section 21.1 21.1 magnets and magnetic fields

21.1 Magnets and Magnetic Fields

Reading StrategyUsing Prior Knowledge Copy the diagrambelow and add what you already know aboutmagnets. After you read, revise the diagrambased on what you learned.

Key ConceptsHow do magnetic poles interact?

How can a magnetic field affect a magnet that enters the field?

Why are some materialsmagnetic while others are not?

Vocabulary◆ magnetic force◆ magnetic pole◆ magnetic field◆ magnetosphere◆ magnetic domain◆ ferromagnetic

material

Ancient Greeks observed that magnetite, or lodestone, attracts iron.Some time before 200 A.D., the Chinese sculpted magnetite into spoon-shaped compasses. They called these stones “south pointers.” By 1150A.D., Chinese navigators used compasses with magnetized iron needles.But properties of magnets were not well explained until 1600. In thatyear, the English physician William Gilbert published De Magnete.

Magnetic ForcesYou can explore properties of magnets on your own. Either side of amagnet sticks to a refrigerator. Yet if you push two magnets together,they may attract or repel. Magnetic force is the force a magnet exertson another magnet, on iron or a similar metal, or on moving charges.Recall that magnetic force is one aspect of electromagnetic force.

Magnetic forces, like electric forces, act over a distance. Look at thesuspended magnets in Figure 1. If you push down on the top two mag-nets, you can feel the magnets repel. Push harder, and the forceincreases. Magnetic force, like electric force, varies with distance.

Gilbert used a compass to map forces around a magnetite sphere. Hediscovered that the force is strongest at the poles. All magnets have twomagnetic poles, regions where the magnet’s force is strongest. One endof a magnet is its north pole; the other end is its south pole. The directionof magnetic force between two magnets depends on how the poles face.

Like magnetic poles repel one another, and opposite magneticpoles attract one another.

Propertiesof magnets

a. ?

c. ? d. ?

b. ?

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Figure 1 The green magnet andlower red magnet attract eachother. The lower red magnet andthe yellow magnet repel eachother. Predicting What wouldhappen if the upper red magneton the pencil were flipped over?

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FOCUS

Objectives21.1.1 Describe the effects of

magnetic forces and magneticfields and explain howmagnetic poles determine thedirection of magnetic force.

21.1.2 Interpret diagrams ofmagnetic field lines around oneor more bar magnets.

21.1.3 Describe Earth’s magnetic fieldand its effect on compasses.

21.1.4 Explain the behavior offerromagnetic materials interms of magnetic domains.

Build VocabularyWord-Part Analysis Have studentsresearch the origin of the word magnetand write a short paragraph explaininghow the word originated. (The wordmagnet is derived from the nameMagnesia, a region that was once part of ancient Greece. This area wasknown for its magnetite ore mines.)

Reading Strategya. Can be temporary or permanentb. Have north and south poles; like polesrepel, unlike poles attract c. Only a fewmaterials can be magnets. d. Magnetsaffect objects with iron but don’t affectmost materials, such as paper, cotton,and so on.

INSTRUCT

Magnetic ForcesBuild Reading LiteracyKWL (Know/Want to Know/Learned)Refer to page 124D in Chapter 5,which provides the guidelines for a KWL strategy.

Have students label three columns on a sheet of paper K, W, and L. Have them write in the K column what theyknow about magnetic forces, and in the W column questions they would likeanswered about the forces exerted bymagnets. Then, have students read theparagraphs on this page and record inthe L column the answers to as many oftheir questions as possible. Verbal, Interpersonal

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Section 21.1

Print• Laboratory Manual, Investigation 21B • Reading and Study Workbook With

Math Support, Section 21.1• Transparencies, Chapter Pretest and

Section 21.1

Technology• Interactive Textbook, Section 21.1• Presentation Pro CD-ROM, Chapter Pretest

and Section 21.1

Section Resources

Magnetic FieldsA magnetic field surrounds a magnet and can exert magnetic forces. InFigure 2, iron filings are used to show the shape of the magnetic fieldaround a bar magnet. A magnetic field, which is strongest near amagnet’s poles, will either attract or repel another magnet that entersthe field. The field lines begin near the magnet’s north pole and extendtoward its south pole. The arrows on the field lines indicate what direc-tion a compass needle would point at each point in space.Where lines areclose together, the field is strong. Where lines are more spread out, thefield is weak.

Magnetic Fields Around Magnets You can use iron filingsto visualize how magnetic fields of two magnets interact. Figure 3Ashows the north pole of one magnet facing the north pole of anothermagnet. Notice that there are no iron filings in the gap between the mag-nets. Iron filings are not attracted to this area because the combinedmagnetic field is very weak. Figure 3B shows the combined field of twomagnets with opposite poles facing each other. The field lines start atthe north pole of one magnet and extend to the south pole of the othermagnet. The field in the gap between the magnets is very strong, as youcan see from the dense crowding of iron filings in this area.

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A

B

Figure 2 A magnetic fieldsurrounds every magnet. Ironfilings reveal the field lines, whichstart near the north pole andextend toward the south pole.Interpreting Diagrams Inwhich two areas of a bar magnetis the field strongest?

Figure 3 Iron filings reveal the combined magnetic fieldof two interacting magnets. A When like poles of twomagnets come together, the magnets repel each other.B When opposite poles of magnets come together, themagnets attract each other.

Magnetic Fields Integrate Earth ScienceEarth’s magnetic field is produced by the motions of hot, liquefied iron withinits core. Induced electric currents in the iron give rise to magnetic fields,which affect the flow of the iron andcause the resulting magnetic fields tobecome stronger.

