Electric Theory English.docx
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Electric Theory English.docx
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ElectricTheoryEnglish
ElectronTheory
Elementsofanatom
Allmatteriscomposedofatoms.AtomiscomposedofProtons,NeutronsandElectrons.Protonshaveapositivecharge.Electronshaveanegativecharge.Neutronsareneutral.Thenegativechargeoftheelectronsisbalancedbythepositivechargeoftheprotons.Thesearereferredtoasboundelectrons.
Conductors,InsulatorsandSemi-conductors
Conductors
Materialsthatpermitmanyelectronstomovefreelyarecalledconductors.
e.g.Copper,Iron,aluminum.
Insulators
Materialsthatallowfewfreeelectronstomovearecalledinsulators.
e.g.Plastic,Rubber,glass.
Semi-conductors
Materialscanbeusedtomanufacturedevicesthathavecharacteristicsofbothconductorsandinsulators.
e.g.Silicon.
ElectricCharges
Neutralstateofanatom
Anatomwithanequalnumberofelectronsandprotonsissaidtobeelectricallyneutral.
e.g.Aluminumatomhas13electronsand13protons
Positiveandnegativecharges
Thelackofelectronsiscalledpositivechargebecausetherearemoreprotonsthanelectrons.Theexcessofelectronshasanegativescharge.
Attractionandrepulsionofelectriccharges
UnlikechargesattractLikechargesrepel
Current
Electricityistheflowoffreeelectronsinaconductorfromoneatomtothenextatominthesamegeneraldirection.Thisflowofelectronsisreferredtoascurrent.ThesymbolofcurrentisA.
Voltage
Voltageistheforcethatisappliedtoaconductorthatcauseselectriccurrenttoflow.Theforcerequiredtomakeelectricityflowthroughaconductoriscalladifferenceinpotential.Currentflowthroughaconductorassameaswaterflowthroughapipe.ThesymbolofvoltageisVorE.
Resistance
Allmaterialimpedestheflowofelectricalcurrenttosomeextent.Theamountofresistancedependsoncomposition,length,cross-sectionandtemperatureofmaterial.ThesymbolofcurrentisΩ.
SimpleElectricCircuit
Afundamentalrelationshipexitsbetweencurrent,voltage,andresistanceintheelectriccircuit.Theremustbeacompletepathforcurrenttoflow.Otherwise,thecircuitopenandthennotwork.
e.g.Thefollowingdiagramisrepresentansimpleelectricalcircuit.Itisconsistofbattery,resistance,ammeterandvoltmeter.
Ohm’sLaw
Therelationshipbetweencurrent,voltageandresistanceisthatcurrentvariesdirectlywithvoltageandinverselywithresistance.
i.e.Current(I)=Voltage(E)/Resistance(Ω)
Voltage(E)=Current(I)XResistance(Ω)
Resistance(Ω)=Voltage(E)/Current(I)
Thecorrectanswerofthecorrectvaluesis:
●CurrentisexpressedinAmps,A
●VoltageisexpressedinVolts,EorV
●ResistanceisexpressedinOhms,Ω
e.g.Thesimplecircuitbelow,assumethevoltagesuppliedbybattery10Voltage,andresistanceis5Ω
Tofindhowmuchcurrent:
Current(I)=Voltage(E)/Resistance(Ω)
Current(I)=10volts/5Ω
=2A
DCSeriesCircuit
Aseriescircuitisformedwhenanynumberofresistorareconnectedend-to-endsothatthereisonlyonepathforcurrenttoflow.
Resistanceinseriescircuit
Thevaluesofresistanceaddinaseries.Ifitisfiveresistanceinaseriescircuit,thentheformulaisRt=R1+R2+R3+R4+R5
Rt=R1+R2+R3+R4+R5
Rt=11k+2k+2k+0.1k+1k
Rt=16.1kΩ
Currentinseriescircuit
Currentisthesameanywhereitismeasuredinaseriescircuit.Forexample,
I=E/R
I=12/10
I=1.2A
Voltageinseriescircuit
Voltagecanbemeasuredacrosseachoftheresistorsinacircuit.Thevoltageacrossaresistorisreferredtoasavoltagedrop.Inthefollowingdiagram,fourresistorsof1.5Ωeachhavebeenplacedinserieswitha12Vbattery.
First,solvefortotalresistance,
Rt=R1+R2+R3+R4
Rt=1.5+1.5+1.5+1.5
Rt=6Ω
Second,solveforcurrent,
I=E/R
I=12/6
I=1A
Third,solveforvoltageacrossanyresistor(ifwanttocheckthethirdresistor):
E=IXR
E=2X1.5
E=3V
Voltagedivisioninaseriescircuit
Itisoftendesirabletouseavoltagepotentialthatislowerthansupplyvoltage.Todothis,avoltagedivider,similartothefollowingdiagram,canbeused.
First,solvefortotalresistance,
Rt=R1+R2
Rt=5+20
Rt=25Ω
Second,solveforcurrent,
I=Ein/Rt
I=50/25
I=2A
Finally,solveforvoltage,
Eout=IXR2
Eout=2X20
Eout=40V
DCParallelCircuit
Resistanceinparallelcircuit
Theparallelcircuitisformedwhentwoormoreresistancesareplaceinacircuitside-by-sidesothatcurrentcanflowthroughmorethanonepath.Iftherearethreeresistorsinaparallelcircuit,theformulasdeterminetotalresistancebelow:
1/Rt=1/R1+1/R2+1/R3
1/Rt=1/5+1/10+1/20
1/Rt=4/20+2/20+1/20
1/Rt=7/20
Rt=20/4
Rt=2.86Ω
Voltageinparallelcircuit
Whenresistorsareplacedinparallelacrossavoltagesource,thevoltageisthesameacrosseachresistor.Thefollowingdiagram:
Currentinparallelcircuit
Currentflowingthroughaparallelcircuitdividesandflowsthrougheachbranchofthecircuit.i.e.Totalcurrentisequaltothesumofthecurrentineachbranch.
