question_answer 31)

  Consider a metal ring kept on top of a fixed solenoid (say on a cardboard), Fig. 4(EP).6. The centre of the ring coincides with the axis of the solenoid. If the current is suddenly switched on, the metal ring jumps up. Explain.                                            

Answer:

                  In Fig. 4(EP).6, when current is suddenly switched on, magnetic flux (downwards) linked with the solenoid and hence with metal ring increases. Current is induced in the metal ring in counterclockwise direction (as seen from top of the ring). As the flow of current in the ring is in a direction opposite to the current in the solenoid, they will repel each other. Therefore, the ring jumps up.