Earth’s magnetic field undergoes suddenreversals every few hundred thousandyears, although the change does nothappen at regular intervals. Currently the North Magnetic Pole is a south pole, which explains why “North” on a compass points to the north. Evidencefor reversals is found in cooled magma,where iron atoms have aligned withEarth’s magnetic field at the time of cool-ing. Cooled magma in ridges along theocean floor provides a continuous recordof magnetic field reversals over time.Logical

Build Science SkillsInferring

Purpose Students are helped to an understanding of the shape of the magnetic field around two bar magnets.

Materials 2 bar magnets, a smallmagnetic compass

Class Time 20 minutes

Procedure Arrange two magnets asshown in either part of Figure 3. Havestudents place the compass at differentpositions about 1–2 cm away from themagnets and sketch the direction inwhich the compass needle points for eachposition. Ask them how the direction ofthe needle corresponds to the direction ofthe field lines in each location. Have themidentify any patterns they notice.

Expected Outcome The compassneedle will be parallel to the field lines at any location. The needle’s south polepoints along the field lines toward thenorth pole of a nearby magnet.Logical, Visual

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Customize for English Language Learners

Reinforce Science ConceptsThe various concepts, such as magnetic field,magnetic forces, and magnetic domains, eachuse the word magnetic, but mean and refer todifferent things. To help English languagelearners develop a clear understanding of theseconcepts, have them construct a Word Analysis

Chart. Instruct students to write each of thevocabulary words on a separate chart and givea definition of each word. Suggest that theycompare and contrast the vocabulary words.Finally, have them draw a picture or diagramto illustrate each concept.

Answer to . . .

Figure 1 The red magnet would floatabove the blue magnet.

Figure 2 The field is strongest at the poles.

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Magnetic Field Around Earth Earth is like a giant magnetsurrounded by a magnetic field. The area surrounding Earth that is influ-enced by this field is the magnetosphere (mag NET oh sfeer).

A compass points north because it aligns with Earth’s magneticfield. However, as Figure 4 shows, Earth’s magnetic poles are not at thegeographic poles. The geographic North Pole is at 90° N latitude, butthe magnetic North Pole is at about 84° N latitude. Because of this, acompass may point east or west of north. The angle between the direc-tion to true north and to magnetic north is called magneticdeclination. Magnetic declination varies with your location on Earth.

Magnetic MaterialsWithin an atom, electrons move around the nucleus. This movement,along with a property of electrons called “spin,” causes electrons to actlike tiny magnets. In many materials, each electron is paired with anotherhaving an opposite spin. Magnetic effects mostly cancel each other. As aresult, these materials have extremely weak magnetic fields.

Many other materials have one or more unpaired electrons. Theunpaired electrons produce magnetic fields. But the fields usuallydon’t combine because the arrangement of the atoms isn’t quiteright. These materials have weak magnetic fields. In a few materials,such as iron, nickel, and cobalt, the unpaired electrons make a strongmagnetic field. Then the fields combine to form magnetic domains.A magnetic domain is a region that has a very large number of atomswith aligned magnetic fields. A ferromagnetic material (fehr oh magNET ik), such as iron, can be magnetized because it contains magneticdomains. When a material is magnetized, most of its magneticdomains are aligned.

Why does a compass point toward north?

Observing MagneticField Lines

Materialssmall container of iron filings, 2 bar magnets, paper, 2 textbooks, masking tape

Procedure1. Place two textbooks side by

side, about 7 cm apart.

2. Place the magnets betweenthe books, with north polesfacing, about 2 cm apart.Tape the magnets in place.

3. Place the paper over themagnets to form a bridge.

4. Sprinkle iron filings on thepaper until you can see themagnetic field lines. Sketchyour observations.

5. Carefully return the filingsto their container.

6. Repeat Steps 2 through 5with opposite poles facing.

Analyze and Conclude1. Inferring Where was the

magnetic field thestrongest? The weakest?

2. Analyzing Data How didthe fields of like polesfacing differ from those ofunlike poles facing?

3. Predicting What resultwould you expect if youused sawdust instead ofiron filings?

Magneticfield

Geographic North Pole Magnetic

North Pole

Magnetic South Pole Geographic

South Pole

Figure 4 Earth is surrounded by magnetic field lines. These lines are densest at the poles.

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Observing Magnetic Field Lines

ObjectiveAfter completing this activity, studentswill be able to• recognize how the magnetic fields

of two magnets combine.

Skill Focus Observing

Prep Time 10 minutes


Safety Students should wear safetygoggles, not inhale the iron filings, andwash their hands when finished.

Teaching Tips• Dropping or banging magnets causes

them to lose their strength.• Suggest that students tape the paper

in place, so that the pattern is notaccidentally disturbed.

Expected Outcome Students willrealize that the interaction of the fieldlines of two magnets depends on howthe magnets are positioned.

Analyze and Conclude1. The field was strongest in the gapbetween opposite poles and weakest inthe gap between like poles.2. The field lines of two like poles spreadapart and had a gap between the poleswith very few field lines. The field linesof two unlike poles extended in linesconnecting the north and south pole.3. No pattern would appear, becausesawdust is not magnetic. Visual, Group

For EnrichmentHave students repeat the experimentusing different separations between themagnets. Ask, What happens to theiron filings and to the field strengthwhen two opposite poles are movedapart? (The filings are less crowded,indicating the field is weaker.)Visual, Logical

Magnetic MaterialsFYIMagnetic domains are quite small andcan only be imaged using microscopes.A variety of instruments are used, suchas scanning tunneling microscopes,magnetic force microscopes, and lightmicroscopes with polarizing filters.

