lab 8 - magnetic fields - physics charges produce electric fields. ... “tesla” is a large...

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  • Lab8:MagneticFields

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    Objectives:Tomeasureandunderstandthemagneticfieldofabarmagnet.Tomeasureandunderstandthemagneticfieldofanelectromagnet,inparticular,asolenoid.Apparatus:Barmagnetbox,compass,PASCOmagneticfieldsensor,USBwire,PASCOinterface,multimeter,powersupply,redandblackleadsandalligatorclips,rheostat. Electricchargesproduceelectricfields.Magnetsandelectromagnetsproducemagneticfields. Weusedtheelectricfieldmodeltoexplainhowachargeorachargedistributionexertsforcesonotherchargesatadistance. Similarlyweuse themagnetic fieldmodel toexplainhowamagnetexerts forcesonothermagnetsandchargesmovingrelativetothefield.Thereforemagneticfieldisavectorquantity.Magneticfield ismeasured inTesla(T). Tesla isa largeunitwithoneTeslabeingaverystrongmagnetic field.Small magnetic fields like the magnetic field of a small bar magnet are measured in millitesla. Thehorizontalcomponentoftheearthmagneticfieldisabout50microteslaonthesurfaceoftheearth.Welearnedthatpositiveandnegativeelectricchargescanbeseparatedandmonopolesanddipolesoccurnaturally.However, inmagnetsweonlyfindmagneticdipolesinnaturewherewealwaysfindanorthpoleandsouthpoletogether.Hereinthisexperimentwequalitativelyinvestigatethemagneticfieldaroundabarmagnet(permanentmagnet)andanelectromagnetusinga tiny testmagnetcalleda compass. Thenwewillmeasure themagneticfieldofabarmagnetandanelectromagnetusingPASCOmagneticfieldsensorandstudyhowthemagneticfielddependsonposition,current,etc.PartA:PermanentMagnets

    1. Takethetwobarmagnetsoutoftheboxandinvestigatetheirinteraction.Explainhowthepolesinteractintermsofattractionandrepulsion.

  • 2. Now place one magnet on the marked space below and identify north and south poles of themagnet.Thedirectionofthemagneticfieldatapointinspaceisindicatedbythecompassmagnet.Thenusing the compass identify the direction of themagnetic field at the points shown. Thendrawmagneticfieldlinesconnectingappropriatepointstocreateamagneticfieldlinedrawing.

    3. NowconnectthePASCOmagneticfieldsensortothePASCOpassportinterfaceandtheinterfacetothecomputerviaUSBcable.OpenuptheCAPSTONEsoftwareandselectthegraphandmeterwindow.Selecttheverticalaxistobemagneticfieldandthehorizontalaxistobetime.Changethesamplingrateto10samplespersecond.Presstherecordbuttonandseeifitrecordsdataonthe graph. Move the sensor away frommagnets and see if the value changes fromposition topositionandinwhichdirectionthesensorisdirected.Enlargethescaleofthemagneticfieldaxistoreadthevalueinminitesla.(1mT=0.001T).

    Alignthesensorasfollowstomeasurethemagneticfield.DirectionofthemagneticfieldWhenyoualignthesensorasshownReadingispositive.

    Thesensorhastobeperfectlyalignedwiththemagneticfieldtomeasureitaccurately.Makesurethatyoukeepthesensorasshowninthefigure.

  • 4. Keepthemagneticfieldsensorasshownbelowandmeasurethemagneticfieldattheedgeofthemagnet.Itmaybehelpfultoputsomethingunderthemagnetsothatthemagnetandsensorareatthesameheight.Thenmovethesensoralongtheaxisofthemagnettotherightandmeasurethemagneticfieldatfivedifferentpoints.Keepthesensoratafixedpositionandruntheprogramtoseethereadingasafunctionoftime.Gettheaveragereadingandrecorditbelow.

    Magnet Sensor

    SN

    Position(cm) Magneticfieldstrength(mT)0.1 0.5 1.0 1.5 2.0 2.5

    5. TransferdatatoanExcelfileandplotmagneticfieldvs.position.Drawanappropriatetrendline

    anddisplaytheequationonthechart.Namethegraph,labelaxes,anddisplayunits.

