Effect of Temperature on Resistance

The resistance of a material is affected by a change in temperature. The effect of temperature on resistance depends upon the type of material. The resistance of conductors like copper and aluminium increases with the increase in temperature. Since the resistance of these metals is directly proportional to the rise in temperature, they are said to have “positive temperature co-efficient of resistance”. The resistance of electrolytes, semiconductors like germanium, silicon, etc., and insulators like glass, mica, rubber, etc. decreases with the increase in temperature. Hence, these materials are said to have “negative temperature co-efficient of resistance”. Contrary to this, the resistance of alloys increases with the rise in temperature but is irregular and almost negligible.

Conductors, Insulators and Semi-Conductors

Substances through which a flow of current, i.e., a flow of free electrons, can be set up easily are called conductors. The conductivity of a substance depends on the number of free electrons present in the substance. Most metals are good conductors.

Properties of Conductors:

•          Low specific resistance

•          Mechanically rigid

•          Easily available

•          Affordable

•          Ductile (in the case of metals)

•          Resistance that increases when the temperature increases (except in the case of carbon)

•          Ability to create a conduction path for current flow when conducting wires or strips are connected.

Insulators

Substances through which a current cannot be passed under normal conditions are called insulators. Insulators have unusually high resistance. Dry-air has a high resistance, which makes it a good insulator. Glass, mica, abonite etc have less resistance than air. Some common insulators are dry air, asbestos,glass, mica, paper, wax, porcelain, cotton, rubber, etc.

Properties of Good Insulators

For substances to be good insulators they need to have certain properties. Good insulators:

•          Have high specific resistance

•         Have high dielectric strength (Dielectric strength, also called breakdown voltage, is the voltage bearing capacity of a dielectric. It is measured in kilo volts per millimetre.)

•          Have high temperature bearing abilities

•          Have good mechanical strength

•          Have a permanent nature

•          Are moisture and water proof

Semi-Conductors

Substances that are neither good conductors nor good insulators are termed as semi-conductors. Such substances have few free electrons, as compared to the number of electrons found in conductors. For this reason, the resistance of semi-conductors is quite high. Impure semi-conductors, that is, semi-conductors mixed with a small amount of some other substance, have a reduced resistance. Semi-conductors are used for making diodes, transistors etc. Germanium, silicon, carbon and boron are some examples of semi-conductors.