# Homework Help: Electric Potential-finding unknown charge

1. Feb 16, 2010

### abspeers

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

The potential equals 7.52 V at the midpoint between two point charges that are 0.91 m apart. One of the charges is 1.11 x 10-9 C. Find the value of the other charge.

2. Relevant equations

V = k [sum] (q/r)

3. The attempt at a solution

7.52=k[(1.11 x 10-9 / 0.455m ) + ( x / 0.455m )]

This gives me -7.3x10^-10, which is incorrect, but I cannot think of any other way to approach this problem.

2. Feb 16, 2010

### kuruman

Your approach is correct and the numbers match the approach. How do you know it is incorrect? Do you know the correct answer?

3. Feb 16, 2010

### abspeers

I am entering the answers in Webassign and it marked my answer as incorrect. I think there must be a mistake because no one I've showed it to has gotten a different answer.

4. Feb 16, 2010

### kuruman

It is time to ask your teacher. You might do everybody a favor.