TOPIC 4.3: MAGNETIC FIELDSThe student will be able to:S3P-4-20: Define the magnetic field as the region of space around a magnet where

another magnet will experience a force.S3P-4-21: Demonstrate and diagram magnetic fields, using lines of force.

Include: bar magnet, horseshoe magnet, between like poles, between unlike poles

S3P-4-22 Describe the concept of magnetic poles and demonstrate that like polesrepel and unlike poles attract.

S3P-4-23: Describe magnetism, using the domain theory.Include: ferromagnetic materials, the attraction of iron objects to north and south poles

S3P-4-24: Investigate the influence and effects of the magnetic field of the Earth.Include: auroras, magnetic declination and inclination

Topic 4: Fields • SENIOR 3 PHYSICS

Topic 4.3 – 34

SPECIFIC LEARNING OUTCOMES

S3P-4-20: Define the magneticfield as the region of spacearound a magnet where anothermagnet will experience a force.

S3P-4-21: Demonstrate anddiagram magnetic fields, usinglines of force.Include: bar magnet, horseshoemagnet, between like poles, betweenunlike poles

GENERAL LEARNING OUTCOMECONNECTION

Students will…Employ effectivecommunication skills and utilizeinformation technology togather and share scientific andtechnological ideas and data(GLO C6)

Entry-Level KnowledgeStudents have completed treatment of theconcepts of gravitational and electric fields.

Notes to the TeacherThe field concept is useful in describing andrelating magnetic fields to gravitational andelectric fields. By this time, students shouldbe familiar with the basic field concept.However, an introduction to the basics ofmagnetism is required at this point.

While we cannot easily “demonstrate”electric and gravitational field lines, we caneasily map magnetic fields, using magnetsand iron filings. This visual aspect ofmagnetic fields helps to reinforce theprevious discussions of field lines.

In magnetic fields, a line of force indicatesthe direction the force would act on a northpole in the field. The number of lines perunit area represents the intensity of thefield.

Student ActivitiesA lab on basic magnetism is essential. Fieldmapping should be included in the activity.

The cardboard backing from a ream of papercan be placed over a magnet. Iron filings aresprinkled on the cardboard. By tapping thecardboard lightly, the iron filings line upalong the magnetic field lines. Differentconfigurations can be used (e.g., horseshoemagnet, two like poles, two unlike poles).

Teacher DemonstrationPlace a magnet under a piece of Plexiglas®

and sprinkle iron filings around the magneton top of the Plexiglas®. The field can beprojected on an overhead.

SUGGESTIONS FOR INSTRUCTION

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SENIOR 3 PHYSICS • Topic 4: Fields

SKILLS AND ATTITUDES OUTCOME

S3P-0-2c: Formulate operationaldefinitions of major variablesor concepts.

GENERAL LEARNING OUTCOMECONNECTION

Students will…Evaluate, from a scientificperspective, information andideas encountered duringinvestigations and in daily life(GLO C8)

Senior Years Science Teachers’Handbook ActivitiesJournal Entry: Students respond in theirjournals to questions (e.g., in your home,where do you find magnetic fields,gravitational fields, electric fields? Listsome examples of each field.).

Compare and Contrast Frames:Students answer the following questions:What is the same about gravitational,electric, and magnetic fields? What isdifferent?

Laboratory ReportStudents write a lab report includingdiagrams of various fields from a variety ofmagnet types.

SUGGESTIONS FOR INSTRUCTION SUGGESTIONS FOR ASSESSMENT

SUGGESTED LEARNING RESOURCES

AAPT Potpourri of Physics Teaching Ideas,160–168.

Kagan, D. (1993) “Building a MagneticLevitation Toy.” The Physics Teacher 31:432.

Topic 4: Fields • SENIOR 3 PHYSICS

Topic 4.3 – 36

SPECIFIC LEARNING OUTCOMES

S3P-4-22: Describe the concept ofmagnetic poles and demonstratethat like poles repel and unlikepoles attract.

S3P-4-23: Describe magnetism,using the domain theory.Include: ferromagnetic materials, theattraction of iron objects to north andsouth poles

GENERAL LEARNING OUTCOMECONNECTION

Students will…Recognize that scientificknowledge is based on evidence,models, and explanations, andevolves as new evidence appearsand new conceptualizationsdevelop(GLO A2)

Notes to the TeacherStudents may not have had a hands-onopportunity to investigate magnetism.Introduce magnetism, using a hands-on lab.

The domain theory is a simple model ofmagnetism, which states that all materialsare made up of tiny regions called domains.The domains behave like magnets. Whenthey are distributed randomly theirmagnetic effects cancel, and when thedomains become aligned the material ismagnetized.

