PHYSICS · Single correct · A Level · Physics · Fields and their consequences · Fields and their consequences
Problem
A bar magnet is moved rapidly toward a thin, closed conducting loop. The magnet is then suddenly stopped and held stationary at the centre of the loop. Which option correctly describes the work done by the external agent during and after the motion?
(a) Positive work is done while the magnet moves; no further work is needed once it is held because the induced current ceases.
(b) Negative work is done while the magnet moves; additional negative work is needed to hold it stationary because the loop keeps driving the magnet outward.
(c) Equal positive and negative work are done during approach and stopping, so the net external work performed is zero.
(d) Positive work is done while the magnet moves; additional negative work is needed to hold it stationary because the induced currents reverse direction and push the magnet away.