Q.1 Write short notes on dielectric materials.
Ans. The dielectric materials are generally insulators. However, there
is slight difference between dielectric materials and insulators. In practice,
dielectric materials conduct electric current to a negligible extent with the
range of their resistivity from 1010 to
1020 Wm. The insulators however offer no passage
for the flow of current through them. Examples of insulating materials are
glass, ceramics, polymers porcelain, wood, paper etc. The concept of dielectric
materials was introduced for the first time by Farady. He discovered that the
capacitance of a parallel plate capacitor is increased when it is filled by a
dielectric material. The capacitance so increased is k times the capacitance
without any dielectric. The factor k is then the property of dielectric and
called the dielectric constant.
Q.2 What is dielectric constant.
Ans. Static Dielectric
Constant:
Permittivity or dielectric constant
is defined as the ratio of electric flux density D and the electric field
intensity E. i.e.,
Permittivity
of a dielectric material is taken as the product of permittivity of free space Î0 and relative permittivity Îr i.e.,
Î = Î0Îr
In S.I.
units, Î0 =
8.854 ´ 10-12 F/m. The relative permittivity Îr is also known as dielectric
constant. The dielectric constant of air is 1.006 and generally taken as unity.
For any other dielectric material, the value of dielectric constant (Îr) is more than 1. Dielectric constant, being the ratio of Î and Î0, is a dimensionless quantity.
Q.3 What do you mean by polarization of a substance ? Write
different mechanism of polarization in a dielectric. (AKTU. 2008-09)
Related Questions -
Q. Explain briefly the different types
of polarization in dielectrics. (AKTU. 2010 - 11)
Q. What is molecular
polarisability? Explain electronic polarisability.
(AKTU. 2012 - 13)
Ans. Polarization of a Substance: -
When a dielectric material is subjected to
an electric field, it gets polarised i.e., the positive and negative charges
are induced in the dielectric and are of opposite charge to that of applied
electric field. It is well known that the amount of positive and negative
charges on a molecule are equal, the distribution of the two kinds of charges
are such that the centres of positive and negative charge distributions are at
points separated by a distance of molecular dimensions. This forms an electric
dipole given by m = ql.
Different
Mechanism of Polarization: -
(1)
Electronic Polarization: -
The electronic polarization occurs due to the displacement
of positive and negative charges in a dielectric material when an external
electric field is applied. Fig (a) shows the charge distribution of an atom in
the absence of the field while fig. (b) shows the charge distribution in
presence of applied electric field. This process occurs throughout the material
and the material as a whole is polarized.
The
induced dipole moment (Pe) is proportional to field strength (E),
i.e.,
Pe µ E
If there
are n atoms in the dielectric, then
Pe µ nE or Pe = ae nE
where ae is electronic
polarizability. The electronic polarizibility is independent of temperature.
(2)
Ionic Polarization: -
Ionic polarization occurs
only in
those dielectric materials which possess ionic bonds such as NaCl. When such a
material is subjected to an external electric field, the adjacent ions of
opposite signs undergo displacement as shown in fig. (b).
The displacement causes an increase
or decrease in the distance separation between the atoms depending upon the
location of ion pair. This leads to a net dipole moment. This polarization is
also independent of temperature.
(3)
Orientational Polarization: -
Polar dielectrics exhibit orientational or
dipolar polarizability. Even in