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Question
With reference to free-electron theory of conductivity, explain the terms:
(a) Drift speed
(b) Relaxation time
Solution
(a) In metals, we have a large number of free electrons which are in random motion i.e., will move all possible speeds in all possible directions. Dining their random motion, they collide and their velocities change. Due to the random motion, the number of electrons crossing any section of the conductor is zero. As a result no current flows. However,when a potential difference is applied across the ends of the metallic conductor, the electrons drift under the effect of the applied electric field in one direction with a very small velocity called the drift velocity. As a result of which a net current flows through the conductor.
(b) Relaxation time is defined as the average time between two successive collisions of the electrons with the positive ions in the metal.
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