Temperature Effects

How Temperature Affects Resistance.

The value of a resistor changes with changing temperature, but this is not as we might expect, mainly due to a change in the dimensions of the component as it expands or contracts. It is due mainly to a change in the resistivity of the material caused by the changing activity of the atoms of that make up the resistor.

Materials which are classed as CONDUCTORS tend to INCREASE their resistivity with an increase in temperature. INSULATORS however are liable to DECREASE their resistivity with an increase in temperature. It should be said however, that materials used for practical insulators (glass plastic etc) only exhibit a marked drop in their resistivity at very high temperatures. They remain good insulators over all temperatures they are likely to encounter in use.

The reasons for these changes in resistivity can be explained by considering the flow of current through the material. The flow of current is actually the movement of electrons from one atom to another under the influence of an electric field. Electrons are very small negatively charged particles an will be repelled by a negative electric charge and attracted by a positive electric charge. Thus if we apply an electric potential across a conductor (positive at one end, negative at the other) electrons will "migrate" from atom to atom towards the positive terminal.

Only some electrons are free to migrate however. Others within each atom are held so tightly to their particular atom that even an electric field will not dislodge them. We say therefore that the current flowing in the material is due to the movement of "free electrons" and the number of free electrons within any material compared with those tightly bound to their atoms is what governs whether a material is a good conductor (many free electrons) or a good insulator (hardly any free electrons).

The effect of heat on the atomic structure of a material is to make the atoms vibrate, and the higher the temperature the more violently the atoms vibrate.

In a conductor, which already has a large number of free electrons flowing through it the vibration of the atoms causes many collisions between the free electrons and the captive electrons. Each collision uses up some energy from the free electron and is the basic cause of resistance. The more the atoms jostle around in the material the more collisions are caused and hence the greater the resistance to current flow.

In an insulator however, there is a slightly different situation. There are so few free electrons that hardly any current can flow. Almost all the electrons are tightly bound within their particular atom. Heating an insulating material vibrates the atoms, and if we heat it sufficiently the atoms vibrate violently enough to actually shake some of their captive electrons free, creating free electrons to become carriers of current. Thus at high temperatures the resistance of an insulator can fall, sometimes dramatically.

When we have a material in which the RESISTANCE INCREASES WITH TEMPERATURE we say the material has a POSITIVE TEMPERATURE COEFFICIENT.

When RESISTANCE FALLS WITH AN INCREASE IN TEMPERATURE we say that the material has a NEGATIVE TEMPERATURE COEFFICIENT.

Therefore in general, we can say that CONDUCTORS HAVE A POSITIVE TEMPERATURE COEFFICIENT

Whilst (at high temperatures)INSULATORS HAVE A NEGATIVE TEMPERATURE COEFFICIENT.

Materials chosen for the construction of resistors therefore are most likely to be carefully selected CONDUCTORS that also have a very low POSITIVE TEMPERATURE COEFFICIENT. Therefore in use resistors will have only very slight increases in resistance as temperature rises.