Intermediate Physics - concord.edu · The power plant is 50 km from the city (a typical distance), and the 100 km of wire used in the transmission line (to the city and back) has

IntermediatePhysics

PHYS102

Dr RichardH.CyburtAssistantProfessorofPhysics

Myoffice:402cintheScienceBuilding

Myphone:(304)384-6006

Myemail:[email protected]

Mywebpage:www.concord.edu/rcyburt

Inpersonoremailisthebestwaytogetaholdofme.

PHYS102

MyOfficeHoursTWR9:30-11:00amW4:00-5:00pm

Meetingsmayalsobearrangedatothertimes,byappointment

PHYS102

ProblemSolvingSectionsIwouldliketohavehour-longsectionsforworkingthroughproblems.

Thiswouldbeanextracomponenttothecourseandcounttowardsextracredit

TR1-2pm

WF10-11am

S308

Ifyoucan’tmakethese,youcanstillpickuptheproblemworksheet.

PHYS102

Midterm2Thursday,March2duringclass8-9:15Chapters22-25.4,Lectures7-13

Allowedonehalfsheet(8.5x11)pieceofpaperw/notes/formuliCalculatorpencilorblue/blackpen

Review,Wednesday,March17-9pm

Bringquestions!!!

PHYS102

IntermediatePhysics

PHYS102

PHYS102

DouglasAdamsHitchhiker’sGuidetotheGalaxy

Inclass!!

PHYS102

Thislecturewillhelpyouunderstand:Transformers

HouseholdElectricity

BiologicalEffectsandElectricalSafety

PHYS102

PowerTransmission

©2015PearsonEducation,Inc.

Example26.3PracticalpowertransmissionToprovidepowertoasmallcity,apowerplantgenerates40MWofACelectricity.Thepowerplantis50kmfromthecity(atypicaldistance),andthe100kmofwireusedinthetransmissionline(tothecityandback)hasaresistanceof7.0Ω.


Example26.3Practicalpowertransmission(cont.)a. Toprovide40MWofpoweratthegeneratorvoltageof25,000V,

whatcurrentisrequired?b. Whatisthepowerdissipatedintheresistanceofthetransmission

lineforthiscurrent?c. Toprovide40MWofpowerat500,000V,whatcurrentis

required?d. Whatisthepowerdissipatedintheresistanceofthetransmission

lineforthishighervoltage?


Example26.3Practicalpowertransmission(cont.)PREPAREWecantreatthecity—andthewiresthattransmitpowertoit—asaload.Allofthevoltagesarerms valuesandthepowerisanaveragepower,sowecanfindthecurrenttoprovideagivenpowerandthepowerdissipatedusingtherelationshipsinEquation26.9.


Example26.3Practicalpowertransmission(cont.)SOLVE

a. Toprovide40MWatthegeneratorvoltageof25,000V,thecurrentis


Example26.3Practicalpowertransmission(cont.)

b. Passingthiscurrentthroughthetransmissionlineswillresultinpowerdissipationinthe7.0Ωresistanceofthewires.Wedon’tknowthevoltagedropacrossthewires,butwedoknowthecurrentandresistance,sowecancompute

Pdissipated inwires =(Irms)2R =(1600A)2 (7.0Ω)=18MW

Thisisnearlyhalfthepowergenerated,clearlyanunacceptableloss.


Example26.3Practicalpowertransmission(cont.)c. Increasingthetransmissionvoltageto500kVreducesthe

necessarycurrent:

Thisisaremarkablysmallcurrenttosupplyacity.Ifyouuseseveralhigh-powerappliancesatonetime,youcouldeasilyusethismuchcurrentinyourhouse.Butthenecessarycurrentforthecitycanbesosmallbecausethevoltageissolarge.


Example26.3Practicalpowertransmission(cont.)d. Therelativelysmallcurrentmeansthatthepowerdissipatedin

theresistanceofthewireswillbesmallaswell:Pdissipated inwires =(Irms)2R =(80A)2 (7.0Ω)=0.045MW

Thisisonlyabout0.1%ofthepowergenerated,whichisquitereasonable.


Example26.3Practicalpowertransmission(cont.)ASSESS Thefinalresult—thepowerdissipatedinthewiresisdramaticallyreducedforanincreasedtransmissionvoltage—isjustwhattheexamplewasdesignedtoillustrate.


PowerTransmissionTransmittingelectricityathighvoltagesmeansasmallercurrentisrequired,andthereforetheresultingpowerlossismoremanageablethanforlowvoltages(andlargercurrents).Thisiswhyelectricaltransmissionlinesrunathighvoltages.Inordertotransmitelectricityathighvoltages,weneedtousetransformerstoincreasethevoltage,whichrequiresACelectricity.ThisiswhyweuseACpowereventhoughitisslightlymoredangerousthanDCpower.


