A bar magnet is pushed toward the centre of a stationary conducting ring along its…
PHYSICS · Single correct · A Level · Physics · Fields and their consequences · Fields and their consequences
Problem
A bar magnet is pushed toward the centre of a stationary conducting ring along its axis at constant speed. An external agent must supply positive mechanical work to keep the magnet moving. Which statement correctly explains why?
(a) The magnet’s approach increases the magnetic flux through the ring; therefore additional work is needed to create this larger magnetic field.
(b) Charges driven by the induced emf pile up on opposite sides of the ring, and the supplied work is stored as electrostatic potential energy between these charges.
(c) The induced current in the ring sets up a magnetic field that opposes the approaching magnet, so the mechanical work done is dissipated as heat in the ring.
(d) The north pole of the magnet is attracted toward an induced south pole in the ring, so extra work is required simply to keep the magnet from speeding up toward the ring.