atom is zero. 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. This happens due to induced current in accordance with Lenz’s law which you will study in Chapter . Thus, the substance develops a net magnetic moment in direction opposite to that of the applied field and hence repulsion.
Some diamagnetic materials are bismuth, copper, lead, silicon, nitrogen (at STP), water and sodium chloride. Diamagnetism is present in all the substances. However, the effect is so weak in most cases that it gets shifted by other effects like paramagnetism, ferromagnetism, etc. The most exotic diamagnetic materials are superconductors .
These are metals, cooled to very low temperatures which exhibits both perfect conductivity and perfect diamagnetism . Here the field lines are completely expelled! χ = – and µ r = . A superconductor repels a magnet and (by Newton’s third law) is repelled by the magnet.
The phenomenon of perfect diamagnetism in superconductors is called the Meissner effect , after the name of its discoverer. Superconducting magnets can be gainfully exploited in variety of situations, for example, for running magnetically levitated superfast trains. . .
Paramagnetism Paramagnetic substances are those which get weakly magnetised when placed in an external magnetic field. They have tendency to move from a region of weak magnetic field to strong magnetic field, i.e., they get weakly attracted to a magnet. T ABLE . Diamagnetic Paramagnetic Ferromagnetic – ≤ χ < < χ < ε χ >> ≤ µ r < < µ r < + ε µ r >> µ < µ µ > µ µ >> µ FIGURE .
Behaviour of magnetic field lines near a (a) diamagnetic, (b) paramagnetic substance.