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Homework Help: Electron Approaching a Charged Sphere

  1. Jun 7, 2012 #1
    1. The problem statement, all variables and given/known data

    An electron starts from rest 2.80 cm from the center of a uniformly charged sphere of radius 1.83 cm. If the sphere carries a total charge of 1.01×10-9 C, how fast will the electron be moving when it reaches the surface of the sphere?

    2. Relevant equations

    UI + KI = UF + KF

    3. The attempt at a solution

    Rearranging terms I got:

    [([(k*Q*-e)/(r-initial)] - [(k*Q*-e)/(radius)])/(r-initial - radius)](2/m)

    k = 9x10^9
    Q = 1.01×10-9
    e = 1.602 x 10^(-19)
    m = 9.1094 x 10^(-31)
    r-initial = .028m
    radius = .0183m

    I then get v^2 = 6.24x10^15. I square root(√) it to get v and I get:

    v = 7.9x10^7 m/s

    I have tried the negative and positive of this and I cannot solve it. Any help would be appreciated
  2. jcsd
  3. Jun 7, 2012 #2


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    What is the factor, (r-initial - radius) doing in there?
  4. Jun 7, 2012 #3
    SammyS, you are a saint. Thank you!
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