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 !