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ELECTROMAGNETISM

Teacher's Guide

This teacher's guide is designedfor use with the Electromagnetismseries of programs produced byTVOntario, the television serviceof The Ontario EducationalCommunications Authority. Forbroadcast dates, consult theTVOntario schedule in SchoolBroadcasts, which is published inSeptember and distributed to allteachers in Ontario. The series isavailable on videotape to educa-tional institutions and non-profitorganizations. For orderinginformation, see inside back cover.

Canadian Cataloguing in Publication Data

Rawlings, Charles.

Electromagnetism. Teacher's guide

To be used with the television program, Electromagnetism.

Bibliography: p.

ISBN 0-88944-123-5

1. Electromagnetism (Television program)

2. Electromagnetism - Study and teaching (Secondary)

1. Brown, Eric. 11. TVOntario. 111. Title.

QC760.R38 1988 530.1'41 C88-099608-0

The Guide

Writers: Eric Brown and CharlesRawlings

Editor: Elaine Aboud

Reviewer: George Landry

Designer: Roswita Busskamp

The Series

Producer: David Chamberlain

Writer: William Conrad

Project Officer: John Amadio

Consultants: Eric BrownCharles Rawlings

Animation: Cinescan

C Copyright 1988 by The OntarioEducational CommunicationsAuthority.All rights reserved. Printed in Canada. 2723187

INTRODUCTION

Both magnetic and electric forceswere observed in ancient times,but it was not until the nineteenthcentury that scientists came to seethese two seemingly unrelatedphenomena as manifestations ofthe same force. This recognition ofelectromagnetic force representedthe first step towards unification ofthe fundamental forces of nature.

As the end of the twentieth centuryapproaches, we know now thatelectromagnetism is all around us.Our civilization would not be thesame without its compasses,electromagnets, electric motors,generators, and transformers. Andyet these employ magnetic fieldswhose strength is insignificantwhen compared to some magneticfields observed by astronomers.For example, it appears that thecore of our galaxy is the source ofa tremendous magnetic fieldarcing hundreds of light years outfrom the galactic plane.

The Electromagnetism series will:

• discuss the basic properties ofmagnetic fields and poles;

• describe Earth's magnetic field;

• explain the source of themagnetic field in ferromagneticmetals;

• describe how electromagnetismis employed in devices such aselectric motors, generators, andtransformers;

• describe how some livingorganisms use Earth's magneticfield for their own purposes.

The series consists of six pro-grams, each with a correspondingunit in this guide. The first pagesof each unit contain material ofinterest to the teacher:

Objectives - what the studentshould learn from the program.

• Program Description -a briefoutline of the flow of conceptsin the program.

• Before-Viewing Activities -discussions, experiments, andreadings that would provideuseful background informationfor students. These need nottake up much classroom timesince most programs reviewimportant background concepts.

• Notes - additional informationnot presented in the program.

Subsequent pages can be photo-copied and distributed to students:

• Program Questions - basedentirely on the material pre-sented in the program. Studentsshould read the questions beforethe program begins, and jotdown brief answers whileviewing the program. If timepermits, a second viewing maybe necessary.

• Discussion Questions - to helpstudents grasp the major con-cepts, become aware of broaderrelationships, and explore someof the implications of thematerial.

• Key Terms - these includedefinitions of the importantterms used in the program andstatements of the scientific lawspresented.

The guide ends with a list ofImportant Dates and Peopleand aBibliography containing bothtextbook and journal references.

• demonstrate the relationshipbetween electric charge flowand magnetic fields;

• After-Viewing Activities -experiments, demonstrations,and discussions to clarify andinvestigate concepts.

1

PROGRAM 1 EARTH'S MAGNETIC FIELD

OBJECTIVES'

After viewing this program,students should be able to;

l. describe some modern-dayapplications of magnetism;

2. recount some early discoveriesabout and experiments onmagnetism;

3. explain the construction anduse of a compass;

4. describe the shape of magneticfields around a bar magnet andthe Earth;

5. define magnetic pole;

6. illustrate the difference be-tween the geographic north poleand the magnetic pole of theEarth;

7. define magnetic declinationand magnetic inclination.

PROGRAM DESCRIPTION

The program opens with a briefdescription of a world that sud-denly must do without magnetism.Without the benefits of magnet-ism, all present-day electric motorsand generators would not operate.The program continues with thediscovery of magnetite (lodestone)by the Greeks and its eventual useas a compass.

The study of magnetism halted atthis point until Petrus Peregrinus(c.1220-?) chiselled a piece oflodestone into a sphere andexplored the magnetic field aroundit by sprinkling small pieces ofiron on it. He found that the piecesof iron lined themselves up in apattern that seemed to radiate

outwards from one point on thesphere and back to another point.Peregrinus called these pointspoles.

The program goes on to describethe three-dimensional field arounda typical bar magnet. Followingthis is a description of the work ofWilliam Gilbert. He felt that theplanet Earth was an enormous barmagnet with poles located in thepolar regions.

The program concludes with adescription of magnetic declina-tion and inclination.

Figure 1

BEFORE-VIEWING ACTIVITIES

For students to comprehend theprogram content, they shouldunderstand:

• the function of a compass;

• the shape of the magnetic fieldabout a bar magnet (see Before-Viewing Activities, Program 2);

• the law of magnetic poles.

AFTER-VIEWING ACTIVITIES

1. Design and construct your owncompass. One method is to floata magnet on a block of wood inwater. Another is to magnetize asewing needle by stroking itwith a bar magnet (alwaysstroke in the same directionwith the same pole of the barmagnet). Float the needle in abowl of water by initiallyplacing it on top of a floatingpiece of toilet tissue and thenpushing the tissue down andaway from the needle with atoothpick. If this is donecarefully, the surface tension ofthe water will hold up theneedle.

2. In the event that some studentsmay have never used a com-pass, it might be useful to havethem gain some experience.Locate the direction of magneticnorth. What you actually arefinding is the direction of themagnetic field at your presentlocation. Since the field isaffected by local conditionssuch as the presence of nearbymetals or iron ore, your com-pass is not likely to pointexactly towards the magneticnorth pole. Find out the mag-netic declination for your area(this information can be ob-tained by calling a local flighttraining school or airport) andlocate true north from yourcompass readings.

