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Q. 16.  Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?

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The Electric Generator:

An electric generator converts mechanical energy into electrical energy.

Principle: It works on the principle of electromagnetic induction. whenever there is a change in the flux through any coil, there would be the induction of current.

Working:

An electric generator, as shown consists of a rotating rectangular coil ABCD placed between the poles of a permanent magnet. The two ends of this coil are connected to the two rings R1 and R2. The inner side of the ring is insulated. 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. The axle may be mechanically rotated from outside to rotate the coil inside the magnetic field. The outer ends of the two brushes are connected to the galvanometer to show the direction of the flow of current in the 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. 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.  This means that the current in the external circuit flows from B2 to B1. After half a rotation, arm CD starts moving up and AB moving down. As a result, the directions of the induced currents in both arms change, giving rise to the net induced current in the direction of DCBA. The current in the external circuit now flows from B1 to B2. Thus after every half rotation the polarity of the current in the respective arms changes. Such a current, which changes direction periodically, is called alternating current (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

The function of Brushes: Brushes help in transferring the current from inside the generator to the external circuit.

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Pankaj Sanodiya

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