Section26.3HouseholdElectricity


HouseholdElectricityForsinglecircuits,wehaveonlydealtwiththepotentialdifference.Whenyouwanttoconnectdifferentcircuitstogether,likeacomputermonitortothecomputeritself,theconnectedcircuitsneedacommonreferencepointforthepotential.Thisiswhywehaveanelectricground.


GettingGroundedTheearthitselfisaconductor.Ifweconnectonepointfromtwodifferentcircuitstotheearthwithidealwires,thenbothcircuitshaveacommonreferencepoint.Inpractice,wecouldcallthepotentialoftheearthVearth =0V.


GettingGroundedAcircuitconnectedtotheearthissaidtobegrounded.Undernormalcircumstances,thegroundconnectiondoesnotcarryanycurrentbecauseitisnotpartofthecompletecircuit,soitdoesnotalterthebehaviorofthecircuit.


ElectricOutletsAreGroundedParallelCircuitsTheoutletsinyourhouseareconnectedinparallel.Electricityprovidedbythepowercompanyistransmittedtooutletsthroughwiresinyourwalls.Oneterminaloftheelectricsupplyisgrounded;wecallittheneutralside.Theotherhasvaryingpotential;thehot side.Eachoutlethastwoslots,oneconnectedtoeachside.


ElectricOutletsAreGroundedParallelCircuitsWhenyouinsertaplugintoanelectricaloutlet,theprongsconnecttothetwoterminalsoftheelectricsupply.Thedeviceyou’vepluggedincompletesthecircuit,thepotentialdifferenceacrossthedeviceleadstoacurrent,andthedeviceturnson.


ElectricOutletsAreGroundedParallelCircuits


ElectricOutletsAreGroundedParallelCircuitsThemultipleoutletsinaroomareconnectedinparallelsothateachworkswhiletheothersarenotbeingused.Ifmultipleoutletsareused,thenthetotalcurrentinthecircuitincreases.Thewiresinyourhousearenotideal;theyhavesmallresistancesandcanheatupandcauseafirewhenthereistoomuchcurrent.Acircuitbreakerlimitsthecurrentineachcircuit.Ifanammetermeasurestoomuchcurrent,itsendsasignaltoopentheswitchtodisconnectthecircuit.


ElectricOutletsAreGroundedParallelCircuitsWhenaplugisconnectedtotheoutletandyouturnoffthedevice,theswitchdisconnectsthehotwire,nottheneutralwire.Thedeviceisthengrounded,andthussafe,whenswitchedoff.

Theroundholeinanelectricoutletisasecondgroundconnection.Ifadevicehasametalcase,andawirecomeslooseinthedeviceandcontactsthecase,apersonthentouchingthecasecangetshocked.

Ifthedevicewiththemetalcaseisgrounded,thepotentialis0andthereforealwayssafetotouch.Ahotwiretouchingagroundedcasewouldcreatealargecircuitthatwouldtripthecircuitbreaker,whichwoulddisconnectthewire.


Example26.4Willthecircuitbreakeropen?Acircuitinastudent’sroomhasa15Acircuitbreaker.Oneevening,sheplugsinacomputer(240W),alamp(withtwo60Wbulbs),andaspaceheater(1200W).Willthisbeenoughtotripthecircuitbreaker?


Example26.4Willthecircuitbreakeropen?(cont.)PREPAREWestartbysketchingthecircuit,asinFIGURE26.11.Becausethethreedevicesareinthesamecircuit,theyareconnectedinparallel.Wecanmodeleachofthemasaresistor.


Example26.4Willthecircuitbreakeropen?(cont.)SOLVE Thecurrentinthecircuitisthesumofthecurrentsintheindividualdevices:

(Itotal)rms =(Icomputer)rms +(Ilamp)rms +(Iheater)rms


Example26.4Willthecircuitbreakeropen?(cont.)Equation26.9givesthepowerastherms currenttimestherms voltage,sothecurrentineachdeviceisthepowerdividedbytherms voltage:

Thisisalmostbutnotquiteenoughtotripthecircuitbreaker.


Example26.4Willthecircuitbreakeropen?(cont.)ASSESS Generallyalloftheoutletsinoneroom(andperhapsthelightsaswell)areonthesamecircuit.Youhavequitepossiblyusedelectricdeviceswiththismuchtotalpowerinoneroomwithoutproblems,sothisresultseemsreasonable.


