For a magnetic material, \( \mu \) is the absolute permeability, \( \mu_r \) the relative permeability and \( \chi \) the magnetic susceptibility. Which of the following is NOT correct?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → For a ferromagnetic material, \( \chi \) is small and positive.
The three quantities are related by \( \mu_r=1+\chi \) and \( \mu=\mu_0\mu_r \). So if one of them is known, the other two follow. Materials are grouped by the sign and size of their susceptibility.
Option (1) For a ferromagnetic material, \( \chi \) is small and positive. — Not correct. The susceptibility of a ferromagnetic material is large and positive (\( \chi\gg1 \)). So its \( \mu_r \) is much greater than 1. A small positive \( \chi \) marks a paramagnetic material.
Option (2) For any material, \( \mu=\mu_0(1+\chi) \). — Correct statement. Comparing \( B=\mu_0(1+\chi)H \) with \( B=\mu H \) gives this relation, which is the same as \( \mu=\mu_0\mu_r \) with \( \mu_r=1+\chi \).
Option (3) For a diamagnetic material, \( 0\le\mu_r\lt1 \). — Correct statement. A diamagnetic material has \( -1\le\chi\lt0 \), so \( \mu_r=1+\chi \) is less than 1 but never negative. The value 0 is reached only for a superconductor.
Option (4) For a paramagnetic material, \( \mu \) is slightly greater than \( \mu_0 \). — Correct statement. Its \( \chi \) is small and positive, so \( \mu_r \) is a little more than 1 and \( \mu \) a little more than \( \mu_0 \).
NCERT (2026-27):
Magnetic Properties of Materials – “The discussion in the previous section helps us to classify materials as diamagnetic, paramagnetic or ferromagnetic. In terms of the susceptibility χ, a material is diamagnetic if χ is negative, para- if χ is positive and small, and ferro- if χ is large and positive.”