Explain diamagnetism and diamagnetic substance.

Explanation

Diamagnetic substance are those which have tendency to move from stronger to the weaker part of the external magnetic field. These substance are repel by magnet.

Figure shows a bar of diamagnetic material placed in an external magnetic field.
The field lines are repelled or expelled and the field inside the material is reduced.
When placed in a non-uniform magnetic field, the bar will tend to move from high to low field.
The simplest explanation for diamagnetism is as follows. Electrons in an atom orbiting around nucleus possess orbital angular momentum. These orbiting electrons are equivalent to current carrying loop and thus possess orbital magnetic moment.
Diamagnetic substance are the ones in which resultant magnetic moment in an 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. 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 substance but the effect is so weak.
The susceptibility of diamagnetic substance is small and negative.