4.7 Frequency Dependence of Dielectric Properties: Resonance

As briefly shown in the earlier section, polarization mechanisms have frequency dependence and that is because of the mass associated with the charge dipoles and hence inherent inertia to movement. This also means that if the frequency of the field is very high, say above 10¹⁵ Hz, no dipole system will be able to follow the field oscillations and in such a situation, no mechanism will contribute and hence ε_r ∼ 1.

However, at frequencies below 10¹² Hz, the dielectric properties are affected by the frequency of the field.

In case of electronic and ionic polarization, the charge dipoles can be considered behaving as mechanical oscillators where charges are connected with linear springs whose restoring force balances the force induced via the applied electric field. The characteristic of any such mechanically oscillating system is resonance at a certain frequency.

In case of orientation polarization, there is no direct mechanical restoring force. Instead we have many statistical events, that respond in their average results to the driving forces of electrical fields. In other words, if a driving force is present, there is an equilibrium state with an (average) net dipole moment. If the driving force were to disappear suddenly, the ensemble of dipoles will assume a new equilibrium state (random distribution of the dipoles) within some characteristic time called relaxation time. This process does not show any resonance phenomena and is characterized by its relaxation time instead of a resonance frequency.