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, inmagnetsweonlyfindmagneticdipolesinnaturewherewealwaysfinda“northpole”and“southpole”together.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.Selecttheverticalaxistobe“magneticfield”andthehorizontalaxistobe“time.”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

𝐵 = !!!"!

= 𝜇!𝑛𝐼

where𝑛 = !!= !"#$%& !" !"#$%

!"#$%!, 𝜇! = 1.257×10−6 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,𝑛, for thissolenoid.Showyourwork.RemembertowatchexponentssinceyoumeasuredthefieldinmT.𝑛 =____________________________

PartC:ConclusionsandReflection

14. Summarizewhatyoulearnedaboutthemagneticfieldfromabarmagnet.

15. Summarizewhatyoulearnedaboutthemagneticfieldfromasolenoid.

16. Wedidnotconsidertheeffectofearth’smagneticfieldwhendoingourexperiment.Wouldyourresultsbeeffectivelydifferentifweconsideredtheeffectofearth’smagneticfield?Youneedtopayattentiontotheearth’smagneticfieldvector(magnitudeanddirection)whenmakingyourarguments.

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