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 :

electromagnetic induction in the aluminium plate giving rise to electromagnetic damping.

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 PYQ — JEE Main 2012 · Shift 0 · Q52 with Solution

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Correct answer: D

What happens when current is passed through the coil?

The coil is in a magnetic field. When current flows through it, a magnetic torque acts on the coil, so it starts oscillating.
Why does placing an aluminium plate nearby stop the oscillation?

As the coil oscillates, the magnetic flux linked with the nearby aluminium plate changes with time.

By Faraday’s law, a changing magnetic flux induces currents in a conductor: $\mathcal{E} = -\frac{d\Phi_B}{dt}$

Since aluminium is a good conductor, eddy currents are induced in the plate.
Effect of these induced currents

By Lenz’s law, the induced currents produce magnetic effects that oppose the motion causing them. Therefore, the oscillation of the coil is opposed.

This causes electromagnetic damping, due to which the oscillations die out quickly.
Check the options
- A: Development of air current when the plate is placed
  This is not the reason.
- B: Induction of electrical charge on the plate
  Not simply static charge; the effect is due to induced eddy currents.
- C: Shielding of magnetic lines of force as aluminium is paramagnetic
  Aluminium is only weakly paramagnetic; this is not the cause of stopping the oscillation.
- D: Electromagnetic induction in the aluminium plate giving rise to electromagnetic damping
  This is exactly correct.
Final answer

The oscillation stops because of eddy currents induced in the aluminium plate, which produce electromagnetic damping.

View written solutionFree

Correct answer: D

What happens when current is passed through the coil?

The coil is in a magnetic field. When current flows through it, a magnetic torque acts on the coil, so it starts oscillating.
Why does placing an aluminium plate nearby stop the oscillation?

As the coil oscillates, the magnetic flux linked with the nearby aluminium plate changes with time.

By Faraday’s law, a changing magnetic flux induces currents in a conductor: $\mathcal{E} = -\frac{d\Phi_B}{dt}$

Since aluminium is a good conductor, eddy currents are induced in the plate.
Effect of these induced currents

By Lenz’s law, the induced currents produce magnetic effects that oppose the motion causing them. Therefore, the oscillation of the coil is opposed.

This causes electromagnetic damping, due to which the oscillations die out quickly.
Check the options
- A: Development of air current when the plate is placed
  This is not the reason.
- B: Induction of electrical charge on the plate
  Not simply static charge; the effect is due to induced eddy currents.
- C: Shielding of magnetic lines of force as aluminium is paramagnetic
  Aluminium is only weakly paramagnetic; this is not the cause of stopping the oscillation.
- D: Electromagnetic induction in the aluminium plate giving rise to electromagnetic damping
  This is exactly correct.
Final answer

The oscillation stops because of eddy currents induced in the aluminium plate, which produce electromagnetic damping.