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IntermediatePhysics
PHYS102
Dr RichardH.CyburtAssistantProfessorofPhysics
Myoffice:402cintheScienceBuilding
Myphone:(304)384-6006
Myemail:rcyburt@concord.edu
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.
©2015PearsonEducation,Inc.
Example26.3Practicalpowertransmission(cont.)a. Toprovide40MWofpoweratthegeneratorvoltageof25,000V,
whatcurrentisrequired?b. Whatisthepowerdissipatedintheresistanceofthetransmission
lineforthiscurrent?c. Toprovide40MWofpowerat500,000V,whatcurrentis
required?d. Whatisthepowerdissipatedintheresistanceofthetransmission
lineforthishighervoltage?
©2015PearsonEducation,Inc.
Example26.3Practicalpowertransmission(cont.)PREPAREWecantreatthecity—andthewiresthattransmitpowertoit—asaload.Allofthevoltagesarerms valuesandthepowerisanaveragepower,sowecanfindthecurrenttoprovideagivenpowerandthepowerdissipatedusingtherelationshipsinEquation26.9.
©2015PearsonEducation,Inc.
Example26.3Practicalpowertransmission(cont.)SOLVE
a. Toprovide40MWatthegeneratorvoltageof25,000V,thecurrentis
©2015PearsonEducation,Inc.
Example26.3Practicalpowertransmission(cont.)
b. Passingthiscurrentthroughthetransmissionlineswillresultinpowerdissipationinthe7.0Ωresistanceofthewires.Wedon’tknowthevoltagedropacrossthewires,butwedoknowthecurrentandresistance,sowecancompute
Pdissipated inwires =(Irms)2R =(1600A)2 (7.0Ω)=18MW
Thisisnearlyhalfthepowergenerated,clearlyanunacceptableloss.
©2015PearsonEducation,Inc.
Example26.3Practicalpowertransmission(cont.)c. Increasingthetransmissionvoltageto500kVreducesthe
necessarycurrent:
Thisisaremarkablysmallcurrenttosupplyacity.Ifyouuseseveralhigh-powerappliancesatonetime,youcouldeasilyusethismuchcurrentinyourhouse.Butthenecessarycurrentforthecitycanbesosmallbecausethevoltageissolarge.
©2015PearsonEducation,Inc.
Example26.3Practicalpowertransmission(cont.)d. Therelativelysmallcurrentmeansthatthepowerdissipatedin
theresistanceofthewireswillbesmallaswell:Pdissipated inwires =(Irms)2R =(80A)2 (7.0Ω)=0.045MW
Thisisonlyabout0.1%ofthepowergenerated,whichisquitereasonable.
©2015PearsonEducation,Inc.
Example26.3Practicalpowertransmission(cont.)ASSESS Thefinalresult—thepowerdissipatedinthewiresisdramaticallyreducedforanincreasedtransmissionvoltage—isjustwhattheexamplewasdesignedtoillustrate.
©2015PearsonEducation,Inc.
PowerTransmissionTransmittingelectricityathighvoltagesmeansasmallercurrentisrequired,andthereforetheresultingpowerlossismoremanageablethanforlowvoltages(andlargercurrents).Thisiswhyelectricaltransmissionlinesrunathighvoltages.Inordertotransmitelectricityathighvoltages,weneedtousetransformerstoincreasethevoltage,whichrequiresACelectricity.ThisiswhyweuseACpowereventhoughitisslightlymoredangerousthanDCpower.
©2015PearsonEducation,Inc.
Section26.3HouseholdElectricity
©2015PearsonEducation,Inc.
HouseholdElectricityForsinglecircuits,wehaveonlydealtwiththepotentialdifference.Whenyouwanttoconnectdifferentcircuitstogether,likeacomputermonitortothecomputeritself,theconnectedcircuitsneedacommonreferencepointforthepotential.Thisiswhywehaveanelectricground.
©2015PearsonEducation,Inc.
GettingGroundedTheearthitselfisaconductor.Ifweconnectonepointfromtwodifferentcircuitstotheearthwithidealwires,thenbothcircuitshaveacommonreferencepoint.Inpractice,wecouldcallthepotentialoftheearthVearth =0V.
©2015PearsonEducation,Inc.
GettingGroundedAcircuitconnectedtotheearthissaidtobegrounded.Undernormalcircumstances,thegroundconnectiondoesnotcarryanycurrentbecauseitisnotpartofthecompletecircuit,soitdoesnotalterthebehaviorofthecircuit.
©2015PearsonEducation,Inc.
