Homework Help: Point charges and potential electric energy

1. Jan 21, 2010

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.

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

2. Relevant 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

3. 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?

2. Jan 21, 2010

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.

3. Jan 22, 2010

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.

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