Mechanism of Polarization

The mechanisms of polarization have been divided into four general kinds

1. Electronic polarization.
2. Atomic polarization.
3. Permanent dipole polarization.
4. Space-charge polarization.

Electronic polarization is a result of dipole moments induced by the electric field. Atoms are composed of heavy nuclei surrounded by negative electron clouds. In the absence of an external field the movements of the electrons are such that the resultant orbital paths are symmetrical about the nucleus as shown schematically in Fig. 2-22(a). These orbital paths become distorted by an external electric field because the paths of the electrons tend to shift against the direction of the applied field as shown in Fig. 2-22(b). A dipole is thus induced; this dipole is in effect positive on the left-hand side for the assumed direction of the field.

Figure 2-22. Electronic polarization; (a) no field; (b) electric field

Figure 2-23. Molecular structure of HCl illustrating atomic dipoles.

Atomic polarization results from an unsymmetrical sharing of electrons when two different atoms combine to form a molecule. This type of polarization is illustrated in Fig. 2-23 for an HCl molecule in which the hydrogen atom and the chlorine atom are both polarized. In this case the electron of the hydrogen atom is attracted into the unfilled outer shell of the chlorine atom.

Permanent dipole polarization. The molecule illustrated in Fig. 2-23 is a permanent dipole because it exists as such even in the absence of an electric field. Such dipoles, however, are oriented at random when there is no resultant field in the dielectric material. However, upon the application of an electric field these dipoles will develop torque and will tend to align themselves in the direction of the electric field as indicated in Fig. 2-21(b).

Space-charge polarization is produced by charge carriers, such as free ions that can migrate for some distance through the dielectric. When the movement of such carriers is impeded by interfaces such as occur where two different dielectrics meet, or if the carriers are arrested in the material, space charges result. When there is no applied electric field the arrangement of these space charges is random and there is no net effect. However, in the presence of an electric field, the positive charge carriers will tend to take positions, relative to the negative charge carriers, in the direction of the field as shown in Fig. 2-24. Fig. 2-24(a) shows the positions of the charge carriers when there is no field. On the one hand, a vertical line through the center of the charges in Fig. 2-24(a) will have zero net charge to the right and left of it. On the other hand, a vertical line through the center of Fig. 2-24(b) will have net positive charge to the left and net negative charge to the right.