3. You can check the direction oftrue north in another way bycoming to the school at nightand locating the north star

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(Polaris) in the sky. This star isvery nearly straight north.Compare its direction to the oneobtained by correcting thecompass. This would also be agood way to determine yourlocal declination without callingan airport.

4. If pieces of lodestone areavailable, repeat some ofPeregrinus's experiments.Explore the magnetic fieldaround the lodestone with ironfilings or compasses. Locate thepoles of the material. By float-ing the lodestone on a piece ofwood, you will find that it actsas a compass.

5. Heat the needle from Activity 1over a Bunsen flame until itbecomes red hot. Allow it tocool, float it in water again, andlook for changes in its magneticproperties. Make sure theneedle cools down far awayfrom any strong magnets in theroom.

NOTES

1. The first mention of compassesis attributed to Shen Kua, aChinese mathematician. In 1050A.D. he wrote about using amagnetic needle to indicatedirection for land travel. Soonafter 1100 A.D., another Chi-nese, Chu Yu, reported that thecompass was used by foreignsailors travelling betweenCanton and Sumatra.

2. The magnetic pole in thenorthern hemisphere is cur-rently located around 78°Nlatitude and 100°W longitudenorthwest of Resolute, N.W.T.

3. Areas where concentrations ofmagnetic materials such as ironare greater than normal willdistort the Earth's magneticfield. Magnetic anomaly detec-tors on low-flying aircraft canfind distortions and predictionsof ore deposit locations can bemade based on these distortions.This method is also used tolocate submerged submarines,sunken ships, etc.

4. Interestingly enough, the northpole of a magnet had previouslybeen defined as the end thatpointed to the geographic northpole of the Earth. Knowing thisand the fact that opposite polesattract, one can see that themagnetic pole that is closest toour north geographic pole is thesouth magnetic pole of theEarth. William Gilbert carriedout further experiments thatinvolved heating magnets, andhe discovered that this causedthe loss of their magnetism.

Since the core of the Earth isnow considered to be largelymolten rock (with a relativelysmall, hot, solid centre), theEarth is no longer thought of asan enormous but simple barmagnet.

5. The locations of the magneticpoles move slowly as timepasses. Since compasses pointin the direction of the localmagnetic field, the directionthey indicate is slowly changingalso. In Toronto, magnetic northhas changed from being 8° westof true north to 9° west of truenorth in the last 20 years or so.(This means that true north asmeasured from a compass inToronto is located 9° to theright of where the compasspoints.) This is magnetic decli-nation. Also, in most locationsthe magnetic field has a verticalcomponent (or dip). This iscalled magnetic inclination.Precisely at the magnetic polesthe field is vertical. At themagnetic equator it would behorizontal. The magneticequator is not a perfect circleand is not necessarily located atthe geographic equator.

3

PROGRAM QUESTIONS

While viewing the program, lookfor answers to the followingquestions:

1. What might happen if magnet-ism suddenly did not exist?

1. Runways at airports are num-bered according to the compassdirection to which they point.For example, at Pearson Intema-tional Airport near Toronto,runway 6 points 60° east ofnorth. Why would the runwaysbe numbered according to thecompass (magnetic) direction?

ver to London, England, by theshortest route. The shortestroute can be determined bystretching a piece of stringbetween the cities, using a globeof the world.

KEY TERMS

2. What were the original, natu-rally occurring magnets called?

3. Name the instrument producedwhen a magnet is placed on apiece of wood and the wood-magnet combination is floatedon water.

4. What are the names of thepoints from which iron filingsseem to radiate when they aresprinkled onto a magnetizedsphere?

5. What did Gilbert concludeabout the entire Earth?

6. Which magnetic pole is locatedin Canada's North?

7. What happens to a magnet if itis heated until it becomes redhot?

8. What term is used to describethe difference in directionbetween true north and thedirection a compass indicates?

9. What is the name of the anglebetween the local magnetic fieldand the surface of the Earth?

DISCUSSION QUESTIONS

Answer these questions afterviewing the program. In somecases, you will have to obtainreference material from the library.

2. On page 10 of the June 1987issue of Discover magazine, it isstated that an "assassin detec-tor" created for an emperor inChina about 2200 years agoconsisted of enormous andpowerful magnetic doors.Anyone who tried to walkthrough the doors with a metaldagger or armor would getpinned to the doors. Could thisbe possible? How would thedoors have been made? Designand perform some scaled-downexperiments to answer thesequestions.

3. Suppose that the door describedin question 2 existed. Describeseveral ways in which anassassin could smuggle weaponsthrough the door.

4. Bring a compass close tovarious metal objects. Does itrespond? Many compasses arelocated in large metal objectssuch as ships, aircraft, and evencars. Does this affect theiroperation? How could thesecompasses be "calibrated" togive the correct reading?

5. What happens to the molecules(or atoms) of a substance whenit is heated? Why might thisdestroy any magnetism the sub-stance may have?

6. Describe the direction(s) anaircraft's compass would showas the plane flies from Vancou-

Compass - a magnet freelypivoted horizontally so that it canalign itself along the local mag-netic field.

Law of magnetic poles - oppositepoles (e.g., north and south) attracteach other while like poles (e.g.,north and north or south andsouth) repel each other.

Lodestone - a ferrous-ferric

Magnetic declination - thedifference between the direction towhich the magnetic field pointsand true north.

Magneticfteld - a region ofspace that is under the influence ofmagnetic forces. Any point in thefield can have associated with it aforce and direction (of the force)that a unit north "pole" wouldexperience if placed there.

Magnetic inclination - the anglebetween the horizontal and thedirection of the magnetic field.

Magnetic pole - the place to-wards which lines of the magneticfield converge or from which theydiverge.

North pole - that end of a magnetwhich is attracted towards themagnetic pole in the Earth's arcticregion.

4

magnetic properties in its naturallyoccurring form. Also calledmagnetite.

