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Homework Help: Finding the voltage of an oil droplet, Millikan's experiment

  1. Jan 16, 2012 #1
    1. The problem statement, all variables and given/known data

    The density of the oil used to form droplets in the Millikan experiment is 9.20 × 102 kg/m3 and the radius of a typical oil droplet is 2.00 μm. When the horizontal plates are placed 18.0 mm apart, an oil drop, later determined to have an excess of three electrons, is held in equilibrium. What potential difference must have been applied across the plates?

    2. Relevant equations

    V = (4/3)(3.14)r^3

    q = (mgd)/V

    where q = charge
    V = voltage

    3. The attempt at a solution

    step 1. find the mass of the droplet.


    step 2
    then multiply that by the viscosity which is 92 kg/m^3 which is .007

    step 3
    get V by itself

    V = mgd/q

    step 4
    find the charge (q)

    1.6 * 10^-19 * 3 = 4.8*10^-19

    step 5
    plug the numbers into the above equation


    the correct answer is 1.3 * 10^4, my answer is way off
  2. jcsd
  3. Jan 16, 2012 #2


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    Homework Helper

    How do you calculate the volume of a droplet from the radius?

  4. Jan 16, 2012 #3
    you have forgotten r^3
    Sorry ehild.... did not see your post in time
    Last edited: Jan 16, 2012
  5. Jan 16, 2012 #4
    Good, now I got it. Thanks for your help.
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