From a homework assignment, there are 4 spheres spaced 1cm apart. Each of the spheres are charged to +10nC and weigh 1 gram. The question wants us to find the final speed of the charges once they've drifted far apart.
I've found the answer, and I realized where my calculations were incorrect. What I don't understand is why my calculations are incorrect. To find the solution to this question, I realized I'd need to find the potential energy of the entire system. I assembled the potential energy with the following formula: $$ \Delta U=\int -F_edr = \frac{kq_1q_2}{r}$$ Which lead me to this incorrect equation: $$U_f-U_i=0-4(\frac{kQ}{L} + \frac{kQ}{L} + \frac{kQ}{\sqrt2 L})$$ The intuition behind this formula was the interaction of three spheres on one sphere would be $\frac{kQ}{L} + \frac{kQ}{L} + \frac{kQ}{\sqrt2 L}$ so the interaction of all spheres on each other would be 4 times that quantity. However, it turns out there are only 6 total interactions in the entire system: $$U_f - U_i = 0 - (\frac{kQ}{L} + \frac{kQ}{L} + \frac{kQ}{L} + \frac{kQ}{L} + \frac{kQ}{\sqrt2 L} + \frac{kQ}{\sqrt2 L})$$ But wouldn't this count interactions going in only one direction? Specifically, if the spheres were named A, B, C, and D––order doesn't matter. Then the only interactions that could have been noted would be $A\,\to\,B, A\,\to\,C, A\,\to\,D, B\,\to\,C, C\,\to\,D, B\,\to\,D$. But what about the half of the interactions: $B\,\to\,A, C\,\to\,A, D\,\to\,A, C\,\to\,B, etc.$ Does the other half not matter in the context of potential energies?