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HomopolarMotor

Grade(s):MSHS Topic:PhysicalScienceStandards:PerformanceExpectationsMS-PS2-3Askquestionsaboutdatatodeterminethefactorsthataffectthestrengthofelectricandmagneticforces.HS-PS2-5Planandconductaninvestigationtoprovideevidencethatanelectriccurrentcanproduceamagneticfieldandthatachangingmagneticfieldcanproduceanelectriccurrent.HS-PS3-3Design,buildandrefineadevicethatworkswithingivenconstraintstoconvertoneformofenergyintoanotherformofenergy.DisciplinaryCoreIdeasMS-PS2.BElectricandmagnetic(electromagnetic)forcescanbeattractiveorrepulsiveandtheirsizesdependonthemagnitudesofthecharges,currentsormagneticstrengthsinvolvedandonthedistancesbetweentheinteractingobjects.HS-PS2.BForcesatadistanceareexplainedbyfields(gravitational,electric,andmagnetic)permeatingspacethatcantransferenergythroughspace.Magnetsorelectriccurrentscausemagneticfields;electricchargesorchangingmagneticfieldscauseelectricfields.HS-PS3.A“Electricalenergy”maymeanenergystoredinabatteryorenergytransmittedbyuelectriccurrents.HS-PS3.AEnergyisaquantitativepropertyofasystemthatdependsonthemotionandinteractionsofmatterandradiationwithinthatsystem.Thatthereisasinglequantitycalledenergyisduetothefactthatasystem’stotalenergyisconserved,evenas,withinthesystem,energyiscontinuallytransferredfromoneobjecttoanotherandbetweenitsvariouspossibleforms.ScienceandEngineeringPractices:AskingquestionsanddefiningproblemsPlanningandcarryingoutinvestigationConstructingexplanationsanddesigningsolutions.Objectives:Todemonstrateenergycanbetransferredfromobjecttoobject.Todeterminewhatelementscausehomopolarmotorstomove.Tocreateahandsoffhomopolarmotor.__________________________


Materials:Neodymiummagnet(AluminumfoilBarecopperwireBattery(AAAorother1.5V)Washer(tostabilizesmallermagnetsonly)

HomopolarMotor

AsPresentedbyLIGOScienceEducationCenter

SynopsisMotorsrelyonmovingelectricalchargesinteractingwithamagneticfield.Thissimpleconceptcanbeusedtocreateverysimplemotorsusingjustabattery,arareearthmagnetortwoandawire.Inthisexercisewecanmakeascrewspin,apieceofcopperwirespin,andwecanevenmakeabatteryspin.


HomopolarMotorAsPresentedbyLIGOScienceEducationCenter

NecessaryMaterials:Battery,rareearthmagnets(canbedonewithregularmagnets,butverydifficult),

Wirewithstrippedends(orbarewire)AdditionalMaterials:ExtraVarietyofbatteries,magnets,wires,wirestripper,needlenosepliers,other

conductingmaterialssuchasscrewsEngage:

Havestudentstouchaconductivestrippedendsofwiretoabattery–whathappenstothewire?WARNING:Weareessentiallyshort-circuitingthebattery–thiscanresultinthebatteryandthewirebecomingextremelyhot.Thereiseventheslimpossibilityofthebattery“exploding”(notinaverydangerousway–justmessy)duetotheheat.DoNOTpinthewirestothebatterywithyourbarefingers,ortheymaygetburned.

Explain:Youjustletthechemicalpotentialenergybecomeheatenergy.WecanalsoconvertthisenergyintoKineticenergyviaamotor.(Showanexampleofamotor.)Youcancreateamotoraswell.Allmovingchargesarepushedonbymagneticfields(unlesstheymoveparalleltothemagneticfield):Force=ChargexVelocityXMagneticFieldstrength(F=qVXB).Whatisanelectriccurrent?It’smovingelectriccharges,sobymovingelectricchargesthroughamagneticfield(butnotinlinewiththefield),thechargesarepushedbythemagneticfield.Motorsusethisprinciple.WearegoingtocreateasimplemotorcalledaHomopolarmotor.Havethestudentsgrabanailorscrew,arare-earthmagnetandapieceofwirewithatleasttheendsexposed.Arrangethemagnetonthebottomstucktothesmallnail/screw,whichisthenstucktothebottomofthebattery–allheldinplacebythemagneticforce.Next,attachapieceofwiretothetopofthebattery(useelectricaltape).Finallytouchthiswiretoeitherthenail/screworthemagnet(ifit’sconductive).Themagnetandnail/screwshouldstartspinning!

