Basic Electrical Engineering

Temperature has a significant effect on the resistance of any electrical element. There are three types of electrical substance: 

Conductors: In metals or in any good conductor there are many free electrons. Actually for these free electrons electron flow can occur and conduction of current happens. But when we heat a conductor or raise its temperature the molecules of the conductors start to move and vibrate at a much higher rate than before. So the free movements of the free electrons are hampered and current cannot pass as easily as before. Thus the resistance of the conductor increases. In summary:

                Increase in temperature will increase the resistance of a conductor. Therefore conductors have a positive temperature coefficient. 

 

Conductors have a positive temperature coefficient

Semiconductors: In semiconductors like silicon increase in temperature will help to increase the number of free electrons or carriers. So increase in temperature will decrease the resistance of the semiconductor. Therefore they have a negative temperature coefficient. 

 

Insulators: In insulator there is no or very less number of free electrons. Increase in temperature will help some of the electrons to get free from their outer shell and conduct electricity. So increase in temperature of an insulator will decrease its resistance. Thus it has a negative temperature coefficient.