Question

For protecting a magnetic needle it should be placed,
A. In an iron box
B. In a wooden box
C. In the metallic box
D. None of these

Answer 1

Hint: Before we proceed with the problem, it is important to know about ferromagnetic materials. They have large, positive susceptibility to an external magnetic field which exhibits a strong attraction to magnetic fields, and are able to retain their magnetic properties even after the external field has been removed. When it is placed in an external magnetic field, these materials are strongly magnetized in the direction of the field.

Complete step by step solution:
For protecting the needle, it should be kept in an iron box because the iron box will divert the magnetic field lines generated by the needle, containing the field lines within the box. instead of in the air.

As we know, iron is a ferromagnetic substance. As it is a ferromagnetic material the magnetic lines of force do not exist. Suppose you want to protect sensitive material from the external magnetic field, then you have to place it inside the iron box.

When the magnetic needle is placed in the iron box, the iron box gets magnetized and the magnet remains unmoved. That is, it gets magnetized by itself and protects the needle.
Therefore, in order to protect a magnetic needle, it should be placed inside the iron box.

Hence, option A is the correct answer.

Note:Some of the properties of ferromagnetic material are as follows.
1. The atoms of ferromagnetic substances have permanent dipole moments.
2. Atomic dipoles in ferromagnetic substances are oriented in the same direction as the external magnetic field.
The magnetic dipole moment of these materials is large and is in the direction of the magnetizing field.