# Homework Help: Two Charges - Electric Potential Question

1. Jan 19, 2012

### pierretong

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

Two charges q = 4.0μ C are fixed in space a distance d = 4.0 cm apart, as shown in the figure. With V = 0 at infinity, what is the electric potential at point C?

You bring a third charge q = 4.0μC from infinity to C. How much work must you do?

What is the potential energy U of the three-charge configuration when the third charge is in place?

Diagram shows (let me use x-y coordinate terms to describe this). Charge q at the origin. Another charge q at (4 cm, 0 cm) and point C at (2 cm, 2 cm)

2. Relevant equations

Electric Potential = kQ/r

3. The attempt at a solution

That's the right equation for the first part correct? I get 1.27*10^4 for the electric potential for one of the charges on point C. When you double that, you get 2.5 * 10^4 V and that should be the answer? (online homework system says that it is wrong).
What am I doing wrong here?

Also I don't understand the second part. If you are moving a charge from infinity, how can you calculate the work? You don't know the distance?

2. Jan 19, 2012

### SammyS

Staff Emeritus
Hello pierretong. Welcome to PF !