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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

Asked in JEE Main 2012 · Eddy currents and damping

• (1) induction of electrical charge on the plate
• (2) shielding of magnetic lines of force as aluminium is a paramagnetic material
• (3) electromagnetic induction in the aluminium plate giving rise to electromagnetic damping
• (4) development of air current when the plate is placed

Step-by-step solution

Why the other options are wrong

• (1) comes from no static charge is involved; the effect needs a changing flux
• (2) comes from aluminium does not shield magnetic field lines, and paramagnetism is irrelevant here
• (4) comes from air currents are far too feeble to stop a coil quickly

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