# Two point charges

1. Sep 21, 2010

### Mikesgto

1. The problem statement, all variables and given/known data
A charge of 0.611 nC is placed at the origin. Another charge of 0.383 nC is placed at x1 = 8.1 cm on the x-axis.
At which point on the x-axis does this potential have a minimum?

2. Relevant equations
U=(kq1q2)/r

3. The attempt at a solution
I really have no idea how to even start this problem. I've been thinking about it for a couple hours now. Any help would be appreciated.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

Last edited: Sep 21, 2010
2. Sep 21, 2010

### rl.bhat

Post the complete question.

3. Sep 21, 2010

### Mikesgto

I edited the post. My apologies.

4. Sep 21, 2010

### Mikesgto

And also once I messed up enough the hint was that
"A necessary condition for the potential to have a minimum is that its derivative is 0."

But the derivative of what? The only thing I can think of is the V(r)=kQ/r

5. Sep 21, 2010

### rl.bhat

Potential at a point distance x from q1 is given by

V = k*q1/x + k*q2/(d-x) where d is the distance between the charges.

To find the minimum potential between the charge, find dV/dx and equate it to zero.
And find x.

6. Sep 21, 2010

### Mikesgto

Thanks a lot! It was kind of a messy derivative but I am so thankful you helped me out. :)