高级技师英语secret.docx
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高级技师英语secret
Passage1
SwitchesandFuses
Anelectricswitchisoftenonawallnearthedoorofaroom.Twowiresleadtothelampintheroom.Theswitchisfixedinoneofthem.Theswitchcancauseabreakinthiswire,andthenthelightgoesout.Theswitchcanalsojointhetwopartsofthewireagain;thenwegetalight.
Switchescancontrolmanydifferentthings.Smallswitchescontrollampsandradiosetsbecausethesedonottakealargecurrent.Largerswitchescontrolelectricfires.Otherswitchescancontrolelectricmotors.
Passage2
TheComputerClassroom
Insomeschoolsthereisacomputerclassroom.Forexample,studentscandotheirmathematicswithacomputer.Thecomputerwritesquestionsonthescreensinfrontofthestudents,andthestudentsanswerontheirkeyboards.
Thisispartofalessonwithagirl:
Computer:
(writingonscreen)Hello!
What’syournameandnumber?
Girl:
(pressingbuttons)Mary,208.
Computer:
Hello,Mary!
Lookatthis:
2x+4y=8(Twoxplusfouryequalseight.)
2x-4y=0(Twoxminusfouryequalsnought.)
Girl:
(afterthinking)x=1;y=2
Computer:
No.Tryagain.
Girl:
x=2;y=1
Computer:
That’sright.Verygood,Mary!
Nowlook……
ThecomputerknowsMary.Thisisnottheirfirstlesson.ThecomputercangiveMarytherightlessonforher,neithertoofast,nortooslow.Andthecomputercandothiswithmanystudentsatthesametime.
Passage3
ForAlmostEverything
Acomputercandoalotofwork.Itcandothousandsofthingsathighspeed.Forexample,thepoliceusecomputers.Thecomputershavealltheinformationabouttraffic,etc.Thepolicecanlookatthisinformationatanytime.Ifapolicemanfindsacarinthecountryandthereisnobodyinitornearit,acomputercanhelphim.Whosecarisit?
Wheredoesitcomefrom?
Hecanaskthecomputerthesethings.Isthereanyinformationinthecomputeraboutthecar?
Yes,somebodymanymilesawaylostittwoweeksago.
Passage4
Communications
Inthebasicelectricsense,theterm“communications”referstothesending,receptionand
processingofinformationbyelectricalmeans.Radiocommunicationwasmadepossiblebytheinventionofthetriode.Ithassubsequentlybecomeevenmorewidespreadandrefinedthroughtheinventionanduseofthetransistor,integratedcircuitsandothersemiconductordevices.
Amoderncommunicationssystemisfirstconcernwiththecollation,processingandstorageofinformationbeforeitstransmission.Theactualtransmissionthenfollows.Finallywehavereception.
Inlong-distancecommunications,atransmitterisrequiredtoprocesstheincominginformationsoastomakeitsuitablefortransmissionandsubsequentreception.
Passage5
TimeConstant
Whenavoltageisappliedacrosstheterminalsofacircuitwithcapacitanceandresistancethevoltagedoesnotappearacrossthecapacitorinstantaneously.Ittakestimefortheplatesofacapacitortoreachtheirfullcharge.
Thetimeforthecapacitortobecomefullychargeddependsontheproductofcircuitresistanceandcapacitance.Thisproduct,RC,orresistancetimescapacitance,iscalledthetimeconstantofcapacitivecircuit.TheRCtimeconstantgivesthetimeinsecondsforthevoltagetoreach63%(actually63.2%)ofitsmaximumvalue.Thegreaterthetimeconstant,thelongerthetimeforthecapacitortoreachitsmaximumvoltage.
Passage6
Resistivity
ThecurrentdensityJinaconductordependsontheelectricfieldEandonthenatureoftheconductor.Ingeneral,thedependenceofJonEcanbequitecomplex.Forsomematerials,especially,however,itcanberepresentedquitewellbyadirectproportionality.ForsuchmaterialstheratioofEtoJisconstant.
Wedefinetheresistivityρofaparticularmaterialastheratioofelectricfieldtocurrentdensity:
E
ρ=—
J
Thatis,resistivityistheelectricfieldperunitcurrentdensity.A“perfect”conductorwouldhavezeroresistivity,anda“perfect”insulatorwouldhaveinfiniteresistivity.Metalsandalloyshavingthelowestresistivitiesarethebestconductors.
Passage7
SeriesCircuit
Ifseveralelectriccomponents,suchasresistors,areconnectedsothatthecurrentisthesameineveryone,thecomponentsaresaidtobeinaseriescircuit.Considerthesimpleseriescircuitcomprisingthebatteryandthreeresistors.ThecurrentIresultinapotentialdifferencebetween
theterminalsofeachresistor.Thatis,
V1=R1I,V2=R2IandV3=R3I
Clear,thesumofthesevoltageisequaltothebatteryemf,or
V=V1+V2+V3
Theequationabovestatesthatthealgebraicsumofthepotentialdifferencesaroundanycompletecircuitisequaltozero.Theequivalentresistanceofanynumberofresistorsconnectedinseriesequalsthesumoftheirindividualresistance.