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Section 21.1 (continued)

Strong and Weak Magnetic Fields Earthhas the strongest magnetic field of the rockyinner planets. Earth has an average fieldstrength at its surface of between 30 �T and 60 �T. (A tesla, T, is the unit by which

magnetic fields are measured.) This is some100 times stronger than the magnetic field ofMercury, roughly 1000 to 5000 times strongerthan the field of Mars, and about 100,000times stronger than the field of Venus.

Facts and Figures

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Electric Charge Review electric charge inSection 20.1. Compare the attraction andrepulsion of positive and negative chargeswith the behavior of two bar magnetsplaced near one another.

Section 21.1 Assessment

Reviewing Concepts1. Describe the interaction of magnetic poles.

2. What two things can happen to a magnetentering a magnetic field?

3. What makes a material magnetic?

4. Describe what happens to the fields of two barmagnets when you bring their north polestogether.

Critical Thinking5. Predicting What happens if you suspend a

bar magnet so that it can swing freely?

6. Relating Cause and Effect How areelectrons responsible for magnetism?

7. Predicting What will happen if you hit amagnet with a hammer? Explain.

8. Designing Experiments How could youtest the effects of heating and cooling on themagnetization of a bar magnet?

Nonmagnetized Materials The fact that a material is ferromagnetic does not mean it is a magnet. If the domains of a ferromagnetic material are aligned randomly, the magnetization of thedomains is cancelled, and it is not a magnet. An iron nail is an exam-ple of a nonmagnetized material. It is ferromagnetic, so the domainshave the potential to be aligned, but normally they are not. Figure 5Ashows the random orientation of domains in nonmagnetized iron.

Magnetized Materials You can easily magnetize a nonmag-netized ferromagnetic material by placing it in a magnetic field. Forexample, if you put a nonmagnetized iron nail near a magnet, youwill turn the nail into a magnet. Figure 5B shows the alignment ofmagnetic domains in magnetized iron. The applied magnetic fieldcauses magnetic domains aligned with the field to grow larger. Thismagnetization can be temporary. If the magnet is moved away fromthe nail, the motion of the atoms in the nail causes the magneticdomains to become randomly oriented again. In some ferromag-netic materials, the domains stay aligned for a long time. Thesematerials are called permanent magnets. They are not truly per-manant, because heat or a jarring impact can realign the domains.

If you cut a magnet in half, each half will have its own north poleand south pole because the domains will still be aligned. If you cutthe pieces in half again, each half will again have a north pole and asouth pole. No matter how many times you cut the magnets, eachpiece will have two different poles. A magnet can never have just anorth pole or just a south pole.

Figure 5 A magnetic field canmagnetize ferromagnetic materials. A Before magnetization, domains arerandom. B Domains aligned with thefield grow during magnetization.Unaligned domains can shrink.

A

B

Students may be visualizing the motionof the electron as if it were a planetrotating on its axis. Emphasize that the“spin” of an electron is not like the spinof a ball, any more than the orbitalmotion of an electron around an atom’snucleus is like the motion of a planetaround the sun. The term spin is appliedto electron behavior that mathematicallyresembles that of a spinning object.Remind students of how electrons inatoms are modeled as “clouds” wherethey are most likely to be located.Logical

ASSESSEvaluate UnderstandingAsk students why a refrigerator magnetsticks to the door of a refrigerator. Besure they explain which material is apermanent magnet, and what happensat the atomic level in the magnetizedmaterial. (The atoms in the refrigeratormagnet, which is made of ferromagneticmaterial, are aligned in the variousmagnetic domains, and so give themagnet a permanent field. When themagnet is attached to the unmagnetizeddoor of the refrigerator, the atoms of the door are aligned, and so becomemagnetized temporarily.)

ReteachUse Figure 3 to explain the shape anddirection of a magnetic field around abar magnet.

Like charges and like poles repel, whileopposite charges and opposite polesattract. In contrast to electric charges,magnetic poles can’t be separated.

If your class subscribesto the Interactive Textbook, use it toreview key concepts in Section 21.1.

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5. The north end of the magnet will swingtoward north, aligning with Earth’s magneticfield just as a compass does.6. The spin and orbital motion of electrons inan atom give the atom a magnetic field.7. The motion of the atoms can cause themagnetic domains to become randomlyaligned. The material loses its magnetization.8. Students’ suggestions for experiments should include some method for testing themagnetization of a bar magnet before and afterit is heated and before and after it is cooled.


1. Magnetic poles that are alike repel oneanother, and magnetic poles that are differentattract one another.2. A magnetic field will either attract or repelanother magnet that enters the field.3. For a material to be magnetized, most of itsmagnetic domains must be aligned.4. The fields interact, and the field betweenthe magnets becomes very weak.

Answer to . . .

The north end of acompass points north

because a freely suspended bar magnetaligns with Earth’s magnetic field.


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Library securityPowerful magnets are used todeactivate tags in library booksbefore borrowing. If the tag isnot deactivated, the alarm willgo off at the library exit.

Transmitterpedestal

Changingelectromagneticfield

Changingelectromagneticfield

Flashingalarm light

Activated tag An activated tagis slightly demagnetized. When it

passes through the pedestal’s EMfield, the tag’s magnetic domains lineup with the field. This change inmagnetic domain emits a signal that ispicked up by the receiver, which setsoff the alarm.

Deactivated tagA deactivated tag is fully

magnetized. When it passes throughthe exit, the tag’s domains do notchange. Because no signal is emitted, the alarm is not set off.

The pedestals The transmitterpedestal contains a wire loop that

produces a changing EM field in the regionbetween the pedestals. The receiver pedestalpicks up any signal produced by the tag.

Anti-Theft Security DevicesAnti-theft security devices are found in stores across theworld. One of the best of these devices is the electro-magnetic (EM) tag system. This system is based on theinteraction between a small piece of magnetic material(a tag) and an EM field created between two pedestalsat the store exit. Applying concepts Which is morehighly magnetized, an activated or a deactivated tag?

