Define current density and relaxation time. Derive an expression for resistivity of a conductor in terms of number density of charge carriers in the…

Definition ofCurrent density is defined as the current flowing per unit area of cross section of a conductor.Relaxation time is the average time interval between two successive collisions for drifting electrons in a conductor. From \(\displaystyle \mathrm{I}=n A e v_{d}\) \[\text { but } \begin{aligned} v_{d} & =\frac{e E}{m} \tau \\ \therefore & \mathrm{I}=n A e . \frac{e E}{m} \tau \end{aligned} \]