An Introduction to Electromagnetism · 2013. 5. 31. · The book’s figures were created by Kim Alberto, Linda Olliver, and Tracey Shipley, from originals by Larry Schafer. The NSTA

An Introductionto ElectromagnetismBy Larry E. Schafer

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Charging Ahead: An Introduction to Electromagnetism

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ISBN 0-87355-188-5

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Acknowledgments ....................................................................................................... ivOverview ....................................................................................................................... vA Learning Map on Electricity and Magnetism ...................................................... viiiGuide to Relevant National Science Education Content Standards ..................... xiisciLINKS......................................................................................................................... xiii

Activity l: A Bonus from Electrical Flow—Magnetism

Student Worksheet ..................................................................................................... 1 Teacher’s Guide to Activity 1 ................................................................................... 9

Activity 2: Coils and Electromagnets


Activity 3: Making an Electric Motor—Electromagnetism in Action


Activity 4: Motion, Magnetism, and the Production ofElectricity


Glossary .................................................................................................................. 65

Contents


NATIONAL SCIENCE TEACHERS ASSOCIATIONiv

Acknowledgments

Larry E. Schafer, the author of Charging Ahead: An Introduction to Electromagnet-ism, teaches physical science and elementary science methods courses at SyracuseUniversity, where he has also chaired teaching and leadership programs. His previ-ous work for the National Science Teachers Association (NSTA) was the student-activity book Taking Charge: An Introduction to Electricity (1992, 2000). He hasdirected many funded projects designed to help teachers improve the science edu-cation in their schools, has worked with the New York State Education Departmentto create a statewide system of elementary science mentors, and has co-authoredbooks for middle school science teachers and their students.

The book’s reviewers were Chris Emery, a physics teacher at Amherst RegionalHigh School, Amherst, Massachusetts; Dale Rosene, a science teacher at MarshallMiddle School in Marshall, Michigan; Daryl Taylor, a physics teacher atWilliamstown High School in Williamstown, New Jersey; and Ted Willard, seniorprogram associate at the American Association for the Advancement of Science’sProject 2061.

The activities in the book were field-tested by Mark M. Buesing and SuzanneTorrence, both physics teachers at Libertyville High School, Libertyville, Illinois,and Jay Zimmerman, a physics teacher at Brookfield Center High School, Brookfield,Wisconsin.

The book’s figures were created by Kim Alberto, Linda Olliver, and Tracey Shipley,from originals by Larry Schafer.

The NSTA project editors for Charging Ahead: An Introduction to Electromagnet-ism were Judy Cusick and Anne Early. Linda Olliver designed the book and thecover. Catherine Lorrain-Hale coordinated production and printing of the book.


vCHARGING AHEAD: AN INTRODUCTION TO ELECTROMAGNETISM

Overview

C harging Ahead: An Introduction to Electromagnetism is a set ofhands-on activities designed to help teachers introducemiddle-level and general high school students to electro-magnetism, one of the most fascinating and life-changingphenomenon humankind has witnessed. In 1820, Hans Chris-

tian Oersted, a Danish physicist and schoolteacher, discovered that an elec-trical current produces magnetism. Little did he know that his discoverywould have an impact on modern day lives in profound ways: that electri-cal motors would start cars, turn CDs and disk drives, run can openers,food processors, refrigerators, and clocks, operate pumps for maintaininglife support, and run nearly all of the machines that produce and manufac-ture the many goods upon which we rely. Little did he know that this con-nection between electricity and magnetism would lead others (Michael Fara-day and Joseph Henry) to discover ways of creating electricity from motionand magnetism and in so doing make it possible for human beings the worldover to move about, heat and light their environments, and instantly andconveniently communicate.

Charging Ahead uses readily available materials to introduce studentsto electromagnetism, to the factors that determine the magnetic strength ofelectrical coils, to the application of electromagnetism in the construction ofan electrical motor, and to the production of electricity through the con-struction of a generator. Throughout Charging Ahead, students are introducedto historical perspectives and to technological applications (circuit break-ers, mag-lev trains, superconducting generators, etc.) of electromagnetism.

