Ncert Exercises & Solutions

Consider a metal ring kept (supported by a cardboard) on top of a fixed solenoid carrying a current I. The centre of the ring coincides with the axis of the solenoid. If the current in the solenoid is switched off, what will happen to the ring?

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

Step 1: When the current is switched off, the magnetic flux linked through the ring decreases. According to Lenz's law, this decrease in flux will be opposed and the ring experience downward force towards the solenoid.

Step 2: This happens because the decrease in flux will cause a clockwise current (as seen from the top in the ring in the figure) i.e., the same direction as that in the solenoid. This makes the opposite sense of the flow of current in the ring (when viewed from the bottom of the ring) and solenoid forming opposite magnetic pole in front of each other.

Hence, they will attract each other but as the ring is placed at the cardboard it could not be able to move downward.

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