Question

When no current is passed through a conductor

• A. the free electrons do not move
• B. the average speed of a free electron over a large period of time is zero
• C. the average velocity of a free electron over a large period of time is zero
• D. the average of the velocities of all the free electrons at an instant is zero

Answer 1

Answer: (C), (D)

The correct options are
C the average velocity of a free electron over a large period of time is zero
D the average of the velocities of all the free electrons at an instant is zero

When no current pass through the conductor i.e. when there is no electric field is applied to the conductor, each electron moves along a straight path at constant speeds and collide with the lattice ions(positive). With each collision the direction of electrons is changing randomly. The resulting path of any electron over along period of time, covering many collisions is a random sequence of straight segments. Due to this, the average number of electrons crossing any small area is nearly equal(in any direction). Thus the average displacement of electrons along any direction during any long period of time is also zero. Hence, the average velocity component in any direction is zero.