外文翻译--能量转换和守恒.doc

河南理工大学毕业设计（论文）1附录1.EnergyconversionandconservationTheconversionofmechanicalenergytoheatisbynomeansnewtous.Wearealsofamiliarwithothertransformationsofenergy.Chemicalenergyisconvertedintoheatwhenfuelburns.Electricalenergyistransformedintoheatandlightinelectricallampsandelectricalstoves.Radiantenergyturnsintoheatwhensunlightstrikesanobjectwhichabsorbsit.“Allcontradictorythingsareinterconnected；notonlydotheycoexistinasingleentityingivenconditions,butinothergivenconditions,theyalsotransformthemselvesintoeachother.”Inaword,allenergiesmaybeconvertedfromoneformtoanotherandwhatismore,theyallcantransformintoheatbythemselves.Heatisanenergyofirregularmotionofparticlesinasubstance,atordinarytemperatureitislessunablethananyoftheotherenergies.However,athightemperaturesheatenergymaybeconvertedintoenergyofmoreusableforms.Somepeoplehavemadedifferentkindsofmachinestoconvertheatintomechanicalenergy.Dieselandgasolineenginesaredesignedtoconvertheatthatisdevelopedbytheburningoffuelintomechanicalenergyforrunningtractors,trucks,andcars.Themechanicalenergytransformedfromheatinasteamturbineismadetooperategenerators.Andthegenerators,inturn,convertthemechanicalenergy.Allthesetransformationsaretakingplaceeveryminuteandeverywhereinourdailylifeandproduction.Inanyenergytransformation,thereissomeloss,butnoenergyisdestroyed.Thepartthatislostissimplywasted.Ifalloftheenergies河南理工大学毕业设计（论文）2thatarewastedwereaddedtothatused,thetotalwouldbefoundtobeequaltothetotalsupplied.Theformmaybechanged,buttheamountremainsunchanged.Thefactthatenergycanbechangedfromoneformtoanother,butcanneitherbecreatednordestroyed,constitutesoneofthemostimportantlawsinscience,thelawofconservationofenergy.Nooneformofenergycanbelongconserved,butthetotalisconservedatanytime.Amachinemaybedesignedtoliftamuchlargerweightthantheforcethatisapplied,butitcanneverproducemoreworkthanwassuppliedtoit.Inotherwords,amachinecannothaveanefficiencygreaterthanone.Sincemancannotcreateordestroyedenergy,hemustusetheenergythatisavailabletohim.Somedevicesweredesignedforthepurposeofdoingworkwithouttheneedofsupplyingenergy.Thesearetheso-calledperpetual-motionmachines.Wesaythatsuchmachinesareimpossiblebecausetheyviolatethelawofconservationofenergy.Theattempthasneverbeensuccessful.Anditwillneverbesuccessful.2.Generatorandelectricity(1)FaradayandhisGeneratorTheelectriccurrentinourhomesisproducedinpowerstationswhichusuallycontainseveralgenerators.Thesearemachineswhichgenerateelectriccurrentwhentheyareturned.Sotherehastobesomekindofenginetoturnthem.Whatkindofenginecanweuse?Steamenginesaresuitable,andsoareoilengines.Sometimesthewaterofagreatrivercanturnthegenerators,andsopowerstationsareoftenbuiltneardams.Thewaterwhichisstoredbehindadamflowsoutwithgreatforce河南理工大学毕业设计（论文）3whenitisallowedtodoso.Wecanusethisforcetoturnmachineswhicharecalledturbines.Thewaterisledthroughbigpipestotheturbines,andthentheyturnthegenerators.Thesesupplythecountrywithusefulcurrent.MichaelFaraday(1791-1867)madethefirstgenerator.Hewasagreatscientist.Hestudiedgasesandchangedsomeofthemintoliquids.Hemademanydiscoveriesinchemistryandelectricity.Beforehistimescientistsgottheirelectriccurrentfromelectriccells.Severalcellstogetherformabattery.AnItalian,Volta,madethefirstbatteryanditproducedasmallcurrent.Moderncellsareboxeswhichcontainacidsandothermaterialssuchasmetalsorcarbonrods.FaradayknewaboutVoltaswork,buthewantedtoproduceanelectriccurrentbyusingmagnets.Anelectriccurrentwhichflowsthroughacoilofwireroundanironrodproducesmagnetisminit.Faradaywantedtodotheopposite:hewantedtoproduceacurrentinawirebyusingmagnetism.Hetriedtodothisforalongtime,buthefailedcompletelyuntilhemovedawirenearthemagnets.Thenhisinstrumentsshowedthatasmallcurrentwasflowinginthewire.Eitherthemagnetorthewirehadtomove.Hemadeasmallmachinetoturnacoilofwirenearthemagnets,andthisgeneratedacurrent.Itwasthefirstgeneratorintheworld.AllmoderngeneratorsdependonFaradayswork.Themagnetsinthemareusuallyelectromagnets；eveninanelectromagnetalittlemagnetismremainsintheironafterthecurrentisswitchedoff.Assoonasthegeneratorturns,asmallcurrentappears.Thisincreasesthemagnetism,andsothecurrentincreases.Thisagainincreasesthe河南理工大学毕业设计（论文）4magnetism,andsoon.Inafewsecondsthereisquiteabigcurrentflowinginthewires.Ifariverturnstheturbines,itdoesallthenecessarywork,andnofuelisneeded.Thosecountrieswhichhavebigandpowerfulriversareluckybecausetheycangetalotofelectricpowerfromthem.(2)DirectandAlternatingCurrentsAdirectcurrentis,ofcourse,useful.Theelectricsysteminacarusesthedirectcurrent.Besides,directcurrentisalsousedtomeetsomeofheindustrialrequirements.However,atpresent,mostcitiesmakeuseofanotherkindofelectriccurrentgoingfirstinonedirectionandtheninanother,wegiveitthenameofanalternatingcurrent.Inspiteofitsbeingveryusefuladirectcurrentsystemhasonegreatdisadvantage；namely,thereisnoeasy,economicalwayinwhichonecanincreaseordecreaseitsvoltage.Thealternatingcurrentdoesnothavethisdisadvantage,itsvoltagemaybeincreasedordecreasedwithlittleenergylossbytheuseofatransformer.Usingatransformeritispossibletotransformpoweratlowvoltageintopowerathighvoltage,andviceversa.Inthatmanner,currentcanbegeneratedatavoltagewhichissuitableforanygivenmachine.Inlargepowerstations,thebestsuitedvoltageisoften