Relation between Polarization density and Electric field in various materials

Description

In classical electromagnetism, polarization density is the vector field that expresses the density of permanent or induced electric dipole moments in a dielectric material. When a dielectric is placed in an external electric field, its molecules gain electric dipole moment and the dielectric is said to be polarized. The electric dipole moment induced per unit volume of the dielectric material is called the electric polarization of the dielectric.
In a homogeneous linear and isotropic dielectric medium, the polarization is aligned with and proportional to the electric field. In this class of materials the polarization density is always parallel to the applied electric field.