21.3 Electrical Energy Generation and Transmission

Reading StrategySequencing Copy the flowchart below. Asyou read, complete it to show how a step-uptransformer works. Then make a similarflowchart for a step-down transformer.

Key ConceptsHow is voltage induced ina conductor?

Name two types ofgenerators.

How can a transformerchange voltage and current?

What are some sources ofelectrical energy in theUnited States?

Vocabulary◆ electromagnetic

induction◆ generator◆ transformer◆ turbine

Alternating current in

smaller coila. b.? ?

642

Figure 12 Photographs of large cities, such as Seattle,Washington, are visible reminders of how much people rely on electrical energy.

Think about how electrical energy affects a city. Trafficlights change colors to control the flow of cars. Flashingneon lights advertise businesses. Subways use electricalenergy to move from place to place. People use electricalenergy to warm their homes, cook their suppers, and washtheir clothes. At night, lights shine from the windows oftall buildings as shown in Figure 12. Where does all theelectrical energy come from?

Generating Electric CurrentAll of the electrical energy that moves subway trains, lightsbuildings, and powers factories comes from the two aspectsof the electromagnetic force. You already know that an elec-tric current produces a magnetic field. However, you maynot know that a magnetic field can be used to produce anelectric current. Electromagnetic induction is the processof generating a current by moving an electrical conductorrelative to a magnetic field. Recall that electrical conductorsare materials through which charge can easily flow.

642 Chapter 21

FOCUS

Objectives21.3.1 Describe how electric current is

generated by electromagneticinduction.

21.3.2 Compare AC and DCgenerators and explain howthey work.

21.3.3 Analyze factors that determinethe output voltage and currentproduced by a transformer.

21.3.4 Summarize how electricalenergy is produced, transmitted,and converted for use in the home.

Build VocabularyParaphrase Have students explainwhat the vocabulary terms mean byparaphrasing their definitions. Havethem write a sentence in which the wordis used, then have the word followed bythe phrase in other words. This exerciseallows students to be certain that theyunderstand the meaning of a given term.

Reading Strategya. Produces changing magnetic fieldb. Induces current in larger coil

INSTRUCT

Generating ElectricCurrentBuild Science SkillsClassifying Have students recall the process of charge induction from Chapter 20. Ask, How iselectromagnetic induction similar to electric charge induction? (Bothtypes of induction involve manipulatingcharges by using fields, either electric ormagnetic, without any other contact with the charges.) By grouping theprocesses of using fields to manipulatecharges in conductors, the general idea of induction will become moreconcrete for students.Verbal, Logical

L2

2

L2

L2

Reading Focus

1

Section 21.3

Print• Laboratory Manual, Investigation 21A• Guided Reading and Study Workbook

With Math Support, Section 21.3 andMath Skill: Calculating Voltage

• Transparencies, Section 21.3

Technology• Probeware Lab Manual, Lab 9• iText, Section 21.3• Presentation Pro CD-ROM, Section 21.3• Go Online, NSTA SciLinks, Transformers;

PHSchool.com, Data sharing

Section Resources

The English scientist Michael Faraday (1791–1867)discovered electromagnetic induction in 1831, openingthe way for many practical uses of electromagnetism.

According to Faraday’s law, a voltage is induced in aconductor by a changing magnetic field. For example,changing the magnetic field through a coil of wire inducesa voltage in the coil. But a current results only if the coilis part of a complete circuit.

You can see this process at work by placing a magnetinside a coil of wire attached to a galvanometer, as shownin Figure 13. If you hold the magnet still, the galvanome-ter will detect no current in the wire. However, if youquickly move the magnet out of the coil, the current flows briefly,and then immediately drops back to zero. Moving the magnet in andout of the coil causes an electric current first in one direction andthen in the other. The same alternating current occurs if you move thecoil and keep the magnet still. As long as the magnet and coil aremoving relative to one another, the galvanometer will record a current.

GeneratorsMoving the magnet in the coil shown in Figure 13 produces only a smallamount of electric current. Most of the electrical energy used in homesand businesses is produced at large power plants using generators. Agenerator is a device that converts mechanical energy into electricalenergy by rotating a coil of wire in a magnetic field. Electric current isgenerated by the relative motion of a conducting coil in a magneticfield. The two types of generators are AC generators and DCgenerators. Although both types have been used, most power plantstoday use AC generators.

AC Generators Figure 14 shows a simplified AC gen-erator. An actual generator has many loops of wire. Thegenerator produces alternating current, in which chargesflow first in one direction and then in the other direction.As you can see, the generator looks very similar to theelectric motor you previously studied. While a motor con-verts electrical energy into mechanical energy, a generatordoes the opposite.

