Chapter 24 lecture - Chapter 24 Capacitors and Capacitance...

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Chapter 24 Capacitors and Capacitance Exam info on ctools
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Capacitor defined: Any 2 conductors with equal but opposite charges. Fine print: 1. One conductor can be replaced with ∞ 2. We could charge conductors with non-equal charges, but we require +Q and -Q→ use a battery to transfer charge 3. Might have an insulator/dielectric in between (more later) DEMO
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Capacitance defined Capacitance = in Coulombs/Volt = FARAD, F V Q V Q C = = Q V C (slope) Capacitance is a measure of how much charge is separated per Volt of potential difference. A 1 F capacitor is BIG!
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Capacitors: What are they good for? Capacitors store charge-they tend to stay charged Capacitors store ENERGY—(mechanical analogy is the spring) Demos: Capacitor and light bulb/LED killer capacitor (next time)
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Calculating Capacitance (vacuum gap, no dielectric yet) Strategy: 1. Put +Q and –Q on the conducting electrodes 2. Calculate E between them 3. Determine 1. Q’s cancel in C = Q/V - = path l d E V Let’s consider some simple examples……..
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Thee Fundamental Capacitor (parallel plate capacitor) Question 1 The electric field between the plates is (neglect edge effects): 1. Q/(2Aε 0 ) 2. Q/(Aε 0 ) 3. 0 Answer: By superposition or realizing E = σ c 0 = Q/(Aε 0 ) and is ~uniform. Then ΔV = V = Ed = Qd/(Aε ) ►►►► C = Q/V = ε 0 A/d = C parallel plate
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Spherical and Cylindrical Capacitors a b b a b a r r r r C r r kQ V - = - = 0 4 ) 1 1 ( πε ) / ln( 2 ln 2 2 0 0 0 a b a b r r L C r r V r
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