JEE Main 2023 · 11 April · evening · Single correct · +4 / −1
Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R
Assertion A: A bar magnet dropped through a metallic cylindrical pipe takes more time to come down compared to a non-magnetic bar with same geometry and mass.
Reason R: For the magnetic bar, Eddy currents are produced in the metallic pipe which oppose the motion of the magnetic bar.
In the light of the above statements, choose the correct answer from the options given below
- (a) A is true but R is false
- (b) A is false but R is true
- (c) Both A and R are true but R is NOT the correct explanation of A
- (d) Both A and R are true and R is the correct explanation of A
Answer and solution
Answer (d) Both A and R are true and R is the correct explanation of A
Solution
Physics sir · 4 steps
A moving magnet means changing flux in the pipe walls, and Lenz's law makes those currents brake the magnet.
- Changing flux. As the bar magnet falls, the magnetic flux through each ring of the metal pipe keeps changing. A changing flux induces an emf, and in a solid conductor this emf drives circulating eddy currents in the pipe wall.
- Lenz's law. These currents flow so as to oppose the cause that produced them, which is the relative motion of the magnet. The ring below the magnet pushes it up and the ring above pulls it back, so there is a net upward force on the magnet.
- Non-magnetic bar. A non-magnetic bar of the same mass and shape produces no flux change, so no eddy currents and no retarding force. It falls with acceleration close to g and reaches the bottom sooner.
- So. A is true, and R is true and is exactly why A happens: the eddy-current force slows the magnet. The answer is (d).
The idea to keep. Whenever a magnet moves near a conductor, ask what flux changes. The induced eddy currents always oppose the relative motion, so they act as a brake on the motion.
Answer from the NTA final answer key. Solution by PrepWiser.