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Homework Help: Displacement of an electron between two plates

  1. Apr 1, 2012 #1
    1. The problem statement, all variables and given/known data
    In an oscilloscope, an electron is accelerated from rest through a voltage of 1.14x103 V. It then passes through deflecting plates 2.00 cm apart, across which there is a voltage of 412 V. Its velocity when it enters the plates is perpendicular to the electric field of the plates. When the electron leaves the plates, what distance will it have moved toward the positive plate? The length of the plates are 4.00 cm.

    The answer is 7.23x10-3 m

    2. Relevant equations

    vi= 0
    Va= 1.14x103 V
    Vd= 412 V
    length of plate = 4 cm = 0.04 m
    d= 2 cm= 0.02 m

    1/2mv2= Vaq
    t=length of plate/v
    F= Eq
    a= F/m

    3. The attempt at a solution

    So, because it's an electron
    q= 1.6x10-19
    m= 9.11x10-31

    Using the above equations and plugging in the givens, I've gotten:

    v= 2.001x107 m/s

    t=1.0x10-9 s

    E= 20600 v/m

    F= 3.296x10-15 N

    a= 3.618x1015 m/s2

    x= 1.809x10-3 m

    which is not the right answer :( I'm not very good at physics so I'm having a hard time figuring out what I'm doing wrong.
  2. jcsd
  3. Apr 1, 2012 #2


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    Homework Helper


    here is a mistake: The electron travels along the length of the plate with the v=2.00 m/s so the time is 2.0x10-9 s

  4. Apr 1, 2012 #3
    Wellp, I'm just a big ol' dummy! I can't believe I made that oversight. Thanks so much!!!
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