Q.5 Describe the langevin’s theory for
paramagnetic material.
Ans. Langevin’s Theory of Paramagnetism: -
Langevin explained the property of paramagnetism. He
derived the expression of susceptibility for a paramagnetic gas using kinetic
theory of gases. A paramagnetic gas when subjected to an external magnetic
field, the molecular dipoles try to align along the field but as a result of
thermal agitation, they orient in a direction making an angle with the field.
Let pm be
the magnetic moment of a molecule and q, the angle made by the magnetic moment
with the direction of magnetic field B.
The potential energy (U) of the
molecule along the direction of the field is
U = -pm B cos q ..........(1)
Let n is the number of molecules
per unit volume and dn is the number of molecules having energy U at an
Now, the total magnetic moment per unit volume may be obtained by integrating the magnetic moment for n molecules.
