电气工程及其自动化专业英语.docx

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