Fitting Charging Ahead into Your CurriculumCharging Ahead is a companion guide to NSTA’s Taking Charge: An Intro-

duction to Electricity. While students would benefit from experiencing theactivities in Taking Charge, it is not necessary that students complete TakingCharge before attempting the activities in this book. Students will neverthe-less need a basic understanding of electrical circuits to understand the ideaspresented in Charging Ahead.

Topic: electromagnetismGo To: www.scilinks.orgCode: CH001

Topic: Hans ChristianOersted

Go To: www.scilinks.orgCode: CH002


NATIONAL SCIENCE TEACHERS ASSOCIATIONvi

Key relationships are developed from what students experience in theactivities. Abstract formulations and mathematical descriptions, althoughimportant, are minimized in Charging Ahead. The activities therefore serveas “end points” for middle school students and “starting points” for highschool students who are on the path toward understanding abstract formu-lations of electromagnetism and electromagnetic induction.

Charging Ahead addresses the National Science Education Standards in anumber of ways. Students learn about energy forms and energy transfer,engineering design and troubleshooting, and science-technology relation-ships. Students are challenged to solve problems and to think critically andcreatively. See p. xii for a Guide to Relevant National Science EducationContent Standards.

OrganizationThe activities in Charging Ahead use an inquiry approach to guide stu-

dent understanding of the concept goals. Each student activity includes anintroduction, a description of the materials needed, a statement of whatstudents will learn, and procedures to follow. None of the activities require“high tech” equipment. Wires, flashlight batteries and bulbs, magnets, andmagnetic compasses are the basic materials used in the activities.

The procedure section of each activity is designed so that students canperform the activity without the teacher’s constant involvement and direc-tion. The procedure section presents students with problems to solve, ques-tions to answer, and tasks to accomplish. It should be clear that studentswill occasionally face difficulty as they work through the procedures. Un-derlying the design of these activities is the idea that students will moremeaningfully understand the concepts and relationships if they are chal-lenged to figure some things out for themselves.

Each activity is accompanied by a teacher’s guide to the activity. Theguide is written so that the teacher acquires a brief overview of what willhappen in the activity, directions for the construction of equipment and/orthe selection of materials, time management recommendations, cautionarynotes, ideas for extended activities, and answers to questions.

Assessment MethodsThe teacher can use both formative and summative assessment with Charg-

ing Ahead. The answers that students give to the questions in each activity pro-vide a formative record of their thinking and learning—showing students andthe teacher what students understand, what is still fuzzy or missing, andwhether students can now use what they know. The suggestions for furtherstudy at the end of each activity can be used to extend—and then test—stu-


viiCHARGING AHEAD: AN INTRODUCTION TO ELECTROMAGNETISM

dents’ learning. These extensions are authentic applications of the conceptsstudents have just investigated. You may wish to build an assessment rubricfor one or more of the extensions and use it as a summative assessment of yourstudents’ mastery of electromagnetism concepts.

Special ConsiderationsThe first and second activities are fairly straightforward. They call on

students to examine the relationship between electrical flow and magnetismand investigate how to increase the magnetic forces created by a current-carrying wire. The third and fourth activities challenge students to build anelectric motor and an electric generator. Electrical motors and generators builtfrom readily available materials are somewhat temperamental. While eachdesign has been thoroughly tested (75 percent of sixth graders had an electri-cal motor going in 30 minutes), neither students nor teachers should expectsuccess without some “troubleshooting.” Success can be greatly improved byusing the recommended materials and by carefully following the directionsand suggestions. The need to “troubleshoot” to get things to work should betaken as an opportunity to help students value the creative and persistentwork done by engineers who design and debug the devices that reliably work.

Initial construction of motor and generator parts will take some time.Students can help with the construction of those parts. Once the parts areconstructed, they can be used repeatedly by different classes of students.

