Case: Read the passage and answer the question.
In terms of the susceptibility, we can classify materials as diamagnetic, paramagnetic or ferromagnetic. Diamagnetic substances are those which have a tendency to move from the stronger to the weaker part of an external magnetic field. In other words, unlike the way a magnet attracts metals like iron, it would repel a diamagnetic substance. Paramagnetic substances are those which get weakly magnetised when placed in an external magnetic field. They have a tendency to move from a region of weak magnetic field to a region of strong magnetic field, i.e., they get weakly attracted to a magnet. Ferromagnetic substances are those which get strongly magnetised when placed in an external magnetic field. They have a strong tendency to move from a region of weak magnetic field to a region of strong magnetic field, i.e., they get strongly attracted to a magnet.
A rod of a material is placed in a magnetic field of intensity 4000 A m−1. Its magnetisation is found to be −0.08 A m−1. The magnetic susceptibility of the material and its type are
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(-2\times10^{-5}\), diamagnetic
The magnetic susceptibility \( \chi \) shows how a material responds to an applied field. It is the ratio of the magnetisation \( M \) to the magnetic intensity \( H \).
Given:
\( H=4000\ \mathrm{A\,m^{-1}} \)
\( M=-0.08\ \mathrm{A\,m^{-1}} \)
\( \begin{aligned} \chi &= \frac{M}{H} \\ &= \frac{-0.08}{4000} \\ &= -2\times10^{-5} \end{aligned} \)
The susceptibility is small and negative. So the magnetisation of the rod points opposite to the applied field. Such a rod would move from the stronger to the weaker part of a non-uniform field.
Therefore, \( \chi=-2\times10^{-5} \) and the rod is diamagnetic.
NCERT (2026-27):
Magnetisation and Magnetic Intensity – “… This influence is mathematically expressed as”
\( M=\chi H \)\( \qquad (5.13) \)
“where χ, a dimensionless quantity, is appropriately called the magnetic susceptibility. … It is small and negative for materials, which are termed diamagnetic. …”