Why conductors have positive temperature co-efficient of resistance

Why conductors have positive temperature co-efficient of resistance?


Increase in temperature of a substance to cause two effects: 

a. This causes the molecules to vibrate faster, thus preventing the movement of free electrons in the material. 

b. It can break some bonds, releasing free electrons. With the rise in temperature of wins, no increase in the number of free electrons and the sole effect of temperature rise is to increase the resistance due to the increased molecular vibration. Winners then have a positive temperature co-efficient of resistance. Example resistance increases with temperature.