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Homework Help: Finding electric potential difference to stop a electron between 2 plates

  1. May 11, 2008 #1

    Question: an electron with a velocity of 5.0x10[tex]^{}6[/tex] m/s is injected into a parallel plate apparatus through a hole in the positive plate. it moves across the vacuum between the plates. what would the potential difference have to be in order to stop the electron 25% of the way across the gap?

    2. Relevant equations
    conservation of energy eqations and electromagnetic equations

    3. The attempt at a solution
    if we set the base line at the positive plate:
    E[tex]_{}kinetic 1[/tex] + E[tex]_{}electric potential 1[/tex] = E [tex]_{}electric potential 2[/tex]

    Vq + 1/2 mV[tex]^{}2[/tex] = Vq(0.75) <---- (to account for the 25%)

    V(-1.6 x 10[tex]^{}-19[/tex] ) + 1/2 (9.1 x 10[tex]^{}-31[/tex] )(5x10[tex]^{}6[/tex]) [tex]^{}2[/tex] = V(-1.6x10[tex]^{}-19[/tex])(.75)

    when i do this my answer does not come out to what it is supposed to (which is 285 j/c)
    what am i doing wrong
  2. jcsd
  3. May 11, 2008 #2


    User Avatar
    Homework Helper
    Gold Member

    I get 285V when I work through your numbers. Your doing the problem correctly, but you may have made an error in computing the numbers. Check your calculations for an error.
  4. May 12, 2008 #3
    kewl thanks for confirming that