Elec. Energy and Capacitance

# Elec. Energy and Capacitance - PHY 2049C General Physics B...

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PHY 2049C – General Physics B Fields, Circuits, Waves, Light Today: 1) Potential and Field 2) Electric Energy of Configuration 3) Capacitor

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Demo: E = V/d Voltage constant, d decreased => E increased +V -V
Potential Energy of Configurations of Charges Potential energy of a system of charges: How much work is required to bring N charges together ? 1 2 3 1 21 2 12 31 32 3 13 23 123 33 211 2 3 11 2 2 0 1 22 2 1 2 tot W qq Wk R k RR WW W W k k k RRR qqq q qk k q k k q k k R R qV = = =+ =++ = + + ⎛⎞ ++ ⎜⎟ ⎝⎠ +

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General: System of N Charges Formula for the final state: The potential configuration-energy of a system of charges equals half the sum of “potential due to other charges” times “charge itself” As opposed to : “potential energy of one charge in the field of others” U = 1 2 i = 1 n Q i V i U = qV 1 2 3
Example: Configuration Potential Energy A system of four charges in fixed positions: Q 1,2 = +Q Q 3,4 = -Q What is the total potential energy ? Guess: is it >0, =0, <0 ? 1 2 3 4

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Is there a “Bottle” for electricity ? “Leyden” Jar: Trying to charge a bottle of water. Observation: Can store a large amount of charge from a high-voltage generator, much higher than e.g. a sphere. We will see, how and why !
Capacitance We have seen that charges possess PE in an electric field. A device that stores energy by locating charges in a field is called “Capacitor” The battery provides work by bringing to and + to + The more charge it stores at a given voltage , the more energy is stored Capacitance Unit 1 F “Farad”= C = Q V 1 Coulomb 1 Volt

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Elec. Energy and Capacitance - PHY 2049C General Physics B...

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