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.

• A. Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
• B. Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
• C. Assertion (A) is true, but Reason (R) is false.
• D. Both Assertion (A) and Reason (R) are false.

Hint:

Above the Curie temperature, ferromagnetic materials lose domain alignment and become paramagnetic due to thermal agitation.

Answer 1

The Correct Option is B

Concept: Ferromagnetic materials possess strong spontaneous magnetisation because magnetic dipoles align in domains. When the temperature reaches the Curie temperature, thermal agitation destroys the ordered alignment of domains and the material becomes paramagnetic.

Step 1: Examine the Assertion. Ferromagnetic materials such as iron, cobalt and nickel lose their ferromagnetic properties above the Curie temperature. Beyond this temperature, thermal motion prevents permanent alignment of magnetic domains. Therefore, they behave as paramagnetic substances. Hence, the Assertion is true.

Step 2: Examine the Reason. Near the Curie temperature, spontaneous magnetisation falls rapidly and disappears at the Curie point. Thus, the disappearance of magnetisation is often described as abrupt rather than gradual. Hence, the Reason is also true.

Step 3: Check whether the Reason explains the Assertion. The actual reason for conversion into a paramagnetic state is thermal agitation destroying domain alignment. The statement regarding abrupt disappearance of magnetisation does not explain why the substance becomes paramagnetic. Therefore, both statements are true but the Reason is not the correct explanation. \[ \boxed{\text{(B)}} \]