JEE MAIN - Physics (2012 - No. 8)
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 :
development of air current when the plate is placed
induction of electrical charge on the plate
shielding of magnetic lines of force as aluminium is a para-magnetic material.
electromagnetic induction in the aluminium plate giving rise to electromagnetic damping.
Explication
Because of the Lenz's law of conservation of energy.