Dielectrics are non-conducting substance. dielectric materials there are no free charges/electrons, Hence there is no possibility of movement of free charges. However, when a dielectric material is subjected to an external electric field, the dipole moment is induced arising out of stretching and reorientation of molecules of dielectrics. This collective effect of all molecular dipole moments is the accumulation of net charges on the surface of the dielectric, which produces a field that opposes externally applied fields. This decreases the net field in the dielectric. Solid dielectrics: Ceramics, glasses, plastics, rubber, mica, asbestos. Liquid dielectrics: Mineral oil, silicone oil, magnesia. To throw light on this phenomenon, let us study dielectrics at a molecular level.
The molecules of a polyatomic substance may be a
(a) polar molecule
(b) non-polar molecule
A polar molecule is one in which the “center of gravity of positive nuclei and revolving electrons do not coincide (or are separated). Polar molecules have a permanent electric dipole moment (Fig. 12.8) e.g. HCI, HO molecules, N2O molecules. Polar substances behave like a tiny electric dipole.
A non-polar molecule is one in which the “center of gravity” of positive nuclei and revolving electrons coincide. Non-polar molecules do not have a permanent electric dipole moment because of their symmetry. e.g. Oxygen (O2), Hydrogen (H2) Fig. 12.8), Carbon dioxide (CO2), Polyethelene, polystyrene.
If such a material is subjected to an external electric field, the positive and negative charges in non-polar molecules are displaced in opposite direction. This displacement is continued till external force on constituent charges and restoring force due to internal molecular field are balanced.
The non-polar molecules thus acquire induced dipole moment. The dielectric is said to be polarised in the external field Fig. 12.9). The induced dipole moments of different molecules add up and give rise to the net dipole moment
In a polar dielectric, the different tiny electric dipoles are randomly oriented (in absence of an electric field) because of thermal agitation. Hence the total dipole moment of the polar dielectric is zero. When a substance with polar molecules is subjected to an external electric field, the tiny electric dipoles get aligned in the direction of the field. Thus there is a net dipole moment in the direction of the field. (Fig. 12.10)
In a nutshell, a dielectric with polar or nonpolar molecules develop a net dipole moment in the direction of the field, in the presence of the field.
Polarization (P) is defined as dipole moment per unit volume and is given by
where X is electric susceptibility of dielectric material.
This relation is true for linear isotropic dielectrics. Linear isotropic dielectrics are those substances in which induced dipole moment is induced in the direction of the field and is proportional to field strength.
Consider a thin slab of the dielectric of permittivity (epsilon) placed in a uniform external electric field Dielectric becomes polarised when kept in an external field, irrespective of the fact that dielectric consists of polar molecules or non-polar molecules. Because of polarization, molecules are oriented such that the negative charges are on the left and positive charges on right. Thus leaving net negative charge at the left surface of dielectric and net positive charge on the right surface of dielectric Fig. 12.11). The net electric charge within the dielectric is zero. Since the dielectric slab as a whole is an electrically neutral, positive induced charge must be equal to the magnitude of negative induced charge. The charges so obtained on the surface of the dielectric slab are called polarization charges. (Fig. 12.11)
Thus a polarized dielectric is equivalent to two charged surfaces with induced charges (polarization charges). The induced surface charges (polarization charges) oppose the external electric field and thereby weaken the original field within the dielectric.
Polarization may be defined as the amount of induced surface charge per unit area or the surface density of polarization charges appearing at right angles to the applied external electric field.