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A dielectric material (dielectric for short) is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material as they do in a conductor, but only slightly shift from their average equilibrium positions causing dielectric polarization. Because of dielectric polarization, positive charges are displaced toward the field and negative charges shift in the opposite direction. 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 polarized, 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.〔Arthur R. von Hippel, in his seminal work, ''Dielectric Materials and Applications'', stated: "''Dielectrics''... are not a narrow class of so-called insulators, but the broad expanse of ''nonmetals'' considered from the standpoint of their interaction with electric, magnetic, or electromagnetic fields. Thus we are concerned with gases as well as with liquids and solids, and with the storage of electric and magnetic energy as well as its dissipation." (Technology Press of MIT and John Wiley, NY, 1954).〕 Dielectrics are important for explaining various phenomena in electronics, optics, and solid-state physics. ==Terminology== While the term ''insulator'' implies low electrical conduction, ''dielectric'' typically means materials with a high polarizability. The latter is expressed by a number called the relative permittivity (also known in older texts as dielectric constant). The term insulator is generally used to indicate electrical obstruction while the term dielectric is used to indicate the energy storing capacity of the material (by means of polarization). A common example of a dielectric is the electrically insulating material between the metallic plates of a capacitor. The polarization of the dielectric by the applied electric field increases the capacitor's surface charge for the given electric field strength.〔Quote from Encyclopædia Britannica: "''Dielectric, insulating material or a very poor conductor of electric current. When dielectrics are placed in an electric field, practically no current flows in them because, unlike metals, they have no loosely bound, or free, electrons that may drift through the material''." *〕 The term "dielectric" was coined by William Whewell (from "dia-electric") in response to a request from Michael Faraday.〔James, Frank A.J.L., editor. The Correspondence of Michael Faraday, Volume 3, 1841–1848, The Institution of Electrical Engineers, London, United Kingdom, 1996. ISBN 0-86341-250-5〕 A ''perfect dielectric'' is a material with zero electrical conductivity (cf. perfect conductor), thus exhibiting only a displacement current; therefore it stores and returns electrical energy as if it were an ideal capacitor. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「dielectric」の詳細全文を読む スポンサード リンク
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