A2 Daily A Level Physics question
In a lab, a conducting aluminium ring is dropped coaxially toward the open end of a vertical solenoid that carries a steady DC current. Air resistance is negligible. Compared with an identical drop when the solenoid current is switched off, which statement must be true about the ring’s speed as it reaches the solenoid’s mouth? (Focus on field and energy reasoning.)
Answer
The correct answer is D.
Correct: D — Less than in free fall, because increasing flux through the moving ring induces a current whose field opposes the motion. A overstates a common misconception: Lenz’s law gives a repulsive effect that resists the increase in flux, so it does not pull the ring in faster. B is wrong because motion through a steady field still changes magnetic flux, so an emf and induced current arise. C is incorrect because there is no mechanism that restores the speed to the free-fall value at the mouth; mechanical energy is diverted into resistive heating, leaving a net speed deficit. D correctly applies Lenz’s law and energy conservation: the induced current produces an upward magnetic force, reducing the ring’s kinetic energy below the free-fall case.