A magnet dropped down a long vertical copper tube falls unusually slowly because
Correct answer: C. induced eddy currents in the tube produce a field opposing the magnet's motion
- A. copper is magnetic and attracts it
- B. air resistance inside the tube is very large
- C. induced eddy currents in the tube produce a field opposing the magnet's motion
- D. the magnet loses its magnetism as it falls
Explanation
The changing flux induces circulating eddy currents in the tube wall, and by Lenz's law their magnetic field opposes the motion, retarding the fall. Copper is not itself magnetic, so the effect disappears entirely if a plastic tube is used, which is the standard control for this demonstration. The same principle gives eddy current braking in trains.
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About Lenz's Law
Lenz's law states that the induced current flows in a direction that opposes the change in magnetic flux producing it. Questions connect this rule with Faraday's law, magnetic flux, induced emf and the direction of motion or current, while distinguishing opposition to flux change from opposition to the magnetic field itself.
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