27)
r thus depends inversely both on the number n of free electrons per unit
volume and on the average time t between collisions As we increase
temperature, average speed of the electrons, which act as the carriers of
current, increases resulting in more frequent collisions The average time
of collisions t, thus decreases with temperature In a metal, n is not dependent on temperature to any appreciable
extent and thus the decrease in the value of t with rise in temperature
causes r to increase as we have observed