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SP212 Lab: One Electrostatics Version: January 6, 2014
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Physics II
Lab 1
SP212
Electrostatics
I. Introduction
A. Physicsisanexperimentalscience,whichmeansthatsomephysicistsdiscoverlawsandprinciplesthroughcarefullyconductedexperimentsanddataanalysis.Oftentheresultisamathematicallydescribedphysicalprinciple.
B. CharlesAugustindeCoulombwasonesuchscientist.Inthe18thcentury,heandmanyofhiscolleaguessuspectedthatthemagnitudeoftheelectricalforcebetweentwochargedobjectsdecreasesasafunctionoftheirseparationsimilarlytothegravitationalforceasdescribedbyNewton’slawofuniversalgravity.
C. In1785,hepublishedreportsonthetheoryofelectricity,whichcontainedwhatisnowknownasCoulomb’sLaw.
D. Coulomb’sLaw:Themagnitudeoftheelectrostaticforcebetweentwochargedparticlesisdirectlyproportionaltotheproductofthemagnitudeofthechargesandinverselyproportionaltothesquareofthedistancebetweenthem.
1 2
2
q qF k
r=
v
E. Oneprimarydifferencebetweengravity(whichstudentslearnedaboutinCH13,PhysicsI)andelectrostaticforce,however,isthattheelectrostaticforcecanbeeitherattractive(likegravity)orrepulsive.
F. Thisrepulsiveforceisreadilyapparentinadeviceknownasanelectroscope.
G. Anelectroscopeisasimplescientificinstrumentinventedintheearly17thcenturytodetectthepresenceandrelativemagnitudeofthenetelectricalchargeonanobject.ElectroscopesdetectchargebytherelativedisplacementofareferenceobjectduetotheelectrostaticforceofrepulsiongovernedbyCoulomb’sLaw
SP212 Lab: One Electrostatics Version: January 6, 2014
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H. TheprimarypurposeofthislabistouseCoulomb’slawtomeasuretheelectricchargetransferredfromoneobjecttoanotherviaasimpleelectroscope.Asecondarypurposeistoobservethisrepulsivebehavior…tohelpstudentsrememberthatVECTORSareimportantandthatDIRECTIONMATTERSgreatly!
II. Discussion
A. Twoclassicaltypesofelectroscopesstillinusetodayarethegoldleafelectroscopeandthepithballelectroscope.
B. Evensince1900,homemadeelectroscopesweremadefromitemstypicallyfoundaroundthehousetoaidscientistsintheirlaboratoryexperiments.Duringtoday’slab,studentswillfollowsuitandbuildelectroscopesfromcommoneverydayobjectstoemphasizethatphysicsoccursintheworldaroundthem.
C. Tobuildtheelectroscopestodaystudentswillusecopperwireandaluminumfoil(oraluminumpaint)astheirconductors.Conductorsarematerialsthroughwhichchargecanmovefreely;examplesincludemetals(suchascopperwireandaluminumfoil),thehumanbody,andtapwater
D. Tohelpinsulatepartsoftheirelectroscope,studentswilluseplastic,glass,anddryairastheirnonconductors.Nonconductors—alsocalledinsulators—arematerialsthroughwhichchargecannotmovefreely;examplesincluderubber,plastic,glass,acrylic,dryair,andchemicallypurewater.
E. Yourinstructorwillexplainhowtobuildeachtypeofelectroscopethatwewillusetodaytomeasureelectrostaticobservablesandhowtoestimatetheuncertainties.
III.Objectives
Attheendofthisactivity,thestudentshould:
A. BeabletoexplainCoulomb’sLaw,includingtheeffectofeachtermontheresultantforce.
B. Observethatlikechargesrepelandunlikechargesattract.
C. Beabletodiscusshowanelectroscopeworkstodetecttherelativemagnitudeofthenetchargeonanobject.
D. Observeandbeabletoexplainthedifferencebetweenchargedbyinductionandchargedbyconduction.
SP212 Lab: One Electrostatics Version: January 6, 2014
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E. BeabletomanipulatetheCoulomb’slawequationandafreebodydiagramtosolvefortheamountofchargeonpointchargesseparatedbyafixeddistanceduetotheelectrostaticforce.
