Question

The thermistors are usually made up of
a) metals with low temperature coefficient of resistivity
b) metals with high temperature coefficient of resistivity
c) metal oxides with high temperature coefficients of resistivity
d) semiconducting materials having low temperature coefficient of resistivity

Answer 1

Thermistors are substances whose resistance changes with the change in temperature. The resistance of the material with high temperature coefficient of resistivity changes more rapidly, than that of other materials having low coefficient of resistivity. Hence accordingly we will determine which of the above substances are more preferable to be used as thermistors
Formula used:
$R={{R}_{\circ }}(1+\alpha \Delta T)$

Complete answer:
Let us say we have a substance whose resistance is ${{R}_{\circ }}$ at some temperature. Let us say further we increased the temperature such that the increase in the temperature is $\Delta T$ . If $\alpha$ is the temperature coefficient of resistivity, then the resistance ‘R’ of the substance after the change in temperature is given by,
$R={{R}_{\circ }}(1+\alpha \Delta T)$
From the above expression we can imply that the materials with high temperature coefficient of resistivity, the resistance of such materials will change with even small changes in the temperature. Metal oxides have a very high temperature coefficient of resistivity than that of their respective metals. Hence thermistors are mostly made of metal oxides.

Note:
It is to be noted that metal oxides have very less electrons in their conduction band. And hence when the temperature increases there is more effective Brownian motion of these electrons. As the Brownian motion increases the electrons hit each other with smaller relation time. Hence the flow of electrons across the material is restricted thereby decreasing the flow of current in the circuit. It is also to be noted that all metals have the temperature coefficient of resistivity as positive while the semiconducting materials have temperature coefficient of resistivity as negative.