Case: Read the passage and answer the question.
The magnetic susceptibility of a substance is a measure of how readily it responds to an external magnetic field. It gives a measure of the ability of a substance to acquire magnetism. It has no units. Mathematically, it is defined as the ratio of the magnetisation to the magnetic intensity. The susceptibility of a certain substance is \( 2.3\times10^{-5} \).
A bar of this substance is placed in a uniform external magnetic field. Which of the following describes the field lines and the field inside the bar?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → The field lines get concentrated inside, and the field inside is slightly enhanced.
Inside a material, \( B=\mu_0(1+\chi)H \). A positive \( \chi \) makes the field a little larger than it would be in vacuum. Here \( \chi \) is only \( 2.7\times10^{-5} \), so the change is very small.
Option (1) The field lines are completely expelled, and the field inside becomes zero. — Incorrect. Complete expulsion happens in a superconductor, for which \( \chi=-1 \). The given \( \chi \) is positive.
Option (2) The field lines are pushed out a little, and the field inside is slightly reduced. — Incorrect. Field lines are pushed out of a diamagnetic bar, whose susceptibility is negative.
Option (3) The field lines are highly concentrated inside, and the field inside becomes very large. — Incorrect. Field lines crowd strongly into a ferromagnetic bar, whose susceptibility is large. A value of \( 2.7\times10^{-5} \) is far too small for that.
Option (4) The field lines get concentrated inside, and the field inside is slightly enhanced. — Correct. The substance is paramagnetic. The lines bend into the bar, and the field inside rises by only about three parts in \( 10^{5} \).
NCERT (2026-27):
Paramagnetism – “… Figure 5.7(b) shows a bar of paramagnetic material placed in an external field. The field lines gets concentrated inside the material, and the field inside is enhanced. In most cases, this enhancement is slight, being one part in 105. …”
FIGURE 5.7 Behaviour of magnetic field lines near a (a) diamagnetic, (b) paramagnetic substance.