# Homework Help: Change in energy between system of point charges

1. Jun 10, 2012

### z0k

1. The problem statement, all variables and given/known data
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?

2. Relevant equations

3. 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 $$U' - U = 10J.$$

2. Jun 10, 2012

### BruceW

I think you have gone about it almost the right way. I think the problem is where you did 1/2 + 1/2 + 1 = 5 This is not right.

PS welcome to physicsforums! :)

EDIT: I am tired and didn't read your post through properly. Sorry. Your maths was 2 + 2 + 1 = 5 which is of course correct. I think your answer is all OK, why do you think it is wrong?

3. Jun 10, 2012

### z0k

Sorry, I think my typesetting make the fractions look confusing. I'm still learning to use Tex properly.
I just wanted to make sure that my set up was correct.
Thank you for your help and for the welcome!

4. Jun 10, 2012

### BruceW

yep, no worries :)