Polarisation vector

POLARISATION VECTOR 

It is defined as the average dipole moment per unit volume of a dielectric. If N is the number of atoms per unit volume and  is average dipole moment per atom, then

Polarisation vector

Its unit is coulomb/m2

Electric displacement vector 

Since the electric field intensity depends on the medium, we define another electric vector which depends only on magnitude of charge and its distribution. It is independent on the nature of medium. This electric vector is known as electric displacement vector.

The electric displacement vector is used for analysing electrostatic fields in the presence of dielectrics.

It is given by   ... (1)

We know that electric field intensity,

From equations (1) and (2), we have

where xe is electrical susceptibility.

Electrical susceptibility (Xe)

It is found that for a large number of dielectrics, the polarisation vector  is proportional to the applied electrical field  for field strengths that are not too large.

Thus,

where Xe is a scalar constant. It is known as electrical susceptibility. It is a characteristic property of every dielectric.