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Homework Help: Vertical Displacement of Electron (Millikan's Oil Drop)

  1. Mar 22, 2012 #1
    1. The problem statement, all variables and given/known data

    An electron moving horizontally at 2.50 x 106 m/s enters a 150 N/C electric field exactly half way between two horizontal parallel plates and follows a parabolic path downwards. Both plates are 14 cm long and spaced 10 cm apart. Determine the vertical displacement of the e- as it travels between the plates.

    2. Relevant equations

    E = Fe/q , F=ma , Ep = mgh

    3. The attempt at a solution

    I found Fe by E/q and got 2.40 x 10-17 J. then I plugged the values into F=ma and found the acceleration to be 2.63 x 1013 m/s2. I then divided the velocity by acceleration and found time to be 9.49 x 10 -18 s. I am now unsure how to find the vertical distance. I used Ep = mgh but got a very large value, so I think that is wrong. Did I make a mistake earlier or should I be using a different equation?
    Last edited: Mar 22, 2012
  2. jcsd
  3. Mar 22, 2012 #2

    Doc Al

    User Avatar

    Staff: Mentor

    The velocity is horizontal but the acceleration is vertical. Instead, ask how long does it take something moving that fast to cover the needed distance?

    Once you have the time, figure out its vertical displacement in that time using kinematics. What is the electron's initial vertical speed?
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