c07GeneratingElectricity 166 9 June 2016 5:11 PM

REMEMBER

Before beginning this chapter, you should be able to:

■ describe how a magnetic � eld exerts a force on a current

■ describe the operation of a simple DC motor, including the role of the commutator.

KEY IDEAS

After completing this chapter, you should be able to:

■ determine the amount of magnetic � ux passing through an area

■ explain how a moving conductor in a magnetic � eld generates a voltage drop

■ describe how the magnetic � ux through a rotating coil changes with time

■ explain how a rotating loop in a magnetic � eld generates a voltage that varies as a sine wave — that is, an AC voltage

■ determine the average induced voltage in a loop from the � ux change and the time in which the change took place

■ determine the direction of the induced current in a loop, using Lenz’s Law

■ calculate the average induced voltage for more than one loop

■ describe and determine the following properties of an AC voltage: frequency, period, amplitude, peak-to- peak voltage, peak-to-peak current, RMS voltage and RMS current

■ interpret RMS in terms of the DC supply that delivers the same power as the AC supply

■ describe the operation of an alternator with the use of slip rings to produce AC, and the operation of a generator with a split-ring commutator to produce � uctuating DC.

CHAPTER

7 Generating electricity

A generator inside a wind turbine

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voltage that varies as a sine wave — that is, an AC voltage

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voltage that varies as a sine wave — that is, an AC voltage

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AC voltage: frequency, period, amplitude, peak-to-

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AC voltage: frequency, period, amplitude, peak-to-peak voltage, peak-to-peak current, RMS voltage and

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peak voltage, peak-to-peak current, RMS voltage and RMS current

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RMS current interpret RMS in terms of the DC supply that delivers the

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interpret RMS in terms of the DC supply that delivers the same power as the AC supply

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same power as the AC supply

describe the operation of an alternator with the use

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describe the operation of an alternator with the use

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of slip rings to produce AC, and the operation of a PA

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of slip rings to produce AC, and the operation of a generator with a split-ring commutator to produce PA

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generator with a split-ring commutator to produce � uctuating DC.PA

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� uctuating DC.

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FSthe � ux change and the time in which the change took determine the direction of the induced current in a loop,

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FS determine the direction of the induced current in a loop,

calculate the average induced voltage for more than one

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describe and determine the following properties of an PR OO

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describe and determine the following properties of an AC voltage: frequency, period, amplitude, peak-to-PR

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AC voltage: frequency, period, amplitude, peak-to- peak voltage, peak-to-peak current, RMS voltage and PR

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peak voltage, peak-to-peak current, RMS voltage and

167CHAPTER 7 Generating electricity

c07GeneratingElectricity 167 9 June 2016 5:11 PM

Making electricity Chapter 6 describes how a magnetic � eld exerts a force on a moving charge, either in a wire as part of an electric current or as a free charge. � is chapter applies this idea to new situations to produce or generate electricity. In doing this, a new concept, magnetic � ux, will be developed to explain how a gener- ator works.

Generating voltage with a magnetic � eld What should happen when a metal rod moves through a magnetic � eld? Imagine a horizontal rod falling down through a magnetic � eld as shown in the � gure at left.

As the rod falls, the electrons and the positively charged nuclei in the rod are both moving down through the magnetic � eld. As was explained in the last chapter, the magnetic � eld will therefore exert a magnetic force on the elec- trons, and on the nuclei.

In which direction will the magnetic force act on the electrons and the nuclei?

� e hand rules from chapter 6 can be used for both the electrons and the nuclei, keeping in mind that the hand rules use conventional current, so elec- trons moving down are equivalent to positive charges moving up.

� e force on the electrons will be towards the far end of the rod, while the force on the nuclei will be to the near end of the rod, as is shown in the � gure below.

+ +

+ +

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– – –

–

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(a)

(b)

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The magnetic � eld forces electrons to the far end of the falling rod.

� e atomic structure of the metal restricts the movement of the positively charged nuclei. � e negatively charged electrons, on the other hand, are free to move. � e electrons move towards the far end of the rod, leaving the near end short of electrons and thus positively charged.

Not all electrons move to the far end. As the far end becomes more negative, there will be an increasingly repulsive force on any extra electrons. Similarly, there will be an increasingly attractive force from the positively charged near end, attempting to keep the remaining electrons at that end. � is process is similar to the charging of a capacitor.

� e movement of the metal rod through the magnetic � eld has resulted in the separation of charge, causing a voltage between the ends. � is is called induced voltage. As long as the rod keeps moving, the charges will remain

N SN S

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A metal rod falling down through a magnetic � eld

Induced voltage is a voltage that is caused by the separation of charge due to the presence of a magnetic � eld.

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