Question

Curie-Weiss Law is obeyed by iron
A. At curie temperature only
B. At all temperatures
C. Below curie temperature
D. Above curie temperature

Answer 1

Hint: the Curie-Weiss Law relates the susceptibility of ferromagnets like iron to their temperature. $\chi = \dfrac{C}{{T - {T_c}}}$. Observe the relation and try to recollect if you have ever seen an iron bar repel in a magnetic field.

Complete step by step answer:
The magnetic susceptibility of a material is a measure of its ability to strengthen or converge a magnetic field. Ferromagnetic materials have the highest $\chi $ value ~100 whereas paramagnetic and diamagnetic materials have a very low $\chi $.

The susceptibility of a ferromagnetic material is dependent on temperature. This is because at higher temperatures, the domains would be more randomised and have more thermal energy. So with increase in temperature, it would be harder to magnetise a material. So we expect an inverse relation between susceptibility and Temperature.
The Curie - Weiss law predicts this relation between $\chi $ and the absolute temperature $T$. The Curie-Weiss law states that: $\chi = \dfrac{C}{{T - {T_c}}}$
Here $C$ is the Curie constant and dependent only on the material.

To answer the question, this law gives a negative value for $\chi $ if the temperature is less than ${T_c}$. This suggests that below a certain temperature, A ferromagnetic substance is capable of opposing the magnetic field in which it is placed.
This is never observed in nature. Ferromagnetic materials like Iron never repels a magnetic field.
So The Curie - Weiss law is applicable only for temperatures greater than Curie temperature.

Additional Information:
The Curie temperature for Iron is ${770^o}C$. So for normal room temperatures, the Curie Weiss relation is not valid.
Though the relation does not say anything about what happens below this temperature, we now know that The ferromagnetic material spontaneously becomes magnetised. This is what we see in our daily lives.

Note: The Curie-Weistrass law is also not applicable at the curie temperature. It fails at this temperature, as it predicts a point of discontinuity. This also does not happen when experiments are conducted.