Statement I : The ferromagnetic property depends on temperature. At high temperature, ferromagnet becomes paramagnet.
Statement II : At high temperature, the domain wall area of a ferromagnetic substance increases.
In the light of the above statements, choose the most appropriate answer from the options given below :

Solution

Physics sir · 4 steps

Heat a ferromagnet past its Curie temperature and the domains do not grow walls, they disappear.

1. Why ferromagnets magnetise. In a ferromagnet, neighbouring atomic dipoles line up with each other in small regions called domains. Thin boundaries called domain walls separate one domain from the next.
2. Statement I. Heating makes the dipoles vibrate randomly, and this works against their alignment. Above the Curie temperature T_C the alignment is lost and the material behaves as a paramagnet, so Statement I is true.

   χ = C/(T - T_C) (T > T_C)

3. Statement II. As the temperature rises, thermal agitation breaks down the ordered domain structure itself. Above T_C there are no domains left, and so no domain walls. Heating does not increase the domain wall area, so Statement II is false.
4. So. Statement I is true and Statement II is false, which is option (b).

The idea to keep. Ferromagnetism needs ordered domains. Heat destroys that order, and above the Curie temperature the domains and their walls are gone, leaving a paramagnet that obeys the Curie–Weiss law.

Answer from the NTA final answer key. Solution by PrepWiser.