From this equation,
resistivity of a material is given by
2
1
m
n e
ρ
σ
τ
=
=
(3 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