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Lecture 13 - Energy storage and dielectrics

Lecture 13 - Energy storage and dielectrics - Energy stored...

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1 Lecture 13 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 dU    Vdq q dq C 0 Q ext q W dq C Total work: 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. 2 2 Q C ACT: Variable capacitor (II) 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 d d /2 A. Increases B. Decreases C. Stays the same C 2 C V is constant, 2 1 2 U CV U 2 U ACT: Variable capacitor (III) 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 d d /2 A. Increases B. Decreases C. Stays the same C 2 C Q is constant, 2 1 2 Q U C U U/2
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2 Where is the energy stored?
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