It=I1+I2+I3
Forexample,tofindthetotalcurrentinthefollowingcircuitdiagram.
I1=E/R1
I1=12/40
I1=0.3A
I2=E/R2
I2=12/20
I2=0.6A
It=I1+I2
It=0.3+0.6
It=0.9A
Series-ParallelCircuits
Series-ParallelCircuitsarecompoundcircuit.Atleastthreeresistorsarerequiredtofromaseries-parallelcircuit.Thefollowingdiagramsshowtwowaysaseries-parallelcombinationcouldbefound.
First,itissimplifyingaseries-paralleltoaseriescircuit
Second,itissimplifyingaseries-paralleltoaparallelcircuit
Power
Wheneverforcesofanykindcausemotion,workisaccomplished.Inthediagrambelowworkisdonewhenamechanicalforceisusedtoliftaweight.IfIforcewereexertedwithoutcausingmotion,thennoworkisdone.
Electricpower
Inanelectricalcircuit,voltageappliedtoaconductorwillcauseelectrontoflow.Voltageistheforceandelectronflowisthemotion.TherateatwhichworkisdoneiscalledpowerandisrepresentedbythesymbolP
Powerformula
Thisisexpressedasvoltagetimescurrent.
P=EI
P=I2R
P=E2/R
Forexample,thefollowingdiagrampowercanbeusinganyofthepowerformulas,
P=EI
P=I2R
P=E2/R
P=12X2
P=22X6
P=122/6
P=24W
P=24W
P=24W
Magnetism
Theprinciplesofmagnetismareanintegralpartofelectricity.Allmagnetshavetwocharacteristics.Theyattractandoldiron.Iffreetomove,likethecompassneedle,themagnetwillassumeroughlyanorth-southposition.Thethreecommonformsofmagnetsarehorseshoe,barandcompassneedle.
Magneticlineofflux
Everymagnethastwopoles,onenorthpoleandonesouthpole.Thisisthepointatwhichmaximumattractionoccurs.Invisiblemagneticlinesoffluxleavethenorthpoleandenterthesouthpole.Bydrawinglinestheway,thefollowingdiagramisobtained.
Thefieldlinesexistoutsideandinsidethemagnet.Themagneticlinesoffluxarealwaysfromclosedloop.
Interactionbetweentwomagnets
Whentwomagnetsarebroughttogether,themagneticfluxfiledaroundthemagnetcausesomeformofinteraction.Twounlikepolesattracteachother.Twolikepolesrepeleachother.
Electromagnetism
Left-handruleforconductors
Anelectromagneticfieldisamagneticfieldgeneratedbycurrentflowinaconductor.Everyelectriccurrentgeneratesamagneticfield.Adefiniterelationshipexitsbetweenthedirectionofcurrentflowandthedirectionofmagneticfield.Theleft-handruleforconductorsdemonstratesthisrelationship.Thelefthandwiththethumbpointinginthedirectionofelectronflow,thefingerswillpointinthedirectionofthemagneticlinesofflux.
Current-carryingcoil
Acoilofwirecarryingacurrentactslikeamagnet.Individualloopofwireactsassmallmagnets.Theindividualfieldaddstogethertoformonemagnet.Thestrengthofthefieldcanbeincreasebyaddingmoreturnstothecoilandincreasethecurrent.
Left-handruleforcoils
Aleft-handruleexitsforcoilstodeterminethedirectionofthemagneticfield.Thefingersofthelefthandarewrappedaroundthecoilinthedirectionofelectronflow.Thethumbpointstothenorthpoleofthecoil.
Electromagnets
Anelectromagnetiscomposedofacoilofwirewouldaroundacore.Thecoreisusuallyasoftironthatconductsmagneticlineofforcewithrelativeease.Whencurrentispassedthroughthecoil,thecorebecomesmagnetized.Alargevarietyofelectricaldevicessuchasmotors,circuitbreakers,relay,andstartersuseelectromagneticprinciples.
IntroductiontoAC
Thesupplyofcurrentforelectricaldevicesmaycomefromadirectcurrent(DC),oranalternatingcurrent(AC).InDCelectricity,electronsflowcontinuouslyinonedirection.InACelectricity,electronsflowfirstinonedirectiontheninanother.TheACgeneratorreversesterminalpolaritymanytimesasecond.
ACsinewave
Alternatingvoltageandcurrentvarycontinuously.ThegraphicrepresentationforACisasinewave.Asinewavecanrepresentcurrentandvoltage.Whenthewaveformisabovethetimeaxis,currentisflowinginonedirection.Thisisreferredtoasthepositivedirection.Whenthewaveformisbelowthetimeaxis,currentisflowinginanotherdirectionandnegativedirection.
Single-phaseandthree-phaseACpower
ACisdividedintosingle-phaseandthree-phasetypes.Single-phaseisusedforsmallelectrical.Three-phasepowerisusedwherelargeblocks.Three-phasepowerisacontinuousseriesofthreeoverlappingACcycle.Eachwave
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