Based on the phenomenon of electromagnetic induction, the experiments studied above generate induced current, which is usually very small. This principle is also employed to produce large currents for use in homes and industry.

In an electric generator, mechanical energy is used to rotate a conductor in a magnetic field to produce electricity.

An electric generator, as shown in Fig. 13.19, consists of a rotating rectangular coil `ABCD` placed between the two poles of a permanent magnet. The two ends of this coil are connected to the two rings `R_1 ` and `R_2`.

The inner side of these rings are made insulated. The two conducting stationary brushes ` B_1` and `B_2` are kept pressed separately on the rings `R_1` and `R_2`, respectively.

The two rings `R_1` and `R_2` are internally attached to an axle. The axle may be mechanically rotated from outside to rotate the coil inside the magnetic field.

Outer ends of the two brushes are connected to the galvanometer to show the flow of current in the given external circuit.



When the axle attached to the two rings is rotated such that the arm `AB` moves up (and the arm `CD` moves down) in the magnetic field produced by the permanent magnet.

Let us say the coil `ABCD` is rotated clockwise in the arrangement shown in Fig. 13.19. By applying Fleming’s right-hand rule, the induced currents are set up in these arms along the directions `AB` and `CD`.

Thus an induced current flows in the direction ABCD. If there are larger numbers of turns in the coil, the current generated in each turn adds up to give a large current through the coil. This means that the current in the external circuit flows from `B_2` to `B_1`.

After half a rotation, arm CD starts moving up and AB moving down. As a result, the directions of the induced currents in both the arms change, giving rise to the net induced current in the direction DCBA.

The current in the external circuit now flows from `B_1` to `B_2`. Thus after every half rotation the polarity of the current in the respective arms changes.

Such a current, which changes direction after equal intervals of time, is called an alternating current (abbreviated as AC). This device is called an AC generator.

To get a direct current (DC, which does not change its direction with time), a split-ring type commutator must be used. With this arrangement, one brush is at all times in contact with the arm moving up in the field, while the other is in contact with the arm moving down.

We have seen the working of a split ring commutator in the case of an electric motor (see Fig. 13.15). Thus a unidirectional current is produced. The generator is thus called a DC generator.

The difference between the direct and alternating currents is that the direct current always flows in one direction, whereas the alternating current reverses its direction periodically.

Most power stations constructed these days produce `AC.` In India, the `AC` changes direction after every `1//100` second, that is, the frequency of `AC` is `50 Hz`. An important advantage of `AC` over `DC` is that electric power can be transmitted over long distances without much loss of energy.

In our homes, we receive supply of electric power through a main supply (also called mains), either supported through overhead electric poles or by underground cables.

One of the wires in this supply, usually with red insulation cover, is called live wire (or positive). Another wire, with black insulation, is called neutral wire (or negative). In our country, the potential difference between the two is `220 V.`

At the metre-board in the house, these wires pass into an electricity meter through a main fuse. Through the main switch they are connected to the line wires in the house. These wires supply electricity to separate circuits within the house.

Often, two separate circuits are used, one of `15 A` current rating for appliances with higher power ratings such as geysers, air coolers, etc. The other circuit is of `5 A` current rating for bulbs, fans, etc.

The earth wire, which has insulation of green colour, is usually connected to a metal plate deep in the earth near the house. This is used as a safety measure, especially for those appliances that have a metallic body, for example, electric press, toaster, table fan, refrigerator, etc.

The metallic body is connected to the earth wire, which provides a low-resistance conducting path for the current.

Thus, it ensures that any leakage of current to the metallic body of the appliance keeps its potential to that of the earth, and the user may not get a severe electric shock.



Figure 13.20 gives a schematic diagram of one of the common domestic circuits. In each separate circuit, different appliances can be connected across the live and neutral wires.

Each appliance has a separate switch to ‘ON’/‘OFF’ the flow of current through it. In order that each appliance has equal potential difference, they are connected parallel to each other.

Electric fuse is an important component of all domestic circuits. We have already studied the principle and working of a fuse in the previous chapter.

A fuse in a circuit prevents damage to the appliances and the circuit due to overloading. Overloading can occur when the live wire and the neutral wire come into direct contact. (This occurs when the insulation of wires is damaged or there is a fault in the appliance.)

In such a situation, the current in the circuit abruptly increases. This is called short-circuiting. The use of an electric fuse prevents the electric circuit and the appliance from a possible damage by stopping the flow of unduly high electric current.

The Joule heating that takes place in the fuse melts it to break the electric circuit. Overloading can also occur due to an accidental hike in the supply voltage. Sometimes overloading is caused by connecting too many appliances to a single socket.