With the help of neat diagrams, explain how the nonpolar dielectric material is polarised in external electric field of increasing intensity. Define polarisation in dielectrics.

Explanation

Consider a rectangular slab of a linear isotropic dielectric placed in a uniform external electric field.
(1) In case of a nonpolar dielectric, the applied field slightly separates the centres of negative and positive charge in a molecule, thus inducing an electric dipole moment in the direction of the field.
In case of a polar dielectric, the permanent dipole moments of its molecules are partially aligned with the field. In either case, the dielectric is said to be polarised.
(2) In a uniformly polarised dielectric, the charges of adjacent interior dipoles add to zero. But due to the unbalanced positive ends of dipoles at one face of the slab, bound positive charge appears on that exterior surface. Similarly, bound negative charge appears on the opposite exterior surface of the slab. These bound surface charges are called polarisation charges. there is, however, no excess charge in any volume element within the slab and the slab as a whole remains electrically neutral.
(3) The external electric field polarises the dielectric, but does not cause an electric current through it as there are no free charges available for motion. Within the dielectric, the field due to the polarisation charges is Opposite to the applied field. Hence, the net electric field within the dielectric is less than the applied field.