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  4. A coil is suspended in a uniform magnetic field, with the plane of the coil parallel to the magnetic lines of force. When a current is passed through the coil, it starts oscillating. It is very difficult to stop, but if an aluminium plate is placed near to the coil, it stops. This is due to

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Q. A coil is suspended in a uniform magnetic field, with the plane of the coil parallel to the magnetic lines of force. When a current is passed through the coil, it starts oscillating. It is very difficult to stop, but if an aluminium plate is placed near to the coil, it stops. This is due to

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Solution:

When an aluminium plate is placed near to the coil, electromagnetic induction in the aluminium plate gives rise to electromagnetic damping that helps to stop the motion.
We know that due to the current in the coil, the magnetic field will exert a force on the coil and due to this force the coil will start to oscillate.
Due to the oscillation of the coil, it will create a varying magnetic field. For a varying magnetic field, an emf will be induced in a conductor. This emf produced in the coil will oppose the varying magnetic field in the coil and thus produce a torque. Due to this torque electromagnetic induction will be produced and this electromagnetic induction will cause electromagnetic damping in the coil.
Thus the oscillation will come to an end.