Electricity and MagnetismWord下载.docx

Electricity and MagnetismWord下载.docx

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Electricityandmagnetismisallaroundus.

Wehaveelectriclights,Electricclocks.

Wehavemicrophones,calculators,televisions,VCRs,radio,computers.

Lightitselfisanelectromagneticphenomenonasradiowavesare.

Thecolorsoftherainbowintheblueskyaretherebecauseofelectricity.

AndIwillteachyouaboutthatinthiscourse.

Cars,planes,trainscanonlyrunbecauseofelectricity.

Horsesneedelectricitybecausemusclecontractions（肌肉收缩）requireelectricity.

Yournervesystemisdrivenbyelectricity.

Atoms,molecule,allchemicalreactionsexistbecauseofelectricity.

Youcouldnotseewithoutelectricity.

Yourheartwouldnotbeatwithoutelectricity.

Andyoucouldnoteventhinkwithoutelectricity,thoughIrealizethatevenwithelectricitysomeofyoumayhaveaproblemwiththat.尽管我认为即使不缺少电，某些同学也成问题

Themodernpictureofanatomisanucleus,whichisverysmallcomparedtothesizeoftheatom.

Thenucleushasprotons,whicharepositivelychargedandithasneutrons,whichhavenocharge.

Themassoftheprotonisapproximatelythesameasthemassoftheneutron.

It'

saboutsixpointseventimestentotheminustwenty-seventhkilograms.

Onepointseven.

Thepositivechargesherewiththenucleons（核子）,withtheneutrons,andthenwehaveelectronsinacloudaroundit.

Andiftheatomisneutral,thenumberofelectronsandthenumberofprotonsisthesame.

Ifyoutakeoneelectronoffyougetapositiveion.

Ifyouaddanelectronthenyougetanegativeion.

Thechargeoftheelectronisthesameasthechargeoftheproton.

That'

swhythenumberisthesameforneutralatoms.

Themassoftheelectronisabouteighteenhundredthirtytimessmallerthanthemassoftheproton.

sthereforenegligiblysmallinmostcases.

Allthemassofanatomisinthenucleus.

IfItakesixbillionatomslineduptouchingother,Itakesixbillionbecausethat'

saboutaboutthenumberofpeopleonearth.

Thenyouwouldonlyhavealengthofsixtycentimeters.

Givesyouanideaofhowsmalltheatomsare.

Thenucleushasasizeofabouttentotheminustwelvecentimeters.

Andtheatomitselfisabouttenthousandtimeslarger.

Thecloudofelectrons,whichisabouttentotheminuseightcentimeters.

Andifyoulinesixbillionofthoseupyouonlygetthismuch.

AlreadyinsixhundredBC,itwasknownthatifyourubamberthatitcanattractpiecesofdryleaves.

AndtheGreekwordforamberiselectron.

Sothat'

swhereelectricitygotitsnamefrom.

Inthesev-sixteenthcenturythereweremoresubstancesknowntodothis.

Forinstanceglassandsulfur.

Anditwasalsoknownandwrittenthatwhenpeoplewereboredatpartiesthatthewomenwouldrubtheiramberjewelryandwouldtouchfrogs,whichthenwouldstartjumpingofdesperation,whichpeopleconsideredtobefun,notunderstandingwhatactuallywashappeningtotheambernorwhatwashappeningtothefrogs.

Intheeighteenthcenturyitwasdiscoveredthattherearetwotypesofelectricity.

Oneifyourubglassandanotherifyourubrubberoramberforthatmatter.

Let'

scalloneAandtheotherB.

ItwasknownthatArepelsAandBrepelsBbutAattractsB.

AnditwasBenjaminFranklinwithoutanyknowledgeofelectronsandprotonswhointroducedtheideathatallsubstancesarepenetratedwithwhathecalledelectricfluid,electricfire.

Andhestatedifyougettoomuchofthefirethenyou'

repositivelychargedandifyouhaveadeficiency（缺点，缺乏）ofthatfire,thenyou'

renegativelycharged.

Heintroducedthesignconventionandhedecidedthatifyourubglassthatthatisanexcessoffireandhecalledthatthereforepositive.

Youwillseelaterinthiscoursewhythischoicehehadfiftypercentchanceisextremelyunfortunatebutwehavetolivewithit.

SoifyoutakethisfluidaccordingtoBenjaminFranklinandbringitfromonesubstancetotheother,thentheonethatgetsanexcessbecomespositivelychargedbutautomaticallyasaconsequenceofthattheonefromwhichyoutakethefluidbecomesnegativelycharged.

Andsothat'

sthewholeideabehindtheconservationofcharge.

Youcannotcreatecharge.

Ifyoucreateplusthenyouautomaticallycreateminus.

Plusandplusrepeleachother.

Minusandminusrepeleachother.

Andplusandminusattract.

AndBenjaminFranklinwhodidexperimentsalsonoticedthatthemorefireyouhavethestrongertheforces.

