Using E = V/l, I = nevd A, and Ohm’s law, one obtains
2
d
eE
ne v
m
ρm
=
The proportionality between the force eE on the electrons in a metal
due to the external field E and the drift velocity vd (not acceleration)
can be understood, if we assume that the electrons suffer collisions
with ions in the metal, which deflect them randomly If such collisions
occur on an average at a time interval t,
vd = at = eEt/m
where a is the acceleration of the electron This gives
2
m
ne
ρ
τ
=
9 In the temperature range in which resistivity increases linearly with
temperature, the temperature coefficient of resistivity a is defined as
the fractional increase in resistivity per unit increase in temperature