1. An electric generator is a device where mechanical energy is used to rotate a conductor in a magnetic field to produce electricity.
2. An electric generator 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 R1 and R2. The two conducting stationary brushes B1 and B2 are kept pressed separately on the rings R1 and R2, respectively. The two rings R1 and R2 are internally attached to an axle.
3. The axle is 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.
4. When the axle is rotated such that the arm CD moves up (and the arm AB moves down) in the magnetic field produced by the permanent magnet. Coil ABCD is rotated anticlockwise in the arrangement. By applying Fleming’s right-hand rule, the induced currents are set up in these arms along the directions AB and CD. The induced current flows in the direction DCBA.
5. 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 B1 to B2. After half a rotation, arm AB starts moving up and CD 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 ABCD. The current in the external circuit now flows from B2 to B1.
6. 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.
7. 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 used at all times in contact with the arm moving up in the field, while the other is in contact with the arm moving down. Thus, a unidirectional current is produced. The generator is thus called a DC generator.
Difference between DC and AC
8. 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.
9. An important advantage of AC over DC is that electric power can be transmitted over long distances without much loss of energy.