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Question
Figure shows a horizontal solenoid connected to a battery and a switch. A copper ring is placed on a frictionless track, the axis of the ring being along the axis of the solenoid. As the switch is closed, the ring will __________ .
Options
remain stationary
move towards the solenoid
move away from the solenoid
move towards the solenoid or away from it depending on which terminal (positive or negative) of the battery is connected to the left end of the solenoid
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Solution
move away from the solenoid
For the circuit,
`E=-L(di)/(dt)`
The current will increase in the solenoid, flowing in clockwise direction in the circuit. Due to this increased current, the flux linked with the copper ring with increase with time, causing an induced current. This induced current will oppose the cause producing it. Hence, the current in the copper ring will be in anticlockwise direction. Now, because the directions of currents in the solenoid and ring are opposite, the ring will be repelled and hence will move away from the solenoid.
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