A magnetic substance has a susceptibility -0.05 at 300 K. The magnetic susceptibility at 600 K is:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → -0.05
The magnetic susceptibility \( \chi \) measures how strongly a material is magnetised by a field, \( M = \chi H \). A negative susceptibility means the material is diamagnetic: the field induces a moment opposite to it.
Diamagnetism comes from the change in the orbital motion of electrons in the applied field, not from the alignment of permanent atomic dipoles, so thermal agitation has nothing to disturb. The susceptibility of a diamagnetic material is independent of temperature.
\( \chi_{600\,\mathrm{K}} = \chi_{300\,\mathrm{K}} = -0.05 \)
Why the other options are incorrect
Option (1) -0.025 — Incorrect. This applies Curie's law \( \chi \propto 1/T \), which holds for paramagnetic materials, not for a diamagnetic one.
Option (2) -0.1 — Incorrect. Doubling the susceptibility on heating has no basis for any magnetic material.
Option (4) -0.5 — Incorrect. The susceptibility of a diamagnetic material does not change with temperature, so it cannot become ten times larger.