DIELECTRICS AND POLARIZATION
Dielectrics are non-conducting substances. In contrast to conductors, they have no or negligible number of charge carriers.
In a dielectric, free movement of charges is not possible. It turns out that the external field induces dipole moment by stretching or re-orienting molecules of the dielectric.
The collective effect of all the molecular dipole moments is net charges on the surface of the dielectric which produce a field that opposes the external field.
The molecules of a substance may be polar or non-polar. In a non-polar molecule, the centres of positive and negative charges coincide. The molecule then has no permanent (or intrinsic) dipole moment.
Examples of non-polar molecules are oxygen (O2) and hydrogen (H2) molecules which, because of their symmetry, have no dipole moment.
On the other hand, a polar molecule is one in which the centres of positive and negative charges are separated (even when there is no external field). Such molecules have a permanent dipole moment.
An ionic molecule such as HCl or a molecule of water (H2O) is examples of polar molecules.
The non-polar molecule thus develops an induced dipole moment. The dielectric is said to be polarised by the external field.
Substances for which this assumption is true are called linear isotropic dielectrics.
The induced dipole moments of different molecules add up giving a net dipole moment of the dielectric in the presence of the external field.
Thus in either case, whether polar or non-polar, a dielectric develops a net dipole moment in the presence of an external field. The dipole moment per unit volume is called polarisation and is denoted by P. For linear isotropic dielectrics