Dielectric

Dielectric, or electrical insulator, substance that is a poor conductor of electricity. Two oppositely charged bodies placed on either side of a piece of glass (a dielectric) will attract each other with a force that is less than it would be if there were only a vacuum between them. If a conductor, such as a sheet of copper, is interposed between the two bodies, the electrical force between them is cancelled out completely.

In most instances the properties of a dielectric are caused by the polarization of the substance. When the dielectric is placed in an electric field, the electrons and protons of its constituent atoms reorient themselves, and in some cases molecules become similarly polarized. As a result of this polarization, the dielectric is under stress, and it stores energy that becomes available when the electric field is removed. The polarization of a dielectric resembles the polarization that takes place when a piece of iron is magnetized. As in the case of a magnet, a certain amount of polarization remains when the polarizing force is removed. A dielectric composed of a wax disc that has hardened while under electric stress will retain its polarization for years. Such dielectrics are known as electrets.

The effectiveness of dielectrics is measured by their relative ability, compared to a vacuum, to store energy, and is expressed in terms of a dielectric constant, with the value for a vacuum taken as unity. The values of this constant for usable dielectrics vary from slightly more than 1 for air up to 100 or more for certain ceramics containing titanium oxide. Glass, mica, porcelain, and mineral oils, often used as dielectrics, have constants ranging from about 2 to 9. The ability of a dielectric to withstand electric fields without losing insulating properties is known as its dielectric strength. A good dielectric must return a large percentage of the energy stored in it when the field is reversed. The fraction lost through so-called electric friction is called the power factor of the dielectric. Dielectrics, particularly those with high dielectric constants, are used extensively in all branches of electrical engineering, where they are employed to increase the efficiency of capacitors. See Insulation.