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(38) Electric Energy and Capacitors with Dielectrics

# (38) Electric Energy and Capacitors with Dielectrics -...

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Lecture 38 Electric energy Capacitors with dielectrics

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Energy stored in a capacitor Work to add a chargedq to a capacitor with capacitance C and charge q: ext elect ric dW dW = - = external work needed to charge it 0 Q ext q W dq C = Total work: Energy stored in a capacitor Energy stored in a capacitor 2 2 1 1 2 2 2 Q U CV QV C = = = DEMOs: Dischargeof a capacitor through a bulb Capacitors store BIG energy. ( 29 dU = - - Vdq = q dq C = 2 2 Q C =
ACT: Variable capacitor (II) ACT: Variable capacitor (II) A. Increases B. Decreases C. Stays thesame A parallel platecapacitor is connected to a battery. While still connected, thedistancebetween the plates is halved. Due to this, the energy stored in thecapacitor: 0 A C d ε = d d/2 C 2 C V is constant, 2 1 2 U CV = U 2 U

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ACT: Variablecapacitor (III) ACT: Variablecapacitor (III) A. Increases B. Decreases C. Stays thesame A parallel platecapacitor is connected to a battery. After it is charged, we disconnect it. Then thedistance between the plates is halved. Dueto this, the energy stored in thecapacitor: 0 A C d ε = d d/2 C 2 C Q is constant, 2 1 2 Q U C = U U/2
Where is the energy stored? The energy is used to create theelectric field that appears between the conductors stored in the electric field .

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(38) Electric Energy and Capacitors with Dielectrics -...

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