Question

The response of polar substances (e.g., HCl, H_2O, etc) to applied electric fields can be described in terms of a classical model which attributes to each molecule a permanent electric dipole moment p. (a) Write down a general expression for the average macroscopic polarization (dipole moment per unit volume) (p) = -(partial differential A/partial differential epsilon)_V, T, N for a dilute system of N molecules at temperature T in a uniform electric field epsilon. Here A is the Helmholtz free energy. (b) Calculate explicitly an approximate result for the average macroscopic polarization at high temperatures.

Answer 1

The molecules in which the arrangement or geometry of the atoms is such that one end of the molecule has a positive electrical charge and the other side has a negative charge.The centers of gravity of the positive and negative charge distributions do not coincide.They have a finite electric dipole moment.E.g. H2O, NH3, HCl, SO2, H2S.The polarization of the dielectric is caused by the E.F. which causes alignment of dipoles.Therefore polarization should depend on net E.F. i.e. polarization is directly proportional to the net E.F.

so, P is proportional to E.

In polar dielectric molecules, permanent dipoles are present & also direction of induced dipole moment is same as external E.F.