A wire coil in the generator is attached to metal bandscalled slip rings. The slip rings are in contact with metalbrushes that are in turn attached to a circuit. As the loopof wire is rotated, perhaps by someone turning it, the mag-netic field induces a current in the wire. This current is inone direction, and then when the loop turns halfwayaround, the current reverses direction.

Movement of magnet

Coil

Galvanometer shows that the current is flowing.

Figure 13 According to Faraday’slaw, the moving magnetic fieldinduces a current in the coil.Predicting If you increase thenumber of turns in the coil, andmove the magnet at the samespeed, will the current increase or decrease?

Slip rings

BrushesDirection in which the loop is turned

Wire loop

Figure 14 In a simple ACgenerator, an external forcerotates the loop of wire in themagnetic field. This induces acurrent in the wire. Forming Hypotheses Could you also induce a current if yourotated the magnets instead of the wire loop?

Magnetism 643

Use VisualsFigure 13 Stress that the upwardmovement of the magnet produces the same results as moving the coildownward over the magnet. In bothsituations, the relative change of themagnetic field with respect to the coil is identical. Have students examine thefigure carefully. Ask, In which directionwould the galvanometer needledeflect if the magnet were moveddownward? (To the left side of thegauge) Ask, Would the result be thesame if the coil were moved upwardover the magnet? (Yes)Logical, Visual

Generators

GeneratingAlternating CurrentPurpose Students will observe how a generator produces an alternatingcurrent that varies with the speed ofrotation of the generator coils.

Materials a hand-operated generator, a galvanometer, insulated wire (2 strands)

Procedure Connect the outputterminals of a hand-cranked generator(preferably the demonstration typefound in school labs) to the wires, andthe other ends of the wires to theterminals of the galvanometer. Point outthe various parts of the generator thatare shown in Figure 14 (loop or coil, sliprings, and brushes). Turn the generatorcrank slowly and smoothly, and allowstudents to observe the changes in thegalvanometer needle. Increase thespeed of cranking to demonstrate howthis increases the current.

Safety Be sure to use insulated wire.Follow lab safety for use of electricaldevices.

Expected Outcome The galvano-meter needle should move back andforth, indicating the changing directionof the electric current produced by thegenerator.Visual, Group

L2

L1

Magnetism 643

Customize for English Language Learners

Reading/Learning LogThe concept of electromagnetic induction issubtle, and is the basis for understandinggenerators, transformers, and electric powertransmission. Be sure that English languagelearners have a clear understanding of thisconcept by having them construct a

Reading/Learning Log. Have students writewhat they understand in the left column, and what they still have questions about in the right column. Allow time for groups of students of mixed language abilities to share their logs.

Answer to . . .

Figure 13 Increase

Figure 14 Yes, induction takes placeregardless of whether the loop ormagnets are moved.

644 Chapter 21

You can buy a small AC generator to power elec-trical devices during a power outage or to use in areasnot served by a power company. Figure 15 shows anAC generator that can produce 3300 watts of power.This is enough for a household or small business touse. Power plants use AC generators that are hugecompared to the generator shown here.

DC Generators A DC generator produces adirect current. Its design is very much like the designof an AC generator except that a commutator replacesthe slip rings. As the loop rotates, an alternating cur-rent is induced in the wire. First, one side of the

commutator contacts a brush. When the loop rotates, current isinduced in the other direction, but now the other side of the commu-tator contacts that brush. For this reason, the current that leaves thegenerator flows in only one direction.

TransformersThe electricity produced by power plants is transmitted through powerlines at very high voltages. These voltages are too high to be handledsafely in homes. The voltage must first be changed, or transformed. Atransformer is a device that increases or decreases the voltage and cur-rent of two linked AC circuits. A series of transformers changeshigh-voltage current in power lines into 240-volt current that entersyour home.

A transformer works only with alternating current because onlyalternating current induces a constantly changing magnetic field.

A transformer changes voltage and current by inducing achanging magnetic field in one coil. This changing field theninduces an alternating current in a nearby coil with a differentnumber of turns.

Why Transformers Are Needed Early power plants usedDC generators because the power plants were close to the customers.As the demand for electric power increased, power plants had to trans-mit power much farther. Remember that an electric charge movingthrough a wire heats the wire. Over long distances, the resistance ofthe wire causes large losses of power. Power losses can be reduced byusing lower current transmitted at a higher voltage. However, voltageand current can be transformed only with alternating current.

