高级技师英语secretWord格式文档下载.docx

高级技师英语secretWord格式文档下载.docx

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Computer:

（writingonscreen）Hello!

What’syournameandnumber?

Girl:

（pressingbuttons）Mary,208.

Hello,Mary!

Lookatthis:

2x+4y=8（Twoxplusfouryequalseight.）

2x-4y=0（Twoxminusfouryequalsnought.）

（afterthinking）x=1;

y=2

No.Tryagain.

x=2;

y=1

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