hence derive an expression for the resistivity of a conductor in terms of number density of free electrons and relaxation time.

Question

Derive an expression for the resistivity of a conductor in terms of
number density of free electrons and relaxation time.

Answer 1

Let  E be the electric field applied across a conductor. If e- drifts a distance l in time t.
Then the drift velocity is given by
v_d=l/t
where t = average time between successive collisions.