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# Classification of Electrical Materials

In some electrical materials, there is a large number of free electrons or loosely bound valence-band electrons present. These electrons are easily knocked out of their orbit and easily constitute a large current. Such materials are known as conductors. Almost all metals and some liquids are good conductors.

In some materials, no free electrons arxe available; the valence-band electrons are tightly bound to the nucleus. Such materials are known as insulators. Examples of some insulators include glass, mica, plastics, etc.

In between the limits of these two major categories is a third general class of materials called semiconductors; where there are no such free electrons present, but free electrons can easily be created by adding some impurities. Examples of some semiconductors include germanium, silicon, etc. For example, germanium, a semiconductor, has approximately one trillion times (1 Ã— 1012) the conductivity of glass, an insulator, but has only about one thirty-millionth (3 Ã— 10-8) part of the conductivity of copper, a conductor.