Tag signal

Receiver pedestal

Activated tagattached to item

Deactivatedtag (fullymagnetized)

Magneticdomain

Wire loop carryingalternating current

Activated tag(demagnetized)

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Anti-Theft Security DevicesElectromagnetic tag systems were firstdeveloped in the 1960s, along withother similar RFID (Radio FrequencyIdentification) technology. This type of system uses electromagnetic waves to identify objects that have beentagged with magnetic material.

Electromagnetic waves consist ofchanging electric fields and changingmagnetic fields that are at right angles to each other and to the direction of the wave. The EM tag system uses themagnetic component of an electro-magnetic wave to temporarily magnetizean activated tag as it passes between the pedestals. This change in the tag’smagnetic domains produces a smallelectromagnetic wave with a particularfrequency. The wave is detected by areceiver, causing an alarm to sound.

A deactivated tag, however, is fullymagnetized, so no change occurs in the magnetic domains when the tagpasses between the pedestals. Thus, no electromagnetic wave is produced,and the tag passes through theelectromagnetic field undetected.

The magnetic properties of the tagcause it to become temporarilymagnetized more easily than ordinarysteel objects. This is why a screwdriveror box of paper clips can pass throughthe system without setting off the alarm.

Other systems make use of thin wirecoils in the tags that act as antennas, as well as small circuit elements.Electromagnetic waves emitted from the pedestal at a particular frequencyinduce a current in the tag’s antenna,and this induced current produces an electromagnetic wave with acharacteristic frequency. This wave isthen detected by the receiver pedestal.

Applying Concepts The deactivatedtag is more highly magnetized than theactivated tag. Logical

For EnrichmentStudents can make a multimedia presen-tation about the EM tag system, as wellas other RFID systems. Articles on thesubject can be found on the Internet andin science and engineering periodicals.Verbal, Portfolio

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21.2 Electromagnetism

Reading StrategyIdentifying Main Idea Copy the tablebelow. As you read, write the main idea of the text that follows each topic.

Key ConceptsHow can an electric chargecreate a magnetic field?

How is an electromagnetcontrolled?

How do galvanometers,electric motors, andloudspeakers work?

Vocabulary◆ electromagnetic

force◆ solenoid◆ electromagnet◆ galvanometer◆ electric motor

Topic Main Idea

Electricity and magnetism

Direction ofmagnetic fields

Direction of electric currents

Solenoids and electromagnets

Electromagnetic devices

a. ?

b. ?

c. ?

d. ?

e. ?

You know that unlike electric charges attract one another and thatlike electric charges repel one another. It is easy to discover a similareffect with the north and south poles of two magnets. However, it’smuch more difficult to figure out the relationship between electric-ity and magnetism. In fact, the connection was discoveredaccidentally by the Danish scientist Hans Christian Oersted in 1820.

One evening Oersted, pictured in Figure 6, was conducting scien-tific demonstrations for his friends and students in his home. Onedemonstration used electric current in a wire, and another used a com-pass needle attached to a wooden stand. As Oersted turned on thecurrent for the electricity demonstration, he saw the compass needlemove. When he turned off the current, the needle moved back to itsoriginal position. Further investigation showed that the current in thewire produced a magnetic field. Oersted had discovered a relationshipbetween electricity and magnetism.

Electricity and MagnetismElectricity and magnetism are different aspects of a single forceknown as the electromagnetic force. The electric force results fromcharged particles. The magnetic force usually results from themovement of electrons in an atom. Both aspects of the electro-magnetic force are caused by electric charges.

Figure 6 In 1820 Hans Oersteddiscovered how magnetism andelectricity are connected. A unit of measure of magnetic fieldstrength, the oersted, is namedafter him.

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FOCUS

Objectives21.2.1 Describe how a moving electric

charge creates a magnetic fieldand determine the direction of the magnetic field based onthe type of charge and thedirection of its motion.

21.2.2 Relate the force a magneticfield exerts on a moving electriccharge to the type of chargeand the direction of its motion.

21.2.3 Explain how solenoids andelectromagnets are constructedand describe factors that affectthe field strength of both.

21.2.4 Describe how electromagneticdevices use the interactionbetween electric currents andmagnetic fields.

Build VocabularyConcept Map Have students make aconcept map comparing the devices inthe vocabulary list.

Reading Strategya. Electricity and magnetism are differentaspects of electromagnetic force.b. Magnetic fields are produced at rightangles to an electric current. c. Electriccurrents are deflected perpendicular to amagnetic field. d. Changing the currentin an electromagnet controls the strengthand direction of its magnetic field.e. Electromagnetic devices changeelectrical energy into mechanical energy.

INSTRUCT

Electricity andMagnetismBuild Reading Literacy

Predict Refer to page 66D in Chapter 3, which provides theguidelines for predicting.

Have students read the first twoparagraphs on p. 635. Ask them topredict what Oersted discovered aboutthe relationship between electricity andmagnetism. (Predictions should indicatethat an electric current produces amagnetic field.) Logical

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Print• Reading and Study Workbook With

Math Support, Section 21.2• Transparencies, Section 21.2

Technology• Interactive Textbook, Section 21.2• Presentation Pro CD-ROM, Section 21.2• Go Online, NSTA SciLinks, Electromagnets

Section Resources

Section 21.2

PPLS

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Force deflecting the charge

Velocity of charge+

Magnetic Fields Around Moving Charges Oersted’sdiscovery about the relationship between a current-carrying wire anda magnet established an important physics principle. Movingelectric charges create a magnetic field. These moving charges maybe the vibrating charges that produce an electromagnetic wave. Theymay also be, as in Oersted’s experiment, the moving charges in a wire.Figure 7 shows how to remember the direction of the magnetic fieldthat is produced. The magnetic field lines form circles around astraight wire carrying a current.

