If you are a student of first or second year or under graduate in physics or relevant subjects, it is common to hear about negative/positive/zero temperature coefficient of a substance. What do you mean by this? If you are still confused about it, then I am here to help you. Let's know:
Temperature co-efficient refers the change of resistance with respect to temperature of a substance. If it's resistance increased with temperature, then the characteristics of the substance is called as positive temperature coefficient. For example: any metallic conductor (Cu, Fe). On the other hand if it's resistance decreased with temperature, then the characteristics
of the substance is called as negative temperature coefficient. For example: insulators and semiconductors. Otherwise, If it's resistance doesn't vary with temperature, then the characteristics
of substance is called as zero temperature coefficient. Actually there is no absolute zero temperature coefficient substance. But resistance of materials like eureka, manganin, constantan etc. have changed negligible (almost zero) with temperature.
This is disgusting. You copy the book! I'm not here for this! That's interesting. Let's know the real scenario.
If the temperature of a substance is increased, it becomes hot! Actually you feel it but what is happened inside of the substance? It causes two effects:
- Molecular vibration: providing more zigzag path to the movement of electrons. (increases resistance)
- Breaking bonds: generating more free electrons. (increases conductance)
These are the reasons! With the rise in temperature of a conductor, there is no increase in the number of free electrons. Because all the valance electrons have become already free. Thus only impact of rising temperature is molecular vibration which hinders the movement of electrons. This means, their resistance increases with the rise in temperature. Therefore, conductors have positive coefficient of resistance.
With the rise in temperature of a semiconductor or an insulator, breaking bonds or excited valance electrons causing an increase in the number of free electrons than in the cooler state. However, the molecular vibration don't grow enough due to covalent bonds. In another sense, in initial state this substance has zero or negligible free electrons causing zero conduction property. However, generated electrons flow to give current conduction. Therefore, semiconductors and insulators have negative coefficient of resistance.
In case of some high resistance alloys like eureka, manganin, constantan etc. the rise of temperature causes negligible increase in resistance. They have almost zero temperature coefficient of resistance. It is because, the two effects of temperature almost cancel each other.