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# Lect_37 - Capacitors Lecture 37 Capacitance A capacitor is...

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Lecture 37 Capacitance Capacitors Capacitors A capacitor is a device whose purpose is to store electric energy which can be released in a controlled manner. Usually made of two conductors with charges + Q and – Q… + - … separated by an insulator (dielectric). Can be air or vacuum . Capacitance Capacitance + - C V = Capacitance: = Usually written as + V = + Potential difference between the conductors Units: Farad 1 F = 1 C/V Symbol: Example: Parallel plate capacitor Example: Parallel plate capacitor Determine the capacitance of two parallel conducting large plates of area A separated by a small distance d . Can be considered infinite planes

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Let the plates have charges + Q and – Q. 0 2 EE σ ε +− == GG Surface charge density: A = E + G G 0 0 total G total 0 = G Between the plates Potential difference between the plates: total 00 Qd VEd d εε ∆= = = G Capacitance of a parallel plate capacitor: 0 C V 0 = The charge cancels out… The capacitance of a system: Does not depend on: •charge , • potential difference V. Depends on: • the geometry of the system, • the dielectric between the conductors (because is always proportional to ) How can I “imagine” capacitance? What makes a good one? Tension is too strong, walls rip, explosion. If you store too much… “Price” Use to store… Balloon Capacitor Device
ACT: Variable capacitor ACT: Variable capacitor A parallel plate capacitor is connected to a battery that produces a constant potential difference across its plates. If we push the

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Lect_37 - Capacitors Lecture 37 Capacitance A capacitor is...

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