Explain the underlying principle and working of an electric

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

solution:
An electric generator is a device which converts mechanical energy to electrical energy using the phenomenon of electromagnetic induction.

Principle involved : When a coil is rotated about an axis perpendicular to the direction of uniform magnetic field, an emf is induced in it (electromagnetic induction).

Construction : It consists of a rectangular coil $\mathrm{ABCD}$ having a large number of turns of a conducting insulated wire wound on a soft iron core. The coil is rotated between the poles of a strong permanent magnet with its axis perpendicular to the magnetic field lines. The ends of the coil are connected to two slip rings $R_{1}$ and $R_{2}$ respectively. The slip rings $R_{1}$ and $\mathrm{R}_{2}$ are internally attached to an axle. The axle is mechanically rotated from outside to rotate the coil inside the magnetic field. The slip rings $R_{1}$ and $R_{2}$ are in sliding contact (moving contact) with two metallic (or carbon) brushes $\mathrm{B}_{1}$ and $\mathrm{B}_{2}$.

Working

(1) Let initially coil ABCD is in horizontal position and it is rotated in between the poles of the magnet. Due to the rotation of coil, the arm AB moves down while the arm CD moves up. Thus, magnetic field lines through them changes and an electric current is induced in AB and CD.

(2) By Fleming's right hand rule, in $\mathrm{AB}$ current flows $\mathrm{B}$ to $\mathrm{A}$ and in $\mathrm{CD}$ current flows $\mathrm{D}$ to C. As a result, an electric current flows through the whole circuit i.e., B $_{2}$ to B $_{1}$ in external circuit. As the coil rotates, the induced current varies in magnitude as well as direction. After half rotation, arms $\mathrm{AB}$ and $\mathrm{CD}$ of the coil interchange their position. Now, the arm $\mathrm{AB}$ is on right and $\mathrm{CD}$ on left side. Thus, the directions of induced currents in $\mathrm{AB}$ and CD are reversed. As a result, an electric current flows through the circuit in reverse direction i.e., $\mathrm{B}$ to $\mathrm{B}_{2}$ in external circuit [see fig.41(b)].

(3) The polarities of two ends of coil changes after every half rotation of the coil. In one complete cycle (rotation), the direction of current changes twice. Such a current, which changes direction after equal intervals of time is called an alternating current (AC).

Brush provide the contact between coil and split rings. One brush is at all time in contact with the arm moving up and the other in the contact with the arm moving down.

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