Generators
Introduction

A potential difference is induced in a wire when it cuts through magnetic field lines. If the wire forms part of a complete circuit, then an induced current will flow.

Electricity is usually produced using a generator. Generators can be found in bicycle dynamos and all power stations.

Alternating current generators
The simplest way of generating a continuous supply of electricity is with an
alternating current
Alternating current (often abbreviated to a.c.) is when the flow of electric current changes direction periodically. The mains is a source of alternating current.
alternating current
(
a.c.
The common abbreviation for alternating current is a.c. This is when the flow of electric current changes direction periodically. The mains is a source of alternating current.
a.c.
)
generator
A generator is a device that creates electrical energy.
generator
. Rather than moving a single wire between two magnets, a generator has a coil of wire rotating in a
magnetic field
Around every magnet there is an invisible magnetic field.
magnetic field
. A model of such a
generator
A generator is a device that creates electrical energy.
generator
is shown in Fig.1.

Before you run the generator, think about which way you expect the current to flow through the wires. If the coil rotates clockwise, the red wire will periodically move upwards. Use Fleming's right-hand rule to predict which way the current will flow in the red wire when it moves upwards.

When the red wire moves upwards, which way will the current in it flow?
  • Try again. You made a mistake using Fleming's right-hand rule.
  • Well done. You applied Fleming's right-hand rule correctly.
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Figure 1.   Generating electricity.
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Rotate the coil by clicking on the buttons. The induced current is shown on the ammeter. The needle points to the right when the current is positive. The direction of the
conventional current
Conventional current flows from positive to negative around a circuit. However, electrons in the wire actually flow from negative to positive.
conventional current
flow is indicated by the arrows next to the wires and the symbols on the coil. A dot means the current is coming towards you (like the tip of an arrow), while a cross means the current is flowing away from you (like the end of an arrow).

Which statement below describes the direction of current flow in the blue and red wires.
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Watch the pointer on the ammeter as the coil rotates. The current is not constant – it varies between flowing in one direction and the other.

In which of these positions of the coil is the current at its highest positive value?
In which of these positions of the coil is the current zero?


The current flowing through the circuit is an alternating current (a.c.). This type of generator is often called an
alternator
An alternator is a type of generator that creates alternating current.
alternator
.

Alternating current
The magnitude of the current generated by the alternator depends on how fast the coil rotates. In Fig.2 below, the
speed
Speed is a measure of how fast something is moving. It is calculated by dividing the distance travelled by the time taken.
speed
of rotation of the coil is controlled by the slider.

Figure 2.   Plotting a.c. current.
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How does the maximum induced current depend on the speed of rotation?
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Set the rotation speed to the maximum positive value. Observe the shape that the crosses on the graph make.

Which of these graphs would best fit the points of the graph in figure 6?
  • Try again. This is a triangular wave. It would not fit the points.
  • Try again. This is a rectangular wave. It would not fit the points.
  • Correct. This curve is called a sine wave. It would fit through the points.
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The experiment in Fig.3 below draws a smooth wave. Use it to see the exact shape of the a.c. current wave.

Figure 3.   Graphing a.c. current.
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The electricity in your house varies in just the same way as the electricity produced by the alternator.

This is because the large generators in
power
Power is the rate of doing work, or the rate of energy transfer. The unit of power is the watt, W.
power
stations are very similar to the alternator. The main difference is that in the power station, the coils are fixed in place around the rotating magnet.

Summary


A device that creates electrical energy is called a generator.

When a coil of wire rotates in a magnetic field, an alternating current (a.c.) is produced. This is why this type of generator is called an alternator.

The amplitude of the current produced by an alternator depends on the speed of rotation of the coil.

The current produced by an alternator has the form of a wave.

Exercises
1. What type of electric current (a.c. or
d.c.
The common abbreviation for direct current is d.c. This is when the flow of electric current is in a constant direction. Batteries are a source of direct current.
d.c.
) would you obtain from …
  • a battery?
    an alternator?
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2. Which one of the graphs below shows an alternating current?
3. What is the term for a device that …
  • creates electricity?
    creates a.c.?
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4. Decide whether changing the factors below will alter the magnitude and/or
frequency
In general, the frequency of an event describes how often it occurs. When talking about waves, the frequency is a measure of how many waves go past a fixed point in a given time.
frequency
of an alternating current.

  • Factor Magnitude altered? Frequency altered?
    Rotation speed
    Number of coils
    Strength of magnet
    Direction of rotation

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5. Which of the graphs shows the current produced by an alternator?
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