Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?

Explanation

Principle : An electric generator works on the principle of electro-magnetic induction phenomenon. According to it, whenever a coil is rotated between the poles of a magnet, an induced current is set up in the coil, whose direction is given by Fleming.s right-hand rule.

Working : 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 prdouced by the permanent magnet. The coil ABCD is rotated clockwise 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. 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.
Function of brushes : A brush is a device which conducts current between stationary wires and moving parts, most commonly in a rotating shaft. A carbon brush is a sliding contact used to transmit electrical current from a static to a rotating part in a motor or generator, and , as regards DC machines, ensuring a spark-free communication.