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Homework Help: Speed of the Proton

  1. Feb 17, 2017 #1
    1. The problem statement, all variables and given/known data
    In the figure, a proton, mass 1.67×10^−27 kg, is projected horizontally midway between two parallel plates that are separated by 0.20cm, with an electrical field with magnitude 7.8×10^5N/C between the plates. If the plates are 5.40cm long, find the minimum speed of the proton that just misses the lower plate as it emerges from the field.
    2. Relevant equations
    q(E) = ma
    x = vt
    y = 1/2a(t^2)

    3. The attempt at a solution

    This is what I did:
    1.6*10^-19(7.8*10^5) = 1.67*10^-27 (a)
    solving for a I get: 7.47*10^13 m/s^2

    then i solve for t
    2*10^-3 = (1/2)(7.47*10^13)(t^2)
    t = sqrt(7.47*10^13/((2)(2*10^-3))
    I get 7.32*10^-9 s

    then i plug that in the x = vt eqn
    0.054/7.23*10^-9 = v
    v = 7.4*10^6 m/s

    But that is wrong
  2. jcsd
  3. Feb 17, 2017 #2
    It says the proton comes in midway, which means y is 0.1cm, not 0.2cm
  4. Feb 17, 2017 #3
    Oh I see.Thank you!
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