(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. 

(a) The statement ‘polarity of the induced emf is such that it opposes a change in magnetic flux’ is given by Lenz law.

The given activity demonstrates the above statement. The amount of magnetic flux linked with the coil increases, when the north pole of a bar magnet is brought near the coil. Current in the coil is induced in a so as to 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  associated with this induced emf has north polarity, towards the north pole of the approaching bar magnet.
Similarly, magnetic flux linked with the coil decreases when the north pole of the bar magnet is moved away from the coil. Inorder to oppose 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 would result in an attractive force which opposes the motion of the magnet and the corresponding decrease in magnetic flux.

(b) (i) Since ϕ = L I ;

where,   
I = Strength of current through the coil at any time,

ϕ =  Amount of magnetic flux linked with all turns of the coil at that time, and

L = Constant of proportionality called coefficient of self-induction.
 

(ii) Induced emf, e=− =  

i.e., e = − 
 
(iii) Since magnetic potential energy is given by ,