Forces Acting on Moving Charges Recall that an electricfield exerts a force on an electric charge. The force is either in the samedirection as the electric field or in the opposite direction, depending onwhether it is a positive or negative charge.

The effect of a magnetic field on a moving charge is different, asshown in Figure 8. A charge moving in a magnetic field will bedeflected in a direction perpendicular to both the magnetic field andto the velocity of the charge. If a current-carrying wire is in a magneticfield, the wire will be pushed in a direction perpendicular to both thefield and the direction of the current. Reversing the direction of thecurrent will still cause the wire to be deflected, but in the oppositedirection. If the current is parallel to the magnetic field, the force iszero and there is no deflection.

What are two kinds of moving charges thatcan create a magnetic field?

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Figure 8 A moving positive charge is deflected ata right angle to its motion by a magnetic field. Inferring In what direction would the particlebe deflected if it had a negative charge instead ofa positive charge?

Directionof current

Directionof electron

flow

Directionof magnetic

field

Current-carrying wire

Figure 7 If you point the thumb ofyour right hand in the direction of the current, your fingers curve in thedirection of the magnetic field.Inferring How can you determinethe magnetic field direction from the direction of electron flow?

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Magnetic Field from Electric CurrentPurpose Students observe how anelectric current produces a magnetic field.

Materials insulated wire, cardboard (10 cm � 10 cm), a burner tripod, a variable DC power supply, 4–6 compasses

Procedure Punch a small hole in thecenter of the cardboard and thread thewire through the hole. Lay the cardboardflat on the burner tripod’s ring support sothat the wire passes through the tripodcenter, perpendicular to the cardboardand extending in a straight line 10 cm oneither side. (A ring stand and clamp maybe needed to support the upper end of the wire.) Connect both ends of thewire to the terminals of the power supply.Place the compasses on the cardboard at a distance of 3–4 cm from the wire.Turn on the power supply and increasethe current until the compass needlesbegin to deflect. Have students noticehow the needles deflect with respect tothe wire. Remove the compasses, turn off the power supply, reverse the wireconnections, and repeat thedemonstration.

Safety Use insulated wire. Followprocedures for electrical safety.

Expected Outcome When the topend of the wire is connected to thepositive terminal of the power supply,the magnetic field will be in a counter-clockwise pattern around the wire,according to the right-hand rule. Thiswill cause the poles of the compasses toalign themselves along the edge of acircle around the wire. The south poleswill form a clockwise pattern. When theconnections are reversed, the direction inwhich the compasses point will reverse. Visual, Group

Use VisualsFigure 8 Explain that the right-handrule also applies to Figure 8. Ask, Howcould you use your hand to determinethe deflection of an electron movingthrough the magnetic poles? (Use yourright hand with your thumb in the directionof the current, which will be opposite thedirection of the electron’s travel.)Visual

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Customize for Inclusion Students

Visually ImpairedThe right-hand rule can be used by studentswith visual impairments to understandmagnetic fields and forces. Explain howstudents can use their right hand to predictthe directions of magnetic fields for an electriccurrent in a straight wire and a solenoid. Asstudents may have difficulty using Figure 7,instruct them using a wire, so that they canunderstand how the right-hand thumb and

fingers are oriented for a positive current andits magnetic field. Then, have students adaptthe rule for positive charges moving in amagnetic field, as shown in Figure 8 (that is,the thumb points in the direction of themoving charge, the fingers extend in thedirection of the magnetic field, and the forceon the charge points outward from the palm).Encourage those students who successfullymaster the rule to explain it to the class.

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Making an Electromagnet

Materialsiron nail, 20 small metal paper clips, 20-cm lengthand 1-m length of insulated wire with strippedends, 6-volt battery, switch

Procedure1. Make a circuit using the nail, wire, battery,

and switch. Use the shorter wire to connectone terminal of the battery to the switch.Connect the longer wire to the otherterminal of the battery. Wrap this wirearound the nail 10 times. Then connect the longer wire to the switch.

2. Hold the head of the nail over the pile ofpaper clips. Close the switch. Record howmany paper clips the nail can pick up.

3. Open the switch. CAUTION If the switch is leftclosed, the wire will become very warm. Wrapthe longer wire 40 more times around the nailin the same direction as before.

4. Close the switch. Record how many paperclips the nail can pick up now.

5. Open the switch and disconnect the circuit.

Analyze and Conclude1. Observing How did your ability to pick up

paper clips with the nail change when youincreased the number of turns in the coil?

2. Drawing Conclusions Why did the nailbecome a magnet when a current-carryingwire was wrapped around it?

Solenoids and ElectromagnetsBefore you can use electromagnetic force, you need to be ableto control it. Using electromagnetic force requires some simpletools. Figure 9A shows a current-carrying wire with a loop in it.The magnetic field in the center of the loop points right to leftthrough the loop, as shown in Figure 9A.

Suppose you loop the wire many times to make a coil, asshown in Figure 9B. Then the magnetic fields of the loopscombine so that the coiled wire acts like a bar magnet. Thefield through the center of the coil is the sum of the fieldsfrom each turn of the wire. A coil of current-carrying wirethat produces a magnetic field is called a solenoid.

If you place a ferromagnetic material, such as an iron rod,inside the coil of a solenoid, the strength of the magnetic field increases. The magnetic field produced by the currentcauses the iron rod inside the coil of the solenoid to become a magnet. An electromagnet is a solenoid with a ferromag-netic core. Changing the current in an electromagnetcontrols the strength and direction of its magnetic field.You can also use the current to turn the magnetic field onand off. People use many devices every day, such as hairdryers, telephones, and doorbells, that utilize electromagnets.

