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1、上海电力学院自动化英语Anenergysource(aprimaryorsecondarycell,agenerator,andthelike)convertschemical,mechanical,thermalorsomeotherformofenergyintoelectricenergy.Anenergyconverter,alsocalledload(suchasalamp,heatingappliance,orelectricmotor),convertselectricenergyintolight,heat,mechanicalwork,andsoon电源(如原生电池、再生电
2、池和发电机等)将化学能、热能或其他形式的能量转换成电能。能量转化器(也称作负载、比如灯泡、取暖器及电机等)将电能转换成光、热和机械等能量。Eventsinacircuitcanbedefinedintermsofe.m.f.(orvoltage)andcurrent.Whenelectricenergyisgenerated,transmittedandconvertedunderconditionssuchthatthecurrentsandvoltagesinvolvedremainconstantwithtime,oneusuallyspeaksofdirect-current(D.C
3、)circuits.电路的工作情况可以用电势(或电压)和电流来描述。当电能在产生、传输和变换时,若电路中相关的电流和电压不随时间而变化,我们便称其为直流电路。Withtime-invariantcurrentsandvoltages,themagneticandelectricfieldsoftheassociatedelectricplantarealsotime-invariant.对于时不变的电流和电压,与电器设备相联系的电场和磁场也是时不变的。Theterminalsofthesourceandloadareinterconnectedbyconductors(generallybu
4、tnotalwayswires).电源和负载端有导体(通常是导线,但少数情况下也有例外)连接。Thissimplecircuitmadeupofasource,aloadandtwowiresisseldom,ifever,metwithinpractice.这种由一个电源、一个负载和两根导线组成的简单电路,在实际中即使能碰见也是很少的。Whateveritsorigin(thermal,contact,etc.),thesourcee.m.f.Eisnumericallyequaltothepotentialdifferencebetweenterminals1and2withtheext
5、ernalcircuitopen,thatis,whenthereisnocurrentflowingthroughthesource:无论电动势E的原动力是什么(即不论是热、摩擦还是其他形式产生的),电源电动势E在数值上等于1、2两端之间的开路电压,即当电源中无电流通过时:Thesourcee.m.f.isdirectedfromtheterminalatalowerpotentialtothatatahigherone.Indiagram,thisisshownbyarrows.电源电动势的方向是从低电位点指向高电位点,在电路图中用箭头表示。Inthegeneralcase,theloa
6、dresistancedependssolelyonthecurrentthroughtheload,whichissymbolizedbythefunctionR(I).通常,负载电阻仅取决于通过负载的电流,这一点可用R(I)的函数符号表示。Fig.14showsvolt-amperecurvesforametal-filamentlamp,V1(I),andforacarbon-filamentlampV2(I).Asisseen,therelationbetweenthevoltageandthecurrentineachlampisotherthanlinear.Theresista
7、nceofthemetal-filamentlampincreases,andthatofthecarbon-filamentlampdecreaseswithincreaseofcurrent.图1.4中的曲线V1(I)和V2(I)分别表示一金属丝灯泡和碳丝灯泡的伏安特性曲线。如图所示每个灯泡的电流和电压之间的关系并不是线性的。随着电流的增加,金属丝灯泡的电阻是增加的,而碳丝灯泡的电阻是减小的。Mostpr