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Homework Help: Acceleration in electric field

  1. Mar 10, 2012 #1
    1. The problem statement, all variables and given/known data

    An electron with initial velocity vx0 =1.0 *10^4 m/s enters a region of width 1.0 cm where its electrically accelerated. it emerges with velocity vx = 4.0 *10^6
    what was its acceleration, assumed constant?

    2. Relevant equations

    F = ma
    v^2 = u^2 + 2as

    3. The attempt at a solution

    Well I thought I would try this

    (v^2 - u^2)/2s =a

    but this did not work.

    Im guessing I am going to need to find E?

    But dont know how to do this with given information.

    can someone point me in the direction of the formula needed for the question?

    Last edited: Mar 10, 2012
  2. jcsd
  3. Mar 10, 2012 #2
    I don't think you need to actually find the field. You can think of this one as a conservation problem where it's final kinetic energy is using the final velocity and so on. You do know the mass of the electron afterall?

    As far as the 1cm goes, do they specify that to be the width, as in, the x-component (the direction the electron is traveling?) or is it 1cm as in, the width of a loop or wire or something that it travels through? In which case, it's length would be mostly arbitrary and you could treat it as an instantaneous point where the velocity changes.
  4. Mar 10, 2012 #3
    not sure about the width I think it is in the x direction
    as for the kinetic energy and conservation how do I link 1/2 m v^2 to acceleration?
  5. Mar 11, 2012 #4
    Well, what's potential energy in an electric field?
  6. Mar 11, 2012 #5
    I feel that you approach is correct. It's a kinematics type problem only. Some where you might be making mistake in units.
  7. Mar 11, 2012 #6
    U = kQq/r
    Still not understanding how that factors into 1/2 mv^2?
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