F. (TimePermitting)Usetwoflexiblestrawstofeeltheelectrostaticforceofrepulsion.
G. (TimePermitting)Beabletocalculatethenumberofconductionelectronsthattransferredfromtheacrylicrodtothepithball.
IV. NeededEquipment
A. AMasonjar,about30cmofbarecopperwire,scotchtape,blankcompactdisk(CD),acrylicrod,clothorfur,aluminumfoil,twopithballs,astanchionforattachingthepiths,calculator,ruler,meterstick,calipers,twoflexibledrinkingstraws,apaperclip,andapencil/pen.
V. Procedure
A. UsingasimplefoilelectroscopetoobserveCoulomb’slaw
A.1.Buildingthefoilelectroscope
a) CutasmallsquareofselfadhesivecorklargeenoughtocovertheholeinthecenterCD.
b) CarefullyremovethebackingandattachthecorktheholeinthecenterCD.Thepapercomesofftheglue,becarefulnottopullthecorkoffoftheglue.
c) Bendoneendofthepieceofbarecopperwireintoasmallspiralwithtotaldiameterbeingabout3cm.Leavetheotherendstraight.Thestraightendshouldbeabout9or10cmlong.Thebelowdrawingarenottoscale.
d) Punctureaholeinthecenterofthecorkusingthepaperclip,thenpushthenonbentendthroughcorkedholeinthecircularpieceofplastic.Leaveabout3cmabovethecircularpieceofplasticandwrapapieceofscotchtapearoundthewiretopreventthewirefromfallingdownthroughthecork.
SP212 Lab: One Electrostatics Version: January 6, 2014
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e) Onthenonbentend,bendtheendtomakea“J”orhookshape.
f) Cuttwostripsofthealuminumfoilapproximately1cminwidthand4cmlong.
g) Foldoverapproximately1cmononeendofeachstriptoreinforceandthenplacethefoilstripsontopofeachother.Thenusingamechanicalpencil(orsimilarlysharpobject,liketheendofthepaperclip)pierceaholeinthecenterofthefoldedoverpiece.
h) Smoothoutthealuminumstripstoensuretheyarenotstucktogether.
i) Nowplacethefoilstripsoverthe“J”youformedinthewireandinstallthemechanismontopoftheMasonjar.
j) Yourfinaldeviceshouldlooksimilartothediagrambelow.
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A.2.Usingtheelectroscopetoshowchargedbyinduction.
a) Here’show:Firsttransferchargesfromyourclothorfuratyourstationtotheacrylicrodbyrubbingthecloth/furovertherodtightlyacoupleoftimes.Thefrictionalchargingprocessinvolvesthetransferofelectronsbetweentwoobjectsbyrubbingthemtogether.
b) Now,let’sseeiftherodhasbeenchargedbyfrictionwhilealsotestingtheprocessofchargingtheelectroscopebyinductionfirst.Holdtheacrylicrodupto(BUTDONOTTOUCH)thecopperspiral.Noticethatthefoilleavesmoveapart.Let’sseewhy.First,assumeforthemomentthattherodhasanetpositivechargeasshowninthefigurebelow.Whentherodisnearthecopperspiral,negativelychargedparticlesinthecopperandthefoilareattractedupintothespiral,whilethepositivelychargedparticlesarerepelleddownintothefoilleaves.Sincebothleaveshavethesamesigncharge,theyrepeloneanother,andsomoveapart.
Figure1
IllustrationfromWikipedia:http://en.wikipedia.org/wiki/Electrostatic_induction:Yourchargesmightbereversedependingonwhetheryouchargedyouracrylicrod
withfurorcloth.
c) Slowlymovetherodawayandnoticethatthefoilleavesstarttomoveclosertogetherbecauselesschargeisinducedinthefoilleaves.
