Electromagnetic Induction

is not merely of theoretical or academic interest but also of practical utility. Imagine a world where there is no electricity – no electric lights, no trains, no telephones and no personal computers. The pioneering experiments of Faraday and Henry have led directly to the development of modern day generators and transformers. Today’s civilisation owes its progress to a great extent to the discovery of electromagnetic induction.

The discovery and understanding of electromagnetic induction are based on a long series of experiments carried out by Faraday and Henry. We shall now describe some of these experiments.

Figure 6.1 shows a coil $C_1$* connected to a galvanometer G. When the North-pole of a bar magnet is pushed towards the coil, the pointer in the galvanometer deflects, indicating the presence of electric current in the coil. The deflection lasts as long as the bar magnet is in motion. The galvanometer does not show any deflection when the magnet is held stationary. When the magnet is pulled away from the coil, the galvanometer shows deflection in the opposite direction, which indicates reversal of the current’s direction. Moreover, when the South-pole of the bar magnet is moved towards or away from the coil, the deflections in the galvanometer are opposite to that observed with the North-pole for similar movements. Further, the deflection (and hence current) is found to be larger when the magnet is pushed towards or pulled away from the coil faster. Instead, when the bar magnet is held fixed and the coil $C_1$ is moved towards or away from the magnet, the same effects are observed. It shows that it is the relative motion between the magnet and the coil that is responsible for generation (induction) of electric current in the coil.

In Fig. 6.2 the bar magnet is replaced by a second coil $C_2$ connected to a battery. The steady current in the coil $C_2$ produces a steady magnetic field. As coil $C_2$ is

* Wherever the term ‘coil’ or ‘loop’ is used, it is assumed that they are made up of conducting material and are prepared using wires which are coated with insulating material.

Josheph Henry [1797 – 1878] American experimental physicist, professor at Princeton University and first director of the Smithsonian Institution. He made important improvements in electro-magnets by winding coils of insulated wire around iron pole pieces and invented an electromagnetic motor and a new, efficient telegraph. He discoverd self-induction and investigated how currents in one circuit induce currents in another.

moved towards the coil $C_1$, the galvanometer shows a deflection. This indicates that electric current is induced in coil $C_1$. When $C_2$ is moved away, the galvanometer shows a deflection again, but this time in the opposite direction. The deflection lasts as long as coil $C_2$ is in motion. When the coil $C_2$ is held fixed and $C_1$ is moved, the same effects are observed. Again, it is the relative motion between the coils that induces the electric current.

The above two experiments involved relative motion between a magnet and a coil and between two coils, respectively. Through another experiment, Faraday showed that this relative motion is not an absolute requirement. Figure 6.3 shows two coils $C_1$ and $C_2$ held stationary. Coil $C_1$ is connected to galvanometer G while the second coil $C_2$ is connected to a battery through a tapping key K.

It is observed that the galvanometer shows a momentary deflection when the tapping key K is pressed. The pointer in the galvanometer returns to zero immediately. If the key is held pressed continuously, there is no deflection in the galvanometer. When the key is released, a momentory deflection is observed again, but in the opposite direction. It is also observed that the deflection increases dramatically when an iron rod is inserted into the coils along their axis.

Faraday’s great insight lay in discovering a simple mathematical relation to explain the series of experiments he carried out on electromagnetic induction. However, before we state and appreciate his laws, we must get familiar with the notion of magnetic flux, ${\Phi }_B$. Magnetic flux is defined in the same way as electric flux is defined in Chapter 1. Magnetic flux through