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  4. When a potential difference is applied across a conductor, the free electrons in the conductor are set in motion. Two velocities are associated with moving electrons-the drift velocity and average velocity, fact is that the two are :

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Q. When a potential difference is applied across a conductor, the free electrons in the conductor are set in motion. Two velocities are associated with moving electrons-the drift velocity and average velocity, fact is that the two are :

J & K CETJ & K CET 2001

Solution:

When the ends of a metallic wire are connected to a battery, a potential difference between the ends of the wire is established. As a result of this potential difference or electric field each free electron experiences an electric force which accelerates the motion of the electron. However, the velocity of the electron does not increase continuously because it collides again and again with the positive ions of the metal and thus, continues to lose the energy taken from the battery. Therefore, the potential difference of the battery does not give accelerated motion to the electrons, but simply gives them a small constant velocity along the length of the wire towards the end at higher potential. This is called drift velocity of the electrons and is superposed on the high velocity of the random motion of the electrons. Hence, drift velocity and average velocity are the same.