(a) Describe a simple experiment (or activity) to show that the polarity of emf induced in a coil is always such that it tends to produce a current which opposes the change of magnetic flux that produces it.
(b) The current flowing through an inductor of self inductance L is continuously increasing. Plot a graph showing the variation of
(i) Magnetic flux versus the current
(ii) Induced emf versus dI/Dt
(iii) Magnetic potential energy stored versus the current.

Explanation

Lenz's law : According to Lenz's law, the polarity of the induced emf is such that it opposes a change in magnetic flux responsible for its production.

When the north pole of a bar magnet is pushed towards the coil, the amount of magnetic flux linked with the coil increases. Current induced in the coil is from a direction such that it opposes the increase in magnetic flux. This is possible only when the current induced in the coil is in anti-clockwise direction, with respect to an observer. The magnetic moment M associated with this induced emf has north polarity, towards the north pole of the approaching bar magnet. Similarly, when the north pole of the bar magnet is moved away from the coil decreases. To counter this decrease in magnetic flux, current is induced in the coil in clockwise direction so that its south pole faces the receding north pole of the bar magnet. This world result in an attractive force which opposes the motion of the magnet and the corresponding decrease in magnetic flux.