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.
Lenz 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 is induced in the coil in 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 `vecM` 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, the magnetic flux linked with 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 would result in an attractive force which opposes the motion of the magnet and the corresponding decrease in magnetic flux.
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