Additionally, the domains that are alreadywith the field can grow in size, while thosenot aligned shrink due to the atoms on theboundary (the domain “wall”).

Student ActivityStudents suspend a magnet from a pivotand bring another magnet nearby todemonstrate attraction and repulsion of likeand unlike poles.

SUGGESTIONS FOR INSTRUCTION

S N

Domains aligned randomly(no magnetic effects)

Domains aligned

Topic 4.3 – 37

SENIOR 3 PHYSICS • Topic 4: Fields

SKILLS AND ATTITUDES OUTCOMES

S3P-0-2c: Formulate operationaldefinitions of major variablesor concepts.

S3P-0-2f: Record, organize, anddisplay data using anappropriate format.Include: labelled diagrams,tables, graphs

GENERAL LEARNING OUTCOMECONNECTION

Students will…Understand how stability,motion, forces, and energytransfers and transformationsplay a role in a wide range ofnatural and constructedcontexts(GLO D4)

Research Report/PresentationStudents complete a research report onmagnetic phenomena, using diagrams toillustrate concepts.

Recognizing the Role of EvidenceStudents discuss their personalunderstanding of what a magnetic fieldrepresents, and resolve competing diagrams.

SUGGESTIONS FOR INSTRUCTION SUGGESTIONS FOR ASSESSMENT

SUGGESTED LEARNING RESOURCES

Appendix 4.3: Student Sampler: MagneticFields

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Topic 4.3 – 38

SPECIFIC LEARNING OUTCOME

S3P-4-24: Investigate theinfluence and effects of themagnetic field of the Earth.Include: auroras, magneticdeclination and inclination

GENERAL LEARNING OUTCOMECONNECTION

Students will…Demonstrate appropriatescientific inquiry skills whenseeking answers to questions(GLO C2)

Notes to the TeacherAncient mariners believed that somewherein the north was a magnetic mountain thatwas the source of attraction for theircompasses. It was not until 1600 thatWilliam Gilbert, in his treatise PhysiologiaNova de Magnete, suggested that the Earthitself behaved like a giant magnet. Insimple terms, the Earth can be thought ofas a bar magnet and the force that attractsthe compass originates inside the Earth.

Magnetic field lines radiate between Earth’snorth and south magnetic poles. Magneticfields are produced by the motion ofelectrical charges (see the next unit onelectromagnetism). The Earth’s magneticfield is not completely understood, but isthought to be associated with electrical

currents produced by the rotational effectsin the liquid outer core of Earth’s interior.Consequently, the Earth’s magnetic fieldreverses itself approximately every 250,000years.

Auroras (the northern and southern lights)are caused by high-energy particles fromthe solar wind trapped in the Earth’smagnetic field. As these particles oscillatealong the magnetic field lines, they enterthe atmosphere near the north and southmagnetic poles. These energetic electronscollide with the oxygen and nitrogenmolecules in the atmosphere. The collisionsexcite the molecules and, when they decayfrom the excited states, they emit the lightthat we see in the auroras.

The magnetic poles are not in the samelocation as the geographic poles. Declinationis the angle between true north andmagnetic north. This angle changes,depending on your location. For example, itis 20°E in Victoria, B.C., but in St. John’s,Newfoundland, it is 23°W. Knowledge ofmagnetic declination is important fornavigational purposes. According to theNational Geomagnetism Program, the northmagnetic pole is a feature unique toCanada, and monitoring its position andmotion is of prime importance to Canadiancartography.

SUGGESTIONS FOR INSTRUCTION

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SENIOR 3 PHYSICS • Topic 4: Fields

SKILLS AND ATTITUDES OUTCOME

S3P-0-1c: Relate the historicaldevelopment of scientificideas and technology to theform and function ofscientific knowledge today.

GENERAL LEARNING OUTCOMECONNECTION

Students will…Understand how stability,motion, forces, and energytransfers and transformationsplay a role in a wide range ofnatural and constructedcontexts(GLO D4)

At each location on the Earth, the magneticfield lines intersect the Earth’s surface at aspecific angle of inclination. Near theequator, the field lines are approximatelyparallel to the Earth’s surface (see diagrambelow) and the inclination angle in thisregion is close to 0°. As one travels northfrom the equator, the angle of inclinationincreases. At the magnetic pole, the fieldlines are directed almost straight down intothe Earth and the inclination is 90°. Thus,the angle of inclination varies with latitude.It has been suggested that some animalshave the ability to distinguish betweenmagnetic inclination angles and therefore“know” their latitude, particularlymigratory animals.

Magnetic inclination (also called themagnetic dip) is the angle that thegeomagnetic field makes with the surface ofthe Earth. Magnetic inclination varies from90° (perpendicular to the surface) at themagnetic poles to 0° at the equator.

Research Report/PresentationStudents research and report on thecharacteristics of magnetic fields of otherplanets in our solar system.

