QQuestion
The following figure shows a small bar magnet falling freely through a copper ring. For the observer at A, the direction of the induced current will be :
(a) clockwise when magnet is above and below the ring
(b) anticlockwise when magnet is above and below the ring
(c) anticlockwise when magnet is above the ring and clockwise when the magnet is below the ring
(d) clockwise when magnet is above the ring and anticlockwise when the magnet is below the ring
✓Answer
(c) anticlockwise when magnet is above the ring and clockwise when the magnet is below the ring
Explanation:
Magnet above the ring (approaching it)
- Flux change — The north pole of the magnet faces the ring, so magnetic field lines enter the ring downwards. As the magnet falls closer, the downward flux through the ring increases.
- Opposition required — To oppose this increase in downward flux, the ring must create an upward field (its own north pole must face the magnet).
- Current direction — Using the right-hand rule, a coil whose top acts as a north pole carries anticlockwise current when viewed from above.
Result: Anticlockwise current while the magnet is above the ring.
Magnet inside and leaving the ring (moving away below it)
- Flux change — Once the magnet has passed through, the same downward field lines are now moving away, so the downward flux decreases.
- Opposition required — The ring tries to maintain the original flux by producing more downward field—i.e., its top must now behave like a south pole.
- Current direction — A coil whose top is a south pole carries clockwise current when viewed from above.
Result: Clockwise current while the magnet is below the ring.
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