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Homework Help: Electric Fields Question, What is the speed of the electron?

  1. Feb 14, 2017 #1
    1. The problem statement, all variables and given/known data
    An electron is released from rest in a weak electric field given by -2.8 x 10^-10 N/C [PLAIN]https://www.flipitphysics.com/Content/smartPhysics/Media/Images/Tipler/Symbols/jhatbold.gif. [Broken] After the electron has traveled a vertical distance of 1.9 µm, what is its speed? (Do not neglect the gravitational force on the electron.) Answer should be in mm/s.

    Any help towards setting up this problem or what formula to use will be greatly appreciated.
    2. Relevant equations

    F= (1/4πE0)(q1q2/r^2)

    Work Done = F*d = 1/2(mv^2)

    3. The attempt at a solution
    I don't really know where to start, this is the last problem on my assignment and I only have a few attempts left, we did not go over this in class yet and our class uses "Flipit Physics" videos instead of a book.

    I tried Fe=qE
    =(1.602x10^-19)(-2.8x10^-10) = -4.4856x10^-29
    then a = Fe/m
    = -4..572477064 x 10^-31 which was wrong

    I also tried Work Done = F*d = 1/2(mv^2) and solved for v but that was also wrong.
    Last edited by a moderator: May 8, 2017
  2. jcsd
  3. Feb 14, 2017 #2
    E=1/2mv^2 is equal to the total work done on the particle. You have two forces: the Lorentz force F=q(E+v*B) and gravitational force F=mg, both in the same direction (I assume j-hat is the unit vector in the y direction).
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