ElectricOutletsAreGroundedParallelCircuitsTheoutletsinyourhouseareconnectedinparallel.Electricityprovidedbythepowercompanyistransmittedtooutletsthroughwiresinyourwalls.Oneterminaloftheelectricsupplyisgrounded;wecallittheneutralside.Theotherhasvaryingpotential;thehot side.Eachoutlethastwoslots,oneconnectedtoeachside.
©2015PearsonEducation,Inc.
ElectricOutletsAreGroundedParallelCircuitsWhenyouinsertaplugintoanelectricaloutlet,theprongsconnecttothetwoterminalsoftheelectricsupply.Thedeviceyou’vepluggedincompletesthecircuit,thepotentialdifferenceacrossthedeviceleadstoacurrent,andthedeviceturnson.
©2015PearsonEducation,Inc.
ElectricOutletsAreGroundedParallelCircuits
©2015PearsonEducation,Inc.
ElectricOutletsAreGroundedParallelCircuitsThemultipleoutletsinaroomareconnectedinparallelsothateachworkswhiletheothersarenotbeingused.Ifmultipleoutletsareused,thenthetotalcurrentinthecircuitincreases.Thewiresinyourhousearenotideal;theyhavesmallresistancesandcanheatupandcauseafirewhenthereistoomuchcurrent.Acircuitbreakerlimitsthecurrentineachcircuit.Ifanammetermeasurestoomuchcurrent,itsendsasignaltoopentheswitchtodisconnectthecircuit.
©2015PearsonEducation,Inc.
ElectricOutletsAreGroundedParallelCircuitsWhenaplugisconnectedtotheoutletandyouturnoffthedevice,theswitchdisconnectsthehotwire,nottheneutralwire.Thedeviceisthengrounded,andthussafe,whenswitchedoff.
Theroundholeinanelectricoutletisasecondgroundconnection.Ifadevicehasametalcase,andawirecomeslooseinthedeviceandcontactsthecase,apersonthentouchingthecasecangetshocked.
Ifthedevicewiththemetalcaseisgrounded,thepotentialis0andthereforealwayssafetotouch.Ahotwiretouchingagroundedcasewouldcreatealargecircuitthatwouldtripthecircuitbreaker,whichwoulddisconnectthewire.
©2015PearsonEducation,Inc.
Example26.4Willthecircuitbreakeropen?Acircuitinastudent’sroomhasa15Acircuitbreaker.Oneevening,sheplugsinacomputer(240W),alamp(withtwo60Wbulbs),andaspaceheater(1200W).Willthisbeenoughtotripthecircuitbreaker?
©2015PearsonEducation,Inc.
Example26.4Willthecircuitbreakeropen?(cont.)PREPAREWestartbysketchingthecircuit,asinFIGURE26.11.Becausethethreedevicesareinthesamecircuit,theyareconnectedinparallel.Wecanmodeleachofthemasaresistor.
©2015PearsonEducation,Inc.
Example26.4Willthecircuitbreakeropen?(cont.)SOLVE Thecurrentinthecircuitisthesumofthecurrentsintheindividualdevices:
(Itotal)rms =(Icomputer)rms +(Ilamp)rms +(Iheater)rms
©2015PearsonEducation,Inc.
Example26.4Willthecircuitbreakeropen?(cont.)Equation26.9givesthepowerastherms currenttimestherms voltage,sothecurrentineachdeviceisthepowerdividedbytherms voltage:
Thisisalmostbutnotquiteenoughtotripthecircuitbreaker.
©2015PearsonEducation,Inc.
Example26.4Willthecircuitbreakeropen?(cont.)ASSESS Generallyalloftheoutletsinoneroom(andperhapsthelightsaswell)areonthesamecircuit.Youhavequitepossiblyusedelectricdeviceswiththismuchtotalpowerinoneroomwithoutproblems,sothisresultseemsreasonable.
©2015PearsonEducation,Inc.
KilowattHoursKilowatthours istheunitofenergytheelectricpowercompaniesusetomeasuretheenergyusedeachmonthatahome.Theconversionbetweenkilowatthoursandjoulesis
1kWh=(1.00´ 103 W)(3600s)=3.6´ 106 J
©2015PearsonEducation,Inc.
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¢.
©2015PearsonEducation,Inc.
ExampleProblemThefollowingdevicesarepluggedintooutletsonthesame120Vcircuitinahouse.Thiscircuitisprotectedwitha15Acircuitbreaker.Doesthecircuitbreakerblow?
Device PowerComputer 250WHeater 900WLamp 100WStereo 120W
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Section26.4BiologicalEffectsandElectricalSafety
©2015PearsonEducation,Inc.
