Ncert Exercises & Solutions

Consider a metal ring kept on top of a fixed solenoid (say on cardboard) (figure). 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.

Hint: The force on the coil is the result of the rate of change of magnetic flux.

Step 1: When the current is switched on, magnetic flux is linked through the ring. Thus, an increase in flux takes place. According to Lenz's law, this increase in flux will be opposed and it can happen if the ring moves away from the solenoid.

This makes happens because the flux increases will cause a counter clockwise current (as seen from the top in the ring in figure.) i.e., opposite direction to that in the solenoid.

Step 2: This makes the same sense of flow of current in the ring (when viewed from the bottom of the ring) and solenoid forming the same magnetic pole infront of each other. Hence, they will repel each other and the ring will move upward.

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