1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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


    User Avatar
    Science Advisor
    Homework Helper

    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!!

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook