17), we have
vd = 
τ
e E
m
Rationalised 2023-24
Current
Electricity
89
Hence,
τ
µ =
vd=
e
E
m
(3 25)
where t is the average collision time for electrons 3 6  LIMITATIONS OF OHM’S LAW
Although Ohm’s law has been found valid over a large class
of materials, there do exist materials and devices used in
electric circuits where the proportionality of V and I does not
hold