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**Electrostatics -- point charges and work**

## Homework Statement

A 3.0 μC point charge and a 9.0 μC point charge are initially infinitely far apart. How much work

does it take to bring the 3.0 μC point charge to x = 3.0 mm, y = 0.0 mm and the 9.0 μC point charge to x = -3.0 mm, y = 0.0 mm? (The value of k is 9.0 × 109 N∙m2/C2.)

q1 = 3.0 uC, q2 = 9.0 uC, k = 9.0 × 109 N∙m2/C2

## Homework Equations

F = k(q1)(q2)/r^2

## The Attempt at a Solution

I know the answer is 41 J (this is just a practice problem). I'm assuming that the formula F = k(q1)(q2)/r^2 is supposed to be used because I have values for most variables, but I'm not sure what the radius would be. If one point charge is at x = 3mm and the other point charge is at x = -3mm, then is the radius 6 mm (I got this value by counting along the x-axis)? Also, how would the answer end up being in Joules?