## Thursday, July 05, 2007

### A Question on Electromagnetic Induction

The following questions will be easy for you if you have understood Lenz’s law clearly: The variation of induced emf (E) with time (t) in a coil if a short bar magnet is moved along the axis with a constant velocity is best represented as This question appeared in IIT Screening 2004 question paper.
When the magnet is moved towards the coil, the magnetic flux linked with the coil goes on increasing and the magnitude of the induced emf goes on increasing as long as the rate of increase of flux is increasing. The rate of increase of flux then has to decrease and hence the magnitude of the induced emf decreases. The rate of increase of flux then becomes zero. Thereafter, the rate of decrease of flux becomes greater and greater, becomes a maximum and then becomes smaller and smaller and finally reduces to negligibly small value. The induced emf therefore follows these variations in the opposite direction (since it is the rate of decrease of flux that occurs now).
The variation of emf is therefore given by curve (B).