Activity: How do generators produce electricity?

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Key Learning Points:
1.   Electromagnetic induction
2.   AC generator
3.   Outline of AC generator in power plant
4.   Communication


Fig. 1   Simple AC generator found in a school laboratory

In this activity, you will

  • do an experiment with a simple AC generator found in a school laboratory to display the waveform of the electromotive force on a CRO
  • identify the different parts of the generator and explain their functions

AC generators in power plants are much more complex than those found in school laboratories. However, we can still gain a heuristic understanding of their basic structure and operation in this activity.

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Part I

Video of the experiment  Watch videoWatch videoDownload video: 19.7mb


Apparatus: AC generator in school laboratory, CRO.

Fig. 2   The generator connected to a CRO
  1. Set up the apparatus as shown in Fig.1.
  2. Turn the handle of the generator and note the brightness of the light bulb. Turn the handle faster then slower to see how the brightness changes.
  3. Connect the output of the generator to a CRO. Turn the handle and adjust the time base of the CRO until a waveform is displayed on screen. Take a photo of the waveform and put it on the student worksheet.
  4. On the worksheet, identify different parts of the AC generator on the photo with those labeled on the schematic diagram.
  5. View the Flash animation below and prepare a short presentation to explain to the class how the generator outputs the AC current, and whether what you observed on the animation is consistent with what you observed on the CRO.
    Flash animation: Simple AC generator


  1. What happens to the brightness of the light bulb when you turn the handle more rapidly? Explain this in terms of the principles of electromagnetic induction.
  2. What does the waveform on the CRO look like? What happens if you turn the handle more rapidly?
  3. Is the waveform you observed on the CRO the same as that in the animation? Why and why not?
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Part II

Looking at a power plant AC generator

In Part I, an AC current was generated by turning the coils of the generator in a magnetic field. The magnetic field was produced by two stationary permanent magnets.

Fig. 3   Schematic diagram showing how a practical generator produces electricity. The rotating electromagnet produces a changing magnetic field which induces a current in the surrounding coil. The windings of a real generator are much more complex.

In a power plant generator, however, the conducting coils are stationary and it is the magnets that are turned. Also, the magnetic field in power plant generators is produced by electromagnets (instead of permanent magnets) with current supplied by an external power source. The functioning of the coils and electromagnets in power plant generators to produce a high AC output are very complex and will not be discussed here.

Fig. 3 is a schematic diagram of a power plant generator. Work out how it functions and explain this to your classmates in a short presentation. (Fig. 4 shows an illustration of a real power plant generator.)

Fig. 4   Illustration of a real power plant generator. Stator refers to the part that does not move, and rotor refers to the part that is rotated inside the stator. The rotor is a large electromagnet with windings inside to produce a magnetic field. As it rotates inside the stator, induced current is generated at the windings of the stator