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1. Eletricalfields
2. ElectricalPoten3al
. Capicitanceanddielectrics
4. Currentandresistance
5. Directcurrentcircuit6. Magne3cfields
7. Sourceofthemagne3cfields
8. Faraday’slaw
9. Inductance
10. Alterna3ngcurrentcircuits11. Electromagne3cwaves
Topics
GeneralphysicsPHYS104
MainReference:PHYSICSFORSCIENTISTSANDENGINEERSwithmodernphysics,8
th
edi3on,R.A.Serway
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Chargesofthesamesignrepeloneanotherandchargeswithopposite
signsaXractoneanother
Anotherimportantaspectofelectricitythatarisesfromexperimentalobserva3ons
isthatelectricchargeisalwaysconservedinanisolatedsystem.Thatis,when
oneobjectisrubbedagainstanother,chargeisnotcreatedintheprocess.
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In1909,RobertMillikan(1868–195)discoveredthatelectricchargealways
occursasintegralmul3plesofafundamentalamountofchargee
Electricchargeqissaidtobequan3zed,whereqisthestandard
symbolusedforchargeasavariable.Thatis,electricchargeexistsasdiscrete“packets,”
andwecanwriteq=-±Ne,whereNissomeinteger.
Electronhasacharge-eandtheprotonhasachargeofequalmagnitudebutoppositesign+e.Somepar3cles,suchastheneutron,haveno
charge.
hXp://www.chem4kids.com/files/atom_structure.html
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Materialssuchasglass,rubber,anddrywoodfallintothecategoryofelectrical
insulators.Whensuchmaterialsarechargedbyrubbing,onlythearearubbed
becomeschargedandthechargedpar3clesareunabletomovetootherregionsof
thematerial.
Incontrast,materialssuchascopper,aluminum,andsilveraregoodelectrical
conductors.Whensuchmaterialsarechargedinsomesmallregion,thecharge
readilydistributesitselfovertheen3resurfaceofthematerial.
Semiconductorsareathirdclassofmaterials,andtheirelectricalproper3esare
somewherebetweenthoseofinsulatorsandthoseofconductors.Siliconandgermanium
arewell-knownexamplesofsemiconductorscommonlyusedinthefabrica3on
ofavarietyofelectronicchipsusedincomputers,cellulartelephones,and
hometheatersystems.Theelectricalproper3esofsemiconductorscanbechanged
overmanyordersofmagnitudebytheaddi3onofcontrolledamountsofcertain
atomstothematerials.
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Tounderstandhowtochargeaconductorbyaprocessknownasinduc3on,consider
aneutral(uncharged)conduc3ngsphere
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Charginganobjectbyinduc3onrequiresnocontactwiththeobjectinducing
thecharge.Thatisincontrasttocharginganobjectbyrubbing(thatis,byconduc3on),
whichdoesrequirecontactbetweenthetwoobjects.
Aprocesssimilartoinduc3oninconductorstakesplaceininsulators .Inmostneutralmolecules,thecenterofposi3vechargecoincideswiththecenterofnega3ve
charge.Inthepresenceofachargedobject,however,thesecentersinsideeach
moleculeinaninsulatormayshiVslightly,resul3nginmoreposi3vechargeonone
sideofthemoleculethanontheother.
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CharlesCoulombmeasuredthemagnitudesoftheelectric
forcesbetweencharged
objectsusingthetorsionbalance,whichheinvented
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Theconstantε0(GreekleXerepsilon)isknownasthe
permivityoffreespaceandhasthevalue
Magnitudeoftheelectricforce(some3mescalledtheCoulombforce)between
twopointchargesisgivenbyCoulomb’slaw.
ThisconstantisalsowriXenintheform
TheCoulombconstantkeinSIunitshasthevalue
Thesmallestunitoffreechargeeknowninnature,thechargeonan
electron(-e)oraproton(+e),hasamagnitude
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Comparisonbetweentheelectricforceandthegravita9onalforce
betweentheelectronandtheprotoninsidehydrogenatom
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wherer12isaunitvectordirectedfrom q1towardq2asshowninFigure
BecausetheelectricforceobeysNewton’sthirdlaw,theelectricforceexerted
byq2onq1isequalinmagnitudetotheforceexertedby q1onq2andinthe
oppositedirec3on;thatis
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Tobedoneinclass
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Theelectricfielddepictstheforceexertedonotherelectrically
chargedobjectsbytheelectricallychargedpar3clethefieldis
surrounding.TheconceptofanelectricfieldwasintroducedbyMichaelFaraday.
MichaelFaraday,(22September1791–25August1867)wasan
Englishchemistandphysicist(ornaturalphilosopher,inthe
terminologyofthe3me)whocontributedtothefieldsofelectromagne3smandelectrochemistry.
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Ifqisposi3ve,theforceisinthesamedirec3onasthefield.Ifqisnega3ve,the
forceandthefieldareinoppositedirec3ons.
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AconvenientwayofvisualizingelectricfieldpaXernsistodrawlines,calledelectric
fieldlinesandfirstintroducedbyFaraday,thatarerelatedtotheelectricfieldina
regionofspaceinthefollowingmanner:
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Representa3veelectricfieldlinesforthefieldduetoasingleposi3vepoint
chargeareshown
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Homework
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2.Electricpoten3al
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?
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c
H k
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Homework
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.Capacitanceanddielectrics
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4.Currentandresistance
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5 Direct Current circuits
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5.DirectCurrentcircuits
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6.Magne9cfields
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Many historians of science believe that the compass, which uses a magne3c
needle,wasusedinChinaasearlyasthe1thcenturyBC,itsinven3onbeingof
ArabicorIndianorigin.TheearlyGreeksknewaboutmagne3smasearlyas800
BC.Theydiscoveredthatthestonemagne3te(FeO4)aXractspiecesofiron.
Subsequentexperimentsshowedthateverymagnet,regardlessofitsshape,has
two poles, called north (N) and south (S) poles, that exert forces on other
magne3cpolessimilartothewayelectricchargesexertforcesononeanother.
Thatis,likepoles(N–NorS–S)repeleachother,andoppositepoles(N–S)aXract
eachother.
Isolatedmagne-cpolesdonotexist!
Magne3cpolesarealwaysfoundinpairs.AllaXemptsthusfartodetectan
isolatedmagne3cpolehavebeenunsuccessful.NomaXerhowmany3mesa
permanentmagnetiscutintwo,eachpiecealwayshasanorthandasouthpole.
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