Question

A bar magnet is falling freely inside a long copper tube and a solenoid as shown in figure (i) and (ii) respectively then acceleration of magnet inside the copper tube and solenoid are respectively (acceleration due to gravity = g)

• A. g, g
• B. Greater than g, lesser than g
• C. Greater than g, g
• D. Zero, lesser than g

Answer 1

Answer: (D)

Zero, lesser than g

Answer 2

If bar magnet is falling vertically through the hollow region of long vertical copper tube then the magnetic flux linked with the copper tube (due to 'non-uniform' magnetic field of magnet) changes and eddy currents are generated in the body of the tube by Lenz's law the eddy currents opposes the falling of the magnet which therefore experience 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.

The resistance of copper solenoid is much higher than that of copper tube, hence the induced current in it, due to motion of magnet, will be much less than that in the tube. Consequently the opposition to the motion of magnet will be less and the magnet will fall with an acceleration (1) less than g. (i.e. a < g).