(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: (i) Magnetic flux versus the current. (ii) Induced emf versus dI/dt. (iii) Magnetic potential energy stored versus the current.

(A). Consider a bar magnet fall freely through a constant height h. Record the timing t1. Now, let the bar magnet fall freely through a metallic coil through the same height. Record the timing t2. It is observed t2 > t1 which suggests an opposing mechanism in the second scenario. The following is the justification of the observation. As the magnet falls through the coil, it will increase the number magnetic field lines through the coil in the downward direction. As e = −dΦ/dt, emf will be induced in the coil and hence current will flow through it as shown in the figure. The current will flow such that it will oppose the motion of the bar magnet. (B). Φ = IL