Show diagrammatically the behavior of magnetic field lines in the presence of (i) paramagnetic and (ii) diamagnetic substances. How does one explain this distinguishing feature?

i) The paramagnetic material’s atoms/molecules have a permanent magnetic dipole moment individually. As there is a random thermal motion of the atoms/molecules, so, no net magnetization is seen. In the presence of a strong external field B₀, at low temperatures, the individual atomic dipole moment align and point in the same direction as B₀. Diagram shows a bar of paramagnetic material placed in an external field. The field lines get concentrated inside the material, and the field inside is enhanced.

ii) In a diamagnetic material electron orbiting around nucleus possess orbital angular momentum. These orbiting electrons can be considered as current-carrying loop possessing orbital magnetic moment. The resultant magnetic moment in an atom is zero but when magnetic field is applied, those electrons having orbital magnetic moment in the same direction slow down and those in the opposite direction speed up. Thus, the material develops a net magnetic moment in direction opposite to that of the applied field and hence repulsion.