AIIMS 2007 Physics Curie Temperature Assertion Reason Question

To analyze the given question effectively, we need to understand the concepts of ferromagnetism, paramagnetism, and the significance of the Curie temperature.

Assertion Explanation

Assertion: Ferro magnetic substances become paramagnetic above Curie temperature.

Explanation: Ferromagnetic materials, such as iron, cobalt, and nickel, exhibit strong magnetic properties due to the alignment of magnetic moments (spins of electrons) in the same direction, forming regions called magnetic domains. Below a certain temperature, known as the Curie temperature ($T_C$), these domains are stable and contribute to the net magnetization of the material.

However, when the temperature exceeds the Curie temperature, thermal energy becomes sufficient to disrupt the alignment of these domains. As a result, the material transitions from a ferromagnetic state to a paramagnetic state, where the magnetic moments are no longer aligned and only exhibit magnetism in the presence of an external magnetic field.

Reason Explanation

Reason: Domains are destroyed at high temperature.

Explanation: This statement is accurate as well. At temperatures above the Curie temperature, the thermal agitation is strong enough to overcome the interactions that keep the magnetic domains aligned. This destruction of domain structure leads to a loss of the net magnetization of the material, confirming that the material behaves as a paramagnet.

Conclusion on Correct Answer

Correct Answer: A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

Both the assertion and the reason are true, and the reason effectively explains why the assertion holds. Thus, option A is the correct choice.

Why Other Options are Incorrect

• B) Both Assertion and Reason are true but Reason is not the correct explanation of Assertion: This option is incorrect because the reason accurately explains the assertion about the behavior of ferromagnetic materials above the Curie temperature.

• C) Assertion is true but Reason is false: This option is incorrect as both the assertion and the reason are true. Therefore, it cannot be the case that the reason is false.

• D) Both Assertion and Reason are false: This option is incorrect since both statements are indeed true.

Key Concepts

1. Curie Temperature ($T_C$): The temperature at which a ferromagnetic material loses its permanent magnetic properties and becomes paramagnetic.
2. Magnetic Domains: Regions within a ferromagnetic material where the magnetic moments are aligned. They are stable below $T_C$ but can become disordered above it.

Summary

In summary, the transition of ferromagnetic substances to paramagnetic behavior above the Curie temperature is due to the destruction of magnetic domains caused by thermal agitation. This is why both the assertion and the reason are true and the reason is the correct explanation for the assertion.