) that showsoxide

PROGRAM 2 MAGNETISM AND ELECTRON FLOW

OBJECTIVES

After viewing this program,students should be able to:

1. explain the construction of atypical battery as devised byVolta;

2. describe Oersted's discovery ofthe magnetic effect of anelectron flow;

3. describe the shape of themagnetic field about a straightconductor carrying an electronflow;

4. use the left-hand rule forconductors to determine thedirection of the magnetic fieldproduced by a given electronflow;

5. describe and explain the shapeof the magnetic field about acoil (helix) carrying an electronflow;

6. use the left-hand rule for coilsto predict the direction of themagnetic field about a coil for agiven electron flow.

electromagnetism. One of theinvestigators was Hans ChristianOersted (1777-1851), who acci-dentally discovered the magneticeffect produced by an electronflow. No longer could the electricforce be considered to have norelationship to the magnetic force.

Andrd Ampere (1775-1836) madea detailed study of the magneticfield about a current-carryingconductor. He discovered that thefield was circular and that itsdirection was related to the direc-tion of the electron flow. Thisrelation is summarized in the left-hand rule for conductors.

When a straight conductor istwisted into a loop, this rule isused to predict the doughnut-shaped field produced by anelectron flow through the loop. Asmore loops are added, a coil (orhelix) is formed. A given electronflow through the coil produces amagnetic field whose direction ispredicted by the left-hand rule forcoils.

The program ends by askingwhether the presence of an iron

bar inside the coil might affect thestrength of the magnetic field. Thisquestion leads into Program 3,"Domain Theory."



• the law of magnetic poles;

• the concept of an electron flow;

• the shape of the magnetic fieldabout a bar magnet.

A simple way to investigate thefield about a bar magnet is to placethe magnet flat between two bookson a non-metallic table top. Lay asheet of acetate over the gapbetween the two books so that itrests across the top of the magnet.Sprinkle iron filings uniformlyover the sheet, then tap it gently topermit the iron filings to line upwith the field. This technique canbe used to indicate the presence offields around different magnetsand display their shapes.

PROGRAM DESCRIPTION

The program opens with a briefreview of the magnetic force andcompares it with the electric force.Up to the beginning of the nine-teenth century, these forces wereconsidered to be completelyseparate. The program continueswith a short outline of the develop-ment of the battery by AlessandroVolta (1745-1827). His batteriesprovided the potential differenceand electron flow required formany scientific investigations,including those in the field of Figure 2

Bar magnet Acetate sheet

5

AFTER-VIEWING ACTIVITIES NOTES

1. Make a battery similar to theone devised by Volta. Separatealternating layers of two differ-ent metals with sheets ofabsorbent paper soaked in a saltsolution. Test the strength of thebattery with a voltmeter or byconnecting a flashlight bulb inparallel with the battery. Theninvestigate the relationshipbetween the potential differenceproduced and (a) the combina-tion of metals used, and (b) thenumber of layers. Coins couldprovide a source of differentmetals. (Note that coins canbecome quite corroded if thebattery is allowed to dischargefor some time.)

2. Form a coil with the strongestpossible magnetic field, using agiven length of flexible conduc-tor and a given battery. Youwill have to devise somemethod for comparing fieldstrengths (e.g., amount ofcompass needle deflection ornumber of nails supported).

1. The term electromagnet has notbeen mentioned in this programas it is generally used to refer toa combination of coil andpermeable core. The coil byitself is called a solenoid.

2. Magnetic lines of force are auseful way to depict the inten-sity and direction of a magneticfield. However, they have nomore reality than do the linesthat represent rays of light. As aresult, the program narrativedoes not mention "lines offorce," referring instead to fielddirection.

3. The strength of the field pro-duced by a current-carrying coilvaries directly with the currentand with the number of turnsper unit length of the coil.

3. While electrons flow through acoil, use small compasses toinvestigate the direction of themagnetic field (a) around theoutside of the coil, and (b)inside it.

6

PROGRAM QUESTIONS DISCUSSION QUESTIONS KEY TERMS


1. Who invented the first batteriesthat could be used to provideelectric current?

2. What were the essential parts ofthese batteries?

3. Who discovered that an electronflow produces a magnetic field?

4. Describe the shape of themagnetic field produced whenan electron flow passes througha straight conductor.

5. Which scientist discovered therelationship between thedirection of the electron flowthrough a straight conductor andthe direction of the magneticfield?

6. In the left-hand rule for conduc-tors, what is indicated by:

a) the direction of the thumb?

b) the direction of the curl ofthe fingers?

7. If an electron flow is passingthrough a coil:

a) where is the field strongest?

b) what happens to the fieldbetween the individual loops ofthe coil?

8. When using the left-hand rulefor coils, what is indicated by:

a) the direction of the curl of thefingers?

b) the direction of the thumb?


1. In what ways are the fields of acurrent-carrying coil and of abar magnet similar? In whatways are they different?

2. Which electrical unit wasnamed after Alessandro Volta?

3. In alternating current (AC), thedirection of the electron flowreverses every 1/120 second.Describe the field of a coil con-taining alternating current.

4. If an electron flow were movingdirectly away from you, wouldthe direction of the magneticfield be clockwise or counter-clockwise?

Form small groups to brainstormanswers to questions 5 and 6.

5. What might you do in order toincrease the strength of themagnetic field produced by acurrent-carrying coil?

6. Think of as many uses of acurrent-carrying coil as you canand list them.

7. Write a brief report on the lifeof Hans Christian Oersted.

8. Write a brief report on the lifeof Andre Ampere.

9. How is a coil used in anelectric bell?

10.a) What does a relay do in anelectric circuit?

b) How is a coil used in a relay?

Battery - a device that provides apotential difference (voltage)which can push electrons around acircuit.

Coil - a conductor containing anumber of loops along its length.

Electron - a tiny negative particlethat orbits the nucleus in an atom.

Electron flow - the passage ofnegatively charged particles calledelectrons through a conductor.

Helix - same as "coil."

Left-hand rule for coils - if theleft hand is placed around a coilsuch that the fingers curl in the di-rection of the electron flow, thethumb points in the direction ofthe field within the coil. Thus, thethumb points towards the northpole of the coil.

Left-hand rule for conductors - ifthe left hand is placed around aconductor with the thumb pointingin the direction of the electronflow, the fingers will curl aroundthe conductor in the same directionas the magnetic field.