What kind of current does a DCgenerator produce?

Figure 15 Small generatorsprovide power in areas that arenot served by power companies.These generators may also beused to provide electricity duringa power outage.

For: Links on transformers

Visit: www.SciLinks.org

Web Code: ccn-2213

644 Chapter 21

TransformersIntegrate Social StudiesDuring the early 1880s, electric power inthe United States was distributed as directcurrent. This system was successful at firstbecause electric energy did not have tobe transmitted very long distances.However, as demand and the distancesbetween production and consumptionincreased, direct-current production couldonly succeed by transmitting electricenergy with greater currents. At the sametime, the Europeans were developingalternating-current systems for electricpower. In 1885, George Westinghouseimported an AC generator and trans-formers, and installed an electric powersystem in Pittsburgh, Pennsylvania. Overthe next seven years, alternating-currentgeneration became more widespread,despite strong resistance from ThomasEdison and other supporters andproducers of direct current.Verbal

Students may be confused in thinkingthat a transformer violates the principleof energy conservation. Explain to stu-dents that, for an ideal transformer, theamount of energy that goes into thetransformer each second (the inputpower) is equal to the energy leavingthe transformer each second (the outputpower). Remind students that electricalpower equals voltage times current.Because a great deal of electrical energyis lost by heating, which is dependentupon the resistance of the conductorand current, less energy is lost when the voltage is high and the current islow. Thus, while transformers do notcreate electrical energy, they do help to reduce the loss of electrical energy.Logical

L2

L2

Section 21.3 (continued)

Uniform Direct Current The direct currentproduced by a generator with a commutatormoves in one direction, but it is not a steadycurrent. As with AC, the induced current fromthe generator ranges from a maximum valueto 0 amps, then increases to the maximumvalue again. To remedy this, many DCgenerators use several coils that are mountedon the rotating axle. The commutator consistsof many segments, with each coil attached totwo oppositely positioned segments. The coils

rotate in the magnetic field, and at any givenmoment they are at different places in theirrotation—one may be at a maximum outputwhile another is at a minimum. The differentamounts of current are combined andtransferred to the output circuit. The coils areoriented in a uniform way to the magnets, sothe total output current is nearly the same atall times. In this way, a nearly constant directcurrent is produced.

Facts and Figures

Download a worksheet ontransformers for students tocomplete, and find additionalteacher support from NSTA SciLinks.

Magnetism 645

400turns

Step-down Transformer

Step-up Transformer

Soft iron core

AC Source

AC Source

Primary coil

Secondary coil

Secondarycoil

Low voltage

Low voltageHighvoltage

Highvoltage

100turns

400turns100

turnsPrimarycoil

A

B

Figure 16 Transformers, such asthose at substations of powerplants, change voltage. A A step-down transformer decreasesvoltage and increases current. B A step-up transformer increasesvoltage and decreases current.

Changing Voltage and Current Figure 16 shows two typesof transformers. Notice that each transformer has two sets of coilswrapped around a ring-shaped iron core. When there is an alternatingcurrent in the primary coil, the current creates a changing magneticfield in the iron core. Because the iron core is also inside the second-ary coil, the changing field induces an alternating current in thesecondary coil.

The number of turns in the primary and secondary coils determinesthe voltage and current. To calculate the voltage, divide the number ofturns in the secondary coil by the number of turns in the primary coil.The result is the ratio of the output voltage to the input voltage.

Transformers are very efficient because very little energy is lost asheat. Assuming 100% efficiency, the power (I � V) must be the samein the primary and secondary coils. Therefore, if voltage increases inthe secondary coil, the current must decrease in the same ratio.

Types of Transformers A step-down transformer decreasesvoltage and increases current. Notice in Figure 16A that the primarycoil has 400 turns, and the secondary coil has 100 turns. If the inputvoltage in the primary coil is 120 volts, then the output voltage isreduced to 30 volts.

A step-up transformer increases voltage and decreases current. InFigure 16B, the primary coil has 100 turns, and the secondary coil has400 turns. If the input voltage is 20 volts, the output voltage is 80 volts.

Build Reading LiteracyReciprocal Teaching Refer to page 628D in this chapter, whichprovides the guidelines for reciprocalteaching.

