SP212 Lab: Three Equipotential Surfaces and Electric Fields
Version: February 1, 2015
PHYSICS II
LAB 3
SP212
EquipotentialSurfacesandElectricFields
I. Introduction
Lastweek,weusedacomputersimulationtoinvestigatethestructureofthe
electricfieldcreatedbyvariouschargedistributions.Thisweek,wewantto
makeobservationsofarealphysicalsystem,andsowewillfocusonthe
electricpotential,aneasierquantitytoexperimentallymeasure.Thislab
shouldhelpyouunderstandhowthetwoconceptsarerelatedtoeachother.
II. Procedure
A. ParallelPlates
A.1.ModelPrediction
a)
Ifyouhaven’tdonethepre‐lab,revisitourelectricfieldsimulatorfrom
lastweekathttp://phet.colorado.edu/en/simulation/charges‐and‐fields.If
youhavedonethepre‐lab,skipto(d).
b)
Constructtwolongparallellines:onenegative,theotherpositive.
c)
Thistime,usetheequipotentialdrawingtooltodraw6‐7equipotential
surfacesin‐betweenyourtwolines.
d)
Noticehowtheequipotentialsurfacesrelatetotheelectricfieldvectors.What
istheanglebetweenthetwo?Whichwaydoestheelectricfieldpoint(from
lowVtohighVorfromhighVtolowV)?Wheredoyoufindapotentialof0
volts?
e)
ReadorrereadsectionA2.Howdoesthegeometryofexperimentdifferfrom
thegeometryofthechargedistributionyoucreatedwiththesimulator?Do
youexpecttofindthezeroequipotentialsurfaceatthesamelocationasthe
model?Whyorwhynot?Howelsedoesyourmodeldifferfromtheactual
experimentyouwillbeconducting?Bespecificbutsuccinctinyouranswer.
Page 1 of 4 SP212 Lab: Three Equipotential Surfaces and Electric Fields
Version: February 1, 2015
A.2ExperimentalTest
a)
Nowturntoyourwatertray.Setthetwostraightelectrodesupasshownon
Graph1:Placeonealongthey=1inchlineandtheotheronthey=8inch
line.Placetheelectrodesneartheleftsideofthetray,leavingplentyofspace
ontherightside.
b)
Pouraboutacentimeterofwaterinyourwatertray.
c)
Setyourdigitalfunctiongenerator(DFG)forsinusoidaloutput,andconnecta
long,blackbananacabletoground,or“GND.”Connectalong,redbanana
cableto“Hi.”MakesuretheAmplitudeisturnedallthewaydown,thatis,
allthewaycounterclockwise.
d)
Connecttheotherendoftheblackcabletothey=1inchelectrode,sothatwill
beGND.
e)
Connecttheotherendoftheredcabletothey=8inchelectrode.
f)
SetyourProtekDMMtomeasureACVolts;that’sthe“V”withthetilde(~)
aboveit.
g)
Beingcarefulnottomovetheelectrodes,connectthe“GND”terminalofyour
Protektothe“GND”electrode,andthe“V”terminaltotheotherelectrode. h)
TurnontheProtek,turnontheDFG,andsetthefrequencyto300Hz.
i)
CarefullyincreasetheDFGamplitudeuptoapproximately5.00Volts(theDFG
doesn’thaveavoltageindicator,sowemustusetheprotek),asnearlyasyou
can.Recordtheactualvalue,callingitVmax.
j)
Locateandtraceout6‐7equipotentialsurfacesbetweenthetwoplatesand
measurethedistanceofeachfromthegroundplate.Thinkaboutthisprocess.
Willitbeeasiertochooseintegervaluesofpotential,orintegervaluesof
distancefromthegroundedplate?Recordyour6valuesofV(r)inExcel.
Watchthoseunits!
k)
Compareyourwatertraymeasurementstoyourcomputerpredictions.Does
thesametypeoffunctionfitboth?Howarethefunctionsdifferent?Canwe
concludethatthecomputersimulationisagoodmodelforourdata?
