# Why it's (1/2)? [Electric Potential Energy]

1. Sep 12, 2015

### Antonius

My question is, why it's 1/2 [qiVi]? Shouldn't it just be qiVi?

Thank you ...

2. Sep 12, 2015

### robphy

You want to count "pairs"...
That is, if you include both $\frac{q_1q_2}{r_{12}}$ and $\frac{q_2q_1}{r_{21}}$, you are counting the same "pair" twice.

3. Sep 12, 2015

Oh I see,