电路基础第一章-英文版chapter1

1FundamentalsofElectricCircuits2010.9BasicComponentsandElectricCircuits2Chapter1BasicComponentsandElectricCircuits1.1SystemsofUnits.1.2ElectricQuantities1.3CircuitElements1.4Ohm’sLawandLinearResistor1.5SummaryandReview31.1SystemofUnits(1)SevenbasicunitsQuantityBasicunitSymbolLengthmetermMasskilogramKgTimesecondsElectriccurrentampereAThermodynamictemperaturekelvinKAmountofsubstancemolemolLuminousintensitycandelacd4ThederivedunitscommonlyusedinelectriccircuittheoryDecimalmultiplesandsubmultiplesofSIunits51.2ElectricQuantities1.charge•Chargeisanelectricalpropertyoftheatomicparticlesofwhichmatterconsists,measuredincoulombs(C).•Oneofthemostfundamentalconceptsinelectriccircuitanalysisisthatofchargeconservation.Thechargeeononeelectronisnegativeandequalinmagnitudeto1.60210-19Cwhichiscalledaselectroniccharge.Thechargesthatoccurinnatureareintegralmultiplesoftheelectroniccharge.6ThemotionofchargesformstheelectriccurrentinawireThecurrenthasbothanumericalvalueandadirectionassociatedwithititisameasureoftherateatwhichchargeismovingpastagivenreferencepointinaspecifieddirection(suchastheoneshowninFig.1.1).2.Current(1)7Electriccurrenti=dq/dt.Theunitofcurrentistheampere(A),anditcanbederivedas1A=1C/s.•Adirectcurrent(dc)isacurrentthatremainsconstantwithtime.•Analternatingcurrent(ac)isacurrentthatvariessinusoidallywithtime.(reversedirection)8Current(2)•ThedirectionofcurrentflowPositiveionsNegativeionsFigure1.19Figure1.2(a)Adirectcurrent(dc)(b)Analternatingcurrent(ac)10Current(3)Example1Aconductorhasaconstantcurrentof5A.Howmanyelectronspassafixedpointontheconductorinoneminute?11Current(4)SolutionTotalno.ofchargespassin1minisgivenby5A=(5C/s)(60s/min)=300C/minTotalno.ofelectronicspassin1minisgiven2119300C/min1.8710electrons/min1.60210C/electronx12Weshouldpaycloseattentiontothatthearrowisafundamentalpartofthedefinitionofthecurrent!Thus,totalkaboutthevalueofacurrenti(t)withoutspecifyingthearrowistodiscussanundefinedentity.Forexample,Fig.1.3aistheproperdefinitive,whereasFig.1.3bandcaremeaninglessrepresentationofi(t)symbology.13i(t)i(t)(a)(b)(c)Figure1.3(a)thecorrectdefinitionofi(t)(b,c)incomplete,improperandincorrectdefinitionsofacurrent14-------------------------------------------------Practice1.1:InthewireofFig.1.7electronsmovingfromlefttorighttocreateacurrentof1mA.DetermineI1andI2.----------------------------------------------------------------Ans:I1=-1mA;I2=+1mA.I1I2Figure1.4153.Voltage(1)•Thevoltage(orpotentialdifference)acrossaterminalpairisthework(energy)requiredtomoveaunitchargethroughanelement,measuredinvolts(V).–wisworkinjoules(J)andqischargeincoulomb(C),and1voltisthesameas1J/C.•Mathematically,dqdwvab/(volt)16Electricvoltage,vAB,isalwaysacrossthecircuitelementorbetweentwopointsinacircuit.–vAB0meansthepotentialofAishigherthanpotentialofB.–vAB0meansthepotentialofAislowerthanpotentialofB.Weshouldnotethatavoltagecanexistbetweenapairofelectricalterminalswhetheracurrentisflowingornot.17Figure1.5Ageneraltwo-terminalcircuitelement18Wemustnowestablishaconventionbywhichwecandistinguishbetweenenergysuppliedtoanelement,andenergythatissuppliedbytheelementitself.Thesenseofvoltageisindicatedbyaplus-minuspairofalgebraicsigns.InFig.1.6a,forexample,theplacementof+signinterminalAindicatesthatterminalAisvvoltspositivewithrespecttoterminalB.19Figure1.620•Justaswenotedinourdefinitionofcurrent,itisessentialthattheplus-minuspairofalgebraicsignsdoesnotindicatethe“actual”polarityofthevoltagebutissimplepartofaconventionthatenableustotalkunambiguouslyabout“thevoltageacrosstheterminalpair.”•Note:Thedefinitionofanyvoltagemustincludeplus-minussignpair!•Usingaquantityv1(t)withoutspecifyingthelocationofplus-minussignpairisusinganundefinedterm.21Figure1.7Practice1.2FortheelementinFig.1.7,v1=17V.Determinev2.---------------------------------------------------Ans:v2=-17V----------------------------------------------------------------------22Figure1.823i+–v4.PowerandEnergy(1)•Poweristhetimerateofexpendingorabsorbingenergy,measuredinwatts(W).•Mathematicalexpression:vidtdqdqdwdtdwpi+–vPassivesignconventionP=+vip=–viabsorbingpowersupplyingpower24254.PowerandEnergy(2)•Thelawofconservationofenergy0p•Energyisthecapacitytodowork,measuredinjoules(J).•Mathematicalexpressionttttvidtpdtw0026Example2ComputethepowerabsorbedbyeachpartinFig.1.10Figure1.1027SolutionInFigure1.10a,with+3Aflowingintothepositivereferenceterminal,wecomputeP=(2V)(3A)=6Wofthepowerabsorbedbytheelement.Fig.1.10bshowsaslightlydifferentpicture.Nowwehaveacurrentof-3Aflowingintothepositivereferenceterminal.However,thevoltageasdefinedisnegative.ThisgivesusanabsorbedpowerP=(-2V)(-3A)=6W28ReferencetoFig.1.10c,weagainapplythepassivesignconventionrulersandcomputeanabsorbedpowerP=(4V)(-5A)=-20WSincewecomputedanegativeabsorbedpower,thistellsusthattheelementinFig.1.10cisactuallysupplying+20W(i.e.,it’sasourceofenergy).29Practice1.31.FindthepowerbeingabsorbedbythecircuitelementinFig.1.11a.Figure1.11302.FindthepowerbeinggeneratedbythecircuitelementinFig.2.11b.3.Findthepowerbeingdeliveredtothecircuit