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Question:

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 and it is very difficult to stop. But if an aluminium plate is placed near to the coil, it stops. This is due to:

induction of electrical charge on the plate

shielding of magnetic lines of force as aluminium is a paramagnetic material

electromagnetic induction in the aluminium plate giving rise to electromagnetic damping

development of air current when the plate is placed

Solution:

Oscillating coil produces time variable magnetic field. It cause eddy current in aluminium plate which causes anti-torque on the coil due to which it stops.