Question

Draw a well labelled diagram of an electric generator.

Answer 1

Hint: An electric generator, also known as dynamo or AC generator, converts mechanical energy into electrical. An axle is rotated supplying mechanical energy through the use of water turbines, wind mills or by hand. An AC current is generated in the output.

Complete answer:
Principle:
The functioning of an AC generator is based on Faraday’s law of induction. According to Faraday’s law of induction a changing magnetic flux will produce an emf:
 \[e = - \dfrac{{d\Phi }}{{dt}}\].
Flux is given by: $\Phi = \overrightarrow B .\overrightarrow A $
where $\overrightarrow B $is the magnetic field and $\overrightarrow A $ is the area vector (normal to surface).

Hence, flux can be changed by changing area or magnetic field or orientation of area with respect to the magnetic field. In the case of an AC generator, the orientation is changed (mechanical rotation is given by another source like a turbine) to produce an alternating emf.

Construction:
Diagram:

A permanent constant magnetic field is applied through fixed magnets. A rectangular conducting coil containing N turns is placed between the two poles of the magnet such that it can be rotated about an axis perpendicular to the direction of magnetic field. Two ends of the coil are connected to two different slip rings. The slip rings are placed around an axle. As the axle is rotated, the slip rings rotate along with the rectangular coil. These slip rings are attached to carbon brushes which are further connected to the external circuit.

Working:
The axle is rotated by an external means. This in turn rotates the rectangular coil about an axis perpendicular to the magnetic field. Let $\theta $ be the angle between $\overrightarrow B $ and $\overrightarrow A $. If $\theta = 0$, the flux is maximum:
$\Phi = BA$
When $\theta = 90$, the flux is zero. For a given angle $\theta $, flux passing through the rectangular loop is:
$\Phi = BA\cos \theta $
When the axle rotates by angular velocity $\omega $, the flux passing through the rectangular loop is:
$\Phi (t) = BA\cos \omega t$
If the rectangular loop has N turns then this changing flux will produce an emf given as:
\[e = - N\dfrac{{d\Phi (t)}}{{dt}}\]
$
  e = - NBA\dfrac{{d(\cos \omega t)}}{{dt}} \\
  e = NBA\omega \sin \omega t \\
 $
As $\sin \omega t$ is a periodic function whose value ranges from -1 to +1, the emf will also be a periodic function of time taking both negative and positive values. Hence, an alternating current (AC) will be produced.

Note: 1. In AC generators, Mechanical rotational energy is transformed into electrical energy. In Motor, electrical energy is transformed into mechanical rotation.
2. Quantity that is responsible for generation of electricity from changing magnetic fields and vice versa is magnetic flux and if a have N turns then flux increases by N times.