Pole Pole

Current

Loop of wire

Solenoid

Current

A

B

Figure 9 The magnetic field lines around a solenoid are like those of a bar magnet.Applying Concepts Which of the poles is north?

Solenoids andElectromagnets

Making an Electromagnet

ObjectiveAfter completing this activity, studentswill be able to• predict how the number of turns of

wire affects the strength of theelectromagnet.

Skill Focus Observing, DrawingConclusions

Prep Time 20 minutes

Advance Prep Cut the wires inadvance and use a wire stripper or wire-cutting pliers to remove 2 cm ofinsulation from each end of the wires.


Safety Students should wear safetygoggles and be careful handling the coilof wire, as the wire may become hot.Students should open the switch whenthe electromagnet is not in use.

Expected Outcome Students willlearn that the strength of anelectromagnet, as indicated by thenumber of paper clips picked up, isdirectly related to the number of turnsin the coil of wire. More turns make the magnet stronger.

Analyze and Conclude1. The electromagnet became strongerwith more turns in the coil.2. The current in the coil produced amagnetic field around and through thenail. This caused the magnetic domainsin the nail to align, temporarily strength-ening the magnetic field of the nail.Logical, Group

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Answer to . . .

Figure 7 Use the right-hand rule, but point your thumb in the oppositedirection of the electron flow (whichwill be the direction of the current).

Figure 8 It would be deflected down.

Figure 9 The one on the left becausemagnetic field lines start at the northpole and end at the south pole.

Vibrating charges,flowing charges

in a current

Big Magnets Because the magnetic fieldsproduced by electromagnets can be madestronger by properly designing theelectromagnet, it is not surprising that thestrongest magnetic fields on Earth are producedby specially designed electromagnets. At theNational High Magnetic Field Laboratory

(NHMFL) at Florida State University inTallahassee, electromagnets have beendesigned that produce continuous magneticfields with strengths up to 50 teslas. These fields are about a million times stronger thanEarth’s magnetic field at Earth’s surface.

Facts and Figures


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The strength of an electromagnet depends on the current in thesolenoid, the number of loops in the coil in the solenoid, and the typeof ferromagnetic core. To increase the strength of an electromagnet,increase the current flowing through the solenoid. A greater currentproduces a stronger magnetic field. Increasing the number of turns,while keeping the same current, will also increase the field strength.Cores that are easily magnetized, such as “soft” iron, make strongerelectromagnets.

Electromagnetic DevicesElectromagnets can convert electrical energy into motion that can dowork. Electromagnetic devices such as galvanometers, electricmotors, and loudspeakers change electrical energy into mechanicalenergy. A galvanometer measures current in a wire through the deflec-tion of a solenoid in an external magnetic field. An electric motor usesa rotating electromagnet to turn an axle. A loudspeaker uses a solenoidto convert electrical signals into sound waves you can hear.

Galvanometers Figure 10 shows a galvanometer, a device thatuses a solenoid to measure small amounts of current. A solenoid isattached to a spring and is free to rotate about an iron core. The sole-noid is placed between the poles of two permanent magnets. When

there is a current in the solenoid’s coils, the resulting magnetic fieldattempts to align with the field of the permanent magnets. The

greater the current, the more the solenoid rotates, as shown bythe pointer on the scale. In an automobile fuel gauge, forexample, a sensor in the gas tank reduces the current as thegas level decreases. This causes the needle to rotate towardsthe “empty” mark.

What does the strength of an electromagnetdepend on?

For: Links on electromagnets

Visit: www.SciLinks.org

Web Code: ccn-2212

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3 2 1 0 1 2 34

5

ScalePointer

Wire

Magnet

Figure 10 A galvanometer uses anelectromagnet to move a pointer.One common application is in anautomobile gas gauge. The pointerindicates the amount of current inthe wire. The wire is connected toa sensor in the gas tank.

638 Chapter 21

Students may wonder how the magneticfield of a solenoid can be fairly simplewhen there are magnetic fields aroundeach segment of wire in the coil. Explainthat such fields are present, but that theycombine in such a way that the fieldoutside the solenoid is much weaker thaninside. The fields combine to effectivelyform a magnetic field that is similar tothat of a bar magnet.Logical

ElectromagneticDevices

Electromagnetic ForcePurpose Students observe themagnetic force exerted on a wirecarrying an electric current.

Materials insulated wire, a largehorseshoe magnet, a variable DC powersupply, 2 ring stands with clamps

Procedure Pass the wire through therings of the ring stands, so that itextends horizontally about 5–10 cmabove the table surface. Position themagnet on its side, so that the wirepasses between the magnet’s poles.Connect the wires to the power supplyand turn it on, increasing the currentuntil the wire is deflected. Turn off thepower, reverse the connections, andrepeat the demonstration.

Safety Use insulated wire. Followprocedures for electrical safety.

Expected Outcome Depending onthe orientation of the magnet, the wirewill be deflected either in toward themagnet’s center or away from it. Thedeflecting force is proportional to thecurrent in the wire and the strength ofthe magnetic field.Visual, Group

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

Earth’s Magnetic Field The magnetic fieldthat is generated within the liquid iron outercore is predominantly dipolar, as shown inFigure 4. Because of this, an analogy is oftendrawn with the dipolar field from a bar magnet,shown in Figure 2. However, it turns out that abetter analogy is with the dipolar field from a

solenoid, shown in Figure 9. The combinationof convection and Coriolis forces likely causesspiral gyres of flowing liquid iron in the outercore. These spirals are thought by geophysiciststo act like the coils of current-carrying wireshown in Figure 9, with the similar result of adipolar magnetic field passing through them.

