JEE Main 2022 · 25 June · evening · Single correct · +4 / −1
Given below are two statements :
Statement I : Susceptibilities of paramagnetic and ferromagnetic substances increase with decrease in temperature.
Statement II : Diamagnetism is a result of orbital motions of electrons developing magnetic moments opposite to the applied magnetic field.
Choose the correct answer from the options given below :
- (a) Both Statement I and Statement II are true.
- (b) Both Statement I and Statement II are false.
- (c) Statement I is true but Statement II is false.
- (d) Statement I is false but Statement II is true.
Answer and solution
Answer (a) Both Statement I and Statement II are true.
Solution
Physics sir · 4 steps
Check each statement against Curie's law and Lenz's law, and both of them hold up.
- Paramagnetic. Thermal motion scrambles the atomic dipoles. Cooling reduces this scrambling, so the dipoles line up more easily. Curie's law says exactly this: χ = C/T.
- Ferromagnetic. Above the Curie temperature T_C, the Curie–Weiss law applies. As T comes down towards T_C, χ grows very large. So Statement I is true for both kinds of material.
χₚₐᵣₐ = C/T, χ_(ferro) = C/(T - T_C) (T > T_C)
- Diamagnetic. An applied field changes the speed of the orbiting electrons. By Lenz's law, the change in each orbital moment points opposite to the field, so the material is weakly repelled. Statement II is true.
- So. Both statements are true, so the answer is (a).
The idea to keep. Para and ferro susceptibilities fight thermal disorder, so cooling raises them. Diamagnetism is induced orbital motion opposing the field (Lenz's law), and it barely depends on temperature.
Answer from the NTA final answer key. Solution by PrepWiser.