D are related by the equation
D= ε0εrE
-12
Where ε0=8.854x10 F/m, is the dielectric constant of vacuum and εr
is the relative dielectric constant or relative permittivity for the
material. It has no units.
Dielectric susceptibility
The magnitude of polarization is directly proportional to the intensity
of the electric field.
Thus, P=χ ε0E (for linear dielectrics)
χ (chi) is the proportionality constant and is called the dielectric
susceptibility of the material. It characterizes the ease with which the
dielectric material can be influenced by an external field. P is a
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