KilowattHoursKilowatthours istheunitofenergytheelectricpowercompaniesusetomeasuretheenergyusedeachmonthatahome.Theconversionbetweenkilowatthoursandjoulesis

1kWh=(1.00´ 103 W)(3600s)=3.6´ 106 J


Example26.5ComputingthecostofelectricenergyAtypicalelectricspaceheaterdrawsanrms currentof12.5Aonitshighestsetting.Ifelectricitycosts12¢perkilowatthour(anapproximatenationalaverage),howmuchdoesitcosttoruntheheaterfor2hours?SOLVE Thepowerdissipatedbytheheateris

P =Vrms Irms = (120V)(12.5A)=1500W=1.5kWIn2hours,theenergyusedis(1.5kW)(2.0h)=3.0kWh.At12¢perkWh,thecostis36¢.


ExampleProblemThefollowingdevicesarepluggedintooutletsonthesame120Vcircuitinahouse.Thiscircuitisprotectedwitha15Acircuitbreaker.Doesthecircuitbreakerblow?

Device PowerComputer 250WHeater 900WLamp 100WStereo 120W


Section26.4BiologicalEffectsandElectricalSafety


BiologicalEffectsandElectricalSafetyTherelativesafetyofelectricityisnotgovernedbythevoltagebutbythecurrent.Current—theflowofchargesthroughthebody—iswhatproducesphysiologicaleffectsanddamagebecauseitmimicsnerveimpulsesandcausesmusclestoinvoluntarilycontract.


BiologicalEffectsandElectricalSafetyAgirltouchingaVandeGraaffgeneratorisnotindangerofshockbecausesheisstandingonaninsulatingsurface.

Thehighresistanceoftheplatformmeanslittlecurrentispassingthroughhertotheground.

Evenifshetouchesagroundedobject,thecurrentismodestbecausethechargeofthegeneratorissmall.




BiologicalEffectsandElectricalSafetyTocalculatecurrentsthroughthebody,wemodelthebodyasseveralconnectedresistors.Theskinhasafairlyhighresistance,buttheinteriorofthebodyhasalowresistanceduetoitshighsaltwatercontent.




Example26.6Istheworkerindanger?Aworkerinaplantgrabsabarewirethathedoesnotknowisconnectedtoa480VACsupply.Hisotherhandisholdingagroundedmetalrailing.Theskinresistanceofeachofhishands,infullcontactwithaconductor,is2200Ω.Hewillreceiveashock.Isitlargeenoughtobedangerous?


Example26.6Istheworkerindanger?(cont.)PREPAREWecandrawacircuitmodelforthissituationasinFIGURE26.14a;theworker’sbodycompletesacircuitbetweentwopointsatapotentialdifferenceof480V.Thecurrentwilldependonthispotentialdifferenceandtheresistanceofhisbody,includingtheresistanceoftheskin.


Example26.6Istheworkerindanger?(cont.)SOLVE FollowingthemodelofFigure26.13,thecurrentpathgoesthroughtheskinofonehand,uponearm,acrossthetorso,downtheotherarm,andthroughtheskinoftheotherhand,asinFIGURE26.14b.Theequivalentresistanceoftheseriescombinationis5050Ω,sotheACcurrentthroughhisbodyis


Example26.6Istheworkerindanger?(cont.)FromTable26.1weseethatthisisaverydangerous,possiblyfatal,current.ASSESS Thevoltageishighandtheresistancerelativelylow,soit’snosurprisetofindadangerouslevelofcurrent.


BiologicalEffectsandElectricalSafetyTheresistanceofpreventativeclothing,likeinworkboots,ishigherthanthatoftheskin;itprovidesprotectionfromhighcurrentsandthereforeelectricshock.


GFICircuitsGroundfaultinterrupter outlets(GFI)haveabuilt-insensingcircuitthatcomparesthecurrentsinthehotandneutralwires.Thecurrentinthesewiresshouldbethesame.Ifthesensordetectsthatsomecurrentfromthehotwireisfindinganalternativepathtotheground,suchasthroughaperson,theGFIdisconnectsthecircuit.


TryItYourself:TestingGFICircuitsYoucantestaGFIcircuitbypressingtheblack“Test”button.Thiscreatesanelectricalconnectionbetweenthehotwireandthegroundconnection,sothecurrentsinthehotandneutralwireswillnotbeequal.Youshouldhearaclickasthecircuitdisconnects.Youcanthenresettheoutletbypressingtheredbutton.Ifanoutletdoesnotrespondlikethis,itshouldbereplaced.


ExampleProblemSupposeapersonisstanding,barefoot,onwetground.Theresistanceoftheskinoneachfootis300Ω.HenowgrabsanimproperlywireddevicewithHisrighthand— establishingaconnectiontothehotwireoftheelectricsupplyat120V.Hishandisabitsweaty,sotheresistanceoftheskinisonly1.2kΩ.

A. Whatcurrentwillflowthroughtheperson’sbody?B. Willthisbelargeenoughforthepersontofeel?Largeenoughtobe

dangerous?


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Intermediate Physics - concord.edu · The power plant is 50 km from the city (a typical distance), and the 100 km of wire used in the transmission line (to the city and back) has