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Conductors and Insulators

The conductivity of a material is effectively related to the number of free electrons in it. A distinction is usually made between conductorsinsulators and semiconductors, which have a special role of their own.

Conductors:

Electric current can only occur in materials that contain charge carriers (usually free electrons) that are free to move within the substance. Those materials that contain many such free electrons that are able to move with little resistance are called conductors of electricity. The following graphic depicts the movement of free electrons between the atoms of a conductor.  

Solid conductors are most likely to be metals, such as gold, silver, copper, aluminium or iron. One non-metallic conductor is the graphite form of carbon. Liquids can also conduct electricity, for instance metallic mercury, indeed any molten metal, or aqueous solutions of salts, acids and bases.

Insulators:

Materials that contain very few free electrons are usually called non-conductors or insulators. They can conduct next to no current. The following graphic illustrates why this is. 

Among the solid materials that do not conduct well are glass, porcelain, amber, rubber, paper, cotton and plastics. They are thus suitable for insulating one conductor from another. These substances do, however, possess a certain, albeit slight, conductivity, i.e. there are no perfect insulators. For this reason there is no actual well-defined boundary between conductors and insulators. Instead there exists a continuous spectrum of conductivity.

Semiconductors:

Materials that fall into the category of semiconductors occupy a special position between conductors and insulators. They are particularly important in the manufacture of electronic components such as diodes, transistors and integrated circuits. The main materials with the requisite properties are silicon and germanium. The conductivity of these matrials can be altered by a process called doping, which introduces impurities into a substance that can lead either to a surplus of free electrons or a relative absence of them. The absence of an electron results in a so-called hole in the atomic lattice. Those free electrons that do exist can sometimes fill a hole but this then leaves a gap elsewhere, so that it appears that the holes themselves are moving through the substance. This kind of movement of holes can be thought of as carrying a charge in a similar way to the free electrons themselves. Holes regarded in this way are considered to be carrying a positive charge in contrast to the negative charge carried by electrons. The following graphic shows how the two types of charge carrier move in a semiconductor of this kind. 

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