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henry3369
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Homework Statement
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.
Homework Equations
W = -ΔU
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?