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 :
- (A)development of air current when the plate is placed.
- (B)induction of electrical charge on the plate
- (C)shielding of magnetic lines of force as aluminium is a paramagnetic material.
- (D)electromagnetic induction in the aluminium plate giving rise to electromagnetic damping.✓ Correct
Answer: (D)
Solution
As the coil swings it carries a changing current in a magnetic field, so the flux through the nearby aluminium plate keeps changing. By Faraday's law this induces eddy currents in the plate, and by Lenz's law they flow so as to oppose the change that causes them. The result is a retarding force on the coil that drains its energy as heat in the plate, so the oscillations die down. This damping is set up by electromagnetic induction in the aluminium plate.
JEE Main · 2012 · 29 Apr 2012 · Offline · Single correct option