Name: Magnetic Susceptibility and Relative Magnetic Permeability
Uploaded: 2019-07-19
Duration: 7 min 13 s
Description: Magnetic susceptibility and relative magnetic permeability sound like complicated terms, but they're not. In this video, we will learn what they are and the...

Question

What is paramagnetism ? Explain it on the basis of electron theory.

Accepted Answer

Explanation of Paramagnetism : The property of paramagnetism is found in those substances whose atoms (or ions or molecules) have an excess of electrons spinning in the same direction. Hence, atoms (or ions or molecules) of paramagnetic substances have a net non-zero magnetic moment of their own, and behave like tiny bar-magnets. Even then the paramagnetic substances do not exhibit any magnetic effect in the absence of external magnetic field. The reason is that the individual atomic magnets are randomly oriented figure and so the magnetic moment of the bulk of the substance remains zero.

When a paramagnetic substance is placed in an external magnetic field $\overset{⇀}{B,}$ then each atomic magnet experiences a torque which tends to turn the magnet in the direction of $\vec{B .}$ Hence, the atoms of the substance are aligned in the direction of figure. As such, the substance acquires a net magnetic dipole moment, that is, it is magnetised
in the direction of $\vec{B,}$ and the field lines become more dense inside the paramagnetic substance compared to those outside.

The atoms in a substance undergo thermal agitation. If the substance is a gas, its atoms are in the state of random motion; and if it is a solid, the atoms vibrate. This agitation disturbs the magnetic alignment of the atoms. Hence, normally, the magnetism in paramagnetic substances is very weak. The magnetism increases on increasing the external magnetic field, or on reducing the temperature. Thus, paramagnetism is temperature-dependent.

Curie's Law : In $1895$ , Curie discovered experimentally that the magnetisation $M$ (magnetic moment per unit volume) of a paramagnetic substance is directly proportional to the magnetic intensity $H$ of the magnetising field and inversely proportional to the kelvin temperature $T .$ That is,

$M = C (H / T), . . . . . . (i)$

where $C$ is a constant. This equation is known as Curie’s law and the constant $C$ is called the Curie constant. The law, however, holds so long as the ratio $H / T$ does not become too large.
$M$ cannot increase without limit. It approaches a maximum value corresponding to the complete alignment of all the atomic magnets contained in the substance.
Curie’s law can be expressed in an alternative form. We know that the magnetic susceptibility $X_{m}$ is defined as

$χ_{m} = M / H$ $. . . . (ii)$

Eliminating M in Eqs. (i) and (ii), we get

$χ_{m} = C / T or χ_{m} \propto 1 / T$

What is paramagnetism ? Explain it on the basis of electron theory.

Important Questions on Magnetic Classification of Substances

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