23) 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