The action of a dielectric

When a dielectric material is placed between the plates of a parallel-plate (or other) capacitor the capacitance increases. The reason for this is shown in Figures 1(a) and 1(b).




The charges on the plates of the capacitor induce opposite charges on the two surfaces of the dielectric. This has the effect of reducing the potential difference across the capacitor.


This can be explained as follows. Under the action of the electric field within the dielectric the molecules become polarised and are aligned as shown in Figure 1(b).
Since the capacitance of a capacitor is given by the formula C = Q/V, if the p.d. (V) across the capacitor is reduced the capacitance must be increased.

The capacitance of a parallel-plate capacitor with a material of relative permittivity e_r filling the space between the plates is



where e_r is the ratio of the capacitances when the space between the plates is a vacuum or a dielectric.

The capacitance of a capacitor when only a thickness t of the air space is filled is



The following example should illustrate how to deal with some of the problems of dielectrics and capacitors.