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Question
Consider a metal ring kept (supported by a cardboard) on top of a fixed solenoid carrying a current I (Figure). 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?
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Solution
This problem is based on Lenz’s law and according to this law, the direction of induced emf or current in a circuit is such as to oppose the cause that produces it.
When the switch is opened, current in the circuit of solenoid stops flowing. Initially, there is some magnetic flux linked with the solenoid and now if current in the circuit stops, the magnetic flux falls to zero or we can say that magnetic flux linked through the ring decreases.
According to Lenz’s law, this decrease in flux will be opposed and the ring experiences downward force toward the solenoid.
This happen because the current decrease will cause a clockwise current (as seen from the top in the ring in figure) to increase the decreasing flux. This can be done if the direction of induced magnetic field is same as that of solenoid. This makes the opposite sense of the flow of current in the ring (when viewed from the bottom of the ring) and solenoid form opposite magnetic poles in front of each other.
Hence, they will -attract each other but as ring is placed at the cardboard it could not be able to move downward.
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