# Calculating Electric Potential at a Point

1. Oct 31, 2012

### swrs92

I'm fairly certain I'm doing this problem right.. But my homework assignment program keeps telling me it's incorrect..

1. The problem statement, all variables and given/known data
The drawing shows six point charges arranged in a rectangle. The value of q is 6.78 μC, and the distance d is 0.15 m. Find the total electric potential at location P, which is at the center of the rectangle.

http://imgur.com/YsSCB
Associated picture

2. Relevant equations

Electric Potential = Ʃ(kq/r)

3. The attempt at a solution

So basically, since the +5, -5, +3, -3 charges cancel out, i ignore them. I pretty much plug in (2*7q)/r.

I end up with 5.09*10^6 V.

I don't know what I'm doing wrong.. Can you please let me know whether or not this is the correct answer based on the numbers I provided, and if not, suggestions as to where I went wrong.

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

2. Relevant equations

3. The attempt at a solution

2. Oct 31, 2012

### SammyS

Staff Emeritus
What did you use for r?

3. Oct 31, 2012

### swrs92

Using the Pythagorean (spelling?) theorem, I got 0.1677m (sqrt(0.15^2 + 0.075^2))

4. Nov 1, 2012

Any insight?

5. Nov 1, 2012

### haruspex

Your calculations all look correct to me. But I'm no electrostatics expert, so I might be missing something wrt units. Millions of volts seems like a big answer, but maybe a microC is a big charge.

6. Nov 1, 2012

### swrs92

Thanks!

Like I said, I'm fairly certain that the issue is with the question as opposed to my work. I tried googling a similar problem online, I used their numbers with my reasonings and I got the right answer

7. Nov 1, 2012

### SammyS

Staff Emeritus
The method looks good, and I got the same answer.

8. Nov 1, 2012

### swrs92

That's good to hear I guess ahah, thanks for your help guys