7

Electron flow

Magnetic field

Left-Hand Rule for Coils

Figure 3

Left-Hand Rule for Conductors

Figure 4

PROGRAM 3 DOMAIN THEORY

OBJECTIVES


1. understand that the strength ofthe magnetic field produced bya current-carrying coil increaseswhen a ferromagnetic substanceis used as a core;

2. identify the combination of coiland core as an electromagnet;

3. explain why an iron atombehaves as a magnetic dipole;

4. describe in terms of domainshow a piece of iron becomesmagnetized;

5. describe various ways ofdemagnetizing a magnetthrough disruption of thealignment of the atoms.

PROGRAM DESCRIPTION

The program begins with a reviewof some basic concepts. Theseinclude the use of the north pole ofa compass in order to indicate thedirection of a magnetic field, andthe left-hand rules for both straightconductors and coils.

The magnetic field of a coil isintensified when an iron core isinserted, and this combination ofcoil and core is known as anelectromagnet. This leads into thequestion: why can substances suchas iron be easily magnetized whileothers such as copper cannot? Inaddition, pure iron is shown to loseits magnetism rapidly, while steelretains its magnetism for a longtime.

In order to explain these concepts,the program examines the motionof electrons within an atom. Thesemotions are compared to the flowof electrons around a single loop.Electron spin is the source of themagnetic field for ferromagneticatoms.

The program goes on to show thatthe magnetic properties of a pieceof iron are a result of the presenceof large groups of atoms calleddomains. Domain theory is thenused to explain:

• the intensification of a magneticfield of a coil by an iron core;

• why only certain substances(ferromagnetic) cause such anintensification;

• why a bar magnet loses itsmagnetism through heating orhammering;

• why steel retains its magnetismlonger than pure iron.

The program ends by hinting that amagnet might somehow influencea conductor that is carrying anelectron flow. Analysis of thispossibility continues in Program 4,"The Motor Principle."



• we left-hand rule for conduc-tors;

• the left-hand rule for coils;

• the structure of atoms as far asthe concept that negativeelectrons move around a posi-tive nucleus.

The students should also observethe following:

• the increase in the magnetism ofa piece of iron by stroking theiron in a constant direction witha magnetic pole;

• the decrease in magnetism of apiece of magnetized iron incomparison to a similar piece ofsteel:

• the loss of magnetism in a barmagnet through hammering orheating;

• the formation of two completebar magnets when one is brokenin half.


1. Devise an experiment to deter-mine whether iron or nickelproduces the strongest fieldwhen used as a core in anelectromagnet.

2. Build a device that uses anelectromagnet, such as a relayor a door chime.

3. Form a hypothesis regardingsome technique for demagnetiz-ing a bar magnet that is notmentioned in the program.Perform an experiment to testthis hypothesis.

4. Devise a method for determin-ing if the electrical resistance ofa coil changes when a ferro-magnetic core is inserted.Perform the experiment usingdirect current and then alternat-ing current.

9

NOTES

l. The strengths of differentmagnetic fields can be com-pared by placing each field inturn at right angles to Earth'sfield. The angular deflection ofa compass from true north isthen a measure of the strengthof the field. If the deflection isD, then the strength of the fieldvaries directly with tan D.

2. The existence of magneticmonopoles has been hypothe-sized. These exotic creatureshave yet to be detected.

3. The statement that electronspins in a given atom exist inonly two directions is the solereference to quantum theory inthe series. It should be realizedthat quantum theory gives theterm electron spin to thatproperty of an electron thatmakes it behave as a magneticdipole. It is no longer consid-ered to actually spin on its axis.

4. A paramagnetic substance isattracted into the strongestregion of a magnetic field,while a diamagnetic substanceis repelled from the strongestregion. Palladium, platinum,and gaseous oxygen are para-magnetic. Most salts and manynon-metals are diamagnetic.

1 0

PROGRAM QUESTIONS DISCUSSION QUESTIONS9. a) Find out what is meant by the

term Curie point.


1. What happens to the strength ofthe magnetic field produced bya coil when an iron core is in-serted?

2. What name is given to thecombination of current-carryingcoil and iron core?

3. Where is most of the mass of anatom located?

4. Where are the electrons in anatom?

5. Which type of electron motionis responsible for the magneticfield of iron atoms?

6. Name two ferromagnetic metalsother than iron.

7. What is meant by the termmagnetic dipole?

8. What name is given to a groupof atoms whose magnetic fieldsare all in the same direction?

9. When a piece of iron is beingmagnetized, which domaingrows?

10. Why will a magnetized iron barlose its magnetism as timepasses?

11. Which type of atom helps steelto retain its magnetism?


1. Which type of electron motiondoes not cause the iron atom tobehave as a tiny magnet?Explain your answer.

2. Which type of electron motiondoes cause the iron atom tobehave as a tiny magnet?

3. Describe some uses of electro-magnets in the home or school.

4. a) How are the domains ar-ranged in a non-magnetic pieceof iron?

b) What happens to thesedomains when the iron is placedwithin a magnetic field?

5. A piece of iron can be magnet-ized by stroking it always in thesame direction with one pole ofa bar magnet. Explain why thisworks.

6. Earth has a large magnetic field.Find out what the centre ofEarth is like, then decide ifEarth's magnetic field is theresult of a huge bar magnet thatextends from pole to pole.

7. Find out how magnetism is usedin:

a) audio tapes;

b) State the value of the Curiepoint for iron, cobalt, andnickel.

Divide into small groups in orderto brainstorm answers to thefollowing questions:

10. How could you construct amagnet with three north polesand only one south pole?

11. Design a toy that makes use ofan electromagnet.

12. Briefly describe two ways inwhich you could cause a barmagnet to lose its magnetism.

b) computer disks.

8. a) How is a diamagnetic sub-stance different from a param-agnetic substance?

b) Give one example of eachtype of substance.

1 1

KEY TERMS

Coil - a conductor containing anumber of loops along its length.

Core - the substance aroundwhich the coil is wrapped.

Demagnetization - any process inwhich a magnetic substance losesits magnetism.

Domain - a region in a ferromag-netic substance in which themagnetic fields of the atoms lineup. It behaves like a tiny magnetwithin the substance.

Magnetic dipole - an objectpossessing two magnetic poles.

Magnetization - the process ofmaking an object magnetic. Whena ferromagnetic object is placedwithin a magnetic field, thedomains which are closely linedup with that field grow into theother domains. Eventually, theobject contains only one domain.

