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Homework Help: Solve for charge(q), given radius, electric field, and density

  1. Oct 7, 2011 #1
    1. The problem statement, all variables and given/known data

    In Millikan's experiment, an oil drop of radius 1.73 μm and density 0.865 g/cm3 is suspended in chamber C when a downward electric field of 1.44 × 105 N/C is applied. Find the charge on the drop, in terms of e. (Tolerance of solution is +/- 2%)


    radius, r = 1.73 μm x 10-6 m

    density, σ = 0.865 x 103 kg/m3

    Electric field, E = 1.44 × 105 N/C

    magnitude of charge of electron, e = 1.6 x 10-19

    gravitational constant, g = 9.8 m/s2

    2. Relevant equations

    1. Force due to electric field: F = -qE

    2. Mass of the drop, m = (4pi/3)*r3

    3. Force of gravity, F = mg = (4/3)*pi*r3*σ*g

    3. The attempt at a solution

    (1) I rearranged equations (1) & (3) from above to solve for q, thus:

    q = (-4*pi*r3*σ*g)/3E

    (2) I then plugged in the given values:

    q = [-4*pi*(1.73 x 10-6m)3*0.865 x 103 kg/m3*9.8 m/s2]/(3*1.44 x 105 N/C)

    = -1.28 x 10-18 C

    (3) Now, to put the solution in terms of e, I divided my result from (2) by the magnitude of charge of electron, e:

    q = (-1.28 x 10-18 C/1.6 x 10-19 C)(1e)

    = -8e C

    However, my answer is incorrect, and I'm not sure why. I also tried to reverse the sign of my solution, but that was incorrect as well.
  2. jcsd
  3. Oct 7, 2011 #2
    using the data I am getting

    [tex]q=-3.83\times 10^{-18}\;\mathrm{C}[/tex]

    check math
  4. Oct 9, 2011 #3
    I checked the math again, and it turns out that the correct result was just -8, instead of -8e. Thank you for your reply, issac.
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