Lenz's Law
A bar magnet is dropped with its north pole pointing downward through a vertical copper ring lying in a horizontal plane. While the magnet approaches the ring from above, the induced current viewed from the top of the ring flows in which sense, and why?
Select the correct option:
Solution
Anticlockwise, to oppose the approaching north pole
Lenz's law fixes the direction of an induced current so that the magnetic effect of that current always opposes the change in flux producing it; this is a direct consequence of energy conservation. As the north pole descends toward the ring, the downward flux through the ring increases, so the induced current must generate a magnetic field that points upward inside the ring to resist the rise in flux. By the right-hand rule, an upward field at the centre requires the current to circulate anticlockwise when seen from above, which also means the top face of the ring behaves as a north pole that repels the falling magnet. The option 'clockwise' reverses the required field and would aid the change, violating energy conservation. The option 'anticlockwise to assist' is self-contradictory because an anticlockwise current opposes, not assists, the approaching pole. The 'no current' option ignores that flux is changing during the fall. This mirrors the NCERT demonstration of induced poles in conducting loops. A sanity check confirms the induced repulsion slightly slows the magnet, consistent with work being done against the field.
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About This Question
- Subject
- physics
- Chapter
- electromagnetic induction and alternating currents
- Topic
- lenz's law
- Difficulty
- Medium
- Year
- 2025
Solution
Correct Answer:
Anticlockwise, to oppose the approaching north pole
Lenz's law fixes the direction of an induced current so that the magnetic effect of that current always opposes the change in flux producing it; this is a direct consequence of energy conservation. As the north pole descends toward the ring, the downward flux through the ring increases, so the induced current must generate a magnetic field that points upward inside the ring to resist the rise in flux. By the right-hand rule, an upward field at the centre requires the current to circulate anticlockwise when seen from above, which also means the top face of the ring behaves as a north pole that repels the falling magnet. The option 'clockwise' reverses the required field and would aid the change, violating energy conservation. The option 'anticlockwise to assist' is self-contradictory because an anticlockwise current opposes, not assists, the approaching pole. The 'no current' option ignores that flux is changing during the fall. This mirrors the NCERT demonstration of induced poles in conducting loops. A sanity check confirms the induced repulsion slightly slows the magnet, consistent with work being done against the field.
This medium difficulty physics question is from the chapter electromagnetic induction and alternating currents, covering the topic of lenz's law. It appeared in the 2025 exam.
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