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Download 7.8.1 The parallel plate capacitor
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7.8.1Theparallelplatecapacitor Acapacitorisadevicewhichisusedtostoreelectriccharge.Thesimplest capacitorconsistsoftwometalplatesseparatedbyanairgap.Iftheplatesare connectedtoabattery,electronsareremovedfromoneplateandmovedaround thecircuittotheotherplate.Thisleavesthefirstplatewithapositivechargeand theotherwithanegativecharge.Thereisanequalandoppositechargeonboth plates.Thebiggerthevoltagesuppliedthebiggerthechargethataccumulates. + - - + Symbolfora + - capacitor. + - electronflow Iftheareaoftheplatesisincreased,morechargecanbestoredforthesame appliedvoltage.Thecapacitance(C)isameasureoftheamountofcharge(Q) thatcanbestoredforagivenvoltage(V): 𝑄 𝐶= 𝑉 Capacitanceismeasuredinfarads(F).Thefaradis,practically,quitealarge valueforcapacitorsthatareusedineverydaycircuits.Wearemorelikelytosee valuesinmicrofarads(μF)andpicofarads(pF). (1)$Acapacitorwithavalueof900pFisfullychargedbya9Vbattery.What chargeisstoredonthecapacitor? Awatermodelofacapacitor Ananalogyforacapacitorisarubbermembranestretchedacrossapipe containingwater.Waterflowisanalogoustoelectriccurrent.Currentwill continuetoflowuntilthemembraneisfullystretched.Atthispointcurrentflow willstop. Thepressureofwaterisanalogoustothevoltage.Asthemembranebecomes moreandmorestretchedthepressurebuildsup. ©2016flippedaroundphysics.com Onemethodtoexplorethebehaviourofacapacitoristousethefollowing circuit: Aconstantcurrent V canbemaintainedby adjustingthe variableresistor. Readingsofvoltage canbetakenat differenttimesasthe A capacitorchargesup. Theswitchcanbe openedatcertain timestotakea reading. (2)$Acapacitorischargedbymeansofaconstantcurrentof0.7μAfor60 secondsandavoltageof4.5Visrecorded.Calculatethechargestoredand,hence, workoutthecapacitanceofthecapacitor. A Inthecircuit,left,atwo 2 wayswitchcanbeusedto connectthecapacitorto connector1or2. 1 (3)$Describewhathappenswhentheswitchisinposition1. (4)$Describewhathappenswhentheswitchisinposition2. (5)$Dosomeresearchandfindoutsomeoftheusesofcapacitorsincircuits. ©2016flippedaroundphysics.com Combiningcapacitorsinseriesandparallel Whencapacitorsarecombinedinseries,thetotalcapacitance𝐶% isgivenbythe following: 1 1 1 1 = + + + ⋯ 𝐶% 𝐶' 𝐶) 𝐶* (6)$Workoutthetotalcapacitanceforthefollowingcombination: 3.2μF 4.6μF 1.9μF Whencapacitorsarecombinedinparallel,thetotalcapacitance𝐶% isgivenbythe following: 𝐶% = 𝐶' + 𝐶) + 𝐶* + ⋯ (7)$Workoutthetotalcapacitanceforthefollowingcombination: 4.6μF 6.2μF 3.2μF 1.9μF (8)$Howdoesaddingcapacitorsinseriesandparallelcomparetoadding resistorsinseriesandparallel? ©2016flippedaroundphysics.com