Passage8
Voltage,ResistanceandCurrent
Voltageisthepotentialdifferenceinanelectriccircuit.Theoppositiongivenbyaconductororaninsulatortotheflowofelectronsiscalledresistance.
Therearetwokindsofcurrent:
directcurrent(DC)andalternatingcurrent(AC).Adirectcurrentisacurrentflowinginaconductoralwaysinonedirection.Analternatingcurrentisacurrentperiodicallychangingitsdirectionofflow.
FromOhm’slawweknowthatthecurrentinanelectriccircuitisequaltothevoltagedividedbytheresistance.Inotherwords,wegetthevoltagewhenmultiplyingthecurrentbytheresistance.
Passage9
Ohm’sLaw
In1825Ohmmadethediscoverythatasimplecorrelationexistsbetweenthreeofthebasicelectricquantities,namely:
resistance,currentandvoltage.Insimplestatement,thecurrentflowinginacircuitisdirectlyproportionaltothevoltageandinverselyproportionaltotheresistance.Thisstatementis,generally,spokenofasOhm’slaw.
ItisacommonpracticetogiveashorterformtoOhm’slawbyusingletters,theletterImeaningcurrent,V—voltage,andR—resistance.
Ohm’slawcan,then,bewritten:
V
I=—
R
Thisrelationshipisconsideredtooneofthemostimportantinallworkwithelectricity.However,weshouldkeepinmindthatthisrelationistrueformetallicconductors.
Passage10
Conductors
Justasgoodelectricalconductors,suchasthemetal,arealsogoodconductorsofheat,poorelectricalconductors,suchasceramicandplasticmaterials,arealsopoorthermalconductors.Thefreeelectronsinametalcarryingchargeinelectricalconductionalsoplayanimportantroleinthe
conductionofheat;henceweexpectacorrelationbetweenelectricalandthermalconductivity.
ThediscoverythatthecurrentdensityJisproportionaltotheelectricfieldEforametallicconductoratconstanttemperaturewasmadebyG.S.Ohm(1789—1854)andiscalledOhm’slaw.AmaterialobeyingOhm’slawiscalledanohmicconductor,oralinearconductor.IfOhm’slawisnotobeyed,theconductoriscallednonlinear.
Passage11
PotentialDifference
Todescribethesituationofachargeinanelectricfield,thequantity“potentialdifference”isintroduce.ThepotentialVABbetweentwopointsAtoBisdefinedastheratiooftheworkthatmustbedonetotakeachargeqfromAtoBtothevalueofq:
WAB
VAB=——
q
Inauniformelectricfield,thepotentialdifferencebetweentwopointsistheproductofthefieldintensityandtheirseparationinadirectionparalleltothatofthefield.ApositivepotentialdifferencemeansthatthattheenergyofthechargeisgreateratBthanatA,whilenegativepotentialdifferencemeansthatitsenergyislessatBthanatA.
Passage12
DirectandAlternatingCurrent
Thecurrentthatflowssteadilyinonedirectionisusuallycalledadirectcurrent.Adirectcurrentis,ofcourse,usefully.Itisthekindofcurrentwhichisalwaysassociatedwithbatteries.Weknowthattheelectricalsysteminanautomobileandanairplane,thetelegraph,thetelephoneandthetrolly-bususethedirectcurrent.Directcurrentisalsousedtomeetsomeoftheindustrialrequirements.
Forindustryandmanyotherpurposes,however,mostcitiesmakeuseofanothertypeofelectriccurrentwhichflowsfirstinonedirectionandtheninanother.Itwasgiventhenameofanalternatingcurrent.Weknowthatthealternatingcurrentistheverycurrentthatmakesradiopossible.
Passage13
Thetransformer
Onecannotcallatransformeramachine,forithasnomovingparts.Weknowthetransformertobeanapparatusthatisdesignedforchangingthealternatingvoltagesandalternatingcurrentsbymeansofmagneticinductionwithoutanychangeoffrequency.
Oneofthegreatadvantagesofthealternatingcurrentistheeaseandefficiencywithwhichpoweratlowvoltagemaybetransformedintoanalmostsimilaramountofpowerathighvoltage,andviceversa.Usingatransformer,itispossibletotransmitthealternatingcurrenttoverydistantplacesatwhichthepowerisrequired.
Passage14
PowerSupply
Thepowersupplyisanessentialpartofeveryelectronicequipment.Initssimplestformitmayconsistofnomorethanatransformer,rectifier,andsmoothingcircuit,butfrequentlymuchmoresophisticatedarrangementsarerequired,especiallyintheindustrialfieldofcomputers,digitalinstruments,D.C.amplifier,etc.
Powersupplycanbedefinedascircuitsthattransformelectricalinputpower,eitherA.C.orD.C.,intoD.C.,outputpower.Thisdefinitiondistinguishespowersuppliesfromotherelectronicpowersourceswhicharedealtwithelsewhereunderthefollowingheading:
D.C.-to-A.C.inverters,D.C.-to-D.C.converters,andstaticinverters.Thetermpowersupplyiscommonlyusedwhenreferringtoanelectronicstabilizingcircuit.
Passage15
ElectronicDigitalComputers
Ingeneraltherearet