As a consequence of taking part in electricity activities, some studentsmay become very interested in motors, generators, and other electrical de-vices. They may be inclined to examine these devices on their own in back-yards and basements. The investigation of household electrical devices canlead to serious injury. Therefore, please warn students that they should notinvestigate electrical devices without the help and supervision of a knowl-edgeable adult.

The activities in Charging Ahead are safe since small currents and volt-ages are used. Short circuits are sometimes used in the activities and thesecircuits can produce hot wires. Student should be warned to keep shortcircuits on only for short periods of time (a few seconds). In such shortperiods of time, the wires wil not significantly heat up nor will batteriesquickly wear out.

The four Charging Ahead activities build on each other, connecting sci-ence content as described in the Atlas of Science Literacy map on p. xi. Youcan compare the concept goals at the start of each activity with your owninstructional goals to determine which activity to use.


NATIONAL SCIENCE TEACHERS ASSOCIATIONviii

What Is This Map?The map on page xi is a way of considering and organizing science

content standards. The map uses the learning goals (or parts of them) of theAmerican Association for the Advancement of Science’s Science for All Ameri-cans (1989) and Benchmarks for Science Literacy (1993). Content standards fromthe National Science Education Standards (NSES) (National Research Council1996) overlap nearly completely with those goals. Arrows connecting thegoals imply that understanding one goal contributes to the understandingof another. Goals that deal with the same idea are organized into vertical“strands,” with more sophisticated goals above simpler ones. Descriptivelabels for the strands appear at the bottom of the map.

The science content on the map lists the ideas relevant to students’ un-derstanding of electricity and magnetism that are both important and learn-able. Your students may well learn more, but will learn better after the basicscience literacy described on the map has been achieved. This map tracesthe ideal development of electricity and magnetism knowledge from kin-dergarten to twelfth grade. Horizontal lines represent the level of gradeappropriateness.

Charging Ahead provides instructional methods that primarily achievelearning goals for the map strand labeled “electromagnetic interactions.”The map suggests what ideas students must have before trying to examinethe relationship between electricity and magnetism. Unit activities as pre-sented may not be sufficient for students to become proficient with some ofthe basic or extended ideas in the map strand; checking the progress of yourstudents along the way will help you see how to adapt instruction. Unitactivities may also touch on concepts outside of what the various sciencestandards consider essential for basic science literacy. Therefore, you maydecide to focus activities to make sure your core learning goals are achieved.

A Learning Mapon Electricity andMagnetism


ixCHARGING AHEAD: AN INTRODUCTION TO ELECTROMAGNETISM

How Can I Use the Map?An Atlas map is designed to help clarify the context of the benchmark

or standard: where it comes from, where it leads, and how it relates to otherstandards. With the map as a guide, you can make sure your students haveexperience with the prerequisite learning, and you can actively draw stu-dents’ attention to related content—getting their framework for learningready!

In addition to using the map to plan instruction, you may wish to an-notate the map with common student misconceptions to address or com-mon accurate conceptions that you can invoke to dispel these misconcep-tions. Motivating questions that have worked for you, and phenomena toillustrate points, may also find a place on your annotated map.

The map can help you connect your instruction to your state sciencestandards. As of this writing, 49 of the 50 states in the United States havedeveloped their own standards, most modeled directly on the National Sci-ence Education Standards or the Benchmarks for Science Literacy. The correla-tion between the NSES and Benchmarks in science content is nearly 100 per-cent. So there is a unity of purpose and direction, if not quite a commonlanguage. Fortunately, the National Science Foundation, the Council of ChiefState School Officers, and other groups have funded and developed websitesto guide educators in correlating these national standards with their stategoals (e.g., the ExplorAsource website at www.explorasource.com/educator. Thewebsites of many state departments of education also provide this correla-tion service for educators.

The map can also provide a way to think about the design of studentassessment . The goal of your summative assessment is to determine whetherstudents can apply their learning to new situations—to show you, and toshow themselves, that they have a new tool for understanding.

