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# Lect_38 - Energy stored in a capacitor = external work...

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Lecture 38 Electric energy Capacitors with dielectrics Energy stored in a capacitor Work to add a charge d q to a capacitor with capacitance C and charge q : ext electric 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: Discharge of a capacitor through a bulb Capacitors store BIG energy. ( ) dU = − − Vdq = q dq C = 2 2 Q C = ACT: Variable capacitor (II) ACT: Variable capacitor (II) A. Increases B. Decreases C. Stays the same A parallel plate capacitor is connected to a battery. While still connected, the distance between the plates is halved. Due to this, the energy stored in the capacitor: 0 A C d ε = d d /2 C 2 C V is constant, 2 1 2 U CV = U 2 U ACT: Variable capacitor (III) ACT: Variable capacitor (III) A. Increases B. Decreases C. Stays the same A parallel plate capacitor is connected to a battery. After it is charged, we disconnect it. Then the distance between the plates is halved. Due to this, the energy stored in the capacitor: 0 A C d ε = d d /2 C 2 C Q is constant, 2 1 2 Q U C = U U/2

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Where is the energy stored? The energy is used to create the electric field that appears between the conductors stored in the electric field .
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