Text 5 electromagnetic induction

If a piece of wire is looped round a coil and when the current to the coil is switched either on or off, a current will flow through the loop of wire. This latter current is called an induced current, and it will occur only when the magnetic field round the coil is changing.

Another way of inducing a current is to move a length of wire across a magnetic field so that it cuts the magnetic lines of force. This is the principle of the dynamo, which consists essentially of a coil of wire rotating in a magnetic field. In this way the lines of force of the magnet are cut by the rotating coil and a current is therefore developed in the coil. Thus the mechanical energy which is used to rotate the coil in the magnetic field is converted into electrical energy within the coil. The dynamo, or generator as larger machines are usu ally called, is one of the most economical methods of producing electricity on a large scale, and this is the method adopted in power stations.

The exact reverse of a dynamo is the electric motor, in which the coil of wire is supplied with a current which is broken at regular intervals by a device known as a commutator; the resulting magnetic forces cause the coil to rotate so that electrical energy is transformed into mechanical energy.

In general, any movement of an electrically charged particle, or any electric current, creates a magnetic force, and conversely any movement of a magnetic pole creates an electric force.

The electromagnetic field

The example of an induced current can be extended to the circuit illustrated in the figure presented by the teacher, in which an electric cell supplies a stream of electrons to a coil of wire, A, through a switch S.

This switch is so arranged that it reverses the connections of the cell to the coil A when it is rotated. Now, if the switch is continuously rotated by some mechanical means, the current in the coil A will constantly and regularly change in direction. The field created by the constantly changing current is called an electromagnetic field, and it will surround the coil A.

If a second and similar coil of wire, B, is placed within this field, a constantly changing current will be induced in this coil, so that if an instrument for measuring current is placed across its ends, the needle will swing backwards and forwards, indicating first a negative and then a positive current, depending on the position of the switch S. Such a current, which regularly changes direction, is called an alternating current. However, if a switch was used that gradually increased, decreased, and reversed the current in coil A and readings were taken on the ammeter at very frequent intervals, a graph of these readings would show that the current changes from positive to negative in a smooth and regular manner.

This is a typical wave-form. It is characteristic of the way in which energy is radiated from one piece of matter (coil A) to another (coil B) by an electromagnetic field in space.

In general, any oscillating current, or any vibrating electric charge, will produce an electromagnetic field through which energy will be transmitted in the form of waves at a finite speed. As it is not easy to visualize how energy can be transmitted by these electromagnetic waves, an analogy may help. Consider a long glass trough filled with water, in which at one end a pencil is inserted and vigorously vibrated. The vibration of the pencil will cause waves to run along the surface of the water which have the appearance illustrated in the figure presented by the teacher,— if we are able to assume that the trough is sufficiently long to be able to avoid the complications of waves reflected from the ends.

If the water is examined carefully, it will be seen that no water actually moves from one end of the trough to the other; each particle of water is simply moving up or down. If a cork with a nail sticking out of it is floated in the water in the middle of the trough, it will bob up and down in the waves. The nail sticking into the cork can be made to hit a bell on its upstroke; this illustrates that the mechanical energy of the moving pencil could be transferred by a wave-motion to the cork, as mechanical energy in ringing the bell.

In the trough the waves are supported in the medium of the water; in the case of electromagnetic waves, no medium is required. In the example the two coils, A and B, can both be housed in a container from which all the air had been evacuated Although it is not easy to visualize, electromagnetic waves need no supporting medium— they can travel through empty space.