# lecture13 - Capacitance Examples Text sections 26.3 to 26.5...

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Capacitance Examples Capacitance Examples •Text sections 26.3 to 26.5 Practice: Chapter 26, problems 47, 61, 65, 67, 69. Extra problems for fun: 49, 55, 63 Energy Stored in a Capacitor Energy Stored in a Capacitor +q -q dq Remove dq from lower plate and add to upper plate: Increase in P.E., Vdq dU = dq C q = Start at q = 0, finish at q = Q: C Q C dq q U Q 2 2 0 = = 2 2 2 1 2 1 2 1 CV QV Q U = = =

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Parallel-Plate Capacitor: d A C ο ε = and d E V = () Ad E CV U = = 2 2 2 1 2 1 Volume between plates 2 | | 2 1 E = = volume U u E “Energy density” (Units: J/m 3 ) We can think of this energy as stored in the electric field set up when the capacitor is charged: This also applies to any electric field. Exercise Exercise s Area A +Q -Q Calculate: a) between plates b) between plates c) C between plates d) U of the system e) Force exerted on top plate (tricky. ..) E r V
a b A neutral conductor of thickness a is slid between the plates of a parallel-plate capacitor (area A , separation b ). What is the capacitance between the upper and lower plates? Example

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## This note was uploaded on 12/03/2009 for the course CHEMISTRY 1E03 taught by Professor Britz during the Spring '09 term at McMaster University.

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lecture13 - Capacitance Examples Text sections 26.3 to 26.5...

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