Question

Which of the following substances' magnetic susceptibility $X_m$ is negative ?
A. Diamagnetic
B. Paramagnetic
C. Ferromagnetic
D. All of these

Answer 1

Magnetic susceptibility for a substance is the ratio of magnetization M of the substance to the externally applied magnetic field H, which is also sometimes called magnetic field strength.

Complete step-by-step answer:
The magnetic susceptibility is given as:
$X_m = \dfrac{M}{H}$
also,
$X_m = \mu_r - 1$
where $\mu_r$ is relative permeability of the substance.

Diamagnetism and paramagnetism are considered weak forms of magnetism, whereas ferromagnetic substances have a permanent magnetic moment even in the absence of magnetic field.
In a diamagnetic substance, the electron spins have complete cancellation and on application of the external magnetic field, weak magnetism arises due to change in orbital motion of electrons induced by applied magnetic field. The magnitude of the induced magnetic moment is extremely small and is in a direction opposite to that of the applied field. This means that a diamagnetic substance gets attracted to weaker magnetic field regions.
Therefore, the relative permeability $\mu_r$ for such substances (diamagnetic) becomes less than unity and the magnetic susceptibility $X_m$ becomes negative.

So, the correct answer is “Option A”.

Additional Information: For paramagnetic substances, the relative permeability is slightly greater than 1 whereas for ferromagnetic substances is much greater than 1. Paramagnetism arises due to incomplete cancellation of electron spins.
Ferromagnetic substances are used in making permanent magnets. There are small magnetically ordered regions called magnetic domains that give rise to magnetism in ferromagnetic materials.

Note: From the given set of options, it is only diamagnetic substances that get repelled by magnetic field, paramagnetic and ferromagnetic are attracted towards magnetic field i.e., show attraction to strong magnetic field regions. Therefore this information should be enough to choose the right answer despite being unaware of the proper definition of magnetic susceptibility.