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# Assign3 - PHY 54 Assignment#3 Summer 2011 Reading...

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Reading : Electrostatics 3. Key concepts: Capacitance, energy storage, dielectrics, energy in the E-feld. 1. Questions about charging and discharging capacitors. a. When an uncharged capacitor is connected across a battery, why do equal and opposite charges appear on the plates? b. If the plates have different shapes or sizes, will the charges still be equal and opposite? Explain. c. Where does the energy come from when a capacitor is charged by a battery? Where does it go if an isolated charged capacitor is discharged by connecting a wire between the plates? 2. Questions about altering the capacitance of a parallel plate capacitor. a. Why does moving the plates closer together increase C ? {Consider an isolated charged capacitor; what happens to Δ V ?] b. If you suspend a copper sheet between the plates, parallel to them, what happens to C ? Suppose the copper sheet is resting on the bottom plate; is the change different? c. You have a copper sheet and a sheet of glass, the same size, that you can insert between the capacitor plates. Which one will change C more? Why? 3. An isolated parallel plate capacitor has charge Q . a. If the plates are moved closer together, the stored energy decreases. Where does the “lost” energy go? b. If a dielectric slab is inserted part way into the gap between the plates and released, what happens? Explain. c. If, instead of being isolated, the capacitor remained connected to the charging battery, the energy increases when the plates are moved closer together. Explain this carefully in terms of how much work is done by what agent. 4. For some purposes, such as the effects of inserting slabs of material between capacitor plates, a conductor can be regarded as a dielectric with an in±nite value of κ . Explain why this is so. PHY 54 Assignment #3 Summer 2011

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5. Two capacitors C 1 and C 2 connected in series or parallel can be replaced by a single equivalent capacitor C . Let the equivalent capacitor have charge Q and potential difference Δ V . a.
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Assign3 - PHY 54 Assignment#3 Summer 2011 Reading...

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