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d) Nowputmorechargeonyourrodbyrubbingitagainwiththeclothorfurafewmoretimes.Thenbringtherodupto(BUTDONOTTOUCH)thecopperspiral.Noticethatthefoilleavesmoverelativelyfurtherapartthanthefirsttime;supportingtheideathatthemagnitudeoftheelectricforceincreasesasthenetchargeincreases..Itworksbestifyoudonotletthefoilleavestouchthesideofthejar.Iftheydotouch,youmighthavetomovethemechanismsuchthattheleavesarenolongertouchingtheMasonjarside.
e) CompletelymovetherodawayandnoticethatthefoilleavesDONOTremaincharged.Thenetchargeinthecopperandfoiliszero,butwiththeabsenceoftherod,thepositiveandnegativechargesarenolongerbeingattractedorrepulsedbytherod.
1) Lookingbackatfigure1,howwouldthepicturechangeiftherodisnegativelycharged?Canwetellonlybythisanalysisifourrodispositivelyornegativelycharged?Whatexperimentwouldyousuggesttotellwhetherourrodwaspositivelychargedornegativelycharges?
2) Whenyourubanacrylicrodwithfur,electronsaretransferredtotherod,causingtherodtobecomenegativelycharged.Ifyourubtheacrylicrodwithsilkcloth,electronsaretransferredtothesilkclothandtherodbecomespositivelycharged.
A.3.Usingtheelectroscopetoshowchargedbyconduction.
a) Beforestartingthisnextexperiment,touchthecopperspiralwithyourfinger.Thisprovidesapathforanyexcesschargestogotogroundviayourbody.Itdoesnothurt,andyouprobablywon’tevenfeelanything.
b) Transferchargesfromyourclothorfuratyourstationtotheacrylicrodbyrubbingthecloth/furovertherodacoupleoftimesasyoudidinthepreviousexperiment.
c) Then,todemonstratechargedbyconduction,TOUCHtheacrylicrodtothecopperspiral.Noticethatthefoilleavesmoveapart.
d) Slowlymovetherodawayandnoticethatthistimethefoilleavesstayapart;indicatingthefactthatchargewaspermanentlytransferredtothefoilleavesbyconduction(touching).
e) Nowputmorechargeonyourrodbyrubbingitagainwiththeclothorfurafewmoretimes.Thentouchthecopperspiralwiththerod.Noticethatthefoilleavesmoveevenfurtherapart,againshowingthattheelectrostaticforceisproportionaltocharge.
f) Completelymovetherodawayandthefoilleavesremainapart.Chargewastransferredbyconduction.
SP212 Lab: One Electrostatics Version: January 6, 2014
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A.4.Discussionpoints:
a) Describetheoperationoftheelectroscope.BespecificanduseproperEnglishsentences.
b) DuringstepA.3,howdoesthemagnitudeofelectrostaticforceononeofthefoilleavescomparethatontheother?Drawafreebodydiagramtoassistyourdiscussion.
c) Explainthedifferencebetweenchargedbyinductionandchargedbyconduction.
d) Whatwouldbetheeffectonthefoilleavesifthehumidityintheroomwereraised?Why?Hint:humidairisbetteratconductingelectricitythandryair.
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B.UsingapithballelectroscopetoobserveCoulomb’sLaw
B.1.Buildingthesimplepithballelectroscope
a) Pithisthespongytissueonthestemsofsomeplants.Itisverysoftandlightweight.Thismakesitadvantageousforourpurposesbecauseitsgravitationalforceissmallenoughthattheelectrostaticforceiscomparable.
b) Inourcase,thepithballshavebeenpaintedwithaluminumpaintsothattheelectrostaticchargewilldistributeitselfuniformlyoverthesurfaceofthepithball.Thisisimportantforthisexperiment,becausewewilltreatthepithballasifitwereapointsourcewithallofitsexcesschargewerelocatedatitscenterofmass.
c) Before attaching anything to the stanchion we must make some initial measurements. 1) Measurethelengthofthestringfromthepivotpointtothecenterofthe
pithball(wearemakingtheassumptionthatthestringisnearlymassless
comparedtothepithball,sothecenterofmassofthesystemisthecenterofthepith‐ball.)Recordthislength.
