Point charges and potential electric energy

[Torquescrew]

This is probably child's play for most of you, but a guy like me struggles to understand basic concepts. A kick in the pants to set me off in the right direction would be greatly appreciated.

Homework Statement

How far must the point charges q_1 = 7.60 microCoulombs and q_2 = -24.0 microCoulombs be separated for the electric potential energy of the system to be -110 J?

Homework Equations

E=F/q
F=k(q1*q2)/(r^2)
k=8.988810^9 (Nm^2)/(C^2)
Volt = N/C
W=Fr
r= distance in this case

The Attempt at a Solution

Honestly, I'm not quite sure where to start. Not sure which formula I'm supposed to use. I gave it a try anyway, though. Coulomb's Law seems perfectly reasonable until I realize that I don't have a value for distance. All the same, I multiplied 8.988E9 by 7.6E-6 and 24E-6 and assumed my force would equal 1.63941r^2 N.
Then, since work is measured in joules and so is potential energy, I tried to use the formula W=Force x distance (and I still don't know the distance), giving me something like 1.63941r^3 Joules.
And if I wasn't already lost, I notice that electric potential is supposed to be measured in volts. But since the question doesn't ask for volts, all that's left is for me to solve for r.
I try to solve for r by means of 1.63941r3=110, which gives me 4.06 (4.1) meters.
Since the question wants the answer in cm, I used 4.1x10^2 cm.

Obviously, I got the answer wrong.
It's no surprise, seeing as how I'm just sort of drowning in a vast ocean of numbers.

Can I get a metaphorical life preserver?

 

[kuruman]

None of the equations that you have listed are related to potential energy. Look in the chapter/section of your textbook that talks about potential energy and read carefully what is said. Then try again.

Other misconceptions that you have

1. Work is equal to force*distance only if the force does not change as the distance changes. This is not the case here.

2. Electric potential (measured in Volts) is usually represented by the symbol V and is not the same as potential energy (measured in Joules) that is usually represented by the symbol U.

 

[Torquescrew]

Actually, that was probably the best advice I could have got. Silly as it sounds, I managed to completely overlook that formula as I was writing my lecture notes.
Thanks for the advice.