Electrical Engineering ⇒ Topic : Mechanism of Current Conduction in Metals

Mechanism of Current Conduction in Metals

Every metal has a large number of free electrons which wander randomly within the body of the conductor somewhat like the molecules in a gas. The average speed of free electrons is sufficiently high( 10⁵ ms^-1) at room temperature. During random motion, the free electrons collide with positive ions (positive atoms of metal) again and again and after each collision, their direction of motion changes. When we consider all the free electrons, their random motions average to zero. In other words, there is no net flow of charge (electrons) in any particular direction. Consequently, no current is established in the conductor.

           When potential difference is applied across the ends of a conductor (say copper wire) as shown in Fig. (a), electric field is applied at every point of the copper wire. The electric field exerts force on the free electrons which start accelerating towards the positive terminal (i.e., opposite to the direction of the field). As the free electrons move, they *collide again and again with positive ions of the metal. Each collision destroys the extra velocity gained by the free electrons.

                                                                              figure (a)

 The average time that an electron spends between two collisions is called the relaxation time (t). Its value is of the order of 10^-14 second.

Although the free electrons are continuously accelerated by the electric field, collisions prevent their velocity from becoming large. The result is that electric field provides a small constant velocity towards positive terminal which is superimposed on the random motion of the electrons. This constant velocity is called the drift velocity.

The average velocity with which free electrons get drifted in a metallic conductor under the influence of electric field is called drift velocity (v_d) . The drift velocity of free electrons is of the  order of 10^-5 ms^-1.

Thus when a metallic conductor is subjected to electric field (or potential difference), free electrons move towards the positive terminal of the source with drift velocity v_d. Small though it is, the drift velocity is entirely responsible for electric current in the metal