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Force between 2 charges

  1. May 19, 2009 #1
    Each of two small non-conducting spheres is charged positively, the combined charge being 40C . When the two spheres are 50 cm apart, each sphere is repelled from the other by a force of magnitude 2.5 N. Determine the magnitude of the smaller of the two charges

    my attempt:
    use eq. F=kq1q2/d
    2.5=((9x10^9)(40-q2)(q2))/.5
    this gives me the wrong answer
    what am i doing wrong?
     
  2. jcsd
  3. May 19, 2009 #2

    Pyrrhus

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    Homework Helper

    You forgot r is square.
     
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