ÿþUnit 1: Conductors, Insulator and Semiconductors  

If we connect a battery across a body, there is a movement of free electrons towards the positive end. This movement of electrocs is an electric current. All materials can be classified into three groups according to how readily they permit an electric current to flow. These are : conductors, insulator and semiconductors.  

In the first category are subtances which provide an easy path for an electric current. All metals are conductors, however some metals do not conduct well. Manganin for example, ia a poor conductor. Copper is a good conductor, therefore it is widely used for the cables. A non-metal which conducts well is carbon. Salt water is an example of a liquid conductor.  

A material which does not easily release electrons is called an insulator. Rubber, nylon, porcelain and air are all insulator. There are no perfect insulator. All insulators will allow some flow of electrons, however this can usually be ignored because the flow they permit is so small.  

Semiconductors are mid way between conductors and insulators. Under certain conditions they allow a current to flow to flow easily but under other they behave as insulators. Germanium and silicon are semiconductors. Mixtures of certain metallic oxides also act as semiconductors. There are known as thermistors. The resistance of thermistors fall rapidly as their temperature rises. They are therefore used in temperature-sensing devices.  

>>>Superconductivity<<<  

The resistance of metals varles with their temperature. When they get hot, their resistance increases. When they cool, their resistance falls. The resistance of some metals and alloys steadily decreases as their temperature is lowered, then fall suddenly to a negligible value at temperatures a few degrees above absolute zero (-273C). In other words, these materials have almost perfect conductors. This is called super conductivity. It occurs only with certain materials, for example lead, and only at very low temperatures.  

The practical applications of super conductivity are limited because of the very low temperatures required. A number pf uses, however, have been proposed. If a current is induced by a magnetic field in a ring of super conducting material, it will continue to circulate when the magneic field is remove. In theory this could be made use of in the memory cells of computers. Memory cells made pf super conducting materials could store information indefinitely. Because of the zero resistivity of the cell, the information could be retrieved very quickly, as fast as 10^-8 seconds.  

Ninety percent of the total losses in modern transformers is due to the resistance of the windings. Transformers could be made with windings cooled to the low temperatures at which superconductivity occurs. The resistance of the windings would be zero and the transformer would be almost ideal. Similarly a 100% efficent electric motor has been proposed using the magnetic field of superconducting coils.