ABSTRACT
The dielectric properties of solids arise from the polarization produced by an electric field, and are described in terms of a specific function which must be added to both the field and the electric potential when the laws of electrostatics are applied to matter in the condensed state. As the solid is a large collection of ions and valence electrons, we differentiate between conductors, with a nearly-free flow of electric charge and dielectrics, having a negligible content of free electrons. In conductors the effect of applying an electric field is to redistribute the free charge until the internal field is cancelled, and hence, the net charge inside a conductor is zero under static conditions. When an electric field is applied to a dielectric, each nucleus of charge q = +Ze will tend to be displaced in the direction of the field, and the corresponding cloud of tightly bound electrons will more against the field, as illustrated in Figure 8.1 (a). The relative displacement, given by the position vector r of the centre of the negative cloud with respect to the nucleus, allows the restoring force due to the electrons to balance the force on the nucleus due to the applied field. As a result, an electric dipole moment p - q r is produced in each atom. As a consequence of the induced dipole moments, layers of negative and positive charges develop on the two surfaces of an element of dielectric, normal to the direction of the applied field E , as in Figure 8.1 (b). The elementary charge induced in a layer of thickness r = \r\ and area dS can be expressed in terms of the polarized charge density o p = dql dS as follows:
vv " V J ( v
which can be interpreted using P dS = P endS.
