1. The problem statement, all variables and given/known data Two point charges are located on the x-axis, q1 = -e at x=0 and q2 = +e at x=a. Find the work that must be done by an external force to bring a third point charge q3 = +e from infinity to x = 2a. 2. Relevant equations W = -ΔU 3. The attempt at a solution W = -(Ufinal - Uinitial) Uinitial = 0 because it is at infinity. So W = -Ufinal = -kq3(q1/r1 + q2/r2). After doing this I get: -(e^2)/(8πεa) which has the correct magnitude but wrong sign. My book uses W = U instead of W = -U. Is there a reason for this? Work is equal to change in NEGATIVE potential energy, so why would it be positive?