25.8 : Dielectric Polarization in a Capacitor

The presence of a dielectric medium in a capacitor not only changes the voltage and capacitance but also affects the electric field. In general, dielectrics can be of two types: polar and nonpolar. In a polar dielectric, the positive and negative charges in the molecules are separated by a distance and hence have a permanent dipole moment. In contrast, no such charge separation exists in a nonpolar dielectric, however the nonpolar molecules get polarized in the presence of an external electric field. In both cases, the dipoles are randomly oriented without an electric field.

When a polar dielectric is placed in a uniform electric field, the dipoles align with the field, inducing equal and opposite surface charges in the faces of the dielectric. Nonpolar molecules can become polar in the presence of an external electrical field, called induced polarization. A nonpolar molecule's positive and negative charges are separated by the external electric field, inducing polarization and further aligning them along the field. The induced charges along the faces of the dielectric reduce the voltage across the plates and increase the capacitance.

The induced surface charges produce an induced electric field. The net electric field is the vector sum of the external and induced fields. The net field can be written in terms of the external field using the dielectric constant.

Combining equations (1) and (2) gives an expression for the induced electric field.

If the magnitude of the external field is increased beyond a limit, the molecules in the dielectric get ionized. Free electrons are produced in the dielectric, allowing the flow of current from one plate to another through the dielectric. This phenomenon is called dielectric breakdown.