A magnet is dropped down an infinitely long vertical copper tube

Options

A. the magnet moves with continuously increasing velocity and ultimately acquires a constant terminal velocity
B. the magnet moves with continuously decreasing velocity and ultimately comes to rest
C. the magnet moves with continuously increasing velocity but constant acceleration
D. the magnet moves with continuously increasing velocity and acceleration

Answer

A

Explanation

If bar magnet is falling vertically through the hollow region of long vertical copper tube, then the magnetic flux linked with copper tube (due to 'non-uniform' magnetic field of magnet) changes and eddy currents are generated in the body of the tube by Lanz's law. The eddy currens oppose the falling of the magnet which, therefore experiences a retarding force. The retarding force increases with increasing velocity of the magnet and finally equals the weight of the magnet. The magnet, then attains a constant final terminal velocity, i.e., magnet ultimately falls with zero acceleration in the tube.