Are There Other Maps?These maps are being copublished by AAAS and NSTA in a new two-

volume work, Atlas of Science Literacy. The complete Atlas will contain nearly100 similar maps on the major elementary and secondary basic science top-ics: gravity, cell functions, laws of motion, chemical reactions, ratios andproportionality, and more.

The connected learning goals displayed in Charging Ahead are only partof a map that is—at the time of this printing—subject to revision. As addi-tional maps are developed and tested, they will be linked to the ChargingAhead page on the NSTA website and added to successive editions of Charg-ing Ahead.


NATIONAL SCIENCE TEACHERS ASSOCIATIONx

Map, Assessment, and the ConstructivistProcess

Use the map as an aid to your constructivist teaching methods, allow-ing students to recognize and integrate concepts—either those never learnedor those incompletely remembered—into the big picture of why these con-cepts are useful to know.

Before you undertake any of the four activities in this book, it is impor-tant to know whether your students have mastered the principles in themap that lead to their current grade level. You may, for example, be sur-prised to learn that some of your high school juniors do not really under-stand that “magnets can be used to make some things move without beingtouched,” a concept that, according to the strand map, should be masteredby grade three. Students may also have a mix of true and false understand-ings about electricity and magnetism as they begin the Charging Ahead ac-tivities. It may be wise to ascertain—perhaps by having each student do a“web” of everything he or she can think of about the term “magnetism”and reviewing those webs—to ensure that all students are starting with thebasic information they need to build on in order to understand the conceptspresented in these activities.


1CHARGING AHEAD: AN INTRODUCTION TO ELECTROMAGNETISM

BackgroundWhen you create a closed circuit with a battery, electrons flow through

the wires, the bulb lights up and gets hot, and the wires and battery warmup. Besides the chemical reactions going on inside the battery, is anythingelse happening? It is hard to tell unless you can use some detection device.In this investigation, you will use a compass to detect magnetism. You willuse the compass to investigate the relationship between electrical flow andany magnetism that is produced from that flow.

Concept Goalsn A current-carrying wire produces a magnetic effect (deflects a compass

needle) in the region around the wire. That magnetic effect is calledelectromagnetism.

n Electrons move along a wire from the negative end of the battery to thepositive end of the battery.

n The direction of the electron flow in a wire determines the direction ofthe magnetic field around the wire.

n The strength of the magnetic influence (field) around a wire becomes lessat greater distances from the wire.

n Magnetic fields (regions of magnetic influence) have direction and“strength.”

n The direction of the magnetic field at a particular point in space is thedirection a compass needle would point if the compass were located atthat point.

A c t i v i t y 1S t u d e n t W o r k s h e e t

A Bonus fromElectrical Flow—Magnetism

Topic: electrical circuitGo To: www.scilinks.orgCode: CH003

Topic: magnetic effectGo To: www.scilinks.orgCode: CH004


NATIONAL SCIENCE TEACHERS ASSOCIATION2

n A left hand is an effective model for showing the relationship betweenthe direction of the magnetic field and the direction of electron flow.

Procedure

1 If you have not used a compass recently, you may want to refreshyour memory. The colored or pointed end of the needle usually pointsapproximately toward the Earth’s geographic north. Hold the compassout in front of you, away from any metal objects, and note that the coloredor pointed end of the needle always points in the same direction, evenwhen you rotate the base or case of the compass.

Move your compass close to an iron or steel object and notice that thecompass needle is attracted to the object. It is important, therefore, to keepthe compass away from iron or steel objects when you are using it to detectmagnetism from other objects. Iron or steel under the desktops can influ-ence the direction in which the compass needle points.

The compass needle is nothing more than a small, light magnet thateasily spins about its center when it interacts with other magnets. Thecompass needle is attracted to iron and steel objects because the needleitself causes those objects to become temporarily magnetized.

