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Homework Help: Electric potential energy Please help

  1. Mar 6, 2008 #1
    A particle of positive charge Q is fixed at point P. A second particle of mass m and negative charge -q moves at constant speed in a circle of radius r1, centered at P. Derive an expression for the work W that must be done by an external agent on the second particle to increase the radius of the circle of motion to r2. (Use epsilon_0 for 0, r1 for r1, r2 for r2, and Q, m, and q as necessary.)
    I know the answer to be (Qq/8pi*episolon_0)(1/r1-1/r2)
    I know that i use the equation w=k*q1q2/r and i believe r to be (1/r1-1/r2). I do not understand how to get 8 though could someone please help!!!
  2. jcsd
  3. Mar 6, 2008 #2
    TE= KE+PE
    you forgot calculate the PE
    increase the radius = change of TE
  4. Mar 7, 2008 #3

    Shooting Star

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    Show your calculations briefly so that we can take it up from there.
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