Define relaxation time of the free electrons drifting in a conductor. How is it related to the drift velocity of free electrons? Use this relation to deduce the expression for the electrical resistivity of the material.
The average time elapsed between two successive collisions is known as the relaxation time of free electrons drifting in a conductor.
Relation between 
and vd is given by,
Consider a conductor of length ‘l’, area of cross-section A and current density n.
Current flowing through the conductor is given by, 
Electric field applied across the ends is given by, E = V/l
So current flowing through the conductor becomes,
Then, 
Using ohm's law, we get
Therefore,
