# Homework Help: Work expended moving a charge

1. Jan 19, 2012

### kraigandrews

1. The problem statement, all variables and given/known data
q3---a-----|
| |
| b
| |
q1---------q2

The figure shows three charges q1, q2 and q3 situated at corners of a rectangle of sides a = 15.0 cm and b = 6.0 cm.
For q1 = 5.90 μC, q2 = -5.90 μC, and q3 = 1.60 μC find the electric potential at the center of the rectangle.
V_center = 1.78×105 V

Continuing with the figure above, how much of the electric energy of the system would be expended in moving q3 to infinity while q1 and q2 remain in their positions?

Im at a loss as how to do this part, I have tried to find the field at the center with q3 no longer there and everytime it works out to be 0, so I am just not sure what to do here.

2. Relevant equations
V = kq/r
W = Vq

3. The attempt at a solution

2. Jan 19, 2012

### Staff: Mentor

What's the electric potential energy between a pair of charges?

3. Jan 19, 2012

### kraigandrews

using V=kq1/a + kq2/a = 0... or at least I think.

4. Jan 19, 2012

### Staff: Mentor

Don't mix up potential with potential energy.

See: Electric Potential Energy

5. Jan 19, 2012

### kraigandrews

ok so U = kq1q2/a... correct? Or should I use the center of the rectangle as the radius?

6. Jan 19, 2012

### Staff: Mentor

You should use the distance between the charges. What's the total PE of the system? How does that change when q3 is moved to infinity?