Question

A small hole is made at the center of the magnet then its magnetic moment:
A) Decreases
B) Increases
C) Remains same
D) Depends on the nature of the magnetic material

Answer 1

Hint: This question is from the topic Magnetism and matter. This can be easily solved if you try to recall what a magnetic moment is, how it gets affected by the size and shape of the magnet, if it gets affected and then you need to apply that knowledge here to reach the correct answer.

Complete step by step answer:
We will be trying to approach the solution to this question exactly as we described in the hint section of the solution. This needs us to think about the magnetic moment, what it is, and how it gets affected by the size and shape of the magnet.
If you are having troubles recalling what affects a magnetic moment, let us help you with it. The magnetic moment of a magnet mainly depends upon the magnetic pole strength of the magnet. If the magnetic pole strength of the magnet decreases, the magnetic moment of the magnet will also decrease. On the other hand, if the magnetic pole strength of the magnet increases, the magnetic moment of the magnet will also increase with it.
We also know that the magnetic pole strength of a magnet depends on its shape and size. In the question, it has been given to us that there is a hole made at the center of the magnet.
We already know that outside of the magnet, the magnetic field lines go from North pole to South pole, whilst inside of the magnet, they go from South pole to North pole. If a hole is made in the magnet, Magnetic field lines in the hole will go from North pole to South pole, while in the magnet, magnetic field lines will go from South pole to the North pole. We can easily see that this causes a decrease in the magnetic pole strength of the magnet and thus, the magnetic moment of the magnet also decreases with it.

Hence, the correct option is option (A), i.e. magnetic moment decreases due to the hole that is made at the center of the magnet.

Note: Many students consider magnetic moments as just the product of pole strength of the magnet and the distance between the poles, and forget to give a thought about how pole strength will be affected due to the hole, thus they mark the option “remains same” and lose their marks.