Hello. I'm just stuck on a problem. A point charge Q=+4.60x10^(-9)C is held fixed at the origin. A second point charge q=+1.20x10^(-9)C with mass of 2.80x10^(-4)kg is placed on the x-axis, 0.250m from the origin. The problem asks, "What is the electrical potential energy U of the pair of charges?" Okay, so I tried using the formulas: -(Ub-Ua)=-deltaU and U=((1)/(4*pi*Eo))*((q1*qo)/(r)) Where I'm confused is that q1 and qo appear the same in both Ua and Ub, thus wouldn't Ub-Ua cancel and equal zero? Here's my obviously incorrect work: -(((1)/(4*pi*Epsilono))*(((1.2x10^(-9))(4.6x10^(-9)))/(.25m)) = -1.98x10^(-7) However, the correct answer according to the back of the book is "0.198J." Can anyone with patience please explain what I'm doing wrong? This material is new to me. I'm in a 5 week summer calculus 2 based engineering physics 2 course (PHY 122). We only just started talking about this material yesterday.