Dielectric Polarization in a Capacitor

The dielectric layer in a capacitor can be made of either polar or nonpolar molecules.

The positive and negative charges in polar dielectrics have a net separation and are therefore permanently polarized. Nonpolar dielectrics have no charge separation; however, in the presence of an electric field, these dielectrics get polarized.

In a completely charged capacitor, if a polar dielectric is inserted, the randomly oriented dipoles align with the field, inducing equal and opposite surface charges.

In the case of nonpolar dielectrics, the electric field separates the positive and negative charges, inducing polarization and further aligning them along the field.

The vector sum of the external field and the field due to the induced surface charges gives the net electric field. This is related to the external field by the dielectric constant. Rearranging the terms gives the expression for the induced electric field.

When the electric field magnitude is increased beyond a limit, it ionizes the molecules in the dielectric. The dielectric behaves like a conductor, allowing current flow between the plates, leading to dielectric breakdown.