2) Normallyyouwouldhavetodeterminethemassofthepithball.Sincethescalesintheroomarenotcalibratedtoalowenoughscale,assumethe
averagemassofapithballis40mg(thisisanexampleofpossibleuncertaintyinourexperiment.)
d) Attach both pith balls to your stanchion and make the two pith balls sit parallel to each other (same distance down from pivot point.) Do not tie strings into knots as they must be used by other students!
e) Touch each pith-ball with your finger to remove any net charge from them.
f) The pith balls should be touching each other at this point. Ensure that the strings are not tangled.
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B.2.Nowyouarereadytoplacechargeonthepithballs
a) Wipetheacrylicrodvigorouslywiththeclothorfurthenrubtherodoneachpithballoneatimetotransferchargetothepithballs.Trynottotouchthepithballswithyourfingersasyouwillaccidentlygroundthemcausingthemtodischarge.
b) Whatdoyouexpectthepithballstodoaftertheyarecharged?Why?
c) Redostepa)afewtimestogetthetwopithballstobeseparatedbyapproximately1to2cm.
B.3.Measurementofthedistancebetweenthepithballs
a) Inordertotrytocalculatethechargeonthepithballs,youneedtoknowtheexactdistancebetweenthetwopithballs.
1) Thisisthemostdifficultpartofthislab.Therearemanydifferentwaystoachievethismeasurementwithoutdischargingthetwopith‐balls,trytodiscoverthemethodthatyouthinkworksbesttoachievethisgoal.Note:
JustlikemanytasksassignedtoyouintheNAVY,itisprobablynotbestachievedviaasingleperson!Trynottotouchthepith‐ballswithyourfingersortheruler/calipersasyouwillaccidentlygroundthemcausing
themtodischarge!
2) Whatwouldhappenifyoubroughtupameasuringscalenexttothepithballsinordertomeasurethedistancebetweenthecenters?Tryitifyouarenotsure.Explain.
3) Ifyoudischargeyourpithballs,andthedistancedropstobelow1cm,thenrechargeyourpithballstoachievethedesiredrangetopreventtheuncertaintiesfrombeinglargerthanthemeasurementsyouaretaking.
4) Ifyoucomeupwithagreatidea,shareitwiththeclass.Theprocessof
sharinglessonslearnedsothatyoudonotrecreatethesameerrorsoverandoveragaininthefleetisanencouragedpractice
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B.4.DataAnalysis(Findingthechargeonthepithballs.)
a) Firstlookattheforcesactingonahangingpithball.Labeleachoftheforcesontheprovidedfreebodydiagramoftheleftball.
b) Labelthetrianglebelowformedfromwherethepithballstartedtowhereitendedwiththeaboveforcesasappropriate.
c) Noticethatinreality,thatthereisasmallheightdifferencebetweentheverticalpositionoftheballanditsfinalposition.Callthisheight,h.IfyourememberfrompendulumsinCH15,thevalueofhistrulyverysmallandcanultimatelybeneglected,butlet’snotgetaheadofourselves.
d) Lookatthesimilartrianglethatisformedbetweentheelectrostaticforceandtheforceofgravityemphasizedinthedrawingabove.IfyoucallthelengthofyourstringL,asstatedabovetheheightdifferenceish,andifyoucallthehorizontaldistancefromvertical,x.ThentheratiooftheelectrostaticforcetothatoftheforceofgravitymustbethesameratioasxtoLhbysimilartriangles.Writethisratioequation.
L L‐h
hx
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e) RearrangetheratioandsolveforFeintermsofFg.
f) Youalreadyknowthat:
1 2
2e
q qF k
r=
vand
gF mg=
g) Butsincewecanassumethatq1=q2andthatLhisessentiallyL,thenyoucanrewritethetwoequationsforFeandsetthemequaltoeachother.Youshouldwindupwithasingleequationthathask,q,andrononesideandx,m,g,Lontheotherside.
h) Now,thinkabouthowrisrelatedtoxandthenrearrangetheequationtosolveforq.Afterwalkingyouthroughtheprocessthisfar,youshouldbeabletosolveforqintermsofg,k,m,L,andxallofwhichshouldbeknownvaluesatthispoint.
i) Determinethechargeoneachpithball.Youranswershouldbeinnanocoulombs.
