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Homework Help: Determing electron charge

  1. May 10, 2007 #1
    1. The problem statement, all variables and given/known data
    Robert Millikan received a Nobel Prize for determining the charge on the electron. To do this he set up a potential difference between two horizontal parallel metal plates. He then sprayed a drops of oil between the plates and adjusted the potential difference until drops of a certain size remained suspended at rest between the plates. Suppose that when the potential difference between the plates is adjusted until the electric field is 10,000 N/C downward, a certain drop with a mass of 3.27 x 10^-16 kg remains suspended. what is the magnitude of the charge on this drop?

    2. Relevant equations
    umm... i suppose E = Kc x (q/r^2) and f = Kc x (q1)(q2)/r^2 would apply. i'm not sure if i need anything else though.

    3. The attempt at a solution

    i have got the right answer i think, but i have no idea how what is going on.

    (3.27 x 10^-16)(9.8) = 3.2046 x 10^-15 = qE
    E = 3.2 x 10^-19

    i understand that here, mass is being multiplied by free fall acceleration, and then set equal to charge x electric field, but i don't know where this came from. does it have anything to do with the unit of electric force being Newtons, or kg x m/s^2. some explanation would be greatly appreciated.
  2. jcsd
  3. May 10, 2007 #2
    Draw a free body diagram of the oil drop and work out the conditions for it to remain suspended.
  4. May 10, 2007 #3
    yeah thanks. i figured it out minutes after. it's basically just electric field against gravity. thank you.
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