Biomedical Engineering Reference
In-Depth Information
13.4.1.2 Factors Af ecting the Dielectric Constant of a Material
h e dielectric constant depends on various factors, some of which are
described below.
•
Applied Voltage:
An applied dc voltage reduces the dielec-
tric constant while an applied ac voltage within a reasonable
range tends to increase dielectric constant.
•
Frequency:
h e dielectric constant strongly depends on
the frequency of an applied voltage. An increase in the fre-
quency of an applied voltage decreases the value of dielectric
constant of nonlinear dielectrics.
•
Humidity and Moisture:
h e presence of humidity and
moisture af ects the important parameters of dielectric
materials. It reduces the dielectric strength of the material
to a great extent.
•
Ef ect of Structure on the Dielectric Constant:
h e polar
materials with permanent dipoles have larger dielectric
constants than non-polar materials. For polar structures,
the magnitude of the dipole also af ects the magnitude of
polarization achievable, and hence the dielectric constant.
Conversely, a polar gas tends to have smaller dipoles and
its low density also means there are less dipoles to polarize;
therefore polar gases have lower dielectric constants than
polar solids or liquids. h e density argument also applies for
non-polar gases when compared with non-polar solids or
liquids.
•
Temperature:
Temperature does not af ect the process of
electronic polarization in non-polar dielectrics and the
electronic polarizability of molecules does not depend on
temperature. However, due to thermal expansion of matter,
the ratio of the number of molecules to the ef ective length
λ of the dielectric decreases when temperature increases,
and for this reason dielectric constant decreases with the
increase of temperature. In the case of polar dielectrics
the molecules cannot orient themselves at low tempera-
ture. When the temperature increases, the orientation of
dipoles is facilitated and permittivity increases. h e tem-
perature at which permittivity attains its maximum value
is known as critical temperature/Curie point or transition
temperature.