Q=_______________________nC
B.5.Discussionpoints:
a) Describeatleastfourpossiblesourcesoferrororuncertaintyintroducedduringtheexperimentnotincludinghumanmistakesormathematicalmistakes.
b) Whatrecommendationsdoyouhavetominimizetheaboveerrorsifyouhadtoredothislab?
c) Howdoesthemagnitudeofelectrostaticforceoneachofthepithballscomparewitheachother?
d) Ifourassumptionthateachofthepithballshadequalchargetransferredtoitturnedoutnottobetrue,saytheleftballhad10%moreexcesschargeonitthantherightball,whataffect(ifany)wouldthishaveontherelativemagnitudeofelectrostaticforceoneachofthepithballs?
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C.(TimePermitting)Anexperimenttofeelthatelectrostaticforceofrepulsion.
C.1.Itturnsoutthatifyourubaplasticdrinkingstrawwithclothorfur,thatchargesaretransferredinamannersimilarrubbingtheacrylicrodwiththeclothorfur.
a) Rubaprovidedplasticdrinkingstrawwithclothorfur.
b) Handittoyourlabpartnerwhoshouldholditviathesmallendbeforetheflexiblepart.Trynottotouchtherestofthestrawwithyourfingersasyouwillaccidentlygrounditcausingittodischarge.
c) Rubanotherprovidedplasticdrinkingstrawwiththeclothorfur.
d) Nowyourlabpartnershouldtrytoholdthestrawsasshownbelow.
e) Yourpartner’sfingersshouldclearlybeabletofeeltheforceopposingtheirattemptstobringthetwostrawstogether.
f) Althoughitwouldbenicetodeterminetheamountofchargeonthestraws,thestrawscannotbemodeledaspointcharges.However,itwasagoodqualitativeexperiment.
D.(TimePermitting)Calculatethenumberoffreeelectronsthatwereplacedonthepithball.
a) Usingthefactthattheelementalchargeis1.602x1019coulombs,calculatethenumberoffreeelectronsthatweretransferredfromyouracrylicrodtoasinglepithball.
#offreeelectrons=_______________________
b) Hopefully this now gives you a better feel for just how large a Coulomb of charge actually is, since we only transferred a few nanocoulombs here!
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E.LabReporttohandin:
E.1.SectionA4’sdiscussionquestionanswers.
E.2.SectionCdata,drawings,andcalculations.(Ifaspreadsheetwasused,includespreadsheet.)
E.3.Section C-5’s discussionquestionanswers.
E.4.Part D: Calculation if assigned.
VI. CleanUp
A. GoldenRule:“Dountoothersasyouexpectthemtodountoyou.”
ThisappliesasmuchhereinthelabasitdoesintheFleet.AsfutureNavalOfficers,howcanyouexpectyourenlistedsailorstomaintainacleanworkareaifyourstateroom,workareas,messarea,etcisa“pigsty?”SoasofficersitisimperativethatwecleanupafterourselvesnotonlytofollowtheGoldenRule,butalsotoleadbyexamplefortheenlistedpersonnelunderourcharge.
1. EndofLabCheckout:Beforeleavingthelaboratory,tidyuptheequipmentattheworkstationandquitallrunningsoftware.
2. Thelabstationshouldbeinbetterconditionthanwhenyouarrivedandmoreimportantly,shouldbeofanappearancethatyouwouldbePROUDtoshowtoyourlegalguardiansduringa“ParentsWeekend.”
3. HaveyourinstructorinspectyourlabstationandreceivetheirpermissiontoleavetheLabRoom.
4. YouSHALLfollowthisprocedureduringeverylabforSP212!
ManythankstoDr.KatzandMr.GaryBishopfortheirassistanceinproducingthisLaboratory
procedure;specificreferencescanbesuppliedonrequest.LCDRTimothyShivok