To help students understand scienceconcepts, have them apply the strat-egies of summarize, question, clarify,and predict as they read the sectionunder the heading Transformers. Forinstance, before students begin to read, ask, What do you think atransformer is used for? (Students mayrespond that transformers are used totransform, or change, electric current insome way.) Have students read to verifytheir predictions. To help studentsdetermine what information is impor-tant and to check understanding, askquestions such as Why must voltagefrom power lines be reduced beforeentering homes? (Voltage is stepped upat power plants so that electric current canbe transmitted more efficiently over longdistances. Overheating, and therefore lossof power, can be avoided by transmittinglower current at higher voltage. A step-down transformer reduces voltage andincreases current to levels that can safelybe used to operate appliances in a home.)Clarify the meanings of unfamiliar wordsand concepts. Ask students to sum-marize what has been read. Encouragestudents to assume the role of leader inthese discussions as they becomefamiliar with the use of these strategies.Verbal, Group

Use Community ResourcesSuggest that students learn more aboutdifferent transformers by contacting the Public Information Office of the local electric utility. Have students findout where step-up and step-downtransformers are used in your city, howmany types of each kind are used, andthe number of turns of wire used for the primary and secondary coils of each. Encourage them also to gatherinformation on the efficiency of thetransformers. Have them write a shortreport of their findings.Intrapersonal, Portfolio

L2

L1

Magnetism 645

Answer to . . .

Direct current

Electrical Energy for Your HomeA single electric light uses relatively little electrical energyby itself. A massive amount of electrical energy, however,is needed for the lights and other electrical devices thatare used by people in an entire city. Consumption on that scale requires equally huge production of electricalenergy to meet the demand.

Most of the electrical energy generated in theUnited States is produced using coal as an energy source.Some other sources are water (hydroelectric), nuclearenergy, wind, natural gas, and petroleum. Below each of the generators that are shown in Figure 17 is a large tur-bine, which can convert energy from one of these sourcesinto electrical energy. A turbine is a device with fanlikeblades that turn when pushed, for example, by water orsteam. Burning fossil fuels or nuclear reactions can heatwater to produce steam that spins a turbine. Water pouringover a dam can also turn a turbine. To produce electricalenergy, the turbine may turn the coils of a generator or itmay spin magnets around the coils of wire.

What can push the blades of a turbine?

646 Chapter 21

You have been hired as an electrical engineer atyour local power plant. Your first task is to planhow electrical energy can be transmitted from the power plant to a school that will soon bebuilt. There are many things you need toinvestigate. What is the voltage generated at the power plant? How should it be stepped up fortransmission? How can it be stepped down foruse in the new school?

Defining the Problem Write a few sentencesthat describe your task and the steps you can taketo complete it.

Organizing Information Research the stepsthat are taken to transmit electrical energy fromthe power plant to other schools in your area.

Creating a Solution Decide what steps youwould take to transmit electrical energy to the new school.

Presenting Your Plan Create a poster showinghow electrical energy can be transmitted to thenew school. Include descriptions of the types oftransformers you could use.

Figure 17 A turbine turns the magnet inside the coilof a generator. Predicting What would happen if aturbine turned faster?

Transmitting Electricity to a New School

646 Chapter 21

Electrical Energy for Your Home

Transmitting Electricity to a New School

Defining the Problem As anengineer, I must first find out whatvoltage and current are needed for theschool, and how this compares with thevoltage and current generated at thepower plant. If the power plant is veryfar away from the school, the voltageneeds to be stepped up and down morethan if the school is close.

Organizing Information Studentscan find out from the local electriccompany what the voltage use forschools is, and how far they are fromsubstations. Transformer conversionfactors must also be obtained.

Creating a Solution Once a nearbysubstation has been located and thedistance to a power plant determined,connection of power lines can beplanned. The voltage changes along thepath should ensure that power cantravel the necessary distances, and thatthe final voltage at the school is correct.

Presenting Your Plan Students’posters should include values for thedifferent voltages along the transmissionpath, and should resemble Figure 18 in layout and content. The school shouldhave some devices with input voltages of 120 V and some that operate at220–240 V. The generated voltages will depend on how far the power istransmitted. Locally produced electricitymay need to be stepped up or downonly once, and then only by a factor of about 10. If the school is far from asubstation, student designs may includea power substation specifically for theschool’s use.Logical, Interpersonal

For Extra HelpBe sure that the need for transformers isclear to students. Emphasize that stepped-up voltages are necessary to reduce heatloss for long-distance transmissions.Encourage students to examine andunderstand Figure 18, as the informationand presentation given there is similar to what they are to prepare.Verbal, Visual

L1

L2

Section 21.3 (continued)

Appliance Power Usage Most appliancesin the United States are designed to operatebetween 110 V and 120 V with an alter-nating-current frequency of 60 Hz. However,some larger appliances, such as electricranges, clothes dryers, and air conditionersoperate at 240 V. Special wiring is needed inthe parts of a house where these appliancesare used.

