Capacitors - PHY 2049C General Physics B Fields, Circuits,...

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PHY 2049C – General Physics B Fields, Circuits, Waves, Light Today: 1) Capacitor 2) Energy in Capacitor 3) Dielectrics
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Important Points from Last Lecture Energy of a configuration of charges: Capacitors Q: Charge on plate V: Potential difference across Units Symbol : Parallel Plates: U = 1 2 i = 1 n Q i V i 1 2 3 C = Q / V 1Farad = Coulomb Volt 0 A C d ε =
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Storage of Electric Energy We have seen that capacitors collect more “Charge per Voltage” than wires. In doing so, they store energy. This can also be calculated by Work +V V= 0 2 2 11 1 22 2 Q UQ VC V C == = 2 0 1 2 V dW VdQ dQ CdV Wd W V d Q C V d V C V = = = = ∫∫
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Effect of dielectric 0 0 0 0 0 0 E E E A QA CC Vd d σ ε σσ κκ κ = == = = =
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Question: Inserting a Dielectric into Capacitor A capacitor containing no dielectric is connected to a battery with 12 V potential. Now the battery is disconnected and a dielectric with κ =4 is put between the plates. What will the potential difference between the plates be ? 1) 3 V 2) 6 V 3) 12 V 4) 24 V 5) 48 V
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Energy in the Electric Field What’s the difference between a charged and an uncharged capacitor? Ans: separated charge 2 2 0 2 0 11 1 22 2 so (electric energy density) 2 E A Q UQ VC V C Cd Qd U Uu AA d ε == = = 2 0 2 00 0 1 2 so so 2 1 2 (total stored electric energy) E E QU Q EQ A E u A Ad A uE U E d V σ εε = = = =⇒ =
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Question CT 29.C4
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Capacitors - PHY 2049C General Physics B Fields, Circuits,...

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