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Chapter V Dielectric Study of ……
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5.5 Dependence of Permittivity on Various Factors
The permittivity depends on the external factors such as frequency of
applied voltage to the dielectric material, temperature, pressure, humidity etc
[7]. This has been summarized hereby briefly.
The permittivity of nonpolar dielectrics does not depend on frequency,
when it is changed within very broad range. On the other hand, in the case of
polar dielectrics when the frequency of alternating voltage increase the value
of remains invariable, initially, but beginning with certain frequency f0 the
polarization fails to settle itself completely during one half cycles and begins
to drop at very high frequencies. The value of f 0 can approximately be
obtained if relaxation time () is equated to half period 1/2f 0 of voltage, when
f0 =
kT
8 r
3
201
(5.19)
Where, η - Dynamic viscosity, r – Radius of the molecule, T - Absolute
temperature and k - Boltzmann constant.
In the case of water is estimated to be the order of 10 -11 sec, hence
the value of f0 should be of order of 10 11 Hz. A phenomenon of migrational
polarization is observed in inhomogeneous trielectrics, particularly in
dielectrics with water inclusions. The phenomenon of so called resonance
polarization may cause maxima in at very high frequencies.
Temperature is not expected to affect the process of electronic
polarization in non-polar dielectrics. A relative sharp change in of non-polar
dielectric is observed for temperature variation when crystalline material goes