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Opposite Charged Plates with Sphere

  1. Feb 21, 2007 #1
    1. The problem statement, all variables and given/known data
    A small sphere with mass 2.90 g hangs by a thread between two large parallel vertical plates 0.05 m apart. The plates are insulating and have uniform surface charge densities + \sigma and - \sigma . The charge on the sphere is q = 9.70×10^6 C.

    What potential difference between the plates will cause the thread to assume an angle of 30.0 deg with the vertical

    2. Relevant equations

    [tex] Delta(V) = Ed = \frac {F_{elec} * d} {q} = \frac {(mg)*(d)*tan(30)} {q} [/tex]

    3. The attempt at a solution

    [tex] \frac {(2.90*8.90)*(0.05)*tan(30)} {9.70*10^{-6}} [/tex]

    The online program complains that its wrong, I'm wondering is the equation correct?
  2. jcsd
  3. Feb 21, 2007 #2
    g=9.8 not 8.9?
  4. Feb 21, 2007 #3
    That was actually a typo, I used 9.8 and still no luck.

    The computation comes out to:

    [tex] -9.38*10^{-7} [/tex]

    Can anyone confirm this?
    Last edited: Feb 21, 2007
  5. Feb 21, 2007 #4


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    Gold Member

    Convert 2.90 from g to kg. Also you are using 9.70×10^-6 C in the denominator, but the initial value was 9.70×10^6 C.
  6. Feb 21, 2007 #5
    The mass was what threw it off, actually Q was 10^-6, I guess copying it over from the flash page removed the negative, though I had that in my solution. Also I figured out the answer. THanks!
    Last edited: Feb 21, 2007
  7. Feb 21, 2007 #6
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