机电专业英语 教学课件 作者 杨寿智 刘水平 机电专业英语.Lesson 24.ppt

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1、Lesson24Kirchhoff’sVoltageLawTextKirchhoff’svoltagelawstates:Thesumofallthevoltagesaroundaclosedpathiszeroor,inotherwords,thesumofthevoltagedropsequalsthetotalsourcevoltage.Forexample,inthecircuitofFigure24.1,therearethreevoltagedropsandonevoltagesource.Ifwesumallofthevoltagesa

2、roundthecircuit,wegetVs-V1-V2-V3=0(24-1)Noticethatthesourcevoltagehasasignoppositetothatofthevoltagedrops①.Thus,thealgebraicsumequalszero.Equation(24-1)canbewrittenanotherwaybytransposingofthevoltagedroptermstotherightsideoftheequation:Vs=V1+V2+V3(24-2)Inwords,Equation(24-2)say

3、sthatthesourcevoltageequalsthesumofthevoltagedrops.BothEquations(24-1)and(24-2)areequivalentwaysofexpressingKirchhoff’svoltagelawforthecircuitinFigure24.1.ThepreviousillustrationofKirchhoff’svoltagelawwasforthespecialcaseofthreevoltagedropsandonevoltagesource.Ingeneral,Kirchhof

4、f’svoltagelawappliestoanynumberofseriesvoltagedropsandanynumberofseriesvoltagesources,asillustratedinFigure24.2②.ThegeneralformofKirchhoff’svoltagelawisVs1+Vs2+···+Vsn=V1+V2++···+Vn(24-3)POLARITIESOFVOLTAGEDROPSAlwaysrememberthatthetotalsourcevoltagehasapolarityinthecircuitoppo

5、sitetothatofthevoltagedrops.AswegoaroundtheseriescircuitofFigure24.3intheclockwisedirection,thevoltagedropshavepolaritiesasshown.Noticethatthepositive(+)sideofeachresistoristheonenearestthepositiveterminalofthesourceaswefollowthecurrentpath.Thenegative(-)sideofeachresistoristhe

6、onenearestthenegativeterminalofthesourceaswefollowthecurrentpath③.Letusexaminepolaritiesinaseriesloopfromtheconventionalcurrentstandpoint.RefertoFigure24.3.Thecurrentflowsoutofthepositivesideofthesourceandthroughtheresistorsasshown.Thecurrentflowsintothepositivesideofeachresist

7、orandoutofthenegativeside.Wenaturallytendtothinkofadropasgoingfromahighertoalowervalue.Ifwegofromamorepositivevoltagetoamorenegativevoltage,thisseemstofitourconceptofadropinvoltage,aswasdiscussedinotherchapter④.Thedirectionofconventionalcurrentthroughthecircuitsupportsthisconce

8、pt,becauseitisconsideredtobefromthepositivesidetothemo