Electrical Engineering ⇒ Topic : Molecular Theory of Magnetism

Molecular Theory of Magnetism

The molecular theory of magnetism was proposed by Weber in 1852 and modified by Ewing in 1890. According to this theory, every molecule of a magnetic substance (whether magnetised or not) is a complete magnet in itself having a north pole and a south pole of equal strength.

                                                                              figure (a)

In an unmagnetised substance, the molecular magnets are randomly oriented and form closed chains as shown in Fig. a (i). The north pole of one molecular magnet cancels the effect of the south pole of the other so that the substance does not show any net magnetism.

When a magnetising force is applied to the substance (e.g. by rubbing a magnet or by passing electric current through a wire wound over it), the molecular magnets are turned and tend to align in the same direction with N-pole of one molecular magnet facing the S-pole of other as shown in Fig. a (ii). The result is that magnetic fields of the molecular magnets aid each other and two definite N and S poles are developed near the ends of the specimen ; the strength of the two poles being equal. Hence the substance gets magnetised.

The extent of magnetisation of the substance depends upon the extent of alignment of molecular magnets. When all the molecular magnets are fully aligned, the substance is said to be saturated with magnetism.

When a magnetised substance (or a magnet) is heated, the molecular magnets acquire kinetic energy and some of them go back to the closed chain arrangement. For this reason, a magnet loses some magnetism on heating