Show diagrammatically the behaviour of magnetic field lines in the presence of (i) paramagnetic and (ii) diamagnetic substances. How does one explain this distinguishing feature?
Solution
For a paramagnetic substance the field lines are drawn in and become slightly more crowded inside, because its relative permeability is a little greater than 1; the atomic dipoles partially align along the field and enhance it. For a diamagnetic substance the field lines are slightly expelled and become less crowded inside, because its relative permeability is a little less than 1; the induced moments oppose the field and reduce it. Thus the distinguishing feature follows directly from whether the induced magnetisation aids (para) or opposes (dia) the applied field.
