Question

Earth’s magnetic field inside a closed box of iron, as compared to that of outside is:
A. less
B. more
C. zero
D. same

Answer 1

Hint: Permeability of a material is the measure of resistance offered by that material against the formation of magnetic field. Permeability of a material is its unique property just like permittivity, refractive index, etc. It is not only defined for a material but also defined for free space. The permeability of free space has a value of $1.256 \times 10^{-6} m \ kg \ s^{-2} \ A^{-2}$ and is denoted by $\mu_{\circ}$.

Complete answer:
Different materials have different values of permeability. The value of permeability for metals is very high. Hence in case of iron, which is a metal, the permeability being high, it provides an easy path for the flow of magnetic field i.e. easily allows the formation of magnetic fields. Hence the magnetic field lines prefer to go via iron material than through free space or air. This phenomenon is also called magnetic shielding.
So the earth's magnetic field is less inside.

So, the correct answer is “Option A”.

Additional Information:
The relative permeability is the ratio of permeability of material to the permeability of free space. Pure iron has a relative permeability of 200000. That means it allows the formation of magnetic fields inside it 200000 times more easily than in free space.

Note:
Permeability can also be defined as the degree of magnetization that a material obtains in response to an external magnetic field. It’s important to note that magnetic fields could turn to zero only in ideal cases because relative permeability can’t be infinite. Hence the field can’t be zero inside the box made from any type of material. In the case of iron, it will be about 99%.