Chapter 26

# Chapter 26 - 26 Capacitance and Dielectrics CHAPTER OUTLINE...

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26 CHAPTER OUTLINE 26.1 Definition of Capacitance 26.2 Calculating Capacitance 26.3 Combinations of Capacitors 26.4 Energy Stored in a Charged Capacitor 26.5 Capacitors with Dielectrics 26.6 Electric Dipole in an Electric Field 26.7 An Atomic Description of Dielectrics Capacitance and Dielectrics ANSWERS TO QUESTIONS Q26.1 Nothing happens to the charge if the wires are disconnected. If the wires are connected to each other, charges in the single conductor which now exists move between the wires and the plates until the entire conductor is at a single potential and the capacitor is discharged. Q26.2 336 km. The plate area would need to be 1 0 m 2 . Q26.3 The parallel-connected capacitors store more energy, since they have higher equivalent capacitance. Q26.4 Seventeen combinations: Individual CCC 123 ,, Parallel CCCCCCCCC 12 13 23 ++ + + + ,,, Series-Parallel 11 1 3 CC C + F H G I K J + , 1 2 C + F H G I K J + , 1 1 C + F H G I K J + 3 1 CC C + + F H G I K J , 2 1 + + F H G I K J , 1 1 + + F H G I K J Series 111 1 F H G I K J , 1 + F H G I K J , 1 + F H G I K J , 1 + F H G I K J Q26.5 This arrangement would decrease the potential difference between the plates of any individual capacitor by a factor of 2, thus decreasing the possibility of dielectric breakdown. Depending on the application, this could be the difference between the life or death of some other (most likely more expensive) electrical component connected to the capacitors. Q26.6 No—not just using rules about capacitors in series or in parallel. See Problem 72 for an example. If connections can be made to a combination of capacitors at more than two points, the combination may be irreducible. 77

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78 Capacitance and Dielectrics Q26.7 A capacitor stores energy in the electric field between the plates. This is most easily seen when using a “dissectable” capacitor. If the capacitor is charged, carefully pull it apart into its component pieces. One will find that very little residual charge remains on each plate. When reassembled, the capacitor is suddenly “recharged”—by induction—due to the electric field set up and “stored” in the dielectric. This proves to be an instructive classroom demonstration, especially when you ask a student to reconstruct the capacitor without supplying him/her with any rubber gloves or other insulating material. (Of course, this is after they sign a liability waiver). Q26.8 The work you do to pull the plates apart becomes additional electric potential energy stored in the capacitor. The charge is constant and the capacitance decreases but the potential difference increases to drive up the potential energy 1 2 QV . The electric field between the plates is constant in strength but fills more volume as you pull the plates apart. Q26.9 A capacitor stores energy in the electric field inside the dielectric. Once the external voltage source is removed—provided that there is no external resistance through which the capacitor can discharge—the capacitor can hold onto this energy for a very long time. To make the capacitor safe
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## This homework help was uploaded on 04/13/2008 for the course PHYS 211 taught by Professor Shannon during the Spring '08 term at MSU Bozeman.

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Chapter 26 - 26 Capacitance and Dielectrics CHAPTER OUTLINE...

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