Dielectric constant, breakdown, strength and polarization

Dielectric Constant is defined as the ratio of Eext/Enet = K, where K is called dielectric constant.

· K is also known as relative permittivity (Er) of the material.

· The value of K is always greater than one.

· For vacuum there is no Em and hence K-1

· For air K = 1.00054 = K for vacuum.

· For perfect conductor Em = 0, hence K= infinity.

Dielectric breakdown and dielectric strength

If a very high electric field is created in a dielectric, the outer electrons may get detached from their parent atoms. The dielectric then behaves like a conductor. This phenomenon is known as dielectric breakdown.

The maximum value of electric field (or potential gradient) that a dielectric material can tolerate without its electric breakdown is called its dielectric strength.

S.I. unit of dielectric strength of a material is V/m.

Polarization of a dielectric slab Suppose a dielectric slab is inserted between the plates of a capacitor having charge Q and –Q as shown in the figure.

If charge on plates are +Q and –Q and area of the plates are A Q, is induced charge