Question

(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 $dl/dt$
(iii) Magnetic potential energy stored versus the current.

Answer 1

When the North pole of a bar magnet moves toward the closed coil, the
magnetic flux through the coil increases. This produced an induced emf which produces (or tend to produced if the coil is open) an induced current in the anti-clockwise sense. The anti-clockwise sense corresponds to the generation of North Pole which opposes the motion of the approaching $N$ pole of the magnet. The face of the coil, facing the approaching magnet, then has the same polarity as that of the approaching pole of the magnet. The induced current, therefore, is seen to oppose the change of magnetic flux that produced it.
When a North Pole of a magnet is moved away from the coil, the current (I) flows in the clock-wise sense which corresponds to the generation of South Pole. The induced South Pole opposes the motion of the receding North Pole.