2 In 1820, Hans Christian Oersted, a Danish physicist and schoolteacher,made the observation you are about to make. His discovery set the stage forthe development of many modern conveniences, including electrical mo-

Compass

Wire on top of compass

Needle position when wireis not connectedto battery

Battery

Figure 1.1

MaterialsFor each group:

n one ìDî battery (drycell) and one batteryholder

n one directional,magnetic compasswith a needle that isfree to move easilywithout sticking

n one 60-cm piece of#24 enamel-coated(insulated) wire (withsanded ends) or #22plastic-coated wire(with stripped ends)

tors and the generation of electricityfrom motion.

a Place the compass on the table atleast 15 cm away from the bat-tery. Connect one end of the wireto the battery. Place the wire in astraight line directly over thecompass and in line with theneedle. Briefly touch (no morethan two seconds) the other endof the wire to the battery and ob-serve what happens to the com-pass needle.

Draw an arrow on the com-pass illustration in Figure 1.1 toshow the direction of the needle



Compass

Wire beneath compass

Needle position when wireis not connectedto battery

Battery

when a current-carrying wire ison top of the compass. Thepointed end of the arrow repre-sents the “north-seeking” end ofthe needle. Also draw an arrowon the wire showing the directionin which the electrons are mov-ing in the wire. Recall that elec-trons move along a wire from thenegative end of the battery to thepositive end of the battery.

b Repeat the above activity, but thistime place the wire under thecompass and align the wire withthe compass needle. Draw an ar-row on the compass drawing(Figure 1.2) to record the direc-tion of the needle when a current-carrying wire is under the compass. Also, draw an arrow showing thedirection of electron flow in the wire. Remember to keep the electricityflowing in the wire for only two seconds.

c Note the direction in which the needle moved (“deflected”) in 2b above.With the wire under the compass and without changing the positions ofthe compass or the wire, what can you do to make the deflected needlepoint in the opposite direction? Describe your solution in the spacebelow.

d It should be clear that a current-carrying wire is somehow creating amagnetic influence in the space around it. What can you do to find outhow the “strength” of that influence changes with different distancesfrom the wire? Describe your solution, your conclusion about distanceand “strength,” and how your observations support your conclusion.

Figure 1.2



e A magnetic field is a region of space in which there is a magnetic influ-ence. There is a magnetic field in the space around a magnet. A compasscan detect a magnetic field if the field is strong enough. Because thecompass needle is deflected in the region around the current-carryingwire, you can conclude that there is_____________________________________________________________________around a current-carryingwire.

f Magnetic fields have both “strength” and direction at each point in space.The direction is the direction that a compass will point if it is held at thatpoint in space. The magnetic field both above and below a current-carry-ing wire is: (circle 1 or 2)

1 in line with the wire.

2 across the wire.g To change the direction of the magnetic field above a wire, you would

have to change the __________________ of the electron flow in the wire.Without moving the wire above the compass, you can do this by

______________________________________________________.

h The magnetic field around a current-carrying wire is “stronger”: (circle 1 or 2)

1 closer to the wire.

2 farther away from the wire.

3 You can use your left hand as a model of the relationship between thedirection of the electron flow and the direction of the magnetic field (thedirection the compass would point) created by that flow.



A Left-hand ModelPretend to grasp the wire with your left hand. Wrap your fingers around

the imaginary wire in such a way that your left thumb points in the direc-tion of electron flow (Figure 1.3). Your fingers will then wrap around thewire in the direction of the magnetic field. You can rotate your hand aroundthe wire to see which way your fingers point at any position around thewire (Figure 1.4).

Practice using the left-hand model by answering the following ques-tions associated with Figure 1.5. (circle the correct answer)

a The magnetic field directly above the wire at “a” would point:

1 to the left.

2 to the right.

3 straight up out of the page.

4 straight down into the page.

Direction ofmagnetic field

Direction ofelectron flow

Left hand

Figure 1.3



b The magnetic field directly belowthe wire at “b” would point:

1 to the left.

2 to the right.



c The magnetic field directly to theleft of the wire (neither above norbelow the wire) at “c” wouldpoint:

1 to the left.

2 to the right.



