Consider the following statements about diamagnetic substances and select the correct statements.
(A) In the external magnetic field, they move from the stronger part to the weaker part of the field.
(B) The net magnetic moment develops in the direction opposite to that of the applied magnetic field.
(C) Relative magnetic permeability is greater than one
(D) Magnetic susceptibility is -1 for a perfect diamagnetic substance.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A), (B) and (D) only
In a diamagnetic substance the applied field induces small magnetic moments in the atoms that oppose the applied field. So the magnetisation is opposite to H, the susceptibility \( \chi \) is small and negative, and the relative permeability \( \mu_r = 1 + \chi \) is slightly less than 1.
(A) In the external magnetic field, they move from the stronger part to the weaker part of the field. — Correct. A diamagnetic substance is repelled by a magnet, so it moves towards the region of weaker field.
(B) The net magnetic moment develops in the direction opposite to that of the applied magnetic field. — Correct. The induced moments oppose the change in flux, so the net moment points against the field.
(C) Relative magnetic permeability is greater than one — Incorrect. Since \( \chi \) is negative, \( \mu_r = 1 + \chi \) is slightly less than one; \( \mu_r \gt 1 \) holds for paramagnetic and ferromagnetic substances.
(D) Magnetic susceptibility is -1 for a perfect diamagnetic substance. — Correct. A perfect diamagnet (a superconductor) expels the field completely, so B = 0 inside it.
\( \displaystyle B = \mu_0(1 + \chi)H = 0 \Rightarrow \chi = -1 \)
NCERT (2026-27):
Magnetic Properties of Materials – “… 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. A glance at Table 5.2 gives one a better feeling for these materials. … TABLE 5.2: Diamagnetic –1 ≤ χ < 0, 0 ≤ μr < 1, μ < μ0”
Diamagnetism – “Diamagnetic substances are those which have tendency to move from stronger to the weaker part of the external magnetic field. … Thus, the substance develops a net magnetic moment in direction opposite to that of the applied field and hence repulsion. … The most exotic diamagnetic materials are superconductors. These are metals, cooled to very low temperatures which exhibits both perfect conductivity and perfect diamagnetism. Here the field lines are completely expelled! \( \chi = -1 \) and \( \mu_r = 0 \).”