Download Basic Circuits Notes

BasicCircuitsNotes
(Handwavyandusinganalogiestosomeextenttohelpyouunderstand)
Primaryanalogy:electricalpotentialislikegravitationalpotential.Thingsgo
“downhill”.Thingsmovebecausethereisadifferencein“height.”Forexample,a
ballstaysstillwhenitisonahorizontalsurface,butrollsdownhillwhenplaceona
rampbecausethereisadifferenceinheight.
Firstsome“definitions”
BatteryorDCPowersupply–providesanelectricalpotentialdifferenceina
completecircuit.[Analogy:itprovidesthedifferencein“height”intheunbrokenor
completecircuit.]
à
Voltage,V(volts)
Currentflowswhenthereisanonzeroelectricalpotentialdifference.Currentflows
fromhighelectricalpotential(+)tolowelectricalpotential(-)
[Analogy:flowsdownhilllikewater]
à
Current,I(AmperesorAmps,A)
Note:Whenwesay“current”,wearereferringtoconventionalcurrent(mythical
positivechargesmovinginourwiresorcircuitelements).Inreality,itisthe
negativelychargedelectronsthatarelooselyboundinmetalatomsthatmakeup
ourwiresandmoveintheoppositedirectionofconventionalcurrent.
Aresistorprovidesresistancetotheflowofcurrentinthecircuit.[Analogy:likea
metalgrateormeshacrossapipecontainingflowingwater,thatinterfereswith,but
doesn’tcompletelystoptheflowofwater.]Resistorstakemanyformsandworkin
differentways,someofwhichwewillexploreintheResponseofCircuitElements
Lab.IntheBasicCircuitslab/BasicCircuitsassignment,wewilluselightbulbsas
ourresistors,becausewhenthelightglowswecantellthatthecurrentisflowing
throughthatbulb(thatpartofthecircuit).Wewon’tcare,however,aboutthe
amountofresistanceinthislab.
à
Resistance,R(Ohms,Ω)
Note:Thereisa“voltagedrop”,achangeinelectricalpotentialdifference,across
somethingthathasaresistance.Soaresistorhasapotentialdifferencefromonside
totheothersideoftheresistor.[Analogy:thereisachangein“height”fromone
sideofthelightbulb/resistortotheotherside.]
Wiresorleadsareusedtoconnecttheitemsinthecircuit,liketheDCpowersupply,
lightbulbs,ammeterandvoltmeter.Wepretendthesewiresareperfectconductors
(havenoresistance).Inreality,theyhaveasmallamountofresistance,butthatis
verysmallwhencomparedtotheresistors,soweignoreitinmostsituations.Ifa
wirehasnoresistance,thenthevoltagedoesnotchangeacrossit(fromonesideof
ittotheother)–thereisnovoltagedrop.[Analogy:whengoingalongawire,itis
likewalkingalongahorizontalpath,becausethereisnochangein“height”.]
BasicCircuitNotes
1/13/17CalPolyPomona
Dr.JulieNazareth
Ammeter–meterusedtomeasurecurrent.Itmustbewiredinserieswithwhatitis
measuring.Thatmeanstheammeterisinlineinthecircuitalongthesamepathas
whatyouwanttomeasurethecurrentin.
Voltmeter–meterusedtomeasurethevoltage(thechangeinelectricpotential
fromonesideofaresistortotheotherorfromoneplaceinacircuittoanotherplace
inthecircuit).Becauseyouaremeasuringthedifferenceinelectricpotential
betweentwolocations(evenifitisjustfromonesideofalightbulbtotheotherside
ofthatsamelightbulb),youmustconnectthevoltmeterinparallelwithwhatitis
measuring.Thatmeansyouhookuponeleadfromthevoltmetertoonesideof
whatyouaremeasuringandyouhookuptheotherleadfromthevoltmetertothe
othersideofwhatyouaremeasuring.
SeriesCircuit–providesonlyonepathforcurrent.
Thecurrentisthesameeverywherebecausechargeisconservedsocurrentis
conserved(i.e.,youdon’tloseorgainanycharge).So,ifyouhavetworesistorsin
series(oneaftertheother),andyoumeasurethecurrentbeforeitentersthefirst
resistor,itwillbethesamevalue(withinmeasuringuncertainty)asifyoumeasured
itbeforethesecondresistorintheseries.
à
I1=I2=I
Thevoltageisdifferentacrosseachcircuitcomponent.TheDCPowersupply
providesthepotentialdifferenceforthewholecircuit[Analogy:heightdifference
forwholecircuit].Ifwepretendthewiresareperfectconductors,thentheonly
voltagedropsareacrosstheseriesresistors[Analogy:voltagedropisastepdown
inheight].IfwehavetworesistorsinserieswithaDCPowersupply,thenweonly
havetwovoltagedrops(oneforeachresistor)tomatchthepotentialdifference
fromthepowersupply[Analogy:twostepstogetfromthe“top”tothe“bottom”].
à
V=V1+V2
ParallelCircuit–multiplepathsforcurrenttofollow.
Thecurrentisdifferentalongthedifferentpathsbutaddsuptothetotalcurrent
becausethecurrentisconserved(wedon’tmysteriouslyloseorgainany).
[Analogy:ajunctionwherethecircuitsplitsislikeaforkintheroad.Someofthe
currentfollowsonepath,therestfollowstheother.]
à
I=I1+I2
Thevoltageisthesameacrosseachresistorandisthesameasthepowersupply.
Eachresistorisalongadifferentpath,butastheonlyplaceforavoltagedropto
occuralongthatpathifthewiresareperfectconductors,thatvoltagedrophasto
matchthepotentialdifferenceprovidedbytheDCpowersupply.[Analogy:thereis
onlyonesteptogetfromthe“top”tothe“bottom”.]
à
V=V1=V2
Now,somediagramstotryandemonstratetheconcepts…
BasicCircuitNotes
1/13/17CalPolyPomona
Dr.JulieNazareth