Q78 · Practice questionComplex unfamiliar4 marks
QUESTION 78 (4 marks)
A strong magnet falls axially through a conducting copper ring. Explain why its acceleration can be less than while it approaches and leaves the ring, using Faraday’s law, Lenz’s law and conservation of energy.
WORKED SOLUTION
4 marksPractice marking scheme
ANSWER
Changing magnetic flux induces a current whose magnetic field opposes the change in flux, producing a magnetic force opposing the magnet’s motion. Mechanical energy is transferred mainly to thermal energy in the ring.
Worked solution
As the magnet approaches the ring, the magnetic flux through the ring changes, so Faraday’s law predicts an induced EMF and current. By Lenz’s law, the induced magnetic field acts to oppose the increase in flux, giving an upward force on the descending magnet. As the magnet leaves, the flux decreases; the induced current reverses so that its field opposes that decrease, again tending to oppose the magnet’s departure. The reduced acceleration does not violate energy conservation: some gravitational potential energy becomes electrical energy and then thermal energy in the ring.
Links changing flux to induced EMF/current.
Applies Lenz’s law on approach.
Applies Lenz’s law on departure.
Explains energy transfer/conservation.
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