Wire

Field below wire?

Field above wire?

Field to theright of wire?

Field to theleft of wire?

b

c d

Electron flow in wire

a

Figure 1.5

Direction ofelectron flow

Direction of magnetic field

Left hand

Figure 1.4



Compasses

End of wirecoming out ofpage; electrons flowalong wire, upand out of pageCompasses

Figure 1.6d The magnetic field directly to theright of the wire (neither abovenor below the wire) at “d” wouldpoint:

1 to the left.

2 to the right.



e Observe Figure 1.6 and assumethat the dot in the center is theend of a wire that is coming outof the page. Further assume thatelectrons are flowing along thatwire out of the page directly up-ward from the page. Use yourleft-hand model to determine thedirection of the compass needle(direction of the magnetic field) at each of the compass points aroundthe wire. Draw the compass needles in the four compasses and use thepointed head of the arrow as the “north-seeking” end of the compassneedle.





Caution

Short circuits arecreated when thewire is connected tothe ends of the bat-tery. The short circuitwill heat up the wireand quickly weardown the battery.Caution the studentsto maintain a shortcircuit for only acouple of seconds ata time. They can dothis by connectingone end of the wireto the battery andbriefly touching theother end of the wireto the battery.

T e a c h e r ’ s G u i d e T oA c t i v i t y 1

A Bonus fromElectrical Flow—Magnetism

What is happening?In this activity, students dis-

cover that a current-carrying wireproduces a magnetic field around it.They use a compass to detect thismagnetic field, and they observethat the direction of the field isacross the direction of the electronflow. Furthermore, the studentslearn that the field is “stronger”closer to the wire. In addition, thestudents learn that the direction ofthe magnetic field at a point in spaceis described as the direction thenorth-seeking end of a compasswould point. Students can use theirleft hands to model the relationshipbetween the direction of the electronflow and the direction of the mag-netic field it produces. Studentspractice applying the model to dif-ferent examples.

Time managementOne class period (40–60 minutes)

should be enough time to completethe activity and discuss the results.

PreparationCollect the materials listed on

page 2. Make sure that the batteriesare not dead, that the compasseswork, and that the ends of the wiresare stripped (plastic-coated wire) orsanded (enamel-coated wire). If thestudents have not worked withenamel-coated wire, show them howto use sand paper to sand off theenamel from the ends of the wires.

Students may find that their com-passes point in different directionswithout any current-carrying wires ormagnetic materials nearby. Why don’tall the compasses point north? Whydo the compasses point in different



Wire on top ofcompass

Drawnneedle

Electron flow

Electron flow

Compass

Battery

Figure 1.7

directions when they are movedaround on the desks or in the room?Often the iron or steel in desks, filingcabinets, walls, etc. influences the di-rection of the compasses. For an ac-curate “north reading,” a compassmust be away from all iron and steelobjects.

Suggestions for furtherstudy

Challenge groups to get togetherto see what happens when two cur-rent-carrying wires are held in linewith a compass needle. Studentsshould discover that when bothwires carry electrons in the same di-rection over and in line with a com-pass needle, the needle deflection isgreater than when just one wire isused. Students also should discover

that when the wires carry electronsin opposite directions over and in linewith the compass needle, the needledeflection is less because the mag-netic fields exert forces on the needlein opposite directions.

Students have studied direct cur-rent electricity where the electronsmove in one direction in the conduc-tor. Alternating current electricity isused in our homes. The electrons inthe alternating currents switch direc-tions 60 times each second. If this elec-tron jiggling is going on in the wiresin our homes, what is happening tothe magnetic field surrounding thosewires? Have students consider thisquestion and guide them to under-stand that the magnetic field aroundthe wires in our homes must be jig-gling or changing directions 60 timeseach second. When held near a cur-rent-carrying house wire, a typicalcompass needle does not show deflec-tion. The inertia of the needle preventsthe needle from changing directions60 times each second. Just as theneedle begins to move in one direc-tion, it is forced in the opposite direc-tion.

