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Homework Statement
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?
Homework Equations
U=W= (q1q2)/(4πεr)
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 !