Question: Assertion (A): When a ferromagnetic substance is heated to high temperature it becomes paramagnetic in nature. Reason (R): The disappearance of magnetisation of a ferromagnet is abrupt and not gradual.
Step 1 (Governing Principle): [0.25 Mark]
The magnetic property of a ferromagnetic material is governed by the Curie-Weiss Law. A ferromagnetic substance possesses spontaneous magnetization due to the alignment of atomic magnetic dipoles within domains. When the temperature $T$ exceeds the Curie temperature ($T_c$), the thermal agitation overcomes the exchange coupling forces that maintain the domain structure.
Step 2 (Analysis of Assertion and Reason): [0.50 Mark]
- Assertion (A): As $T > T_c$, the long-range order of the domains is destroyed, and the material transitions into a paramagnetic state. This is a standard physical phenomenon described in NCERT Physics Class 12, Chapter 5. Thus, Assertion (A) is True.
- Reason (R): The transition from ferromagnetic to paramagnetic state at the Curie temperature is a second-order phase transition. While the susceptibility $\chi$ follows the relation $\chi = \frac{C}{T - T_c}$ (where $C$ is the Curie constant), the loss of spontaneous magnetization is a continuous (gradual) process as the temperature approaches $T_c$, not an abrupt "disappearance" in the sense of a discontinuous jump to zero. Thus, Reason (R) is False.
Step 3 (Final Concluded Answer): [0.25 Mark]
Since Assertion (A) is True but Reason (R) is False, the correct option is: (c) Assertion (A) is true but Reason (R) is false.
💡 EL10 Concept Insight: Think of a ferromagnetic material like a group of soldiers standing in perfect formation; heating them up is like making them dance wildly—eventually, they get so energetic that they lose their formation and start moving randomly like gas particles (paramagnetism), but this happens gradually as they get hotter, not all at once like a light switch.