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7/30/2019 Electric fields - P2, week 2
1/23
Electriccharge Electricchargeisapropertyofma.erthatcausesthepar2clesorobjectsthatpossessittoexperienceaforcewhen
nearotherobjectsorpar2clesthatalsopossessit
Thereareonlytwotypesofchargeonabody: -vetoomanyelectrons +vetoofewelectrons
Oppositechargesa4ract,likechargesrepeleachother Theelectricchargeisthesourceofelectricforce(canbea.rac2veorrepulsive)
Chargedbodiesinteractbyexer2ngforcesoneachother1
ElectricchargeisconservedItcanbetransferredbutnotcreatedordestroyed
Anobjectwithequalamountsof+veandvechargeissaidtobeelectricallyneutral(thereisnonetcharge)
7/30/2019 Electric fields - P2, week 2
2/23
ConductorsandInsulators
Conduc2ngmaterials(conductors)suchasmetalscontainalotoffreeelectrons
Insulatorsarematerialsthatdonowallowtheflowoffreeelectrons(egglass,plas2c)
Earthing(orgrounding)referstotheprocessbywhichaconductoremp2esitschargetothegroundandbecomes
electricallyneutral
7/30/2019 Electric fields - P2, week 2
3/23
Electric3ieldlines
Supposewebringasmall+vechargeBclosetoamuchbigger+vechargeA
Bexperiencesarepellingforce Thefieldlinesarethelineswhichposi
7/30/2019 Electric fields - P2, week 2
4/23
CoulombsLaw
Theelectrosta
7/30/2019 Electric fields - P2, week 2
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Electric3ieldstrength
TheelectricfieldstrengthEatapointisequaltotheforceonaunitchargeatthepoint
F=forceonaunitchargeq TheunitofEisN/CUniformfield->Esameatallposi2onsinfield Forceonatestchargeisthesamewhereveritisplaced
Non-uniformfield->Evarieswithposi2on
E=
F
q
5
7/30/2019 Electric fields - P2, week 2
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Electric3ieldstrengthnear
pointcharge TheelectricfieldstrengthEnearapointchargeis
Ifwewanttofindtheelectricfieldstrengthduetomul2plepointcharges,weusetheprincipleofsuperposi
7/30/2019 Electric fields - P2, week 2
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7
7/30/2019 Electric fields - P2, week 2
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Electric3ieldstrengthnearan
electricdipole Electricdipole:Twoequalandoppositechargesseparatedbyadistanced
Whatistheelectricfieldduetothedipole? E=E(+)-E(-)
- +E(-)
- +d
r(-)
r(+)
z
E(+)
dipoleaxisP
8
7/30/2019 Electric fields - P2, week 2
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pistheelectricdipolemoment=qd zisthedistancefromthemidpointofthedipoleaxis
Electric3ieldstrengthnearan
electricdipole
E(-)- +
d
r(-)
r(+)
z
E(+)
dipoleaxisP
E=1
20
pz
3
9
7/30/2019 Electric fields - P2, week 2
10/23
ElectricPotential
Theelectricpoten2alVatapointintheelectricfieldisdefinedasthepoten2alenergyUperunitchargeofaposi2ve
testchargeplacedatthatpoint
Electricpoten
7/30/2019 Electric fields - P2, week 2
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7/30/2019 Electric fields - P2, week 2
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Equipotentials
Equipoten
7/30/2019 Electric fields - P2, week 2
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Potentialgradients
Apoten
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Potentialgradients
Uniformfield Thepoten
7/30/2019 Electric fields - P2, week 2
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Potentialgradients
Non-uniformfield MoveatestchargeasmalldistancertowardsachargeQgenera2nganelectricfield
MustexertaforceF=-qE WorkdoneW=Fr WorkdoneperunitchargeisW/q=Fr/q=-qEr/q=-Er
Thisisequaltothechangeofpoten2alV=-Er
Therefore
r
E= V
r
15
7/30/2019 Electric fields - P2, week 2
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Parallel-plateCapacitors
Twoparallelplatesadistanancedapartarechargedtoapoten2aldifference
Oneplategainsthee-theotheroneloses Chargestoredoneachplateassumingemptyspacebetweenplates
Waystoincreasechargestored:increaseA,increaseV,decreased
Q0 = AV0
d
16
7/30/2019 Electric fields - P2, week 2
17/23
Parallel-plateCapacitors
Thereisanotherwaytoincreasethechargestored:Useadielectric
Dielectrics=insula2ngmaterialsusedtoincreasethecapacityoftheplatestostorecharge
Whenadielectricisplacedbetweenthechargedplates,itsmoleculesbecomepolarized
ForafixedV,inser2ngadielectricwillincreasethechargestoredintheplates
17
7/30/2019 Electric fields - P2, week 2
18/23
Relativepermittivity
Therela2vepermiVvity(ordielectricconstant)risthera2oof(chargestoredwithdielectric)/(chargestoredwithout
dielectric)
Chargestoredoneachplate(withdielectric)
Sometypicalvaluesforr acuum/Air=1,Acrylic(Plexiglass)=45,Beeswax=6 Silk=35,Silicon=1,Pyrexglass=5,Paper=3
Q = rQ0 =AV
0r
d
18
7/30/2019 Electric fields - P2, week 2
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Capacitance(ofparallelplatecapacitor)
ThecapacitanceCofacapacitorisdefinedasthechargestoredperunitpd(voltage)appliedacrossitsplates
Unit:Farad(F)=1Coulombperolt Capacitancevaluesareusuallygivenin
F(1F=10-6F) pF(1pF=10-1F)
LargecapacitancecanbeachievedbyincreasingA,decreasingd,usingdielectrics
C=Q
V
=
A0r
d
19
7/30/2019 Electric fields - P2, week 2
20/23
EnergystoredinaCapacitor
Chargingacapacitorstoresenergyinit Wecandischargeacapacitorinacircuit
Totalworkdoneinchargingthecapacitor=energystoredinthecapacitor
Charge
oltage
0
Q Q+Q
W
0
Q0
EC=
1
2
Q0V0=
1
2
CV0
2=
1
2
Q0
2
C
0
7/30/2019 Electric fields - P2, week 2
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ChargedConductingSpheres
Theelectricfieldofachargedspherebehaveslikethatofapointcharge
Atestchargeinthefieldexperiencesthesameforceasifallthechargewereconcentratedatthecenterofthesphere
Outsidethesphere,theelectricfieldstrengthandtheelectricpoten2alarethesameasifproducedbyanequalchargeat
thecenter
E=1
40
Qr2 V=
1
40
Qr
1
7/30/2019 Electric fields - P2, week 2
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ChargedConductingSpheres
Insidethespherethereisnoelectricfield(E=0) Thepoten2alremainsataconstantvalueinside Nopoten2aldifferencebetweenanypointinsidethesphereandapointatthesurface
Vin=
1
40
Q
R
7/30/2019 Electric fields - P2, week 2
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ToDo
Readchapter13fromthebook[p37-p54Electricfields] HomeworkAssignmentwk:ques2ons13.1,13.3,13.,13.6,13.8and13.10
Handitinnolaterthan:00pmnextWednesday(23Jan)-LATEWORKWILLNOTBEACCEPTED
3