Download (4Mb) - Etheses - Saurashtra University

Chapter V Dielectric Study of ……
-
5.2 Fundamentals of Dielectric Studies
Materials, which are electric insulators or in which an electric field can
be sustained with a minimum dissipation power are known as dielectric
materials. The dielectric includes all materials except condensed states of
metals. A dielectric is characterized by its dielectric constant r, which relates
the charge density to the electric field by the following relation,
D = E = 0.rE (5.1)
Where, = 0.r, 0 is the permittivity of free space and r is the relative
permittivity or dielectric constant of the material (r = /0).
In the earlier experiments Faraday found that by inserting a dielectric
material between the capacitor plates, the capacitance was increased by a
factor of r. The reason is the appearance of charges on the surface of the
dielectric necessitating the arrival of fresh charges from the battery in order to
keep the voltage constant. This is described in a schematic diagram of figure
(5.1).
In vacuum the surface charge density (D0) on the condenser plate is
represented as
D0= = 0E (5.2)
Where, d is the distance between the plates.
In the presence of dielectric, the surface charge density (D) of
dielectrics increase to
D =
= 0.r E (5.3)
The dipole moment per unit volume of dielectric (polarization- P) is
equal to the surface charge density of dielectrics, therefore,
P = 0.r E (5.4)
186