Question

A light disc made of aluminium (a nonmagnetic material) is kept horizontally and is free to rotate about its axis as shown in the figure. A strong magnet is held vertically at a point above the disc away from its axis. On revolving the magnet about the axis of the disc, what happens to the disc? (The figure is schematic and not drawn to scale).

A. Rotate in the direction opposite to the direction of the magnet’s motion
B. Rotate in the same direction as the direction of the magnet’s motion
C. Not rotate and its temperature will remain unchanged
D. Not rotate but its temperature will slowly rise

Answer 1

Hint:Before we proceed with the problem, it is important to know about the Lenz law. The emf will be induced Whenever there is a magnetic flux linked with the coil and the direction of induced emf opposes the change in magnetic flux known as Lenz law.



Complete step by step solution:
Now we can solve the problem step by step as follows.

Image: An aluminium disc with the magnet held above it.

The aluminium disc and the magnet area as shown in the figure. Now, if we start moving the magnet about the axis of the disc, the number of magnetic field lines changes, that is the magnetic field line decreases. Here considering a small area of the disc where the magnetic field line passes. The magnetic flux is defined as the number of field lines passing per unit area, whereas flux and magnetic field are directly proportional to each other, the magnetic field decreases, and the flux also decreases.

Based on Lenz’s law, the disc also tries to move in the same direction as the magnet. On the underside of the disc, due to the movement of the magnet, there is a reduction in flux. This flux change causes the production of eddy currents which leads to the generation of heat. These currents are such that they oppose the relative motion of the disc. So, the disc will rotate in the direction of the rotation of the magnet.

Hence, Option B is the correct answer

Note:Lenz's law will be helpful to understand the concept of stored magnetic energy in an inductor. It is also used in microphones, AC generators, etc.