Electric Theory English.docx

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