The absolute permeability of a substance is less than that of free space. What can we say about it?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → The susceptibility of the substance is less than that of sodium.
The permeability of a substance is \( \mu=\mu_0(1+\chi) \), where \( \chi \) is its magnetic susceptibility. If \( \mu\lt\mu_0 \), then \( 1+\chi\lt1 \), so \( \chi\lt0 \) and the substance is diamagnetic.
Option (1) The susceptibility of the substance is less than that of sodium. — Correct. Sodium is a paramagnetic material, so its susceptibility is small and positive. The given substance has a negative susceptibility, which is less than that of sodium.
Option (2) The substance is ferromagnetic. — Incorrect. A ferromagnet has a large positive susceptibility, so its permeability is far greater than \( \mu_0 \).
Option (3) The substance is lodestone. — Incorrect. Lodestone is a naturally occurring hard ferromagnet that forms a permanent magnet, so its permeability is far greater than \( \mu_0 \).
NCERT (2026-27):
Magnetisation and Magnetic Intensity – “… The magnetic permeability of the substance is μ and it has the same dimensions and units as μ0; μ = μ0μr = μ0 (1+χ).”
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.”
Paramagnetism – “Some paramagnetic materials are aluminium, sodium, calcium, oxygen (at STP) and copper chloride.”