# Homework Help: Electric Potential Difference and charges

1. Feb 26, 2005

### chattkis3

Here's the problem I am having trouble with, even though i think it should be simple...

Four identical charges (+3.5uC each) are brought from infinity and fixed to a straight line. The charges are located .35m apart. Determine the electric potential energy of the group.

So I've tried two different approaches. First, I found every possible combination of the 4 charges (q1q2 q1q3 q1q4 q2q1 q2q3 q2q4 q3q1 q3q2 q3q4 q4q1 q4q2 q4q3) and used the equation U = kqq/r to find the potential for all 12 combos and found the sum of all 12 potentials. This didn't give me the right answer.

So then I tried just the combinations that were totally different (q1q2 q1q3 q1q4 q2q3 q2q4 q3q4) and found the sum of the potentials, and that did not give me the correct answer either...

2. Feb 26, 2005

### MathStudent

You have the right idea here. However, notice that in the equation U = kqq/r it doesn't matter which charge comes first. ie: kq1q2/r = kq2q1/r so really what you did was count the potential twice for each pair of charges. so the correct answer, should be this answer divided by 2 (assuming everything else you did was correct ).

Another thing to watch out for is the distance r. notice that it is not equal for each pair of charges.

This should give you the right answer assuming that you account for the distances correcly. Anothering to check on is your units. did you use 10^-6C for $$\mu C$$ ?

3. Feb 26, 2005

### dextercioby

How come you didn't get the correct result in the second case...?You're adding negative #-s which are scalars.Are u sure you're not screwing up arithmetics...?

Daniel.

4. Feb 26, 2005

### chattkis3

Ok I got it, thanks a lot. I was using the wrong distance for q1q4