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Homework Help: Check physics answers

  1. Dec 13, 2003 #1
    problem #1
    Two point charges of +3.0uC and -7.0uC are placed at x=0 and x=.20m, respectively. What is the magnitude of the electric field at the point midway between them (x=.10m)?

    E3 = (kq)/r^2 = [(9x10^9 )(3x10^-6 C)]/.10m^2 = 2.7x10^6 N/C
    E7 = (kq)/r^2 = [(9x10^9 )(7x10^-6 C)]/.10m^2 = 6.3x10^6 N/C
    E=E3+E7 = 9x10^6 N/C
    solutions manual has the answer as 9x10^5 N/C

    problem #2
    If the distance between two closely spaced parallel planes is halved, the electric field between them: becomes 4 times as large.

    if d=2, E=1
    if d=1, E=4
    so becomes 4 times as large
    solutions manual has the answer as remains the same

    problem #3
    A 12-V battery is connected to a parallel-plate capacitor with a plate area of 0.40 m^2 and a plate seperation of 2.0 mm. How much energy is stored in the capacitor?

    A=0.40 m^2
    d=2.0 mm = .002 m
    V= 12V
    find Uc

    C=(EoA)/d = (8.85x10^-12 x .40 m^2)/.002 m
    C= 1.77x10^-9 F
    Uc=1/2cv^2 = 1/2(1.77x10^-9 F)(12v)^2 = 1.27x10^-7 J

    solutions manual has the answer as 2.5x10^-7 J
  2. jcsd
  3. Dec 14, 2003 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Your answers to #1 and #3 look good to me. I'm not sure what you are doing for #2, but in any case: as long as the charge remains the same, the field within the (closely spaced) plates is independent of separation.
  4. Dec 14, 2003 #3
    thanks Doc, in #2 I was getting confused about the (A)area= d^2 vs the separation distance. Q remains same, E doesn't depend on separation distance between two closely spaced parallel plates.
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