### Frequency and Temperature Dependence Of Polarization Mechanisms:

Frequency dependence

1.  On application of an alternating field across the material, the polarization process occurs as a function of time.

2.  Electronic polarization is extremely rapid and is complete at any instant of time even when the frequency of the voltage is very high in the optical range. Thus it occurs at all frequencies.

3.  Orientational  polarization is lower than the ionic polarization and occurs only at electrical frequencies which are smaller than the infra red frequencies.

4.  Space-charge polarization is the slowest process and occurs only at power frequencies(50-60 Hz).

5.  Thus at low frequencies, the value of the total polarization is very high and at high frequencies(optical frequencies). The value of the toital polarization is very small

Temperature dependence

1.    Usually orientiational polarizability depends strongly on temperature. Whereas the electronic and ionic polraizabilities and practically independent of temperature.

2.    Orientational polarization is inversely proportional to temperature.

3.    Normal temperatures will oppose the permanent dipoles to align in the field direction. But higher temperatures facilitate the movement of ions and molecules so that a given polarization process, which is not possible at higher frequencies at normal temperatures can occur at higher frequencies  at elevated temperatures. Thus at very high temperatures, ionic polarization can occur at infrared frequencies.

Local Field or Internal Field

It is the space and time average of the electric field intensity acting on a particular molecule. The local field intensity ‘Ei’ is larger than the macroscopic intensity E, since ‘Ei’ excludes the molecule’ own field, which is in the direction opposite to E.