Answer:
Principle of an electric generator It works on the phenomenon of electromagnetic induction i.e., when a coil is rotated in a uniform magnetic field, an electric current is induced in a closed circuit of the coil. (1)
Working of an AC generator when the armature coil ABCD rotates in clockwise direction in the magnetic field, the magnetic flux linked with it changes. Hence, the induced current flows in the direction in ABCD as per Fleming's right-hand rule. Thus, the current in the external circuit flows from 
to 
.
Ac this instant arm AB is moving up and CD is moving down. After half rotation, arm AB starts moving down while CD starts moving up. Hence, the direction of current in the coil reverse. Net induced current flows in the direction DCBA. Hence, the current in the external circuit flows from to .
Therefore, the polarity of induced current between the brushes 5, and B^ changes after every half rotation. Such current is called alternating current. (3)
Function of brushes The two carbon brushes provide the path to the induced current to flow from the armature and the slip rings to the external circuit containing load resistance. (1)
Or
A solenoid is a long coil of circular loops of insulated copper wire. Magnetic field lines are produced around the solenoid when a current is allowed to flow through it. The magnetic field produced by it is similar to the magnetic field of a bar magnet.
The field lines produced in a current-carrying solenoid is shown in the following figure (1½)
(2)
In the given figure, when the North-pole of a bar magnet is brought near the end connected to the negative terminal of the battery, the solenoid repels the bar magnet. Since like poles repel each other, the end connected to the negative terminal of the battery behaves as the North pole of the solenoid and the other end behaves as a South pole. Hence, one end of the solenoid behaves as a North pole and the other end behaves as a South pole. (1½)
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