[电气工艺原理].Electrical.Craft.Principles.5th.Edition.Volume1

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[电气工艺原理].Electrical.Craft.Principles.5th.Edition.Volume1

Basicelectricalunitsandcircuits

闸墩滑模施工组织设计Electricinsulators

Electricalcraftprinciples

Table1.3Electricinsulators

Conductorsandinsulators

Semiconductorshaveelectricalpropertieslyinginbetweenthoseofconductorsand insulators. They occupy a very important place in such devices as rectifiers and transistors,whichwillbeconsideredinChapter15.

Beforewecangoontoconsidertheelectricalforcethatresultsinelectrondrift in a conductor,wemust look at theunits usedformeasuringwork and power.Fuller considerationofelectricalenergy,workandpowerisgiveninChapter5

Energyandwork

Energyandwork

Power Power is therate of doing work or of using energy.For instance,an electrician can cutaholeinasteelplateusingahanddrilloranelectricdrill.Withboththeeffective workdonewillbe thesame,buttheelectricdrillwillcut throughtheholemore quicklybecauseitspowerisgreater.Itfollowsthat

workorenergy oower= time

Basicelectricalunitsandcircuits

TheSlunit ofpoweristhewatt(W),whichisarateof doingworkof onejouleper second.

10 Electricalcraftprinciples

Similarly,ifweknowhowmuchpowerisbeingusedandthetimeforwhichit is used,wecanfindthetotalenergyused: workorenergy=powerxtime orjoules=wattsxseconds

TTIPTC Aworkstruckrequiresaforceof180Nto smoved20m,andwhataveragepowe work=distancexforce =20×180joules =3600J work averagepower= time 3600 一 watts 40 =90W

Aworkstruckrequiresaforceof180Ntomoveit.Howmuchworkisdoneifthetruck is moved20m,andwhataveragepowerisemployedif themovementtakes40s? work=distancexforce =20×180joules =3600J work averagepower= time 3600 一 watts 40 =90W

1.6Electromotiveforceandpotentialdifference

Whenanelectriccurrentflows,energyisdissipated.Sinceenergycannotbecreated itmustbeprovidedbythedeviceusedforcirculatingthecurrent.Thisdevicemaybe chemical,suchasabattery;mechanical,suchasagenerator;orit mayhaveoneofa number of other forms.Manyyears ago,electricity was thought tobe a fluid which circulatedastheresultofaforce,andthusthetermelectromotiveforce(EMF), symbolE,cameintouse.TheEMFismeasuredintermsofthenumberofjoulesof worknecessarytomoveonecoulombofelectricityaroundthecircuit,andthushas theunitjoules/coulomb.Thisunit isreferred to asthevolt(symbolV),sothat 1volt=1joule/coulomb

Example1.6 A battery with an EMFof 6V gives a current of 5 A around a circuit for5 min.How muchenergyisprovidedinthistime? totalchargetransferredQ=It=5×5×60C=1500C totalenergysupplied=(joules/coulomb)xcoulombs =voltsxcoulombs =6×1500joules=9000J

InExample1.6,eachcoulombofelectricitycontained6Jofpotentialenergyon leavingthebattery.Thisenergywasdissipatedonthejourneyaroundthecircuit, so that the same charge would possess no energy on its return to the battery.The amount of energy expended by 1 C in its passage between any twopoints in a circuit isknownasthepotentialdifference(PD)betweenthosepoints,andismeasured in joules/coulomb,orvolts.Theinternationalsymbolforsupplyvoltageorvoltagedrop has changed fromVto U,the symbolforthe unit of voltage(thevolt)remaining as V.Aconvenient definitionof thevolt is therefore that it is equal tothedifferencein potential between twopoints if 1 J of energy is required to transfer1C of electricity betweenthem.

引桥盖梁施工方案Example1.7 Howmuchelectrical energyis converted intoheat eachminutebyanimmersion whichtakes13Afroma230Vsupply? energygivenupbyeachcoulomb=230J quantityofenergyflowperminute=It=13×60coulombs=780C therefore energy converted in 1 min =(joules/coulomb)× coulombs =230x780joules 179400Jor179.4kJ

Forametallic conductorwhichiskept ataconstant temperature,itisfound that the ratio

potential difference acrossconductor(volts) resulting currentinconductor(amperes)

isconstant,andthisratioisknownastheresistance(symbolR)oftheconductor. ThisimportantrelationshipwasfirstverifiedbyDrG.S.Ohm,andisoftenreferred to as‘Ohm’s law'.The unit of resistance is the ohm(Greek symbolΩ2,‘omega").A conductorhas aresistance of1Ω2if thePD across its endsis1Vwhenit carries a currentof1A.Adeviceintendedtohaveresistanceiscalledaresistor. TherelationshipexpressedbyOhm'slaw,whichisoffundamentalimportancein electricalengineering,canbesimplywrittenasaformula

thesubjectoftheformulacanbechangedtogive

Basicelectricalunitsandcircuits 11

Electricalcraftprinciples

cfg桩技术交底书(二级)Anelectricalheaterusedona 230Vsupplycarries a current of12A.What isits resistance? U R= T 230 一 ohms 12 =19.22

Example 1.9