Dielectrics

Abstract

An ideal insulator contains no free electrons so that when an electric field is applied to it there is no macroscopic movement of charge; instead, it suffers displacement of its electrons with respect to their parental nuclei, creating thus electric dipoles. The electric field is said to (electrically) polarize the material. The dipole moment of the polarized atom in figure 8.1 is given by μ = qξ, where ξ is the displacement of the charge cloud. μ is directed from negative charge centre to positive charge centre. The units of dipole moment are thus Coulomb-metres (Cm) in SI units, but frequently used is the more conveniently sized unit, the Debye (D), which is 3.33 × 10^-30 Cm (or 10^-18 esu-cm). The polarization, P, is the dipole moment per unit volume (and thus has units of C/m²):

$$ P = {N_v}\mu $$

where N _v is the number of dipoles/m³.