In dielectric materials, the atoms or the molecules do not have any permanent dipole moment. If such a material is placed in an electric field, the electron charge distribution is sligthly shifted opposite to the electric field. This induces dipole moment in each atom or molecule and thus we get a dipole moment in any volume of the material and due to this they develop an electric field inside the material which is opposite to the direction of the external field applied.
be the applied field due to external sources and
be the field due to polarization. The resultant electric field is
. For homogeneous and isotropic dielectrics, the direction of
is opposite to the direction of
. The resultant field
is in the same direction as the applied field
but its magnitude is reduced. We can write
where K is a constant for the given dielectric which has a value greater than one. This constant K is called the dielectric constant or relative permittivity.