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河南理工大学毕业设计(论文)1附录1.EnergyconversionandconservationTheconversionofmechanicalenergytoheatisbynomeansnewtous.Wearealsofamiliarwithothertransformationsofenergy.Chemicalenergyisconvertedintoheatwhenfuelburns.Electricalenergyistransformedintoheatandlightinelectricallampsandelectricalstoves.Radiantenergyturnsintoheatwhensunlightstrikesanobjectwhichabsorbsit.Allcontradictorythingsareinterconnectednotonlydotheycoexistinasingleentityingivenconditions,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.Thesearethesocalledperpetualmotionmachines.Wesaythatsuchmachinesareimpossiblebecausetheyviolatethelawofconservationofenergy.Theattempthasneverbeensuccessful.Anditwillneverbesuccessful.2.Generatorandelectricity1FaradayandhisGeneratorTheelectriccurrentinourhomesisproducedinpowerstationswhichusuallycontainseveralgenerators.Thesearemachineswhichgenerateelectriccurrentwhentheyareturned.Sotherehastobesomekindofenginetoturnthem.WhatkindofenginecanweuseSteamenginesaresuitable,andsoareoilengines.Sometimesthewaterofagreatrivercanturnthegenerators,andsopowerstationsareoftenbuiltneardams.Thewaterwhichisstoredbehindadamflowsoutwithgreatforce河南理工大学毕业设计(论文)3whenitisallowedtodoso.Wecanusethisforcetoturnmachineswhicharecalledturbines.Thewaterisledthroughbigpipestotheturbines,andthentheyturnthegenerators.Thesesupplythecountrywithusefulcurrent.MichaelFaraday17911867madethefirstgenerator.Hewasagreatscientist.Hestudiedgasesandchangedsomeofthemintoliquids.Hemademanydiscoveriesinchemistryandelectricity.Beforehistimescientistsgottheirelectriccurrentfromelectriccells.Severalcellstogetherformabattery.AnItalian,Volta,madethefirstbatteryanditproducedasmallcurrent.Moderncellsareboxeswhichcontainacidsandothermaterialssuchasmetalsorcarbonrods.FaradayknewaboutVoltaswork,buthewantedtoproduceanelectriccurrentbyusingmagnets.Anelectriccurrentwhichflowsthroughacoilofwireroundanironrodproducesmagnetisminit.Faradaywantedtodotheoppositehewantedtoproduceacurrentinawirebyusingmagnetism.Hetriedtodothisforalongtime,buthefailedcompletelyuntilhemovedawirenearthemagnets.Thenhisinstrumentsshowedthatasmallcurrentwasflowinginthewire.Eitherthemagnetorthewirehadtomove.Hemadeasmallmachinetoturnacoilofwirenearthemagnets,andthisgeneratedacurrent.Itwasthefirstgeneratorintheworld.AllmoderngeneratorsdependonFaradayswork.Themagnetsinthemareusuallyelectromagnetseveninanelectromagnetalittlemagnetismremainsintheironafterthecurrentisswitchedoff.Assoonasthegeneratorturns,asmallcurrentappears.Thisincreasesthemagnetism,andsothecurrentincreases.Thisagainincreasesthe河南理工大学毕业设计(论文)4magnetism,andsoon.Inafewsecondsthereisquiteabigcurrentflowinginthewires.Ifariverturnstheturbines,itdoesallthenecessarywork,andnofuelisneeded.Thosecountrieswhichhavebigandpowerfulriversareluckybecausetheycangetalotofelectricpowerfromthem.2DirectandAlternatingCurrentsAdirectcurrentis,ofcourse,useful.Theelectricsysteminacarusesthedirectcurrent.Besides,directcurrentisalsousedtomeetsomeofheindustrialrequirements.However,atpresent,mostcitiesmakeuseofanotherkindofelectriccurrentgoingfirstinonedirectionandtheninanother,wegiveitthenameofanalternatingcurrent.Inspiteofitsbeingveryusefuladirectcurrentsystemhasonegreatdisadvantagenamely,thereisnoeasy,economicalwayinwhichonecanincreaseordecreaseitsvoltage.Thealternatingcurrentdoesnothavethisdisadvantage,itsvoltagemaybeincreasedordecreasedwithlittleenergylossbytheuseofatransformer.Usingatransformeritispossibletotransformpoweratlowvoltageintopowerathighvoltage,andviceversa.Inthatmanner,currentcanbegeneratedatavoltagewhichissuitableforanygivenmachine.Inlargepowerstations,thebestsuitedvoltageisoften6,300or10,500V.Powerbeingtransmittedoverlongdistanceswithlesslossathighvoltagethanatlowvoltage,itismoreeconomicaltoincreasethevoltageto35,000or110,000oreven220,000Vfortransmission.Whereverthepoweristobeused,itisloweredtothevoltagewhichsatisfiesthatparticularpurpose,suchas
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