Nucleus - the positive centre ofthe atom. It contains most of themass of the atom.

Electromagnet - the combinationof coil and core.

Electronflow - the passage ofnegatively charged particles calledelectrons through a conductor.

Ferromagnetic - any substancethat contains atoms which possessan overall magnetic field.

Ferromagnetism - the magnetismpossessed by an object because itsatoms behave like tiny magnets.Each electron in the atom has aproperty called spin, which resultsin a magnetic field. For mostatoms these magnetic fields canceleach other. In a ferromagneticsubstance such as iron, several ofthese fields are not cancelled. Theatom is left with an overall mag-netic field.

Left-hand rule for coils - if theleft hand is placed around a coilsuch that the fingers curl in thedirection of the electron flow, thethumb points in the direction ofthe field within the coil. Thus thethumb points towards the northpole of the coil.

1 2

PROGRAM 4 THE MOTOR PRINCIPLE

OBJECTIVES


1. describe Ampere's discovery ofthe magnetic forces that areexerted on two current-carryingconductors located close to eachother;

2. explain the operation of anelectromagnetic rail gun;

3. state and apply the motorprinciple in explaining typicalelectromagnetic interactions;

4. state and apply the left-handrule for force in determining thedirection of the forces that con-ductors experience in externalmagnetic fields;

The program concludes with aquestion concerning the pos-sible use of a motor as anelectric generator, leading intoProgram 5, "ElectromagneticInduction."


For the students to comprehend theprogram content, they shouldunderstand:

electric current;

magnetic fields and poles;

the left-hand rule for conduc-tors;

magnetic fields around a barmagnet.

NOTES

1. Short but interesting articles onrail guns appear in ScientificAmerican magazine (October1985, p. 80) and Aviation Weekand Space Technology (Decem-ber 21, 1987, p. 29).

2. Applications of the motorprinciple (other than in electricmotors) include electron beamdeflection in televisions, electricmeters such as ammeters andvoltmeters, loudspeakers, mag-netic monorail trains, etc.

5. describe the operation of asimple electric motor;


6. explain the function of commu-tators and brushes in electricmotors.

PROGRAM DESCRIPTION

In the opening scene an electro-magnetic rail gun is observedfiring supplies from the Moon toTitan. The explanation of theoperation of this gun leads into adiscussion of Ampere's experi-ment with current-carrying con-ductors and illustrates the motorprinciple.

Next the program introduces theleft-hand rule for force and uses itto show the direction of the forcesthat exist in electric motors. Theoperation of commutators andbrushes in electric motors is dem-onstrated.

1. Many universities and compa-nies are actively involved indesigning rail and coil guns.Potential uses include produc-ing nuclear fusion through highimpact, launching payloads intospace at low cost, and synthe-sizing new materials. Studentscould make presentations onthese rail and coil gun designs.

2, Design and construct a smallbattery-operated electric motor.

3. Design and construct your ownsmall rail gun as a science classor science fair project. Theteacher must ensure that appro-priate precautions are takenboth with the projectiles andhigh electric currents.

13

PROGRAM QUESTIONS DISCUSSION QUESTIONS KEY TERMS


1. Name the French physicist whodiscovered that a force existsbetween two parallel current-carrying conductors.

2. When electric currents in twoparallel conductors are flowingin the same direction, whatforce exists between them:attraction or repulsion?

3. When electric currents in twoparallel conductors are flowingin opposite directions, whatforce exists between them:attraction or repulsion?

4. When the fields between twocurrent-carrying conductorshave the same direction, whatforce exists between them:attraction or repulsion?

5. When the fields between twocurrent-carrying conductorshave the opposite direction,what force exists between them:attraction or repulsion?

6. What is the name of the prin-ciple involved in questions 1 to5?

7. In the left-hand rule for force, ifthe thumb points in the direc-tion of electron flow, where dothe fingers and palm point?

8. What would happen if a directcurrent electric motor didn'thave a commutator?

9. Why are brushes used inelectric motors?

Answer these questions afterviewing the program. In somecases you will have to obtainreference material from the library.

1. Would reversing the directionof current flow in the rails affectthe direction in which a rail gunfires?

2. How could electricity to rum arail gun be obtained on theMoon?

3. Why are rail guns not generallyconsidered as a way to launchastronauts into orbit?

4. Investigate and describe howthe Van Allen radiation beltsare formed around the Earth.How does the motor principleapply?

5. Investigate and describe how anelectron gun in a televisionoperates.

6. Electricity is normally suppliedto the home in the form ofalternating current (AC). Itchanges in direction 120 times asecond! Investigate and de-scribe the operation of motorsthat use this type of current.How are they different from thedirect current (DC) motorsdescribed in this program?

7. Can you imagine a situation inwhich a DC motor would need aslight turn to get it started whenthe current is turned on? De-scribe the exact location of thearmature and the motor con-struction that would produce it.How could this problem besolved? (Brainstorm possiblesolutions in small groups.)

Brushes - part of the electriccircuit in an electric motor. Theyallow a sliding contact to be madein the circuit.

Commutator - a device thatallows current direction to bereversed at regular intervals in anelectric motor.

Conductor - a material thatallows electrons to flow throughitself easily. This material isusually a copper or aluminumwire.

Electric motor - a machine thatconverts electrical energy intomechanical energy.

Left-hand rule for force - if thethumb of the left hand is pointed inthe direction of the electron flowin a conductor, and the out-stretched fingers point in thedirection of the external magneticfield, the palm will point in thedirection of the force that theconductor experiences.

Motor principle -if a current-carrying conductor is placedwithin an external magnetic field,it will experience a force that isperpendicular to both the directionof the electron flow and theexternal magnetic field.

14

Figure 5

Direction of magnetic field

Left-Hand Rule for Force

PROGRAM 5 ELECTROMAGNETIC INDUCTION

OBJECTIVES'


1. explain the meaning of the termelectromagnetic induction;

2. describe Faraday's discovery ofelectromagnetic induction;

3. describe the conditions requiredfor electromagnetic induction tooccur;

4. explain the operation of asimple transformer;

5. state Lenz's law;

6. use Lenz's law to predict thedirection of an induced current;

7. explain how an electric genera-tor produces a current.

