Current Density and Ohm's Law:

In our earlier discussion we have mentioned that, conductors have free electrons that move randomly under thermal agitation. In the absence of an external electric field, the average thermal velocity on a microscopic scale is zero and so is the net current in the conductor. Under the influence of an applied field, additional velocity is superimposed on the random velocities. While the external field accelerates the electron in a direction opposite to it, the collision with atomic lattice however provide the frictional mechanism by which the electrons lose some of the momentum gained between the collisions. As a result, the electrons move with some average drift velocity . This drift velocity can be related to the applied electric field by the relationship

......................(3.1)

where is the average time between the collisions.

The quantity i.e., the the drift velocity per unit applied field is called the mobility of electrons and denoted by .

Thus , e is the magnitude of the electronic charge and , as the electron drifts opposite to the applied field.