Polar dielectric molecules have a permanent dipole moment, In the absence of an electric field, these dipoles align at random due to thermal agitation. As a result, the total dipole moment is equal to zero. When an electric field is applied, these randomly aligned dipoles try to align in the direction of the electric field. As a result there is a net dipole moment in the direction of the electric field.

──────────────────────────────────────────────────────────────────────────────────────────