PERMITTIVITY

a universal electric constant appearing in the mathematical formulation of two fundamental phenomena, the existence of a physical force between two separated electric charges (see Coulomb force), and the modification of the properties of an electric field attending the introduction into it of a dielectric (see electric displacement). Permittivity is a generalized, or large-scale, description of electric behaviour that does not specify detailed features on the atomic dimension. The permittivity of an insulating, or dielectric, material is commonly symbolized by the Greek letter epsilon, e; the permittivity of a vacuum, or free space, is symbolized e0; and their ratio e/e0, called the dielectric constant (q.v.), is symbolized by the Greek letter kappa, k. In the rationalized metre-kilogram-second (mks) and SI systems, the magnitude of the permittivity of a vacuum e0 is 8.854 10-12. Its units and those of permittivity e are square coulombs per newton square metre. In the mks system, permittivity e and the dimensionless dielectric constant k are formally distinct and related by the permittivity of free space e0; e = ke0. The magnitude and units of the permittivity of free space in the mks system are a necessary consequence of the laws of physics and of the decision to make the practical electrical units in use, such as volt and ampere, compatible with the mechanical units, such as metre and kilogram. In the centimetre-gram-second (cgs) system, the value of the permittivity of free space e0 is chosen arbitrarily to be 1. Thus, the permittivity e and the dielectric constant k in the cgs system are identical; both of them are dimensionless numbers.