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; 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.

Answer 1

Hint: We are given a coil in a uniform magnetic field and the plane of the coil is parallel to the magnetic lines of force. The coil starts oscillating when a current is passed through it and to stop the oscillation we bring an aluminium plate. We know that aluminium is a conductor and when a conductor is in a magnetic field an emf will get produced. Using this concept we can solve the question.

Complete answer:
In the question it is said that there is a coil suspended in a uniform magnetic field. The plane of the coil is parallel to the magnetic lines of force.
It is said that a current is passed through this coil in the magnetic field. We know that due to the current in the coil, the magnetic field will exert a force on the coil and due to this force the coil will start to oscillate.
Due to the oscillation of the coil, it will create a varying magnetic field.
Now it is said that we place an aluminium plate near the coil. We know that aluminium is a good conductor of electricity.
For a varying magnetic field, an emf will be induced in a conductor. This emf produced in the coil will oppose the varying magnetic field in the coil and thus produce a torque. Due to this torque electromagnetic induction will be produced and this electromagnetic induction will cause electromagnetic damping in the coil.
Thus the oscillation will come to an end.

So, the correct answer is “Option D”.

Note:
A material that allows the flow of electrons in it freely, from particle to particle is known as conductor. By this movement of electrons the conductors pass charge through it. Many materials are used as conductors, but copper, aluminium, steel etc are the frequently used conductors.