Paramagnetic Substances:
Paramagnetic substances are those which develop feable magnetisation in the direction of the magnetising field. Such substances are feebly attracted by magnets and tend to move from weaker to stronger parts of a magnetic field.
Examples: Aluminium (Al), chromium (Cr), manganese, (Mn), platinum (Pt), Ca, Sb, Na, Li, Mg Tungsten, copper chloride, salt solution etc.
Explanation of Paramagnetism:
According to Langevin, the atoms or molecules of a paramagnetic material possess a permanent magnetic moment either due to the presence of some unpaired electron or due to the non-cancellation of the spins of two electrons because of special reason. In the absence of an external magnetic field, the atomic dipoles are randomly oriented due to their ceaseless random motion, as shown in figure 8.27 (a). There is no net magnetisation.
When a strong enough field is applied and the temperature is low enough, the field tends to align the atomic dipoles in its own direction, producing a weak magnetic moment in the direction of magnetic field. The material tends to move from a weak field region to a strong field region. This phenomenon is paramagnetism.
When a paramagnetic material is placed in a magnetic field, the lines of force prefer to pass through it than through the surrounding air i. e., the lines of force get slightly more concentrated inside the material, as shown in figure. When a glass *** containing a paramagnetic liquid is placed over two closely lying opposite poles of magnet, the liquid accumulates and elevates in the middle and thins out near the poles. When the poles are moved apart the field at the poles becomes stronger than that at the centre and the liquid moves towards the poles.
Experimentally, the intensity of magnetisation of a paramagnetic substance is proportional to magnetic field B0 and inversely proportional to temperature T.
Diamagnetic Substances:
Diamagnetic substances are those which develop feable magnetisation in the opposite direction of the magnetising field. Such substances are feebly repelled by magnets and tend to move from stronger to weaker parts of a magnetic field.
Examples : Cu, Zn, Sb, **, Hg, H2, N2, Au, Ag, Pb, air water, diamond silicon, quartz, alcohol, marble, glass, helium, argon, salts like sodium choride etc.
Explanation of Diamagnetism:
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 substances are those 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. This happens due to induced current in accordance with Lenz’* law, which you will study later. Thus, the substance develops a net magnetic moment in opposite direction to that of the applied field and hence repulsion.
The most exotic diamagnetic materials are super conductors. These are metals, cooled to a very low temperature which exhibits both perfect conductivity and perfect diamagnetism. Here the field lines are completely expelled. The susceptibility Xm of diamagnetic materials is small and negative. The relative permeability μr(1 + Xm) is positive but less than 1 for a diamagnetic material. A superconductor repels a magnet and is repelled by the magnet. The phenomenon of perfect diamagnetism in superconductors is called the Meissner effect.
When a glass *** containing a diamagnetic liquid is placed between two closely lying (3 – 4 mm apart) poles of a magnet, the liquid is found to move towards the poles causing a depression in the middle. This indicates that the field is stronger in the middle than that near the poles. Now if the poles are moved apart sufficiently, the magnetic field at the middle becomes weaker than that near the poles. Consequently the liquid accumulates in the middle and becomes thin near the poles.
