Potential shift is proportional to electric field. Therefore, the relative capacitance has the following relationship with the electrode polarization rate χe : εr = 1 + χe .

Most dielectrics are insulators. Examples include porcelain (earthenware), mica, glass, plastics, and various metal oxides. Some liquids and gases can be good dielectric materials. Dry air is a good dielectric and is used in variable capacitors as well as certain types of transmission lines. Distilled water is a good dielectric if it is kept free of impurities, and has a relative capacitance of about 80.

Dielectrics have the property of making a space larger or smaller than its actual size. For example, when a dielectric material is placed between two charges, it reduces the force acting between them as if they were moved farther apart. When an electromagnetic wave travels through a dielectric, the speed of the wave is reduced, making it behave as if it had a shorter wavelength.

From an electrical point of view, relative capacitance is a measure of the degree to which a substance concentrates lines of electrostatic flux. More precisely, it is the proportion of electrical energy stored in an insulator when an electrostatic field is applied to it relative to a vacuum, which has a capacitance of 1. Thus, the relative capacitance also becomes the relative electrostatic permittivity.