Question

A cylindrical bar magnet is lying along the axis of a circular coil. If the magnet is rotated about the axis of the coil then
A. emf will be induced in the coil
B. only induced current will be generated in the coil.
C. no current will be induced in the coil
D. both emf and current will be induced in the coil.

Answer 1

In order to induce emf or current in the circular coil, there needs to be a change in the magnetic flux of the coil. The motion of the bar magnet with respect to the coil needs to be such that the number of flux lines around the coil change.

Complete step-by-step solution:
We are given a cylindrical bar magnet which is lying along the axis of a circular coil. Diagrammatically, it looks like the following.

Now Faraday’s law of electromagnetic induction says that whenever there is a change in the magnetic flux linked with a coil, as a result, an electromotive force (emf) is induced in the coil.
We are given that the given cylindrical magnet is being rotated along its axis but this rotation does not lead to any change in the magnetic flux of the coil. If there is no change in the magnetic flux linked with the coil then there will be no induction of emf in the coil and hence, there will be no induced current in the coil.

Based on this reasoning, we can say that the correct answer to the question is option C.

Note: The flux of the magnetic lines of force is equal to the number of lines of force crossing per unit area. When the magnetic rotated about its axis of rotation, the lines of force move symmetrically about the axis which results in no net change in the flux but in case if we also move the magnet along the axis of the cylinder keeping the coil stationary then there shall be changed in the flux which will lead to induction of emf and current in the coil.