If a magnet is dropped through a closed coil of wire, it will fall with an acceleration
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → less than g
As the magnet falls, the magnetic flux through the closed coil changes and a current is induced in it. By Lenz’s law, this current opposes the change in flux, and so it opposes the motion of the magnet that causes the change.
Option (1) more than g — Incorrect. The force of the induced current on the magnet opposes its motion. So it acts upward on the falling magnet and cannot add to its weight.
Option (2) equal to g in the beginning and then more than g — Incorrect. Far from the coil, the flux changes very little and the acceleration is close to g. As the magnet nears the coil, the induced current opposes its fall. So the acceleration drops below g. It never goes above g.
Option (3) equal to g — Incorrect. The magnet would fall freely only if no current flowed, as with an open coil. In a closed coil, the induced current exerts an upward force on the magnet.
Option (4) less than g — Correct. The force of the induced current always acts upward on the magnet.
While the magnet approaches the coil, the face of the coil towards it has the same polarity and repels it. While it moves away, the near face has the opposite polarity and attracts it back.
The net downward force is less than \( mg \), so the acceleration is less than g. Part of the energy of the falling magnet is turned into Joule heat in the coil.
NCERT (2026-27):
Lenz’s Law and Conservation of Energy – “The polarity of induced emf is such that it tends to produce a current which opposes the change in magnetic flux that produced it.”
Lenz’s Law and Conservation of Energy – “… Similarly, if the North-pole of the magnet is being withdrawn from the coil, the magnetic flux through the coil will decrease. To counter this decrease in magnetic flux, the induced current in the coil flows in clockwise direction and its South-pole faces the receding North-pole of the bar magnet. This would result in an attractive force which opposes the motion of the magnet and the corresponding decrease in flux.”
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.”