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8.A: Capacitance (Answers)

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    8.1. \(\displaystyle 1.1×10^{−3}m\)

    8.3. 3.59 cm, 17.98 cm

    8.4. a. 25.0 pF;

    b. 9.2

    8.5. a. \(\displaystyle C=0.86pF,Q_1=10pC,Q_2=3.4pC,Q_3=6.8pC\);

    b. \(\displaystyle C=2.3pF,Q_1=12pC,Q_2=Q_3=16pC\);

    c. \(\displaystyle C=2.3pF,Q_1=9.0pC,Q_2=18pC,Q_3=12pC,Q_4=15pC\)

    8.6. a.\(\displaystyle 4.0×10^{−13}J\); b. 9 times

    8.7. a. 3.0; b. \(\displaystyle C=3.0C_0\)

    8.9. a. \(\displaystyle C_0=20pF, C=42pF\);

    b. \(\displaystyle Q_0=0.8nC, Q=1.7nC\);

    c. \(\displaystyle V_0=V=40V\); d. \(\displaystyle U_0=16nJ, U=34nJ\)

    Conceptual Questions

    1. no; yes

    3. false

    5. no

    7. \(\displaystyle 3.0μF,0.33μF\)

    9. answers may vary

    11. Dielectric strength is a critical value of an electrical field above which an insulator starts to conduct; a dielectric constant is the ratio of the electrical field in vacuum to the net electrical field in a material.

    13. Water is a good solvent.

    15. When energy of thermal motion is large (high temperature), an electrical field must be large too in order to keep electric dipoles aligned with it.

    17. answers may vary


    19. 21.6 mC

    21. 1.55 V

    23. 25.0 nF

    25. \(\displaystyle 1.1×10^{−3}m^2\)

    27. 500 µC

    29. 1:16

    31. a. 1.07 nC;

    b. 267 V, 133 V

    33. \(\displaystyle 0.29μF\)

    34. 500 capacitors; connected in parallel

    35. \(\displaystyle 3.08μF\) (series) and \(\displaystyle 13.0μ\) (parallel)

    37. \(\displaystyle 11.4μF\)

    39. 0.89 mC; 1.78 mC; 444 V

    41. \(\displaystyle 7.5μJ\)

    43. a. 405 J; b. 90.0 mC

    45. 1.15 J

    47. a. \(\displaystyle 4.43×10^{−9}F\);

    b. 0.453 V;

    c. \(\displaystyle 4.53×10^{−10}J\);

    d. no

    49. 0.7 mJ

    51. a. 7.1 pF;

    b. 42 pF

    53. a. before 3.00 V; after 0.600 V;

    b. before 1500 V/m; after 300 V/m

    55. a. 3.91;

    b. 22.8 V

    57. a. 37 nC;

    b. 0.4 MV/m;

    c. 19 nC

    59. a. \(\displaystyle 4.4μF\);

    b. \(\displaystyle 4.0×10^{-5}C\)

    61. \(\displaystyle 0.0135m^2\)

    63. \(\displaystyle 0.185μJ\)

    Additional Problems

    65. a. 0.277 nF;

    b. 27.7 nC;

    c. 50 kV/m

    67. a. 0.065 F;

    b. 23,000 C;

    c. 4.0 GJ

    69. a. \(\displaystyle 75.6μC\); b. 10.8 V

    71. a. 0.13 J;

    b. no, because of resistive heating in connecting wires that is always present, but the circuit schematic does not indicate resistors

    Figure shows a closed circuit with a battery of 400 volts. The positive terminal of the battery is connected to a capacitor of 3 micro Farads, followed by a combination of two capacitors in parallel with each other, followed by a fourth capacitor of value 6 micro Farads, which in turn is connected to the negative terminal of the battery. The capacitors in parallel to each other have values 6 micro Farad and 3 micro Farad.

    73. a. \(\displaystyle −3.00μF\);

    b. You cannot have a negative \(\displaystyle C_2\) capacitance.

    c. The assumption that they were hooked up in parallel, rather than in series, is incorrect. A parallel connection always produces a greater capacitance, while here a smaller capacitance was assumed. This could only happen if the capacitors are connected in series.

    75. a. 14.2 kV;

    b. The voltage is unreasonably large, more than 100 times the breakdown voltage of nylon.

    c. The assumed charge is unreasonably large and cannot be stored in a capacitor of these dimensions.

    Challenge Problems

    77. a. 89.6 pF;

    b. 6.09 kV/m;

    c. 4.47 kV/m;

    d. no

    79. a. 421 J;

    b. 53.9 mF

    81. \(\displaystyle C=ε_0A/(d_1+d_2)\)

    83. proof

    Contributors and Attributions

    Samuel J. Ling (Truman State University), Jeff Sanny (Loyola Marymount University), and Bill Moebs with many contributing authors. This work is licensed by OpenStax University Physics under a Creative Commons Attribution License (by 4.0).

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