Page 2 of 4 SP212 Lab: Three Equipotential Surfaces and Electric Fields
Version: February 1, 2015
B.
PlugandRing
B.1ModelPredictions
a) Ifyouhaven’tdon’tthepre‐lab,revisitourelectricfieldsimulatorfromlast
weekathttp://phet.colorado.edu/en/simulation/charges‐and‐fields.Ifyouhave
donethepre‐labskiptoB.2.
b) Plottheequipotentialsurfacesforasinglepositivepointchargeintheelectric
fieldsimulator.MeasureV(r)for6‐7equipotentialsurfaces,andfitapower
lawtoyoursimulateddatainExcel.Whatistheexponent?Isitwhatyou
expected?
B.2ExperimentalTest
a) AsshowninGraph2,setupthebrasspluginthecenterofyourwatertray,
andplacethelargebrassringaroundit.Groundtheouterring,andattachthe
“hot”(notGND)leadtothecentralplug.Usethesamesettingsyoudidforthe
parallelplates.
b) MeasureV(r)for6‐7equipotentialsurfaces.(Rhetorical:Areyougettingthe
feelingofdéjàvu?)
c) Usingtheadvancedphysicstechniqueof“lookingatit”,Ifigurethissetup
looksalotlikethepointchargeIjustbuiltinthesimulator,soV(r)should
behavethesame…
Wellonsecondthought,the1/rbehaviorofthepointchargepotentialwasa
resultofthe1/r2behaviorofGauss’law.Theelectricfieldfallsofflikethe
boundaryofthevolume,whichin3‐Disthesurfaceareaofasphere.Sincethe
plugistallerthanthewater,thisset‐upismorelikeasetoflongnested
cylinders,likeonpage721ofourtextbook.Inthatset‐up,btakesontherole
ofwhatwehavebeencallingr,thedistancefromtheinnerplug.HowdoesV
dependonbinthatcase?(Actuallybistheradiusoftheoutercylinder,but
thisdetailisnotimportanttoourdiscussionhere.)
Hmmm,twointerestinghypotheses.
d) UseyourdatatodetermineifIwasrightwithmyfirsthypothesisormy
second.
i. Tryfittingapowerlawtothedata,andthentryfittingalogarithmic
functiontothem.Whichonefitsbetter?
ii. Isourplugmodeledwellasapointcharge?Explain.
Page 3 of 4 SP212 Lab: Three Equipotential Surfaces and Electric Fields
Version: February 1, 2015
III.Clean‐Up(ensurenowaterleftondesk!)
A. GoldenRule:“Dountoothersasyoudesirethemtodountoyou.”
B. ThisappliesasmuchhereinthelabasitdoesintheFleet.AsfutureNavalOfficers,
howcanyouexpectyourenlistedsailorstomaintainacleanworkareaifyour
stateroom,workareas,messarea,etcisa“pigsty?”Soasofficersitisimperativethat
wecleanupafterourselvesnotonlytofollowtheGoldenRule,butalsotoleadby
examplefortheenlistedfolksunderourcharge.
C. EndofLabCheckout:Beforeleavingthelaboratory,pleasetidyuptheequipmentat
theworkstationandquitallrunningsoftware.Trustme,it’seasiertospongethe
wateroutofthetrayintothejarthencarrythejartothesink.DON’Ttrytocarrythe
wholetray!
D. Thelabstationshouldbeinbetterconditionthanwhenyouarrivedandmore
importantly,shouldbeofanappearancethatyouwouldbePROUDtoshowtoyour
legalguardiansduringa“ParentsWeekend.”
E. Haveyourinstructorinspectyourlabstationandreceivetheirpermissiontoleave
theLabRoom.
F. YouSHALLfollowthisproceduredoingeverylabforSP212!
Many thanks to Dr. Huddle, Dr. Katz, LCDR Riebel, LCDR Barry, and Dr. Fontanella for their assistance in producing this Laboratory procedure; specific references can be supplied on request. LCDR Timothy Shivok Page 4 of 4