Dielectrics :
Dielectrics are non-conducting substances. In contrast to conductors, they have no (or negligible number of) charge carriers. The free charge carriers move and charge distribution in the conductor adjusts itself in such a way that the electric field due to induced charges opposes the external field within the conductor. This happens until, in the static situation, the two fields cancel each other and the net electrostatic field in the conductor is zero.
In a dielectric, this 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.
Unlike in a conductor, however, the opposing field so induced does not FIGURE Difference in behaviour of a conductor and a dielectric in an external electric field. exactly cancel the external field. It only reduces it. The extent of the effect depends on the nature of the dielectric.
The molecules of a substance may be polar or non-polar.
Non-polar Molecule :
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.
Polar Molecule :
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) are examples of polar molecules.
In an external electric field, the positive and negative charges of a non-polar molecule are displaced in opposite directions. The displacement stops when the external force on the constituent charges of the molecule is balanced by the restoring force (due to internal fields in the molecule).
The non-polar molecule thus develops an induced dipole moment. The dielectric is said to be polarised by the external field.
We consider only the simple situation when the induced dipole moment is in the direction of the field and is proportional to the field strength. The induced dipole moments of different molecules add up giving a net dipole moment of the dielectric in the presence of the external field.
A dielectric with polar molecules also develops a net dipole moment in an external field.
In the absence of any external field, the different 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, i.e., the dielectric is polarised.
The extent of polarisation depends on the relative strength of two mutually opposite factors :
1. The dipole potential energy in the external field tending to align the dipoles with the field
2. And thermal energy tending to disrupt the alignment.
Note :
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, P = χe E where χe is a constant characteristic of the dielectric and is known as the electric susceptibility of the dielectric medium.