PROGRAM DESCRIPTION

The program begins by askinghow the electric current that isprovided to homes and industriesis produced. It then reviews theability of an electron flow toproduce a magnetic field. Thisleads into the possibility that amagnetic field could in turnproduce a current.

Both Joseph Henry (1797-1878)and Michael Faraday (1791-1867)discovered this phenomenon,which is known as electromag-netic induction. The programexamines Faraday's discovery indetail and compares his apparatusto a simple transformer.

Lenz's law is then introduced andapplied to predicting the directionof induced currents. Finally, theprogram presents the concept of

electromagnetic induction toexplain the operation of an electricgenerator.


For students to comprehend theprogram content, they should:

• review Oersted's discovery ofelectromagnetism;

• review the left-hand rule forconductors;

• review the left-hand rule forcoils;

• examine the parts of a simpletransformer;

• examine the parts of a simplegenerator.


l. Take a field trip to a localgenerating station. Pay attentionto:

• the electric generators;

• how the generator armaturesare rotated;

• the energy source used toprovide the rotation of the ar-matures;

• the use of transformers todistribute the electrical energy.

2. Devise a way to use a simpleelectric motor as a generator.

3. Perform an experiment todetermine the potential differ-ence and current properties for atransformer. The apparatusshould consist of a transformer(different number of turns onthe two windings), a source ofalternating current, two ACvoltmeters, and two AC amme-

ters. To ensure your safety,make sure that the potentialdifferences and currents arelow.

NOTES

1. For an ideal transformer (onethat does not lose any energy,and they are rather hard to find):

where "p" refers to theprimary coil and "s" to the sec-ondary coil.

where "N" refers to thenumber of turns in the coil.

2. A transformer that containsmore turns in the secondary coilthan in the primary coil wouldhave a higher potential differ-ence across the secondary coilthan across the primary coil.Such a transformer is called astep-up transformer. However,the law of conservation ofenergy demands that as thepotential difference increases,the current decreases.

A transformer that has fewerturns in the secondary coil thanin the primary coil causes thepotential difference to decrease,and so is known as a step-downtransformer.

3. Lenz's law is actually anapplication of the law of conser-vation of energy. If the inducedfield produced by the coil aidedthe inducing action, then thecoil could generate an electriccurrent without any input ofenergy. It could simultaneouslydraw a magnet into itself while

16

supplying a current to an out-side circuit. Perpetual motionthus could be possible.

4. Joseph Henry discoveredelectromagnetic induction in thesummer of 1831. However, hehad to return to his teachingduties at the Albany Academy,and so put off further researchand publication until the follow-ing summer.

On August 29, MichaelFaraday discovered the sameeffect, pursued his researchrapidly, and announced his find-ings to the Royal Society inNovember 1831. The credit fordiscovery thus went to Faraday.

Although he is not credited forthis discovery, Joseph Henry iswell known for the discovery ofself-inductance and for numer-ous applications in the field ofelectromagnetism.

17

PROGRAM QUESTIONS


1. Which American physicistdiscovered electromagneticinduction?

2. In Faraday's experiment:

a) what name is given to thecoil that is connected to thebattery?

b) what name is given to thecoil in which the current is in-duced?

3. What must be happening to amagnetic field if it is inducingan electron flow?

4. List the three main parts of atransformer.

5. Who developed the first electricgenerator?

6. Which physicist discovered thatthe field produced by theinduced current always opposesthe inducing action?

7. Name the parts of the armaturein a generator.

8. Describe the motion of thearmature while the generator isoperating.

9. Name the type of current thatregularly reverses itself.



1. a) State Lenz's law.

b) If the south pole of a barmagnet moves towards a coil,which magnetic pole will beinduced at the end of the coilclosest to the magnet?

c) If the north pole of a barmagnet moves away from acoil, which magnetic pole willbe induced at the end of the coilclosest to the magnet?

d) A bar magnet with its northpole near the end of a coil is notmoving at all relative to thecoil. Which magnetic pole isbeing induced at the end of thecoil closest to the bar magnet?

2. How could the leads be con-nected to a generator in order toprovide:

a) direct current to an outsidecircuit?

b) alternating current to anoutside circuit?

3. What is the primary source ofenergy for your local electricgenerating station?

4. Describe how the induction coilin a car operates.

5. Explain how a laboratoryinduction coil operates.

6. a) Find out how transformersare used in the distribution ofelectrical energy from the gen-erating station to your home.

b) Why are step-up transformersused in this process? (A step-uptransformer has more turns inthe secondary coil. As a result,the transformer increases thepotential difference but de-creases the current.)

7. Compare the operation ofmotors and generators.

1 8

KEY TERMS

Alternating current - an electronflow that regularly reverses itself.

Armature - the combination ofcoil and core that spins in agenerator.

Core - the substance aroundwhich a coil is wrapped. If nosubstance is placed inside a coil itis said to have an air core.

Electric generator -a device thatproduces a potential difference (orcurrent) by spinning a coil in amagnetic field.

Electromagnetic induction - theprocess in which a changingmagnetic field causes an electronflow.

Galvanometer - an electric meterthat can detect and measure tinycurrents.

Lenz's law - When a current isinduced by a changing externalmagnetic field, the direction of theinduced current is such that itsmagnetic field opposes the changein the external field.

Primary coil - the coil in atransformer that receives a current.

Secondary coil - the coil in atransformer in which a current isinduced.

Transformer - a device consist-ing of two coils linked by acommon core. A changing currentin the primary coil induces acurrent in the secondary coil.

PROGRAM 6 LIFE IN THE FIELD

OBJECTIVES


1. explain what the solar wind is;

2. describe the appearance ofsunspots;

3. state one type of activity thatoccurs in sunspots;

4. describe the production of solarflares and prominences;

5. explain the effect of the Earth'smagnetic field on the chargedparticles of the solar wind;

6. describe the structure andimportance of the Van Allenradiation belts;

7. describe how the aurorae(borealis and australis) areformed;

8. describe a magnetic fteldreversalof the Earth and itspossible effects on life;

9. describe the natural use of theEarth's magnetic field by livingorganisms such as the homingpigeon and mud bacteria.