Many countries, particularly in Europe, have a household voltage between 220 V and240 V with a frequency of 50 Hz. Appliancesused in these countries are manufactured tooperate at these higher voltages. Internationaltravelers often carry converters, devices thatuse a transformer to change voltage andcurrent so that an appliance designed for one system can be used in another system.

Facts and Figures

Generatingplant

Step-uptransformer

11,000 V

240,000 V

Step-downtransformer(substation)

High-voltagetransmissionlines

Step-downtransformer

220–240 V

7200 V

Section 21.3 Assessment

Reviewing Concepts1. How is voltage induced in a conductor?

2. Name two types of generators.

3. How does a transformer work?

4. Name six sources of electrical energy in the United States.

Critical Thinking5. Relating Cause and Effect Explain how

water can be used to create electrical energy.

6. Applying Concepts What is the connectionbetween Faraday’s law and the generation ofelectrical energy?

7. Comparing and Contrasting Describehow AC generators and DC generators arealike and how they are different.

8. Drawing Conclusions Why can’t you use electrical energy directly from a high-voltage line?

9. Calculating An electronic device contains atransformer. Its primary coil has 200 turns, andits secondary coil has 20 turns. If the device is plugged into a 120-volt line, what is theoutput voltage of the device?

Follow the steps shown in Figure 18 from the point where electricalenergy is generated. The power plant on the left generates electrical energythat is stepped up to hundreds of thousands of volts. Transformers, whichare shown in the middle of the diagram, make it possible to bring electrical energy efficiently from the power plant to users. After travelingalong the high-voltage transmission lines, the voltage is stepped down ata substation, to a few thousand volts. The electrical energy is then dis-tributed to neighborhoods. Just before the electrical energy reachespeople’s homes, the voltage is stepped down to between 220 and 240 volts.Heavy duty appliances, like an electric stove, use 240-volt circuits. Mostother appliances in the home use 120 volts.

Magnetism 647

Figure 18 Voltage is increasedfor long-distance transmission,and then decreased near homes,schools, and businesses.Interpreting Diagrams Howmany step-down transformersare shown in the figure?

Compare-Contrast Paragraph Write aparagraph comparing and contrasting what step-up and step-down transformers do. (Hint: Use the terms voltage, primary coil, secondary coil, input, and output.)

Use VisualsFigure 18 Have students look carefullyat the various changes in voltage thatoccur during power transmissionbetween the generating plant and ahome. Ask, What is the stepped-upvoltage for transmission? (Thevoltage is stepped up from 11,000 V to 240,000 V.) Ask, By how much is the voltage stepped down at thesubstation? (The voltage is stepped down from 240,000 V to 7200 V.)Visual

ASSESSEvaluateUnderstandingAsk each student to write two questionsabout generating and transmittingelectric power. Review the questions foraccuracy, and then have students formgroups and quiz each other, using theirapproved questions.

ReteachUse Figure 14 to review how a generatoruses electromagnetic induction toproduce an alternating current.

Student paragraphs should include the following comparisons: both trans-formers have iron cores to channel themagnetic field; both have primary andsecondary coils of wire for the input andoutput of electricity; neither has anymoving parts. Contrasts should include:a step-up transformer increases voltageand decreases current, while a step-down transformer decreases voltage andincreases current; a step-up transformerhas more wire turns in the secondary(output) coil than in the primary (input)coil, while a step-down transformer hasmore wire turns in the primary coil.

If your class subscribesto iText, use it to review key concepts inSection 21.3.

L1

L2

3

L1

Magnetism 647

5. Water can turn a turbine that turns the axleof a generator and produces electricity.6. The relative motion of a magnet and a coilof wire will cause charges to flow in the wire,thus generating an electric current andelectrical energy.7. Both produce an electric current by therotation of a wire coil in a magnetic field. In a DC generator, current flows in only one direction.8. High voltages are dangerous. Householddevices are designed to use much lower voltages.9. 12 V

Section 21.3 Assessment

1. Voltage is induced in a conductor by achanging magnetic field.2. AC, which produces alternating current,and DC, which produces direct current3. A transformer changes voltage and currentby generating a changing magnetic field inone coil. This field then induces a current in anearby coil with a different number of turns.4. Coal, nuclear power, water (hydroelectric),wind, natural gas, and petroleum

Answer to . . .

Figure 17 A faster-turning turbinewould turn the generator faster,generating a higher current.

Figure 18 Two

Steam or water