Capacitance. Applications of Electric Potential. Capacitors in Kodak Cameras 3/17/2014. AP Physics B

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1 3/7/04 apacitance P Physics B pplicatins f Electric Ptential Is there any way we can use a set f plates with an electric fiel? YES! We can make what is calle a Parallel Plate apacitr an Stre harges between the plates! String harges- apacitrs capacitr cnsists f cnuctrs f any shape place near ne anther withut tuching. It is cmmn; t fill up the regin between these cnuctrs with an insulating material calle a ielectric. We charge these plates with ppsing charges t set up an electric fiel. apacitrs in Kak ameras apacitrs can be easily purchase at a lcal Rai Shack an are cmmnly fun in ispsable Kak ameras. When a vltage is applie t an empty capacitr, current flws thrugh the capacitr an each sie f the capacitr becmes charge. The tw sies have equal an ppsite charges. When the capacitr is fully charge, the current stps flwing. The cllecte charge is then reay t be ischarge an when yu press the flash it ischarges very quickly release it in the frm f light. ylinrical apacitr

2 3/7/04 apacitance In the picture belw, the capacitr is symblize by a set f parallel lines. Once it's charge, the capacitr has the same vltage as the battery (.5 vlts n the battery means.5 vlts n the capacitr) The ifference between a capacitr an a battery is that a capacitr can ump its entire charge in a tiny fractin f a secn, where a battery wul take minutes t cmpletely ischarge itself. That's why the electrnic flash n a camera uses a capacitr -- the battery charges up the flash's capacitr ver several secns, an then the capacitr umps the full charge int the flash tube almst instantly Measuring apacitance Let s g back t thinking abut plates! V E, V E, if cnstant E Q Therefre Q V cntant f prprtinality apacitance Q V Q V The unit fr capacitance is the FRD, F. apacitr Gemetry The capacitance f a capacitr epens n HOW yu make it. area f plate istance beteween plates cnstant f prprtinality vacuum permittivity cnstant 8.85x0 Nm

3 3/7/04 apacitr Prblems What is the RE f a F capacitr that has a plate separatin f mm? D 8.85x0 Sies.3x0 8 m 069 m 0.00 Is this a practical capacitr t buil? NO! Hw can yu buil this then? The answer lies in REDING the RE. But yu must have a PITNE f F. Hw can yu keep the capacitance at F an reuce the rea at the same time? a DIELETRI!!! Dielectric Remember, the ielectric is an insulating material place between the cnuctrs t help stre the charge. In the previus example we assume there was NO ielectric an thus a vacuum between the plates. k Dielectric k ll insulating materials have a ielectric cnstant assciate with it. Here nw yu can reuce the RE an use a LRGE ielectric t establish the capacitance at F. sing MORE than capacitr Let s say yu ecie that capacitr will nt be enugh t buil what yu nee t buil. Yu may nee t use mre than. There are basic ways t assemble them tgether Series One after anther Parallel between a set f junctins an parallel t each ther. 3

4 3/7/04 apacitrs in Series apacitrs in series each charge each ther by INDTION. S they each have the SME charge. The electric ptential n the ther han is ivie up amngst them. In ther wrs, the sum f the iniviual vltages will equal the ttal vltage f the battery r pwer surce. apacitrs in Parallel In a parallel cnfiguratin, the vltage is the same because LL THREE capacitrs tuch BOTH ens f the battery. s a result, they split up the charge amngst them. apacitrs STORE energy nytime yu have a situatin where energy is STORED it is calle POTENTIL. In this case we have capacitr ptential energy, c Suppse we plt a V vs. Q graph. If we wante t fin the RE we wul MLTIPLY the variables accring t the equatin fr rea. = bh When we this we get rea = VQ Let s a unit check! Vltage = Jules/ulmb harge = ulmbs rea = ENERGY 4

5 3/7/04 Ptential Energy f a apacitr This energy r area is referre as the ptential energy stre insie a capacitr. Nte: The slpe f the line is the inverse f the capacitance. Since the RE uner the line is a triangle, the ENERGY(area) =/VQ ( mst cmmn frm Q VQ V V ( V) V Q Q ) Q 5