PROGRAM DESCRIPTION

The program opens by illustratingthe need to protect an astronaut onthe moon from the stream ofcharged particles known as thesolar wind. It continues on toillustrate sunspots, flares, andprominences, and their effects onthe solar wind intensity. TheEarth's magnetic field protects lifeby deflecting these charged par-ticles around the Earth and bytrapping some of them in the Van

Allen radiation belts. How thisdeflection is accomplished isexplained by applying the motorprinciple.

The program then describes theformation of the aurorae (borealisand australis) and the Earth'smagnetic field reversals. Theprogram ends by showing how theEarth's magnetic field is usednaturally by creatures as diverse ashoming pigeons and mud bacteriato navigate.



• the motor principle as applied toa flow of charge through aconductor in a magnetic field;

• the Earth's magnetic field.


1. Investigate the sunspot cycleand its effect on (or coincidencewith) climatic variations. Deter-mine where sunspots are in thepresent cycle and make a long-range weather prediction. Graphrecent cycle intensities andmake some extrapolations.

2. Investigate how astronauts areprotected against the solar windparticles.

3. Research and attempt somesolar photography. Your pic-tures will reveal some of thesunspots. Do this over a periodof time to determine suchfactors as the solar rotation rateand sunspot "lifespans."

Note: Never look directly atthe sun with the naked eye oroptical instruments such asbinoculars and telescopes. Useback-projection techniques.

4. Carry out experiments tochange electron flow directionsin Crookes tubes with magnets.Exercise caution as high volt-ages are involved and X-rayproduction is possible espe-cially in "cold cathode" tubes.

5. Experiment with various gasesin Crookes tubes and producelights of various colors ...thereby producing aurorae in atube!

6. Research and describe howother living creatures use theEarth's magnetic field.

7. When comets approach the sunthey develop two tails. Researchand explain:

a) why this occurs;b) why one tail is curved andthe other is straight.

NOTES

1. Flowing outward from the sunis a stream of charged particlescalled the solar wind. Becauseof this charge, they can bedeflected by Earth's magneticfield. Electromagnetic radiationfrom the sun, such as ultravioletlight, is not deflected in thisway. Fortunately, the ozonelayer, which absorbs a lot ofultraviolet radiation, protects us.

2. The sun's magnetic fieldreverses too. Strong magneticfields have been detected insunspots. The direction of afield is uniform through one

1 9

particular sunspot cycle andthen reverses in the next one.The period of the sunspot cycleis about 10 to 11 years. How-ever, the period of a cycle fromthe time of one sunspot maxi-mum with the magnetic field inone direction to the next maxi-mum with the field in the samedirection is about 21 years.

Throughout its history, Earthhas experienced periodic fluc-tuations in its own magneticfield. Geophysicists believe thatEarth's magnetic field is actu-ally produced by complexmovements of charges withinthe molten core of the planet.This flow of charge seems tochange from time to time,resulting in a reversal of thenorth and south poles. The flowof charge within the core slowsdown, reducing the magneticfield to about ten percent of itsnormal strength. With a dra-matically weakened magneticfield, the solar wind is no longerdeflected around the earth, andso the charged particles raindown to the surface. This candisrupt the genetic materialwithin living cells. The subse-quent mutating effect mayaccount for the suddenextinction of many plants andanimals, and the appearance ofwhole new species. Then theweakened magnetic field re-verses, and returns to fullstrength.

3. Over the last 76 million years,171 magnetic field reversalshave been identified. The latestone took place about 700 000years ago. For about one millionyears before that, the field wasreversed at almost all times,except for two 100 000 year

periods in which it was in itspresent direction. The length oftime between reversalsaverages about 450 000 years,but has ranged from 50 000years to three million years.Until we understand more abouttheir causes, reversals willprobably remain fairly unpre-dictable. The field of the Earthdoes seem to have weakened byabout 15 percent since 1670.

4. The aurorae are usually foundin the far north or south becauseonly there does the magneticfield bring the charged particlesof the solar wind down farenough to interact with mole-cules.

5. Experiments seem to indicatethat homing pigeons use thesun's location in the sky and theEarth's magnetic field fornavigation. They use the sunwhen it is visible and the mag-netic field when the sun is notvisible. Experiments show thatpigeons with magnets taped totheir heads (to override theEarth's field) become lost oncloudy days, but not on sunnydays. On cloudy days, withoutthe magnets, the pigeons do notget lost.

20

PROGRAM QUESTIONS


1. What is the name given to theflow of charged particlestravelling out from the sun?

2. What are sunspots?

3. What will happen to the strengthof the solar wind if the numberof sunspots increases?

4. What protects humans from thesolar wind?

5. When the particles of the solarwind collide with atoms andmolecules in the atmosphere,what is observed?

6. What happens to Earth's climateduring periods of low solaractivity?

7. What is a magnetic field rever-sal?

8. Name at least two creaturesother than humans that use theEarth's magnetic field for navi-gation.


Answer these questions afterviewing the program. In somecases, you may have to obtainreference material from the library.

1. Other than light rays andcharged particles, what elsestreams out from the sun?

2. How long is the sunspot cycle?When will the next solarmaximum be reached?

3. If a space shuttle were at atypical orbital height of 200 km,

how many times higher abovethe Earth's surface wouldastronauts have to travel inorder to reach the lowest VanAllen radiation belt?

4. What would happen if theEarth's magnetic field vanishedpermanently? Give some bio-logical and physical conse-quences.

5. Which planets have significantmagnetic fields? Which planetsdo not?

KEY TERMS

Aurora (borealis and australis) -a glow in the upper atmospherethat varies in intensity from aslight haze to large curtains ofmulticolored streamers. It isusually found in regions near themagnetic poles of the Earth. If it iscloser to the north magnetic pole itis called aurora borealis (Northernlights), and if it is closer to thesouth magnetic pole, it is calledaurora australis.

Magnetic field reversal - aswitching of Earth's magneticpoles, south becoming north andvice-versa. This happens ataverage intervals of 450 000years. The cause is still not fullyunderstood.

Radiation - a flow of energy inthe form of waves or particles.

Solar wind - the stream ofcharged particles flowing out fromthe sun.

Sunspots - areas on the sun'ssurface that are distinctly coolerthan the unspotted portions of thesun's surface. Strong magneticfields exist within the sunspots andmay actually cause them.

