Question

A magnet is in the form of a bar. Initially, the axis of the bar magnet is along x-axis. At some distance from the magnet, a coil is kept in the y-z plane with its center along x-axis. Which of the following motions of the bar magnet will NOT induce electric current in the coil?

• A. Translational vibrations perpendicular to x-axis
• B. Rotation of bar magnet about the x-axis.
• C. Magnet swinging back and forth about the x-axis with the south pole fixed on the x-axis.
• D. Bar magnet spinning about a perpendicular bisector as its axis of rotation, with its center on the x-axis.

Answer 1

The correct option is B Rotation of bar magnet about the x-axis.


Current will be induced in the coil when there is a change in the magnetic flux passing through the coil.

$I_{ind}=-\frac{1}{R}\frac{d\varphi }{dt}=-\frac{1}{R}A\frac{dB}{dt}$

In case (b), since the magnetic is rotating about its own axis, there is no change in magnetic field due to the bar magnet and hence no change in magnetic flux.
Hence there is no current induced in the coil in case (b)