Science Journal EntriesStudents research Earth’s “dynamo”mechanism, and use the results to explainwhy Venus has no field and why the Sunhas an oscillating field.

SUGGESTIONS FOR INSTRUCTION SUGGESTIONS FOR ASSESSMENT

SUGGESTED LEARNING RESOURCES

WebsitesGeomagnetism:<http://www.ngdc.noaa.gov/seg/potfld/geomag.shtml>

Geology: Plate Tectonics:<http://www.earth.nwu.edu/people/seth/107/>

<http://www.platetectonics.com/index.asp>

When the Earth Moves—Discoveries inPlate Tectonics and Seafloor Spreading:<http://www.beyonddiscovery.org/>

Plate Tectonics—A Paradigm Under Threat:<http://ourworld.compuserve.com/homepages/dp5/tecto.htm>

Topic 4: Fields • SENIOR 3 PHYSICS

Topic 4.3 – 40

SPECIFIC LEARNING OUTCOME

S3P-4-24: Investigate theinfluence and effects of themagnetic field of the Earth.Include: auroras, magneticdeclination and inclination

GENERAL LEARNING OUTCOMECONNECTION

Students will…Demonstrate appropriatescientific inquiry skills whenseeking answers to questions(GLO C2)

When magnetic minerals are free to reorientthemselves, they can align with the Earth’smagnetic field. As molten rock containingferromagnetic minerals (e.g., magnetite)cools below the Curie Point, the iron-bearing mineral grains preferentially orientthemselves to the prevailing magnetic fieldof Earth. This orientation becomes locked inplace as the rocks cool and harden. Thismagnetic inclination allows geologists tospeculate about where on the globe avolcanic rock might have formed.Unfortunately, only the approximatelatitude at the time of formation of the rockcan be “locked in”—not the longitudinalposition on the globe.

During the 1950s and 1960s, the science ofpaleomagnetics provided crucial novelfacts to the development of new theoriesabout the mobility of the ocean floors.Eventually, it was the rock magnetic datathat turned many geologists away from theacceptance of permanent continents andoceans to the more innovative notions ofcontinental drift, sea-floor spreading, andthe theory of plate tectonics in the early1970s. See the references for further sourceson this connection between physics andgeology.

Student ActivitiesStudents can address several questions withrespect to the study of paleomagnetism.• What magnetic orientations would we

expect to see in rocks at different spots onthe Earth’s surface, such as the northpole, Manitoba’s north, or the Equator?

• What conditions must exist for theminerals to move around and thus be ableto align themselves with the Earth’smagnetic field?

• How does a rock preserve the magneticorientation of its iron-bearing minerals?

• If we were to measure the inclinationangle recorded in volcanic rocks, where onthe Earth would we note a 0-degree angleof inclination? How about 90 degreesdown? How about 90 degrees up?

Senior Years Science Teachers’Handbook ActivitiesStudents research contributions of earlyArctic explorers in locating the truemagnetic north (e.g., Sir John Ross, RoaldAmundsen).

SUGGESTIONS FOR INSTRUCTION

Topic 4.3 – 41

SENIOR 3 PHYSICS • Topic 4: Fields

SKILLS AND ATTITUDES OUTCOME

S3P-0-1c: Relate the historicaldevelopment of scientificideas and technology to theform and function ofscientific knowledge today.

GENERAL LEARNING OUTCOMECONNECTION

Students will…Understand how stability,motion, forces, and energytransfers and transformationsplay a role in a wide range ofnatural and constructedcontexts(GLO D4)

Major Explanatory Stories of ScienceStudents explore the contents of WilliamGilbert’s tract Physiologia Nova de Magneteof 1628, and describe details of his“physiological” approach to understandingmagnetic phenomena. Produce a “tract” orpamphlet for distribution to Grade 3students in your community who also studymagnetism. (See Appendix 4.5: WilliamGilbert and the Earth’s Magnetic Field.)

SUGGESTIONS FOR INSTRUCTION SUGGESTIONS FOR ASSESSMENT

SUGGESTED LEARNING RESOURCES

WebsitesMany interesting photos of auroras can befound in the astronomy picture of the dayarchive at: <http://antwrp.gsfc.nasa.gov/apod/archivepix. html>

For an aurora over Winnipeg, see: <http://antwrp.gsfc.nasa.gov/apod/ap011105.html>.

ReferencesBrush, Steven. (1997) “Whole EarthHistory.” Historical Studies in the PhysicalSciences 17.2: 345-355.<http://punsterproductions.com/~sciencehistory/WEH.htm>

Appendix 4.5: William Gilbert and theEarth’s Magnetic Field

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Topic 4: Fields • SENIOR 3 PHYSICS

NOTES