Relative permittivity and polarisation

Solids have traditionally been divided into three classes when the electrical properties are described. Those that conduct electricity well are called conductors. This group is typified by metals. Those solids that conduct poorly are called semiconductors. This group contains elements such as silicon and germanium, and large numbers of minerals such as the iron sulphide fool s gold, FeS2- Solids that do not conduct electricity are called insulators or dielectrics. Many oxides, such as magnesium oxide, MgO, and most polymers, such as polyethylene, are insulators. 

This division is far too coarse to encompass the wide range of electrical properties that are now known. It is quite feasible to turn an insulating oxide into a very good metallic conductor, and 

Insulators are explained in terms of chemical bonding as those solids in which the outer electrons are unable to move through the structure. They are localised in strong bonds if the material is considered to be a covalent compound, or else are restricted to the region close to an atomic nucleus if the compound is supposed to be ionic. In either case, these electrons are trapped and cannot move from one region to another. 

Insulating materials are often referred to as dielectrics. One of the most important parameters used to describe an insulator is its dielectric constant, properly called the relative permittivity, Si. 

Dielectrics form the working material in capacitors. A capacitor consisting of two parallel metal plates separated by a dielectric (including air) has a 

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Using Equations (11.3)-(11.5) it is possible to derive the most widely used relationship between relative permittivity and polarisability, the Clausius-Mossotti relation. Equation (11.6), usually written ... 

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