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Effects of an electric field on a motion of an electron

  1. Jul 7, 2007 #1
    1. The problem statement, all variables and given/known data

    An electron is projected at an angle of 31.7° above the horizontal at a speed of 8.40×10^5 m/s in a region where the electric field is E = 383j N/C. Neglecting the effects of gravity, calculate the time it takes the electron to return to its initial height.

    2. Relevant equations

    I first found the electric force that the E field exerts on the electron by using F=qE.

    Then I found the acceleration of the electron with a=(q/m)E

    Thought perhaps electron would undergo motion similar to a projectile, so then first found the y component of the velocity sin 31.7=opp/8.40x10^5s.

    Then applied vf=vi + at solving for t (with vf=0 because it will be when the electron has all potential energy at its highest point)

    3. The attempt at a solution


    a=-6.13x10^-14N/9.11x10^-31kg (mass of electron)

    sin 31.7=opp/8.40x10^5m/s
    opp= 4.41x10^5m/s

    vf=vi +at
    0=4.41 + -6.73x10^16m/s^2t
    t=6.55x10^-12s (this would be the time to reach max height, so the amount of time to get back down to its initial height would be double this)

    so t= 1.31x10^-11s
    But this answer is not right... and I'm not show how else to relate t to an electric field. Can anyone point out what i'm doing wrong here??

    Thanks SOOO much!! :)
  2. jcsd
  3. Jul 7, 2007 #2


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    Uh, the exponent of the electron charge is -19, not -16. But there is nothing wrong with your method.
  4. Jul 7, 2007 #3
    omg... wow... I can't that was all it was!!

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