metal ring levitates when AC is used, therefore the net force on it should be zero according to Newton’s second law. The free body diagram of metal ring is shown in figure 13 (b). Weight (w) acts down. To balance it, a force (F) equal in magnitude and opposite in direction should act as shown in figure 13 (b).

In this activity AC is used. AC changes both its magnitude and direction in regular time intervals. We know that the current through the coil, produces a magnetic field so that one end of the coil behaves like north pole and other end behaves like south pole for a certain time interval. For the next interval, coil changes its polarities.

Hence we can say that coil undergoes changes in its poles in the same intervals of time. The levitation of metal ring is possible only when the metal ring behaves like a magnet and should change its polarities in the same time intervals but in a sense opposite to that of the solenoid (coil) as shown in figure 13(c). Assume that the current flows in an clockwise direction in the solenoid as viewed from the top. Then the upper end becomes a south pole. An upward force is applied on the ring only when the upper side of the ring becomes a north pole (i.e. south pole of the ring faces towards the south pole of solenoid). It is only possible when there exists a anticlockwise current (viewed from the top) in the ring. After certain intervals, solenoid changes its polarities, so that the ring should also change its polarities in the same intervals. This is the reason why the metal ring is levitated.

AC is not a constant current. So that, the magnetic induction changes in both magnitude and direction in the solenoid and in the ring.

Here the area of the metal ring is constant. But the field through the metal ring changes so that flux linked with the metal ring changes.

flux linked with metal ring is zero when no current flows through the solenoid. When the current is allowed to flow through the solenoid, it behaves like bar magnet. So the flux is linked to the metal ring when