CBSE Class 10 Answered

A conductor contains large number of loosely bound electrons called free electrons or conduction electrons and the remaining material is a collection of relatively heavy positive ions called lattice.The free electrons in a conductor are always in random motion .This random motion is due to the thermal energy of the conductor. The velocity of a free electron due to thermal energy of the conductor is called thermal velocity and is very large.

The number of free electrons in a conductor is very large and due to their random motion they keep on colliding with the positive ions in the cionductor,The collision is so frequent that the net flow of electrons in any particular direction is zero.

However when some potentiial difference is applied across the two ends of a conductor, an electric field is set up.If 'V' is the potential difference applied and 'l ' the length of the conductor, then the magnitude of electric field set up , E =V / l.

When there is an electric filed inside the conductor , a force acts on each electron in the direction opposite to the field.The electrons get biased in their random motion in favour of the force.As a result the electrons drift slowly in this direction.At each collision electrons start afresh in a random direction with a random speed but gains an additional velocity due to the electric field. This velocity increses with time and suddently becomes zero as the electrons makes a collision with the lattice and starts afresh with a random velocity. As the time 't' between successive collisions is small, the electrons slowly and steadily drifts opposite to the applied field. If the electrons drift a distance 'l' in a long time 't' we can define drift speed as

And the short time for which a free electron accelerates before it undergoes a collision with the positive ion in the conductor , is called relaxation time.

The average velocity with which free electrons in a conductor gets drifted under the influence of an external electric field applied across the conductor is called drift velocity.