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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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