Theclosertheseobjectsaretoeachother,thestrongertheforces.

Andtherearesomesubstancesthathenoticed,whichconductthisfluid,whichconductthisfire,andtheyarecalledconductors.

IfIhaveaglassrodasIhavehereandIrubitthenitgetsthispositivechargethatwejustdiscussed.

SohereisthisrodandIrubitwithsomesilkanditwillgetpositivelycharged.

WhathappensnowtoanobjectthatIbringclosetothisrodandIwillstartoffwith[从（用）…开始,endupwith结束；

以…而结束；

以…告终]takingaconductor.

AndthereasonwhyIchooseaconductoristhatconductorshaveasmallfractionoftheirelectrons,whicharenotboundtoatomsbutwhichcanfreelymovearoundintheconductor.

scharacteristicforaconductor,formetals.

snotthecasewithnonconductors.

Theretheallelectronsarefixedtoindividualatoms.

Soherewehaveacertainfractionofelectronsthatcanwanderaround.

What'

sgoingtohappenthatelectronswanttobeattractedbythesepositivecharges.

Plusandminusattracteachother.

Andsosomeoftheseelectrons,whichcanfreelymovewillmoveinthisdirectionandsotheplusstaybehind.

Thisprocesswecallinduction.

Yougetsortofapolarization.

Yougetachargedivision.

saverysmalleffect,perhapsonlyoneintentothethirteenelectronsthatwasoriginallyherewillendupherebutthat'

sallittakes.

Sowegetapolarizationandwegetalittlebitmorenegativechargeontherightsidethanwehaveontheleftside.

Andsowhat'

sgoingtohappenis,sincetheattractionbetweenthesetwowillbestrongerthantherepellingforcebetweenthesetwobecausethedistanceissmallerandFranklinhadalreadynoticedtheshorterthedistancethestrongertheforce.

Whatwillhappenisthatifthisobjectisfreetomoveitwillmovetowardsthisrod.

AndthisisthefirstthingthatIwouldlikeyoutosee.

Ihavehereaconductorthatisaballoon,helium-filledballoon.

AndIwillrubthisrodwithsilk.

AndasIapproachthatballoonyouwillseethattheballooncomestotherod.

Iwillthentrytorubwiththatrodseveraltimesonthatballoon.

Itwilltakeawhileperhapsbecausetheroditselfisaverygoodnonconductor.

snotsoeasytogetchargeexchangebetweenthetwo.

ButifIdoitlongenoughIcancertainlymakethatballoonpositive.

Thenthey'

rebothpositive.

Andthentheywillrepeleachother.

Butfirst,theinductionpart,wherebyyouwillseetheballooncometotheglassrod.

Theseexperimentsworkbestwhenitisdry,inthewinter.

Theydon'

tworksowellwhenitishumidsoit'

sagoodtimetoteacheightotwointhewinter.

OKtherewegothisshouldbepositivelychargednow.

Andtheballoonwantstocometotheglass.

Youseethat?

Veryclearly.

Comeonbaby.

OK.

SonowIwilltrytogetthisballoonchargedalittlesothereisachangeofelectronsthatgofromtheballoontotheglass.

Andtheglassdoesn'

t,it'

snotaconductoritselfsoitisnotalwayssoeasytogetchargeexchanges.

OKlet'

sseewhetherIhavesucceedednowinmakingtheballoonpositivelychargedaswellastheglassrod.

Ifthat'

sthecasethentheballoonisnotgoingtolikeme.

Theballoonwillnowberepelled.

Andyouseethatveryclearly.

Toshowyounowthatthereareindeedtwodifferentkindsofelectricity.

IfInowrubwithcatfurbytraditionwedothatwithcatfur,Idon'

tknowwhybytraditionweusesilkfortheglass.

Soifwedothiswithcatfurnowthenthisbecomesnegativelycharged.

Rememberthereweretwotypesofelectricity.

Andsincethatballoonispositivelychargednowtheballoonwillcometome.

Andthereitis.

Nowitcomestome.

Soyou'

veseenforthefirsttimenowclearlythattherearetwodifferentkindsofelectricity.

ThepositivechargeischosenbyFranklinontheglassrodandthenegativechargeontherubber.

SonowyoumaythinkthatifIapproachanonconductingballoonwithaglassrodandIhaveanonconductingballoonhereyoumaythinknowthatthisballoonwillnotcometotheglassrodbecausetherearenofreeelectrons.

Sotheseelectronscannotfreelymoveandsoyoudon'

tgetthispolarization.

Youdon'

tgetthisinduction.

Butthatisnotthecase.

Andthisisactuallyquitesubtle.

Youhavetolooknowattheatomicscale.

IfItakeanatomlikeyouhavehere.

Youhavepositivechargeandyouhavetheelectronshereinacloudaroundthepositivenucleus.

IfIbringaglassrodpositivelychargednearby,thentheseelectronswhicharestucktotheatoms,theycannotfreelymovelikeincond