Discussion
Two circuits A and B are placed in close vicinity to each other. Circuit A comprises a galvanometer connected to two ends of the closed circuit. Circuit B comprises a battery and a key connected to two ends of the circuit. Initially the key is not inserted. Which of the following will be observed if the key is inserted and later removed ?
Answer: A
There are momentary galvanometer deflections in opposite directions that die out shortly.
This is electromagnetic induction. When the key is inserted, the current in circuit B rises from zero, so the magnetic field it produces through circuit A changes and induces a brief current in A, and the galvanometer deflects. Once the current in B is steady, the field stops changing and the deflection dies away.
When the key is removed, the current in B falls to zero, the field through A changes the other way, and the induced current flows in the opposite direction. So the deflections are in opposite directions, not the same direction, and there is an effect on removing the key as well as on inserting it.
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