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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