Electrostatics of Dielectric Materials

Abstract

A dielectric is effectively an insulator, meaning that a priori it does not have free and mobile charges. Nevertheless, different types of charges can be found, as shown in Figure 2.1. They are:

• 1. Dipolar charges, which are attached to a molecule that makes up the dielectric; examples are the dipoles attached to HCl molecules where the center of negative electronic charge is displaced towards the Cl atom, while the H has an excess positive charge, thus giving rise to the dipole H⁺Cl⁻. These charges are inseparable, tied to the bonded H and Cl, and are called bound charges.

• 2. Charges due to discontinuities such as interfaces between aggregates. Where the solid dielectrics exhibit defaults, charges can accumulate giving rise to particular electronic phenomena, such as the Maxwell-Wagner-Sillars effect at low frequencies.

• 3. Homocharges, which have the same sign as the electrodes to which they are adjacent.

• 4. Heterocharges, which have the opposite sign to the electrode at which they are near.

• 5. Space charges, which are charges localised within a region of space.

• 6. Free charges, in principle, are little or not present in dielectrics. They can appear, however, when there is a breakdown caused the application of an electric field and a sudden loss in the ability of the material to insulate. When the current is relatively weak it is called a leak current and it can be due to a wide range of causes, such as impurities in the dielectric and so forth.