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Homework Help: Mass of charged sphere suspended between charged plates

  1. Jun 27, 2016 #1
    1. The problem statement, all variables and given/known data
    A small sphere with charge 2.4 micro coulombs is suspended from a thread between 2 charged plates. The plates have a voltage of 62 and the distance between the plates is 3.1 cm. The sphere hangs at 18 degrees to the vertical.

    2. Relevant equations
    E = V/r
    FE = Eq
    Fnetx = FE - FT
    Fnety = FTy - Fg

    3. The attempt at a solution

    E = 62V/0.031m = 2000 N/C

    FE = Eq = (2000 N/C)(2.3 x 10^-6 C)

    FTx = FE = 4.6 x 10^-3 N

    FTy = FE * tan18 degrees
    = 4.6 x 10^-3 N * tan18
    = 1.5 x 10^-3 N

    Fg = mg
    1.5 x 10^-3 N = m(9.8 m/s^2)
    m = 1.5 x 10^-4 kg
     
  2. jcsd
  3. Jun 27, 2016 #2

    TSny

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    Hello. In the problem you state that the charge is 2.4 μC but you use 2.3 μC in the calculation.

    You wrote FTy = FE * tan18o . Is this correct (keeping in mind that the 18o is measured from the vertical)?
     
  4. Jun 27, 2016 #3
    I just realised the problem statement says 2.4 μC but the diagram says 2.3 μC hmmm. Assuming 2.4 is correct..

    FTy = FE*tan72
    = 1.5 * 10^-2 N

    Fg = mg
    1.5 * 10^-2 = m(9.8)
    m = 1.5 ^ 10^-3 kg
     
  5. Jun 27, 2016 #4

    TSny

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    That looks right.
     
  6. Jun 27, 2016 #5
    Ok than you!
     
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