2 1

Van Allen radiation belts - thedoughnut-shaped areas around theEarth where incoming solar windparticles are temporarily trappedas they are deflected by the Earth'smagnetic field.

IMPORTANT DATES AND PEOPLE

c. 624-546 B.C.

Thales: Greek philosopher. Firstto describe experiments withnaturally magnetic minerals and todiscover that they could attracti ron.

1031-1095 A.D.

Shen Kua: Chinese mathemati-cian and instrument maker. First toclearly mention in literature theuse of a magnetic needle for landtravel to indicate direction.

c. 12th century

Chu Yu: Chinese. Reported thatin the period 1086-1099 A.D. thecompass was used by foreignsailors travelling between Cantonand Sumatra.

1492

Voyage of Christopher Columbus.

1519-1522

Ferdinand Magellan and JuanSebastian del Cano accomplishedthe first circumnavigation of theworld.

1544-1603

William Gilbert: British physi-cian and scientist. Correctlydescribed the main features of themagnetic field of the Earth anddiscovered the actions (repulsiveand attractive) between magneticpoles. Discovered that magnetizediron lost its magnetism whenheated to high temperatures.Published De Magnete in 1600.

1745-1827

Count Alessandro Volta: Italianphysicist. Inventor of the voltaicpile (battery) and electroscope.

1749-1752

Benjamin Franklin: Experi-mented with electricity.

1769-1821

Napoleon Bonaparte.

1771-1851

Hans Christian Oersted: Danishphysicist. Discovered magneticaction of an electric current(1820).

1157-1217 1588 1775-1836

Alexander Neckam: Englishcyclopedist. First known Europeanto refer to compasses.

c. 13th century

Petrus Peregrinus de Maricourt:French crusader. Gave firstdetailed description of the compassas an instrument of navigation(1269). Explored the field arounda spherical piece of lodestone withsmall bits of iron, and called thelocations where the magneticforces seemed to concentratepoles.

Defeat of the Spanish Armada.

1643-1727

Sir Isaac Newton: Englishphysicist. Experimented with light,motion, and gravitation.

1736-1806

Charles-Augustin de Coulomb:French physicist. In 1785 estab-lished with some precision theinverse square laws of attractionand repulsion between magneticpoles.

Andre-Marie Ampere: Frenchphysicist. Developed formula forstrength of magnetic field. Sug-gested that magnetism was a resultof tiny circulating currents inmolecules.

1775-1783

American Revolution.

1789

French Revolution.

1275-1292

Marco Polo in China.

22

1791-1867 1831-1879

Michael Faraday: English physi-cist and chemist. In 1831 madewhat was in effect the first (ho-mopolar) electric generator. Thistype of generator was a low-voltage machine of no commercialsignificance because of its highcost and low efficiency. Made thefirst transformers, along withJoseph Henry.

1797-1878

Joseph Henry: American physi-cist. In 1829 discovered self-induction. Believed to be the firstto discover electromagneticinduction in 1830, butFaraday soon after discovered thesame effect and published hisfindings first. Along with Faraday,made the first transformers.

James Clerk Maxwell: Scottishphysicist. In 1864 developed thefour fundamental equations thatcombined electricity and magnetism into one phenomenon -electromagnetism.

1859-1906

Pierre Curie: French physicist.Husband of Marie Curie. Madeimportant investigations onparamagnetism. The Curie point(the temperature at whichferromagnetic materials lose theirmagnetism and become paramag-netic) is named for him.

1866

Laying of first transatlanticcable.

1804-1865 1876Heinrich Lenz: German physicist.Lenz's law governs the directionof electromagnetically inducedcurrents.

Invention of telephone.

1901

1808-1835

Hippolyte Pixii: French inventor.Made the first heteropolar electricgenerator (1832) - a more usefultype than Faraday's homopolargenerator. Invented thecommutator.

Marconi achieves the transmissionof the first transatlantic radiomessage.

1812-1814

War of 1812 (conflict betweenU.S. and Great Britain).

23

BIBLIOGRAPHY

I NTERMEDIATE-LEVELTEXTBOOKS

Heath, R.W. et al. The Fundamen-tals of Physics. Toronto: D.C.Heath Canada Ltd., 1978.

Encyclopaedia Britannica, 1986.MagnetismEarthSunColumbus, ChristopherElectricity

Hirsch, A.J. Physics: A PracticalApproach. Rexdale, Ont.: JohnWiley and Sons Canada Ltd.,1981.

UPPER-LEVEL TEXTBOOKS

Gray, H.J., and Isaacs, A. NewDictionary of Physics. London:Longman Group Ltd., 1975.

McGraw-Hill Encyclopaedia ofScience and Technology. 6th ed.Vol 10. New York: McGraw-HillBook Co., 1987.

Giancoli, D.C. Physics: Principleswith Applications. 2nd ed. Engle-wood Cliffs, N.J.: Prentice-HallInc., 1985.

Kane, J.W., and Sternheim, M.M.Physics. 2nd ed. New York: JohnWiley and Sons Ltd., 1984.

Meyer, H.W. History of Electricityand Magnetism, Cambridge,Mass.: MIT Press, 1971. (out ofprint)

National Geographic Picture Atlasof Our Universe. Washington,D. C.: National Geographic Soci-ety, 1980.

Martindale, D.G. et al. Fundamen-tals of Physics: A Senior Course.Toronto: D.C. Heath Canada Ltd.,1986.

REFERENCE BOOKS

MAGAZINES

Scientific AmericanDec. 1981Magnetic Navigation in Bacteria

Sept. 1983The Earth's Core

Asimov, Isaac. A Choice ofCatastrophes. NewYork: Fawcett-Columbine Books, 1981.

Feb. 1979The Source of the Earth's Mag-netic Field

. Asimov's BiographicalEncyclopedia of Science andTechnology. 2nd revised ed.Garden City, N.Y.: Doubleday &Co. Inc., 1982.

Bowers, Brian. Michael Faradayand Electricity. E. Sussex, U.K.:Wayland Publishers Ltd., 1980.

Considine, M., ed. Van Nostrand'sScientific Encyclopedia. 6th ed.New York: Van Nostrand Rein-hold Co., 1983.

Oct. 1985Coilguns and Railguns

AstronomyMay 1986Voyageur: Discovery at Uranus

National GeographicAug. 1985Earth

24

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