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Homework Help: Finding capacitence, used C = (EoA)/d, but didn't work!

  1. Oct 9, 2005 #1
    This seems like a plug n' chug problem but i missed it somehow..
    A parallel-plate air-filled capacitor having area 38 cm2 and plate spacing 2.0 mm is charged to a potential difference of 450 V. Find the following values.

    (a) the capacitance
    wrong check mark pF
    (b) the magnitude of the charge on each plate
    (c) the stored energy
    (d) the electric field between the plates
    (e) the energy density between the plates

    So i used C = (EoA)/d
    A = .38
    V = 450
    d = .002m
    C = [(8.85e-12)(.38)]/.002
    C = 1.68e-9, but the want it in pF, p is e-12, so i moved the decimal over to amek it 1680pF, but it was still wrong, did i mess up converting it from e-9 to e-12?
  2. jcsd
  3. Oct 9, 2005 #2

    Physics Monkey

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    38 cm^2 = .0038 m^2
  4. Oct 9, 2005 #3
    hm...1 meter = 100 centimeters
    38cm/100cm = .38m
  5. Oct 9, 2005 #4


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    Area is NOT measured in "cm", but in cm x cm .
    as in 10cm x 3.8 cm ... Convert both of them!
  6. Oct 9, 2005 #5
    I know area is m^2, not m, or cm^2, not cm...
    The problem already gives me the Area, so its already cm^2:
    38 cm^2, so i want to conver that into meters, so .38 m^2. 2.0mm is the distance, i don't square that also.
  7. Oct 9, 2005 #6

    Physics Monkey

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    [tex] 38 \, cm^2 = 38 \, cm^2 * \frac{1 \, m}{100 \, cm}* \frac{1 \, m}{100 \, cm} = 38*10^{-4} \, m^2 = .0038 \, m^2 [/tex]
  8. Oct 9, 2005 #7
    ohhhh my bad, sorry i'm slow hah, thanks guys!
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