    6. Qualitativelyexplainhowthestrengthofthemagneticfieldchangeswithdistancefromtheedgeofthemagnetbasedonyourgraph.

    7. Yourdatamightindicateamagneticfieldevenwhenthesensorisfarawayfromthemagnet.Whatarethepossiblereasonsforthissituation?(Slowlymovetheprobearoundotherwires,computersetc.andseeiftheyindicateanymagneticfield)

  • PartB:Electromagnets

    The figure below shows a schematic diagram of a solenoid. The direction of themagnetic fieldcreatedbythesolenoiddependsonhowthewireiswrappedandthedirectionofthecurrentflow(whichend itenters).Thestrengthof themagnetic fielddependson thenumber turnsperunitlengthofthesolenoid,thecurrentinthewire,andthemagneticpermeabilityinthemedium.

    magneticfieldNScurrentincurrentout

    Youaregivenasolenoidanditisanelectromagnet.Carefullyobservehowitismade.Aninsulatedwire iswrappedaroundahollowmetallic cylinder.Whena current flows through thewire thesolenoidbecomesanelectromagnet. In thisexperimentyouwant to investigate thestrengthofthemagneticfieldinsidethesolenoidasthecurrentisvaried.Weneedtosendaknowncurrentthroughthesolenoidwireandmeasuremagneticfield.Connectthecircuitasshownbelowfortheexperiment.

    SensorInterfacecomputerSolenoidPowersupplyAmmeter Rheostat(variableresistor)

    8. Powersupplyissetaround5.0Vforyouandadjusttherheostatuntilthecurrentindicatedbytheammeteris0.25A.Placethemagneticsensorasshownaboveandmakesurethatalignswiththeaxisofthesolenoid.Inserttheprobeaboutoneinchintothesolenoidandreadthemeasurement.Withtrialanderrorfindoutthemaximumreadingforrecording.Recordthereadingbelowwithunits.

    Magneticfield=__________________________________

    9. Switchoffthepowersupply.Thenswitchthedirectionofthecurrentbyswitchingthewires

    connectedtothepowersupply.Switchonthepowersupply.Recordthereadingbelowwithunits.Makesurethecurrentisstill0.25A.

    Magneticfield=__________________________________

  • 10. Whydoesthemagneticfieldhaveadifferentsignnow?Explain.

    11. Now repeat the same measurement as in Question #8 with a range of values for the currentflowingthroughthewiresofthesolenoid.Recordyourdatainthetable.

    Current(A) Magneticfield(mT)0.25 0.50 1.25 1.50 1.75 2.00

    Themagneticfieldinsideasolenoidisexpectedtobegivenbytheequation

    = !!!"!

    = !

    where = !!= !"#$%& !" !"#$%

    !"#$%!, ! = 1.257106 Tm/A

    So this suggests that themagnetic field is linearlydependenton thecurrent. Plot themagneticfieldvs.currentgraphinExcel.Drawalineartrendlineanddisplayequationonchart.

    12. Are your measurements of magnetic field vs. current consistent with a linear relationship?Explainwhyyoucometothatconclusion.

    13. Usetheslopeofyour linear fitting todeterminethenumberof turnsperunit length,, for thissolenoid.Showyourwork.RemembertowatchexponentssinceyoumeasuredthefieldinmT. =____________________________

  • PartC:ConclusionsandReflection

    14. Summarizewhatyoulearnedaboutthemagneticfieldfromabarmagnet.

    15. Summarizewhatyoulearnedaboutthemagneticfieldfromasolenoid.

    16. Wedidnotconsidertheeffectofearthsmagneticfieldwhendoingourexperiment.Wouldyourresultsbeeffectivelydifferentifweconsideredtheeffectofearthsmagneticfield?Youneedtopayattentiontotheearthsmagneticfieldvector(magnitudeanddirection)whenmakingyourarguments.

    Instructionsonhowtosubmitthegraphs:1. Openaworddocumentandtypethenamesofallpresentgroupmembers.2. IfyouneedhelpfindingslopesusingExcel,pleasetalktoyourTA.3. CopyyourExcelgraph(withtitleandaxislabels)toyourWorddocument.4. Printthedocumentandattachittothelab.