BiologicalEffectsandElectricalSafetyTherelativesafetyofelectricityisnotgovernedbythevoltagebutbythecurrent.Current—theflowofchargesthroughthebody—iswhatproducesphysiologicaleffectsanddamagebecauseitmimicsnerveimpulsesandcausesmusclestoinvoluntarilycontract.
©2015PearsonEducation,Inc.
BiologicalEffectsandElectricalSafetyAgirltouchingaVandeGraaffgeneratorisnotindangerofshockbecausesheisstandingonaninsulatingsurface.
Thehighresistanceoftheplatformmeanslittlecurrentispassingthroughhertotheground.
Evenifshetouchesagroundedobject,thecurrentismodestbecausethechargeofthegeneratorissmall.
©2015PearsonEducation,Inc.
BiologicalEffectsandElectricalSafety
©2015PearsonEducation,Inc.
BiologicalEffectsandElectricalSafetyTocalculatecurrentsthroughthebody,wemodelthebodyasseveralconnectedresistors.Theskinhasafairlyhighresistance,buttheinteriorofthebodyhasalowresistanceduetoitshighsaltwatercontent.
©2015PearsonEducation,Inc.
BiologicalEffectsandElectricalSafety
©2015PearsonEducation,Inc.
Example26.6Istheworkerindanger?Aworkerinaplantgrabsabarewirethathedoesnotknowisconnectedtoa480VACsupply.Hisotherhandisholdingagroundedmetalrailing.Theskinresistanceofeachofhishands,infullcontactwithaconductor,is2200Ω.Hewillreceiveashock.Isitlargeenoughtobedangerous?
©2015PearsonEducation,Inc.
Example26.6Istheworkerindanger?(cont.)PREPAREWecandrawacircuitmodelforthissituationasinFIGURE26.14a;theworker’sbodycompletesacircuitbetweentwopointsatapotentialdifferenceof480V.Thecurrentwilldependonthispotentialdifferenceandtheresistanceofhisbody,includingtheresistanceoftheskin.
©2015PearsonEducation,Inc.
Example26.6Istheworkerindanger?(cont.)SOLVE FollowingthemodelofFigure26.13,thecurrentpathgoesthroughtheskinofonehand,uponearm,acrossthetorso,downtheotherarm,andthroughtheskinoftheotherhand,asinFIGURE26.14b.Theequivalentresistanceoftheseriescombinationis5050Ω,sotheACcurrentthroughhisbodyis
©2015PearsonEducation,Inc.
Example26.6Istheworkerindanger?(cont.)FromTable26.1weseethatthisisaverydangerous,possiblyfatal,current.ASSESS Thevoltageishighandtheresistancerelativelylow,soit’snosurprisetofindadangerouslevelofcurrent.
©2015PearsonEducation,Inc.
BiologicalEffectsandElectricalSafetyTheresistanceofpreventativeclothing,likeinworkboots,ishigherthanthatoftheskin;itprovidesprotectionfromhighcurrentsandthereforeelectricshock.
©2015PearsonEducation,Inc.
GFICircuitsGroundfaultinterrupter outlets(GFI)haveabuilt-insensingcircuitthatcomparesthecurrentsinthehotandneutralwires.Thecurrentinthesewiresshouldbethesame.Ifthesensordetectsthatsomecurrentfromthehotwireisfindinganalternativepathtotheground,suchasthroughaperson,theGFIdisconnectsthecircuit.
©2015PearsonEducation,Inc.
TryItYourself:TestingGFICircuitsYoucantestaGFIcircuitbypressingtheblack“Test”button.Thiscreatesanelectricalconnectionbetweenthehotwireandthegroundconnection,sothecurrentsinthehotandneutralwireswillnotbeequal.Youshouldhearaclickasthecircuitdisconnects.Youcanthenresettheoutletbypressingtheredbutton.Ifanoutletdoesnotrespondlikethis,itshouldbereplaced.
©2015PearsonEducation,Inc.
ExampleProblemSupposeapersonisstanding,barefoot,onwetground.Theresistanceoftheskinoneachfootis300Ω.HenowgrabsanimproperlywireddevicewithHisrighthand— establishingaconnectiontothehotwireoftheelectricsupplyat120V.Hishandisabitsweaty,sotheresistanceoftheskinisonly1.2kΩ.
A. Whatcurrentwillflowthroughtheperson’sbody?B. Willthisbelargeenoughforthepersontofeel?Largeenoughtobe
dangerous?
©2015PearsonEducation,Inc.
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