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Homework Statement
The energy stored by any pair of positive charges is inversely proportional to the distance between them, and directly proportional to their charges. Three identical point charges start at the vertices of an equilateral triangle, and this configuration stores 15 Joules of energy. How much more energy, in Joules, would be stored if one of these charges was moved to the midpoint of the opposite side?
Homework Equations
The Attempt at a Solution
We determine our formula to be $$U = \frac{kq^2}{r}.$$ Then from the given data, for three point charges we have $$U = 3 \frac{kq^2}{r} = 15J.$$ To compute the new position:
$$
U' = k(\frac{q^2}{r/2} + \frac{q^2}{r/2} + \frac{q^2}{r}) \\
U' = 5 \frac{kq^2}{r} \\
U' = 25J \\
$$
So the difference is $$U' - U = 10J.$$
I'm not quite sure if I am going about this the right way.