A bar magnet is released from rest along the axis of a very long, vertical copper tube. After some time the magnet will
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → move with almost constant speed
As the magnet falls, the magnetic flux through each section of the copper tube changes, so eddy currents are induced in the tube. By Lenz’s law these currents oppose the motion of the magnet, giving an upward drag force.
The faster the magnet falls, the faster the flux changes and the larger the drag. The magnet speeds up until the drag equals its weight; after that the net force is nearly zero and it falls with an almost constant (terminal) speed. The lost gravitational energy appears as heat in the tube.
Why the other options are incorrect
Option (1) oscillate — Incorrect. The drag only opposes the motion; it never reverses it, so there is nothing to make the magnet move back up.
Option (3) move with an acceleration \( g \) — Incorrect. The magnet falls with acceleration g only in a non-conducting tube (or with no tube), where no eddy currents are induced.
Option (4) stop — Incorrect. The drag exists only while the magnet moves; if it stopped, the drag would vanish and gravity would start it again.
Note: A similar question was also asked in CUET (UG) 2025 on 14-05-2025 (Slot 1).
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), and it also appears to be out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Eddy Currents – “… Take two hollow thin cylindrical pipes of equal internal diameters made of aluminium and PVC, respectively. Fix them vertically with clamps on retort stands. Take a small cylinderical magnet having diameter slightly smaller than the inner diameter of the pipes and drop it through each pipe in such a way that the magnet does not touch the sides of the pipes during its fall. You will observe that the magnet dropped through the PVC pipe takes the same time to come out of the pipe as it would take when dropped through the same height without the pipe. Note the time it takes to come out of the pipe in each case. You will see that the magnet takes much longer time in the case of aluminium pipe. Why is it so? It is due to the eddy currents that are generated in the aluminium pipe which oppose the change in magnetic flux, i.e., the motion of the magnet. The retarding force due to the eddy currents inhibits the motion of the magnet. Such phenomena are referred to as electromagnetic damping. Note that eddy currents are not generated in PVC pipe as its material is an insulator whereas aluminium is a conductor.”