Facts and Figures


Reviewing Concepts1. Besides a magnet, what can create

a magnetic field?

2. How is the magnetic field of anelectromagnet controlled?

3. How are solenoids and electromagnetsused in galvanometers, electric motors, and loudspeakers?

4. How does a ferromagnetic rod inside a solenoid affect the strength of an electromagnet?

Critical Thinking 5. Comparing and Contrasting What is the

effect of a magnetic field on a stationaryelectric charge? On a moving electric charge?

Magnetism 639

Insulators In Section 20.2 you learnedthat electric charge doesn’t flow easilythrough electrical insulators. Use this toexplain why a solenoid has insulated wires.

Electric Motors An electric motor is adevice that uses an electromagnet to turn anaxle. Figure 11 shows how an electric motorworks. In this figure, the wire is connected to abattery. An actual motor has many loops ofwire around a central iron core to make themotor stronger. In the motor of an electricappliance, the wire would be connected to anelectrical circuit in a building.

What makes a motor turn? When current flows through a loop ofwire, one side of the loop is pushed by the field of the permanentmagnet. The other side of the loop is pulled. These forces rotate theloop. If there were no commutator ring, the coil would come to rest.But as the loop turns, each C-shaped half of the commutator connectswith a different brush, reversing the current. The forces now changedirection, so the coil continues to rotate. As long as current flows,rotation continues.

Loudspeakers A loudspeaker contains a solenoid placed aroundone pole of a permanent magnet. The current in the wires enteringthe loudspeaker changes direction and increases or decreases toreproduce music, voices, or other sounds. The changing current pro-duces a changing magnetic field in the solenoid coil. The magneticforce exerted by the permanent magnet moves the coil back andforth. As the coil moves, it causes a thin membrane to vibrate, pro-ducing sound waves that match the original sound.

6. Applying Concepts Why is it a good idea to have the coil of a solenoid wound closely with many turns of wire?

7. Inferring What is the purpose of thecommutator in an electric motor?

8. Relating Cause and Effect What causesthe membrane in a loudspeaker to vibrate?

Commutator

Brush

Loop ofwire

Direction ofrotation

Current

Figure 11 A battery suppliescurrent to a loop of wire throughthe commutator. As thecommutator turns, the directionof current switches back andforth. As a result, the coil’smagnetic field keeps switchingdirection, and this turns the coilabout an axle. Predicting What would happenif you reversed the positive and negative connections on the battery?

Build Science SkillsApplying Concepts Have studentsread the paragraphs on electric motorsand help them apply what they alreadyknow about work, different forms ofenergy, and energy conservation. Ask,What forms of energy are shown forthe electric motor in Figure 11?(Chemical energy, electrical energy, andmechanical energy) Ask, What energytransformations take place whenoperating the motor? (Chemical energyin the battery is converted to electricalenergy. Electrical energy interacts with the magnetic field to do work, and so istransformed into the kinetic energy of the rotating wire loop and into any workthe motor does.)Logical

ASSESSEvaluate UnderstandingAsk students to list three examples ofdevices that use electromagnetic forces(at least one of which is not given in thesection). Have students explain whateach device does and how electricityand magnetism interact in the device.(Could be an electric bell, relay switch, or microphone)

ReteachUse Figures 7 and 9 to review thedirection of magnetic fields produced by electric currents.

The strength of the electromagnetdepends upon the current in thesolenoid. Insulated wires make itpossible to direct the current throughseveral tightly wound loops, enhancingthe strength of the magnetic field. Theinsulated wire prevents a short circuitbetween adjacent coils.


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Magnetism 639

5. A magnetic field doesn’t affect a stationarycharge. A magnetic field deflects a movingcharge in a direction perpendicular to boththe field and the velocity.6. It produces a more uniform field andincreases its strength.7. The commutator reverses the current in theelectromagnet, reversing the magnetic field ofthe electromagnet, and enabling the axle toturn continuously in one direction.8. The interaction of the magnetic field of thepermanent magnet with the changing field of the electromagnet


1. A moving electric charge can create amagnetic field.2. It can be turned on and off. Its strength and direction can be controlled by controllingthe current.3. They change electrical energy intomechanical energy.4. It makes the magnetic field much stronger.When current flows through the coil, it createsa magnetic field that magnetizes theferromagnetic rod.

Answer to . . .

Figure 11 The motor’s axle wouldspin in the opposite direction.

It depends on thestrength of the current,

the number of coils of wire, and thetype of ferromagnetic core.


Peeking Inside the Human BodyMagnetic Resonance Imaging (MRI) is used bydoctors to create more detailed images of the humanbody than are possible with X-rays.

Body tissues vary in their concentration of hydrogen atoms. Fathas a high concentration, as do tissues containing water, becauseof the hydrogen in H2O. The concentration of hydrogen atomsin bone is very low. MRI reveals these differences in great detail,with fat and fluids (including blood) showing up as bright areasand bone as dark areas. MRI scans can even depict thebrain. It produces images of suchdetail that they are used byresearchers studying how the brainworks, as well as by doctorsinvestigating diseases.

Creating an MRI imageThe scanner uses three magneticfields to read data up and downand along slices of the body.This produces an image that isviewed and interpreted bydoctors and radiographers.

Inside the scannerThe varying magnetic fields canmake images of “slices” throughthe body in different planes. Themain magnet produces a magneticfield as much as 30,000 timesstronger than that of Earth.

Head-to-toevariation

Top-to-bottomvariation

Left-to-rightvariation

Main magnet This powerfulmagnet immerses the patientin a stable, intense magnetic

field—the other three magnetscreate a variable field.

