Question

A bar magnet is released from rest along the axis of a very long, vertical copper tube. After some time the magnet

• A. will stop in the tube.
• B. will move with almost constant speed.
• C. will move with an acceleration $g$
• D. will oscillate.

Answer 1

The correct option is B will move with almost constant speed.

As the magnet is moving under gravity, the flux linked with copper tube will change because of motion of the magnet. This will produce eddy currents in the body of the copper tube. According to Lenz's law, these induced current oppose the fall of magnet. So, the magnet will experience a retarding force. This force will continuously increase with increasing velocity of the magnet till it becomes equal to the force of gravity. After this, the net force on the magnet will become zero. Hence, the magnet will attain a constant speed.