the
absence of external electric field, the polar dielectrics exhibit dipole
moment. The orientations of the molecules are random and hence the net dipole
moment is zero.
The dipole orientation is shown in fig.(a). When an external field is applied, the
field tries to align these dipoles along the direction of field as shown in
fig.(b). This type of polarization is known as orientational polarization.
The orientational polarization is strongly
temperature dependent. This decreases with increase of temperature.
(4)
Space-Charge Polarization: -
Space charge polarization occurs due to the accumulation of
charges at the electrodes or at the interfaces in a multiphase materials. As
shown in fig. (b), the ions diffuse over appreciable distance in response to
the applied field. This gives rise to redistribution of charges in the
dielectric medium.
The space-charge polarization is not
an important factor in most common dielectrics. Among the different
polarizations, electronic and ionic polarizations are insensitive to
temperature changes.
The total polarization a of a material
is sum of electronic, ionic and orientational polarization, i.e.,
a = ae + ai + a0.
Q.4 The dielectric constant of helium at 0oC and 1
atmospheric pressure is 1.000074. Find the dipole moment induced in helium atom
when the gas is in an electric field of intensity 100 volt/m. Number of atoms
per unit volume of helium gas are 2.68 ´ 1027. (AKTU. 2008-09)
Ans. Given, er =
1.000074, E = 100 Volt/m, N = 2.68 ´ 1027 atom/unit volume
Dipole
moment,
Q.5 Briefly describe internal fields is liquid
and solids.
Ans. Internal Fields in Solids and Liquids:
When a liquid or solid dielectric material
is subjected to an external field E, each of the atoms of the dielectric
develops a dipolement. This dipole exhibits an electric field apart from the
applied external electric field. As the atoms in liquids and solids are
surrounded on all sides by other polarized atoms, the internal intensity at
point is not only the electric field due to the applied external field (E) but
also due to field created by the neighbouring atoms (E¢). Therefore,
the internal field is given by
Ei = E
+ E¢
Expression
for the Internal Field:
Consider a dielectric material either liquid or solid under
the action of an external electric field E. The dielectric is polarized and let
us consider an infinite string of similar equidistant atomic dipoles as shown
in Figure.
Let us
consider the inter-atomic distances as d and dipole moment of each dipole as m. The resultant
field at X due to all other dipoles can be determined as follows:
The electric field due to a dipole
at any point is known and therefore the electric field at X due to the dipole A1 is given by
Or 
……….(i)
……….(i)