Radio-frequencysource

Motorizedbed

Head-to-toefield magnets

Left-to-rightfield magnets

Top-to-bottomfield magnets

Each scan can takeseveral minutes, sothe patient mustlie very still.

640 Chapter 21

640 Chapter 21

Peeking Inside the Human BodyBackgroundMRI is an example of a procedure calledtomography, where many images of the body are combined to give a com-posite view. MRI uses nuclear magneticresonance, or NMR, to obtain informationfrom hydrogen atoms in the body. NMR was discovered in 1946, and wasoriginally used to identify hydrocarbonmolecules. In the 1970s, the techniquewas combined with computers toproduce images of tissues in the body.

Build Science SkillsUsing Analogies

Purpose Students will simulate how applied magnetic fields can disrupt the magnetic fields of atoms.

Materials a short pencil (about 5 cm long), a cardboard disk (7 cm wide), a steel thumbtack, a bar magnet, paper


Procedure Insert the thumbtack intothe eraser end of the pencil, and punchthe pencil through the center of thecardboard disk to make a “top” that canspin. Make sure the cardboard does notslip along the surface of the pencil. Placethe top on a piece of paper to preventmarking the table. Spin the top with thethumbtack side upward, making sure thatthe top is neither too stable or unstablewhile spinning. Spin the top again, andplace one end of the magnet about 2 cmto the side of the thumbtack. Repeat thetest, placing the magnet slightly closer,until the spinning top is deflected by themagnet. Make sure that the top is notsimply pulled into contact with themagnet. Remove the magnet while thetop is still spinning and note its behavior.

Expected Outcome Because the top isfairly stable while spinning, it is analogousto the spinning hydrogen atoms in thebody. The alignment and deflection ofthese atoms by the magnetic fields isanalogous to the deflection of the top bythe magnet. By observing a large-scalemodel of an atomic process, students canvisualize the atomic process more clearly.Visual

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� Items such as jewelry, watches, coins, keys,and credit cards must be removed beforebeginning an MRI. Research inthe library or on the Internetwhy these items interfere with the procedure or pose a risk to the patient.

� Take a Discovery Channel VideoField Trip by watching“Magnetic Viewpoints.”

Going Further

How MRI worksMRI affects the nuclei of hydrogen atoms in the body. Thenuclei are made to absorb and then re-emit energy by acombination of strong magnetic fields and radio wavepulses. The emitted signals are then used to mapconcentrations of hydrogen in the body.

Hydrogen nucleus Spin axis

MRI spinal cord scanThe bright red patch

here indicates a tumoron the dark green

spinal cord. While bone tissue

itself is not visible, the vertebrae can beseen because of the

marrow they contain.

1. Random axes The spins of hydrogennuclei point inrandom directions.Like tiny magnets,each nucleus has anorth pole and asouth pole.

2. Aligning axes When the main MRImagnet is switchedon, the magnetic fieldmakes the spins ofhydrogen nucleimostly point in thesame direction.

Pulse of radio wavesfrom scanner

Spin axes realignwith magnetic field.

Spin axes change direction. Radio waves

emitted by nuclei.

Spinal cord tumorhighlighted by MRI

Spin axes line up.

4. Realigning axesWhen the pulse stops,hydrogen nuclei emitradio waves as theyreturn to alignmentwith the main magneticfield. With the lessermagnets switched on asnecessary to alter themagnetic field at a locallevel, these waves arepicked up by thescanner, which buildsup an image ofdifferent tissues.

3. Wobbling axes A pulse of radiowaves from the MRIscanner knocks thehydrogen nuclei outof alignment.

Magnetism 641

Video Field Trip

Going FurtherStudent research should indicate thatmost of these items can be attracted bythe powerful magnets in the MRIscanner, and this attraction could resultin injury to the patient or damage to the machine. Credit cards and otheridentification with magnetic strips are indanger of being erased by the magneticfield. Watches with mechanical workscan become permanently magnetized,and so cease to keep correct time. Theelectronics in digital watches may alsobe temporarily or permanently affectedby strong magnetic fields.Verbal, Logical

Magnetism 641

After students have viewed the Video Field Trip,ask them the following questions: What is thepurpose of magnetic resonance imaging (MRI)?(Student answers may include recording images ofinternal body organs, detecting tumors, and obser-ving how the brain works.) How does MRI work?(The patient is bathed in a strong magnetic field thatcauses some nuclei in the body’s atoms to line up likespinning tops. A radio pulse knocks the nuclei out of

alignment, and when the pulse stops the nuclei emita signal as they line up again. A computer analyzesthe signal to form an image.) What advantagedoes MRI have over X-rays in the detection ofcancers? (It can detect some kinds of cancer earlierthan X-rays can, and MRI is safer to use than X-rays.) Give an example of how MRI is used tostudy how the brain works. (Student answers may include that MRI images show the area of thebrain that responds to a sensation such as pain in a particular part of the body. The images can be used to study medical disorders such as epilepsy and schizophrenia.)

Video Field Trip

Magnetic Viewpoints

ttomnets


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

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Reading Focus

1

Section 21.3

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

Math Support, Section 21.3 and Math Skill: Calculating Voltage

• Transparencies, Section 21.3

Technology• Probeware Lab Manual, Lab 9• Interactive Textbook, 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

Generating Alternating 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

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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 electrical energy produced by power plants is transmitted throughpower lines at very high voltages. These voltages are too high to be usedsafely 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 can beused safely in your 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 electrical energyduring a 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

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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

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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

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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


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? (The voltage 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

ASSESSEvaluate UnderstandingAsk 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.


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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


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


section 21.1 21.1 magnets and magnetic fields

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