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 \( 2.3\times10^{-5} \).
Consider the following statements about the substance described in the passage:
(A) Its atoms have permanent magnetic dipole moments.
(B) Its magnetisation \(M\) and the magnetic intensity \(H\) are opposite in direction.
(C) Its susceptibility depends on the temperature of the sample.
(D) With no external field, a sample of it shows a net magnetisation.
Which of the above statements are incorrect?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (B) and (D) only
A susceptibility of \( 2.7\times10^{-5} \) is small and positive. So the substance is paramagnetic.
(A) Its atoms have permanent magnetic dipole moments. — Correct statement. Each atom of a paramagnetic material has a dipole moment of its own.
(B) Its magnetisation \(M\) and the magnetic intensity \(H\) are opposite in direction. — Incorrect statement. With \( \chi \) positive, \( M=\chi H \) points along \( H \). The two are opposite only in a diamagnetic material.
(C) Its susceptibility depends on the temperature of the sample. — Correct statement. For a paramagnetic material, \( \chi \) depends on the material and also on its temperature.
(D) With no external field, a sample of it shows a net magnetisation. — Incorrect statement. Thermal motion keeps the atomic moments pointing at random. So without a field they add up to zero.
NCERT (2026-27):
Paramagnetism – “The individual atoms (or ions or molecules) of a paramagnetic material possess a permanent magnetic dipole moment of their own. On account of the ceaseless random thermal motion of the atoms, no net magnetisation is seen. …”
Magnetisation and Magnetic Intensity – “… χ is small and positive for materials, which are called paramagnetic. It is small and negative for materials, which are termed diamagnetic. In the latter case M and H are opposite in direction. …”
Paramagnetism – “… For a paramagnetic material both χ and μr depend not only on the material, but also (in a simple fashion) on the sample temperature. …”