Dielectric

In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the material as they do in an electrical conductor, because they have no loosely bound, or free, electrons that may drift through the material, but instead they shift, only slightly, from their average equilibrium positions, causing dielectric polarisation. Because of dielectric polarisation, positive charges are displaced in the direction of the field and negative charges shift in the direction opposite to the field. This creates an internal electric field that reduces the overall field within the dielectric itself. If a dielectric is composed of weakly bonded molecules, those molecules not only become polarised, but also reorient so that their symmetry axes align to the field.

The study of dielectric properties concerns storage and dissipation of electric and magnetic energy in materials. Dielectrics are important for explaining various phenomena in electronics, optics, solid-state physics and cell biophysics.

Although the term insulator implies low electrical conduction, dielectric typically means materials with a high polarisability. The latter is expressed by a number called the relative permittivity. Insulator is generally used to indicate electrical obstruction while dielectric is used to indicate the energy storing capacity of the material (by means of polarisation). A common example of a dielectric is the electrically insulating material between the metallic plates of a capacitor. The polarisation of the dielectric by the applied electric field increases the capacitor's surface charge for the given electric field strength.

The term dielectric was coined by William Whewell (from dia + electric) in response to a request from Michael Faraday.

A perfect dielectric is a material with zero electrical conductivity (cf. perfect conductor infinite electrical conductivity), thus exhibiting only a displacement current; therefore it stores and returns electrical energy as if it were an ideal capacitor.

The electric susceptibility ${\displaystyle \chi _{e}}$ of a dielectric material is a measure of how easily it polarises in response to an electric field. This, in turn, determines the electric permittivity of the material and thus influences many other phenomena in that medium, from the capacitance of capacitors to the speed of light.

It is defined as the constant of proportionality (which may be a tensor) relating an electric field ${\displaystyle \mathbf {E} }$ to the induced dielectric polarisation density ${\displaystyle \mathbf {P} }$ such that

$${\displaystyle \mathbf {P} =\varepsilon _{0}\chi _{e}\mathbf {E} ,}$$