1 Road Safety And Automobile Association Patrol In Great Britain

CONDUCTORS AND INSULATORS


A conductor is a material, which has a very low resistance to an electric current.
An insulator is a material, which has a very high resistance to an electric current.
The layer of insulating material in a cable is called insulation.
The difference between a conductor and an insulator, at the level of atoms and molecules, is to do with electrons. One gram of copper and 1 gram of rubber each have about the same number of electrons, about 300x10²⁰. Copper is one of the best electrical conductors and rubber is one of the worst. In copper, there is a very large number of 'free electrons', which are only loosely attached to the copper atoms. These free electrons can move about through the metal and so they can conduct a current, in rubber, almost all of the electrons are tightly bound to the carbon atoms and the hydrogen atoms of the molecules of rubber; there are hardly any free to conduct a current. This is the case in other insulators, too.
In metals, the free electrons can move about through the metal, and so metals are conductors of electricity.
Figure 1 shows the relative resistance of the same sized specimens of different materials. Silver, copper, and aluminum have the lowest resistance. Silver is too expensive to use in cables, although it is used for special contacts. Graphite, a form of carbon, is a conductor with a resistance about 2000 times as great as that of copper, at room temperature.
At the other end of the scale are the insulators, with PVC among the best for practical uses. Its resistance is over one thousand million times the resistance of a piece of copper of the same dimensions. From the point of view of practical use, PVC is also very resistant to water; it is tough and resistant to mechanical damage; and it is relatively cheap, in comparison with other insulators that might be used on wires. So at working temperatures below 70°C, PVC is the most widely used insulation and cover for cables.
Between the conductors and insulators are a group of materials known as semi-conductors. They consist of various metallic oxides and of materials like germanium and silicon, and they find important applications in control devices, such as rectifiers and transistors.
Topic # 16