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.