In this topic, you study Magnetism – Definition & Theory.
The property of a magnet by virtue of which attraction of iron takes place is known as magnetism.
THEORIES OF MAGNETISM
From the period when iron first came into use as a magnetic material, attempts have been made to explain the reasons for its pronounced magnetic properties.
Initially, Ampere suggested that the magnetic property of iron and similar materials might be due to circulation of current round each molecule. This suggestion was obviously based on the magnetic effects of current carrying conductor discovered by Oersted.
Later, molecular theory of magnetism was proposed by Weber and Ewing. According to this theory, the molecules of all ferromagnetic substances are themselves small permanent magnets. When a substance is in a non-magnetized condition, the molecular magnets are arranged in the haphazard manner as shown in Fig. 4.21 (a).
Naturally, their magnetic fields neutralize one another and no resultant magnetic effect is produced. When a magnetomotive force is applied, the molecular magnets all begin to be oriented in the direction Of the magnetomotive force. If the external field that is applied is weak, owing to the relative stability of natural random grouping, only partial alignment of these magnets is resulted (Fig. 4.21 b). The application of fields of increased field strength produces increasing degree of alignment. When a substance is completely magnetized, all the molecular magnets are perfectly aligned as shown in Fig. 4.21 (c) and the resultant magnetic effect is strongest. When completely magnetized, a substance is said to have attained saturation.
Fig. 4.21: Weber-Ewing theory Of magnetism
The molecular theory of magnetism does not explain how the molecules receive theirmagnetism. The modern theory of magnetism offers rational explanation for various magnetic properties Of the material and therefore, supersedes the molecular theory.
A detailed discussion on this theory is beyond the scope of this book. However, brief explanation is given below.
According to the modern theory, in an atom Of an element, electrons (which are charges of negative electricity) revolve in orbits around the nucleus.
Since a current is merely a flow of electrons, an electron travelling round a closed path constitutes an electric current. Consequently, each electron produces a magnetic field directed along the axis of rotation.When atoms combine to form molecules, the magnetic effects due to these orbital motions Of electrons generally neutraize each other. Therefore, these effects do not contribute much to the magnetic properties of the material. However, in addition to the orbital motions, the electrons spin on their own axes, just as the earth spins about its axis. A spinning electron sets up a magnetic field a long its axis of spin and the direction of the field depends on the direction of spin. In any atom, some of the electrons spin in one direction and some in other. There is a resultant magnetic effect only if these spins do not balance Out. The atoms of ferromagnetic substances have a number of uncompensated electron spins which are primarily responsible for their magnetic properties. The groups of molecules (or atoms) which act like elementary magnets magnetized to saturation due to magnetic effects of electron spins are called ‘domains’.In an unmagnetized substance (ferromagnetic), the magnetic axes of different domains are in various directions so that their magnetic effects cancel one another. After the application of magnetomotive force externally, the magnetic axes of the various domains are oriented so that they coincide with the direction of the magnetomotive force, thereby ultimately giving the Strongest resultant magnetic effect. Thus, the domain is similar in many respects to the molecular permanent magnet in the theory proposed by Weber and Ewing.
