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Electriccharge Electricchargeisapropertyofma.erthatcausesthepar2clesorobjectsthatpossessittoexperienceaforcewhen

nearotherobjectsorpar2clesthatalsopossessit

Thereareonlytwotypesofchargeonabody: -vetoomanyelectrons +vetoofewelectrons

Oppositechargesa4ract,likechargesrepeleachother Theelectricchargeisthesourceofelectricforce(canbea.rac2veorrepulsive)

Chargedbodiesinteractbyexer2ngforcesoneachother1

ElectricchargeisconservedItcanbetransferredbutnotcreatedordestroyed

Anobjectwithequalamountsof+veandvechargeissaidtobeelectricallyneutral(thereisnonetcharge)

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ConductorsandInsulators

Conduc2ngmaterials(conductors)suchasmetalscontainalotoffreeelectrons

Insulatorsarematerialsthatdonowallowtheflowoffreeelectrons(egglass,plas2c)

Earthing(orgrounding)referstotheprocessbywhichaconductoremp2esitschargetothegroundandbecomes

electricallyneutral

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Electric3ieldlines

Supposewebringasmall+vechargeBclosetoamuchbigger+vechargeA

Bexperiencesarepellingforce Thefieldlinesarethelineswhichposi

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CoulombsLaw

Theelectrosta

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Electric3ieldstrength

TheelectricfieldstrengthEatapointisequaltotheforceonaunitchargeatthepoint

F=forceonaunitchargeq TheunitofEisN/CUniformfield->Esameatallposi2onsinfield Forceonatestchargeisthesamewhereveritisplaced

Non-uniformfield->Evarieswithposi2on

E=

F

q

5

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Electric3ieldstrengthnear

pointcharge TheelectricfieldstrengthEnearapointchargeis

Ifwewanttofindtheelectricfieldstrengthduetomul2plepointcharges,weusetheprincipleofsuperposi

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7

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Electric3ieldstrengthnearan

electricdipole Electricdipole:Twoequalandoppositechargesseparatedbyadistanced

Whatistheelectricfieldduetothedipole? E=E(+)-E(-)

- +E(-)

- +d

r(-)

r(+)

z

E(+)

dipoleaxisP

8

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pistheelectricdipolemoment=qd zisthedistancefromthemidpointofthedipoleaxis

Electric3ieldstrengthnearan

electricdipole

E(-)- +

d

r(-)

r(+)

z

E(+)

dipoleaxisP

E=1

20

pz

3

9

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ElectricPotential

Theelectricpoten2alVatapointintheelectricfieldisdefinedasthepoten2alenergyUperunitchargeofaposi2ve

testchargeplacedatthatpoint

Electricpoten

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Equipotentials

Equipoten

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Potentialgradients

Apoten

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Potentialgradients

Uniformfield Thepoten

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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

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Parallel-plateCapacitors

Twoparallelplatesadistanancedapartarechargedtoapoten2aldifference

Oneplategainsthee-theotheroneloses Chargestoredoneachplateassumingemptyspacebetweenplates

Waystoincreasechargestored:increaseA,increaseV,decreased

Q0 = AV0

d

16

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Parallel-plateCapacitors

Thereisanotherwaytoincreasethechargestored:Useadielectric

Dielectrics=insula2ngmaterialsusedtoincreasethecapacityoftheplatestostorecharge

Whenadielectricisplacedbetweenthechargedplates,itsmoleculesbecomepolarized

ForafixedV,inser2ngadielectricwillincreasethechargestoredintheplates

17

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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

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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

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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

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ChargedConductingSpheres

Theelectricfieldofachargedspherebehaveslikethatofapointcharge

Atestchargeinthefieldexperiencesthesameforceasifallthechargewereconcentratedatthecenterofthesphere

Outsidethesphere,theelectricfieldstrengthandtheelectricpoten2alarethesameasifproducedbyanequalchargeat

thecenter

E=1

40

Qr2 V=

1

40

Qr

1

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ChargedConductingSpheres

Insidethespherethereisnoelectricfield(E=0) Thepoten2alremainsataconstantvalueinside Nopoten2aldifferencebetweenanypointinsidethesphereandapointatthesurface

Vin=

1

40

Q

R

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ToDo

Readchapter13fromthebook[p37-p54Electricfields] HomeworkAssignmentwk:ques2ons13.1,13.3,13.,13.6,13.8and13.10

Handitinnolaterthan:00pmnextWednesday(23Jan)-LATEWORKWILLNOTBEACCEPTED

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