A bar magnet is pushed at a steady speed towards a closed coil, with its north pole facing the coil. The work done by the person pushing the magnet
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → appears as heat produced by the induced current in the coil
As the north pole approaches, the flux through the coil increases. By Lenz’s law the induced current makes the near face of the coil a north pole. This pole repels the magnet. To keep the magnet moving, the person has to push against this repulsion.
Option (1) is zero, as the magnet moves at a steady speed — Incorrect. A steady speed means the net force on the magnet is zero. The push of the person balances the repulsion. The magnet moves in the direction of the push, so the person does positive work.
Option (2) is negative, as the induced current in the coil pulls the magnet towards the coil — Incorrect. An attraction would help the motion and increase the flux faster, which goes against Lenz’s law. The coil repels the approaching magnet.
Option (3) is stored as the kinetic energy of the magnet — Incorrect. The speed of the magnet stays the same, so its kinetic energy does not change.
Option (4) appears as heat produced by the induced current in the coil — Correct. The work done against the repulsion becomes electrical energy of the induced current. This energy is lost as heat in the coil (Joule heating). This is how Lenz’s law agrees with the conservation of energy.
NCERT (2026-27):
Lenz’s Law and Conservation of Energy – “… In this situation, the bar magnet experiences a repulsive force due to the induced current. Therefore, a person has to do work in moving the magnet. … This energy is dissipated by Joule heating produced by the induced current.”