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# lecture04 - Physics 102 Lecture 04 Capacitors batteries...

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Capacitors (& batteries) Physics 102: Lecture 04

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Phys 102 so far Lecture 1 – electric charge & electric force Lecture 2 – electric field Lecture 3 – electric potential energy and electric potential Lecture 4 – capacitance Lecture 5 – resistance Lecture 6 – Kirchhoff’s rules Lecture 7 – RC circuits Lecture 12 & 13 – AC circuits Basic principles of electricity Applications of electricity – circuits
Recall from last lecture….. Electric Fields, Electric Potential

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Comparison: Electric Potential Energy vs. Electric Potential V AB : the difference in electric potential between points B and A U AB : the change in electric potential energy of a charge q when moved from A to B U AB = q V AB q A B
Electric Potential: Summary E field lines point from higher to lower potential For positive charges, going from higher to lower potential is “downhill” Positive charges tend to go “downhill”, from + to - Negative charges go in the opposite direction, from - to + U AB = q V AB + charge E

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Important Special Case Uniform Electric Field Two large parallel conducting plates of area A +Q on one plate - Q on other plate Then E is uniform between the two plates: E=4 π kQ/A zero everywhere else This result is independent of plate separation This is call a parallel plate capacitor d A E +Q –Q A
Parallel Plate Capacitor: Potential Difference A B E=E 0 d V =V A – V B = +E 0 d + + + + + - - - - - A B d + + + + + - - - - - + + + + + - - - - - E=2 E 0 V =V A – V B = +2E 0 d Potential difference is proportional to charge: Double Q Double V Charge Q on plates Charge 2Q on plates 10 E 0 =4 π kQ/A

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Capacitance: The ability to store separated charge C Q/V Any pair conductors separated by a small distance. (e.g. two metal plates) Capacitor stores separated charge Positive Q on one conductor, negative Q on other Net charge is zero E d + + + + + - - - - - 12 Q=CV U =(½) Q V Stores Energy Units: 1 Coulomb/Volt = 1 Farad (F)
Why Separate Charge?

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lecture04 - Physics 102 Lecture 04 Capacitors batteries...

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