When two materials are placed in an external magnetic field, the behaviour of magnetic field lines is as shown in the figure. Identify the magnetic nature of each of these two materials.
Answer: Material (a): diamagnetic; Material (b): ferromagnetic.
Solution
The way a material redistributes the external field lines reveals its magnetic nature. In (a) the field lines are pushed out of the specimen and the field inside is weaker than outside - this expulsion is the signature of a diamagnetic material (relative permeability slightly less than 1). In (b) the field lines crowd strongly into the specimen and the field inside is much larger than outside - this strong concentration is characteristic of a ferromagnetic material (relative permeability very large).
