电气工程及其自动化专业英语.docx
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电气工程及其自动化专业英语
电气工程及其自动化专业英语
SectionIbasicelectriccircuit
Chapter1IntroductiontoelectriccircuitsNewWordsandExpressions
1.electricalcircuit
n.
电路
2.voltage
n.
电压,伏特
3.current
n.
电流,通用的,流通的,现在的
4.currentflow
n.
电流
5.resistor
n.
电阻,电阻器
6.battery
n.
电池
7.load
n.
负载,负荷
8.performanee
n.
性能
9.circuitdiagram
n.
电路图
10.idealizedmodel
n.
理想模型
Introduction
*Asimplecircuitanditscomponents.
idealizedmodelofthecircuit
*Modelcanbechangedifnecessary.
*summarize
Inelementaryphysicsclassesyouundoubtedlyhavebeenintroducedtothefundamentalconceptsofelectricityandhowrealcomponentscanbeputtogethertoformanelectricalcircuit.Averysimplecircuit,forexample,mightconsistofabattery,somewire,aswitch,andanincandescentlightbulbasshowninFig.1-1.Thebatterysuppliestheenergyrequiredtoforceelectronsaroundtheloop,heatingthefilamentofthebulbandcausingthebulbtoradiatealotofheatandsomelight.
Energyistransferredfromasource,thebattery,toaload,thebulb———Youprobablyalreadyknowthatthevoltageofthebatteryandtheelectricalresistaneeofthebulbhavesomethingtodowiththeamountofcurrentthatwillflow
inthecircuit.Fromyourownpracticalexperieneeyoualsoknowthatnocurrentwillflowuntiltheswitchisclosed.Thatis,foracircuittodoanything,theloophastobecompletedsothatelectronscanflowfromthebatterytothebulbandthenbackagaintothebattery.Andfinally,youprobablyrealizethatitdoesntmuchmatter,whetherthereisonefootortwofeetofwireconnectingthebatterytothebulb,butthatitprobablywouldmatterifthereisamileofwirebetweenitandthebulb.
AlsoshowninFig.1-1isamodelmadeupofidealizedcomponents.Thebatteryismodeledasanidealsourcethatputsoutaconstantvoltage,VB,nomatterwhatamountofcurrent,i,isdrawn.Thewiresareconsideredtobeperfectconductorsthatoffernoresistaneetocurrentflow.Theswitchisassumedtobeopenorclosed.Thereisnoarcingofcurrentacrossthegapwhentheswitchisopened,noristhereanybouncetotheswitchasitmakescontactonclosure.Thelightbulbismodeledasasimpleresistor,R,thatneverchangesitsvalue,nomatterhowhotitbecomesorhowmuchcurrentisflowingthroughit.
Fig.1-1(a)Asimplecircuit
(b)Anidealizedrepresentationofthecircuit
Formostpurposes,theidealizedmodelshowninFig.1-1bisanadequaterepresentationofthecircuit;thatis,ourpredictionofthecurrentthatwillflowthroughthebulbwhenevertheswitchisclosedwillbesufficientlyaccuratethatwecanconsidertheproblemsolved.Theremaybetimes,however,whenthemodelisinadequate.Thebatteryvoltage,forexample,maydropasmoreandmorecurrentisdrawn,orasthebatteryages.Thelightbulb'sresistancemaychangeasit
heatsup,andthefilamentmayhaveabitofinductanceandcapacitanceassociatedwithitaswellasresistancesothatwhentheswitchisclosed,thecurrentmaynotjumpinstantaneouslyfromzerotosomefinal,steadystatevalue.Thewiresmaybe
undersized,andsomeofthepowerdeliveredbythebatterymaybelostinthewiresbeforeitreachestheload.Thesesubtleeffectsmayormaynotbeimportant,dependingonwhatwearetryingtofindoutandhowaccuratelywemustbeabletopredicttheperformaneeofthecircuit.Ifwedecidetheyareimportant,wecanalwayschangethemodelasnecessaryandthenproceedwiththeanalysis.Thepointhereissimple.Thecombinationsofresistors,capacitors,inductors,voltagesources,currentsources,andsoforth,thatyouseeinacircuitdiagramaremerelymodelsofrealcomponentsthatcomprisearealcircuit,andacertainamountofjudgmentisrequiredtodecidehowcomplicatedthemodelmustbebeforesufficientlyaccurateresultscanbeobtained.Forourpurposes,wewillbeusingverysimplemodelsingeneral,leavingmanyofthecomplicationstomoreadvaneedtextbooks.
Chapter2
Definitionsofkeyelectricalquantities
NewWordsandExpressions
charge
n.vt.电荷;充电
nucleus
n.原子核(pl.);nuclearadj.
negative
n.否定,负数,底片adj.否定的,消极的,负的,阴性的
positive
adj.[数]正的adj.[电]阳的
ingeneral
通常,大体上,一般而言,总的说来
algebraic
adj.代数的,关于代数学的
solutiontothecircuitproblemn.关于电路问题的解法
theunitsofpowern.功率的单位
directcurrent(dc)n直流电
alternatingcurrent(ac)n.交流电
sinusoidally
adv.正弦地
transistor
n.晶体管
Part1ChargeandCurrent
Anatomconsistsofapositivelychargednucleussurroundedbyaswarmofnegatively
chargedelectrons.Thechargeassociatedwithoneelectronhasbeenfoundtobe1.602x10-19coulombs;or,statedtheotherwayaround,onecoulombcanbedefinedasthechargeon6.242x1018electrons.Whilemostoftheelectronsassociatedwithanatomaretightlyboundtothenucleus,goodconductors,likecopper,havefreeelectronsthataresufficientlydistantfromtheirnucleithattheirattractiontoanyparticularnucleusiseasilyovercome.Theseconductionelectronsarefreetowanderfromatomtoatom,andtheirmovementconstitutesanelectriccurrent.
