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