Molecular Theory of Magnetism - Facts about Molecular Theory

The theory was first propounded by Weber in 1852 AD and was later developed by Ewing in 1890 AD. Due to this fact, it is also known as Weber Ewing theory. According to this theory, every magnetic substance contains a very tiny magnet called molecular magnet. In a magnetized piece of iron, molecules are present in all possible direction and form (closed chains) random orientation and the result is that one molecular magnet cancels

the effort of other, so the net magnetization is zero. However, in the case of magnet or in the presence of external field, the molecular magnet are turned around and lined up parallel to each other and are oriented towards the magnetic field with their north pole pointing one direction and the south pole pointing in the opposite direction.

In the middle region, north and South Pole of molecular magnet neutralize each other, so that the magnetization in this region is nearly zero. At the ends are free north and south poles so that the two poles have equal magnetic strength.

Facts about Molecular Theory

The following facts are explained by the molecular theory of magnetism:

• Each molecule of a magnetic substance, whether it is magnetized or unmagnetized is an independent magnet.

• In an unmagnetized substance, the molecules are not arranged in any order, so they neutralize the magnetic force with each other.

• In a magnetized substance, the molecules are arranged in an order so as to produce an external effect. In this order, all the north poles of the molecules of the magnetized substances point to one direction and all their south poles point to a direction opposite to which their north poles point. 

• Two poles of magnet cannot be separated. They exist side by side. If magnet is broken in to two or more pieces, each piece will behave like an individual magnet with opposite poles appearing at two ends of it. If any one piece is further broken into two parts, each new piece will again become an independent magnet with two opposite poles at its ends. Thus, it is not possible to separate two poles.

• The two poles of a magnet are equal in strength. This is because the number of North Pole at one end is equal to the number of free South Pole at other end.

• When an iron bar is magnetized, it slightly increases in length due to the setting of molecular magnets along with straight chains.

• When a magnet is strongly heated, the speed of the molecules is accelerated resulting in the decrease of their rigidity. It causes them to go back to the closed chain arrangement. Thus, the magnet is demagnetized on heating.