Asthechargesflowthroughthewire,themagnetpushesonthemwhentheycrossthemagneticfield.AccordingtoNewton’s3rdlaw(Foreveryactionthereisanequalandoppositereaction),the


chargesalsopushonthemagnet.Sincethemagnetandnail/screwaretheonlythingsthatcanmove,theystartmoving!Nowlet’strytobuildahomopolarmotorthathasthewiremoving.Therearemanywaystodothis–thekeypartsare:

1) Tomakesurethewiretouchesthetopofthebatteryatalltimes.2) Tomakesurethewiretouchesthebottomofthebattery(evenindirectly)atalltimes–this

completesthecircuit.3) Tomakesurethewirecrossesthemagneticfield.4) Tomakesurethatthereisn’tmuchfriction!

Theeasiestwaytodothisisprobablytostickthemagnet(ifitisconductive)onthebottomofthebattery,andformabalancedpieceofwirethatlightlysitsonthebatteryandlightlytouchesthesidesofthemagnet.

Itreallyhelpsifthewirehasitscenterofgravityeitherdirectlybelowthepointitbalancesorclosetounderneaththepointitbalanceson.Ifthecenterofgravityistoofaroff,thenthecenterofgravitywilltrytoshifttodirectlyunderthepointthewirehangsfrom,ofteneitherintroducingtoomuchfriction,ormorelikelypullingthetopoffofthebattery.

Extend:Challengestudentstocreateahomopolarmotorthat

1) Staysspinningforthelongesttime2) Spinsthefastest


3) Getsthemostnumberofspinsoutofabattery.4) Hastheleastamountofcopperrevealed(somecopperisnecessarytorevealtocompletethe

circuit–butdoesthewholepieceofcopperneedtoberevealed)?ORChallengethestudentstocreateahomopolarmotorthatdrivesa“vehicle.”Thiscanbedonebyusingasecondrare-earthmagnet,andmakingsurethebatteriesaresmallerdiameterthanthemagnet(orusingwashersaswheels).

Evaluate:Leftuptotheteacher

Abitmorebackground…Aphysicistmightsay:F=qVXBwhereFistheforceonthecharge,qisthecharge,VisthevelocityofthatchargeXmeansyoutakeacrossproductandBisthemagneticfield.Thecrossproductmeansthatonlywhenchargescrossthemagneticfield(notwhentheygointhesamedirectionofit)dotheyexperienceaforceandtheforceisperpendiculartoboththevelocityandthemagneticfield.Anotherwaytosayitis:F=qVBsinΘWhereFistheForce,qisthecharge,Visthevelocity,BisthemagneticfieldstrengthandΘistheanglebetweenthevelocityandthemagneticfield.

Internetresources

http://dangerouslyfun.com/homopolar-motorhttp://www.break.com/usercontent/2008/1/HOW-TO-build-a-Homopolar-Motor-434687.htmlhttp://en.wikipedia.org/wiki/Homopolar_motorhttp://blog.makezine.com/archive/2009/07/homopolar_motor_from_make_volume_01.htmlhttp://www.evilmadscientist.com/article.php/HomopolarMotorhttp://www.absoluteastronomy.com/topics/Homopolar_motorhttp://www.instructables.com/id/Homopolar-Motor-With-Five-Speed-Manual-Stick-Shift/http://atg.ga.com/EMS/defense/dc-motor/index.phphttp://www.arborsci.com/detail.aspx?ID=1283http://www.wondermagnet.com/asourceofrare-earth(particularlyneodymium)magnetshttp://www.youtube.com/watch?v=SWOvtGcP9J4