Explain polarisation of polar molecule in uniform electric field.

Explanation

The permanent dipole moment of polai molecules is zero because the differen permanent dipoles are oriented randomly due to thermal agitation, so the total dipole moment is zero.

When an external field is applied the individual dipole moments tend to align with the field. When summed over all the molecules there is then a net dipole moment in the direction of the external field means the dielectric is polarized which is shown in above figure.
The extent of polarization depends on the relative strength of two mutually opposite factors. The dipole potential energy in the external field tending to align the dipoles with the field and thermal energy tending to disturb the alignment. Generally, the alignment effect ls more important for polar molecules.