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GORESET
PotentialDifference
ThevoltagedifferencebetweenanytwopointsinacircuitisknownasthePotentialDifference,pdorVoltageDropandit
isthedifferencebetweenthesetwopointsthatmakesthecurrentflow.Unlikecurrentwhichflowsaroundacircuitinthe
formofelectricalcharge,potentialdifferencedoesnotmoveitisapplied.Theunitofpotentialdifferenceisthevoltandis
definedasthepotentialdifferenceacrossaresistanceofoneohmcarryingacurrentofoneampere.Inother
words,V=I.R
Ohm''sLawstatesthatforalinearcircuitthecurrentflowingthroughitisproportionaltothepotentialdifferenceacross
itsothegreaterthepotentialdifferenceacrossanytwopointsthebiggerwillbethecurrentflowingthroughit.For
example,ifthevoltageatonesideofa10Ωresistormeasures8Vandattheothersideoftheresistoritmeasures5V,
thenthepotentialdifferenceacrosstheresistorwouldbe3V(8-5)causingacurrentof0.3Atoflow.Ifhowever,the
voltageononesidewasincreasedfrom8Vtosay40V,thepotentialdifferenceacrosstheresistorwouldnowbe40V-
5V=35Vcausingacurrentof3.5Atoflow.Thevoltageatanypointinacircuitisalwaysmeasuredwithrespecttoa
commonpoint,generally0V.
Forelectricalcircuits,theearthorgroundpotentialisusuallytakentobeatzerovolts(0V)andeverythingisreferenced
tothatcommonpointinacircuit.Thisissimilarintheorytomeasuringheight.Wemeasuretheheightofhillsinasimilar
waybysayingthatthesealevelisatzerofeetandthencompareotherpointsofthehillormountaintothatlevel.
Inaverysimilarwaywecancallthecommonpointinacircuitzerovoltsandgiveitthenameofground,zerovoltsor
earth,thenallothervoltagepointsinthecircuitarecomparedorreferencedtothatgroundpoint.Theuseofacommon
groundorreferencepointinelectricalschematicdrawingsallowsthecircuittobedrawnmoresimplyasitisunderstood
thatallconnectionstothispointhavethesamepotential.Forexample:
PotentialDifference
AstheunitsofmeasureforPotentialDifferencearevolts,potentialdifferenceismainlycalledvoltage.Individual
voltagesconnectedinseriescanbeaddedtogethertogiveusa"totalvoltage"sumofthecircuitasseenintheresistors
inseriestutorial.Voltagesacrosscomponentsthatareconnectedinparallelwillalwaysbeofthesamevalueasseenin
theresistorsinparalleltutorial,forexample.
forseriesconnectedvoltages,
forparallelconnectedvoltages,
ExampleNo1
ByusingOhm''sLaw,thecurrentflowingthrougharesistorcanbecalculated.Forexample,Calculatethecurrent
flowingthrougha100Ωresistorthathasoneofitsterminalsconnectedto50voltsandtheotherterminalconnectedto
30volts.
VoltageatterminalAisequalto50vandthevoltageatterminalBisequalto30v.Therefore,thevoltageacrossthe
resistorisgivenas:
V=50v,V=30v,therefore,V-V=50-30=20v
Thevoltageacrosstheresistoris20v,thenthecurrentflowingthroughtheresistorisgivenas:
I=V÷R=20V÷100Ω=200mA
VoltageDivider
Weknowfromtheprevioustutorialsthatbyconnectingtogetherresistorsinseriesacrossapotentialdifferencewecan
produceavoltagedividercircuitgivingratiosofvoltageswithrespecttothesupplyvoltageacrosstheseries
combination.ThisthenproducesaVoltageDividernetworkthatonlyappliestoresistorsinseriesasparallelresistors
produceacurrentdividernetwork.Considerthecircuitbelow.
VoltageDivision
Thecircuitshowstheprincipalofavoltagedividercircuitwheretheoutputvoltagedropsacrosseach
resistor,R1,R2,R3andR4arereferencedtoacommonpoint.Foranynumberofresistorsconnectedtogetherinseries
thetotalresistance,RTofthecircuitdividedbythesupplyvoltageVswillgivethecircuitcurrentasI=Vs/RT,Ohm''s
Law.Thentheindividualvoltagedropsacrosseachresistorcanbesimplycalculatedas:V=IxR.
Thevoltageateachpoint,P1,P2,P3etcincreasesaccordingtothesumofthevoltagesateachpointuptothesupply
voltage,Vsandwecanalsocalculatetheindividualvoltagedropsatanypointwithoutfirstlycalculatingthecircuitcurrent
byusingthefollowingformula.
VoltageDividerEquation
ABAB
AB
Where,Visthevoltagetobefound,Ristheresistanceproducingthevoltage,Risthetotalseriesresistance
andVisthesupplyvoltage.
Thenbyusingthisequationwecansaythatthevoltagedroppedacrossanyresistorinaseriescircuitisproportionalto
themagnitudeoftheresistorandthetotalvoltagedroppedacrossalltheresistorsmustequalthevoltagesourceas
definedbyKirchoff''sVoltageLaw.SobyusingtheVoltageDividerEquation,foranynumberofseriesresistorsthe
voltagedropacrossanyindividualresistorcanbefound.
Thusfarwehaveseenthatvoltageisappliedtoaresistororcircuitandthatcurrentflowsthroughandaroundacircuit.
Butthereisathirdvariablewecanalsoapplytoresistorsandresistornetworks.Powerisaproductofvoltageand
currentandthebasicunitofmeasurementofpoweristhewatt.
InthenexttutorialaboutResistors,wewillexaminethepowerdissipated(consumed)byresistanceintheformofheat
andthatthetotalpowerdissipatedbyaresistivecircuit,whetheritisseries,parallel,oracombinationofthetwo,we
simplyaddthepowersdissipatedbyeachresistor.
(x)(x)T
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