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Electric Potential -- work due to a non-conducting ring

  1. Oct 10, 2015 #1
    1. The problem statement, all variables and given/known data
    A charge of q1=-9.8 nC is uniformly distributed around a thin plastic ring lying in a yz plane with the ring center at the origin. A q2= -8.0 pC point charge is located on the x axis at x = 3.4 m. For a ring radius of 1.2 m, how much work must an external force do on the point charge to move it to the origin?

    2. Relevant equations
    U=W= (q1q2)/(4πεr)

    3. The attempt at a solution
    it says in the solution that W=ΔU= ( q1q2)/(4πε) * ( 1/r - 1/ √(r^2 + x^2) ) ; can someone please explain where did we get the ( 1/r - 1/ √(r^2 + x^2) ) from?
    Thank you !
     
  2. jcsd
  3. Oct 10, 2015 #2

    vela

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    You got it from the solutions....nyuck nyuck nyuck

    What do r and x represent?
     
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