With reference to free electron theory of conductivity, explain the terms (a) Drift speed (b) Relaxation time
(a) Drift speed: The free electron in metals move constantly at random with high velocity inside the metal. They frequently collide with the positive ions present within the metal, thus changing their direction of motion. Hence they have no net motion in any particular direction when a potential difference is applied across a metal the free electrons drift with a small constant velocity in the direction opposite to the applied field. This is drift speed.
(b) Relaxation time: It is the average time interval between two successive collisions of a free electron with the positive ions in the metal.