Electric Potential Energy Question

[leaf345]

Hi, this is my first time posting here but I have a question that looked really simple at first, but the answer I get is wrong from the answer given to us by our teacher.
The question is this:
Consider the diagram below
http://img485.imageshack.us/img485/9282/physics3bn.jpg

a) What is the electric potential energy of the system?
What I did
3x10^-6=q1
1x10^-6=q2
-2x10^-6=q3

Epotental(total)= (Epotential @ q1) + (Epotential @ q2) + (Epotential @ q3)
=[(kq1q2)/0.36 + (kq1q3/0.36)] + [(kq2q1/0.36) + (kq2q3/0.4)] + [(kq3q1/0.36) + (kq3q2/0.4)]

I used the formula E=(kq1q2)/r to get the potential energies at all 3 points and then added them together. I got -0.24 J but the answer the teacher gave us is -0.12J. Is it possible he is mistaken, or am I oversimplifying this question?

BTW: This is grade 12 physics

 

[durt]

Try applying this method to a two-charge system and see if you can figure out what you're doing wrong. Here's how I would do a problem like this: I'd "move" one charge in from an infinite distance and calculate the work done, which is zero. Then I'd bring in the second and fix it in place and calculate the work done. Then I'd bring in the third charge, etc.

 

[leaf345]

I tried what you recommended in your second part and it worked :)
Thanks.