Answers to questions found withinProcedure on pages 2ñ7.

2a. Draw an arrow on the compass inFigure 1.1 to show the direction ofthe needle when a current-carryingwire is on top of the compass. Alsodraw an arrow showing the directionof electron flow in the wire.

One answer is shown in Figure1.7. If the terminals of the battery



Figure 1.8

wire and compass are closer. As-suming that more deflectionmeans a “stronger” interaction,the conclusion is that the mag-netic influence is “stronger”closer to the wire.

2e. When a compass needle is deflectedin the region around a current-car-rying wire, you can conclude thatthere is a magnetic field around thewire.

2f. The magnetic field both above andbelow a current-carrying wire is: (1)in line with the wire or (2) across thewire?

(2) across the wire.

2g. To change the direction of themagnetic field above a wire, youwould have to change the direction

Compass

Electron flow

Electron flow

Drawnneedle

Wire beneathcompass

Battery

were reversed, the drawn arrowwould be deflected to the otherside of the wire.

2b. Draw an arrow on the compass inFigure 1.2 to record the direction ofthe needle when a current-carryingwire is under the compass. Also,draw an arrow showing the directionof electron flow in the wire.

One answer is shown in Figure1.8. If the terminals of the batterywere reversed, the drawn arrowwould be deflected to the otherside of the wire.

2c. Note the direction in which the needlemoved (“deflected”) in 2b above. Withthe wire under the compass and with-out changing the positions of the com-pass or the wire, what can you do tomake the deflected needle point in theopposite direction?

The solution is to keep the wiresand compass the same, butswitch wires on the terminals ofthe battery. This sends the elec-trons in the opposite directionthrough the wire.

2d. What can you do to find out how the“strength” of the magnetic influencearound the current-carrying wirechanges at different distances fromthe wire? Describe your solution,your conclusion about distance and“strength,” and how your observa-tions support your conclusion.

Change the distance between thecurrent-carrying wire and com-pass. Note that there is greater de-flection in the compass when the



of the electron flow in the wire. With-out moving the wire above the com-pass, you can do this by switchingthe ends of the wire on the termi-nals of the battery.

2h. The magnetic field around a current-carrying wire is “stronger”: (1)closer to the wire or (2) farther awayfrom the wire.

(1) closer to the wire.

3a. The magnetic field directly above thewire at “a” would point: (1) to theleft, (2) to the right, (3) straight upout of the page, or (4) straight downinto the page.

(1) to the left.

3b. The magnetic field directly below thewire at “b” would point: (1) to theleft, (2) to the right, (3) straight up

out of the page, or (4) straight downinto the page.

(2) to the right.

3c. The magnetic field directly to the leftof the wire (neither above nor belowthe wire) at “c” would point: (1) tothe left, (2) to the right, (3) straightup out of the page, or (4) straightdown into the page.

(4) straight down into the page.

3d. The magnetic field directly to theright of the wire (neither above norbelow the wire) at “d” would point:(1) to the left, (2) to the right, (3)straight up out of the page, or (4)straight down into the page.

(3) straight up out of the page.

3e. Observe Figure 1.6 and assume thatthe dot in the center is the end of awire that is coming out of the page andthat electrons are flowing along thatwire directly upward from the page.Use the left-hand model to determinethe direction of the compass needle ateach of the compass points around thewire. Draw the compass needles in thecompasses; use the pointed head of thearrow as the “north-seeking” end ofthe compass needle.

The compass directions areshown in Figure 1.9.

Compasses

Compasses

End of wirecoming out ofpage; electrons flowalong wire, up andout of page

Figure 1.9

Note: The left-hand model is the sameas the right-hand rule found in physicstextbooks. Here, the direction of electronflow is used. The right-hand rule usescurrent direction (positive charge flow).


Documents

An Introduction to Electromagnetism · 2013. 5. 31. · The book’s figures were created by Kim Alberto, Linda Olliver, and Tracey Shipley, from originals by Larry Schafer. The NSTA