Dielectric

In physics, a dielectric is an insulating (or very poorly conducting) material. The material can be solid, liquid or gaseous.

When a voltage difference is applied to top and bottom of a cylinder filled with a dielectric, no current will flow inside the cylinder because, unlike metals, a dielectric has no free&mdash;or loosely bound&mdash;electrons that can drift through the material. Instead, electric polarization occurs. The positive charges within the dielectric are displaced minutely in the direction of lower voltage, and the negative charges are displaced minutely in the opposite direction. When the molecules constituting the dielectric are polar (like water molecules), the molecules will align in the field, thus contributing to the electric polarization. Inside the cylinder no net charge density will arise because the charges in adjacent volume elements cancel. However, at the top and bottom of the cylinder an uncanceled surface charge will appear, and this surface charge (positive at the low voltage side and negative at the high voltage side) will oppose the electric field associated with the voltage difference. Thus, the polarization of the dielectric reduces the electric field inside the dielectric.

Dielectric material is characterized by an intrinsic property called relative permittivity, usually denoted by &epsilon;r (formerly this was known as the dielectric constant). The relative permittivity describes the ease of the polarization of the material and determines the size of the surface charge densities at the top and bottom of the cylinder.

The Coulomb force between two permanent electric point charges placed inside a dielectric medium is 1/&epsilon;r smaller than it would be in a vacuum due to the polarization of the dielectric medium by the point charges. The quantity of electric energy stored per unit volume of a dielectric medium is proportional to &epsilon;r. The capacitance of a capacitor filled with a dielectric is a factor &epsilon;r greater than it would be in vacuum.