Electricity and MagnetismMore on Electric PotentialMore on Electric PotentialMore on Electric PotentialCharge and PotentialCharge and PotentialCapacitanceCapacitanceCapacitanceCapacitorParallel Plate CapacitorParallel Plate CapacitorParallel Plate CapacitorParallel Plate CapacitorMar 4 2002Electricity and Magnetism•Electric Potential– Electric Field and Electric Potential– CapacitorsMar 4 2002More on Electric Potential• Where do charges go?++++++++++++++++Mar 4 2002More on Electric Potential++++++++++r0r1Mar 4 2002More on Electric Potential• Practical application– Lightning rod: Pointy tip -> small r!++++++++++r0r1Mar 4 2002Charge and Potential• Potential is proportional to ChargerRV=kQ/R2*kQ/R+++++++RMar 4 2002Charge and Potential• Potential is proportional to Charge– V * constant = Q•True for any object– true for point charge– Superposition principle•Define – Q = C V -> C is called CapacitanceMar 4 2002Capacitance• [Capacitance] = [C/V] = Farad– 1 Farad is huge•Sphere–C = R/k– The bigger the sphere, the bigger the capacitanceMar 4 2002CapacitanceQVQ = C*V => V = Q/Cslope = 1/CC smallC bigMar 4 2002Capacitance• Example C(Earth):C = 6*106m /9*109Nm2/C2 = 540 µFarad– Small capacitance– Potential increases quickly with charge– Hard to put charge on sphere – For sphere, reference point is r=infinity– How to build better device?Mar 4 2002Capacitor• Better capacity– don’t get charges from infinity, but from other object close-by• Capacitor–Def: Two conductors separated by insulator• Example: Parallel plate capacitorMar 4 2002Parallel Plate Capacitor+++++++++++-----------d-Q-QMar 4 2002Parallel Plate Capacitor+++++++++++------------Q -QdMar 4 2002Parallel Plate Capacitor• Depends only on Geometry!– like it did for sphere• To store lots of charge–make A big– make d small+++++++++++-----------d-Q-QC = ε0A/dMar 4 2002Parallel Plate CapacitorC=ε0A/dslope = 1/CV+++++++++++------------Q
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