Inawire,whenonecoulomb'sworthofchargepassesagivenspotinonesecond,thecurrentisdefinedtobeoneampere(abbreviatedA),namedafterthenineteenth-centuryphysicistAndr'eMarieAmp'ere.Thatis,currentiisthenetrateofflowofchargeqpastapoint,orthroughanarea:
i=dq/dt(1.1)
Ingeneral,chargescanbenegativeorpositive.Forexample,inaneonlight,positiveionsmoveinonedirectionandnegativeelectronsmoveintheother.Eachcontributestocurrent,andthetotalcurrentistheirsum.Byconvention,thedirectionofcurrentflowistakentobethedirectionthatpositivechargeswouldmove,whetherornotpositivechargeshappentobeinthepicture.Thus,inawire,electronsmovingtotherightconstituteacurrentthatflowstotheleft,asshowninFig.1-2.
(〉)
dq
/=—
dt
Fig.1-2Byconvention,negativechargesmovinginonedirectionconstituteapositivecurrent
flowintheoppositedirection
W/henchargeflowsatasteadyrateinonedirectiononly,thecurrentissaidtobedirectcurrent,or血Abattery,forexample,suppliesdirectcurrent.Whenchargeflowsbackandforthsinusoidally,itissaidtobealternatingcurrent,orac.IntheUnitedStatestheacelectricitydeliveredbytesofacanddcareshowninFig.1-3.
Time―
(a)
Fig.1-3(a)Steady-statedirectcurrent(de)(b)Alternatingcurrent(ac)
Part2Kirchhoff'sCurrentLaw
TwoofthemostfundamentalpropertiesofcircuitswereestablishedexperimentallyacenturyandahalfagobyaGermanprofessor,GustavRobertKirchhoff(1824-1887).Thefirstproperty,knownasKirchhoff'scurrentlaw(abbreviatedKCL),statesthatateveryinstantoftimethesumofthecurrentsflowingintoanynodeofacircuitmustequalthesumofthecurrentsleavingthenode,whereanodeisanyspotwheretwoormorewiresarejoined.Thisisaverysimple,butpowerfulconcept.Itisintuitivelyobviousonceyouassertthatcurrentistheflowofcharge,andthatchargeisconservative—neitherbeingcreatednordestroyedasitentersanode.Uniesschargesomehowbuildsupatanode,whichitdoesnot,thentherateatwhichchargeentersanodemustequaltherateatwhichchargeleavesthenode.
ThereareseveralalternativewaystostateKirchhoff'scurrentlaw.ThemostcommonlyusedstatementsaysthatthesumofthecurrentsflowingintoanodeiszeroasshowninFig.1-4a,inwhichcasesomeofthosecurrentsmusthavenegativevalueswhilesomehavepositivevalues.EquallyvalidwouldbethestatementthatthesumofthecurrentsleavinganodemustbezeroasshowninFig.1-4b(againsomeofthesecurrentsneedtohavepositivevaluesandsomenegative).Finally,wecouldsaythatthesumofthecurrentsenteringanodeequalsthesumofthecurrentsleavinganode(Fig.1-4c).Theseareallequivalentaslongasweunderstandwhatismeantaboutthedirectionofcurrentflowwhenweindicateitwithanarrowonacircuitdiagram.Currentthatactuallyflowsinthedirectionshownbythearrowisgivenapositivesign.Currentsthatactually
flowintheoppositedirectionhavenegativevalues.
Fig.1-4lllustratingvariouswaysthatKirchhoff'scurrentlawcanbestated
(a)Thesumofthecurrentsintoanodeequalszero
(b)Thesumofthecurrentsleavingthenodeiszero
(c)Thesumofthecurrentsenteringanodeequalsthesumofthecurrentsleavingthenode
Notethatyoucandrawcurrentarrowsinanydirectionthatyouwant—thatmuchisarbitrary—butoncehavingdrawnthearrows,youmustthenwriteKirchhoff'scurrentlawinamannerthatisconsistentwithyourarrows,ashasbeendoneinFig.1-4.Thealgebraicsolutiontothecircuitproblemwillautomaticallydeterminewhetherornotyourarbitrarilydetermineddirectionsforcurrentswerecorrect.
Example1.1UsingKirchhoff'sCurrentLaw
Anodeofacircuitisshownwithcurrentdirectionarrowschosenarbitrarily.Having
pickedthosedirections,i1=-5A,i2=3A,andi3=-1A.WriteanexpressionforKirchhoff'scurrentlawandsolvefori4.
Solution.ByKirchhoff'scurrentlaw,
1+i4
i4=-1A
i1+i2=i3+i4sothat
Thatis,i4isactually1Aflowingintothenode.Notethati2,i3,andi4areallenteringthenode,andi1istheonlycurrentthatisleavingthenode.
Part3Kirchhoff'sVoltageLaw
Electronswon'tflowthroughacircuituniesstheyaregivensomeenergytohelpsendthem