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Homework Help: Finding the minimum speed of a proton traveling between two parallel plates.

  1. Feb 18, 2012 #1
    A proton, mass 1.67 x 10^-27 kg , is projected horizontally midway between two parallel plates that are separated by 0.8 cm , with an electrical field with magnitude 7.2 x 10^5 N/C between the plates. If the plates are 4.70cm long, find the minimum speed of the proton that just misses the lower plate as it emerges from the field.

    q(proton)= + 1.60218 x -19 C

    V(f) = √2ad = √2qEL/m

    I used the formula above to try and solve for the final velocity but got the wrong answer, I converted my distances and lengths into m first.

    My second attempt involved a variation of this formula: v= √Eq/dm x [L^2 + d^2]

    And my third another variation: v= L x √qE/2dm.

    d= 0.008m, L= 0.047m, E = 7.2 x 10^5, m= 1.67 x 10^-27.

    I'm not sure how to use the first equation while taking into account the distance between the plates. Any advice?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Feb 19, 2012 #2


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    Homework Helper

    Read the problem carefully. Does it ask the final velocity of the proton?
    What does it mean that the proton is projected horizontally between the plates? Make a drawing of the problem.

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