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Homework Help: Point charges and potential electric energy

  1. Jan 21, 2010 #1
    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
    k=8.988810^9 (Nm^2)/(C^2)
    Volt = N/C
    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. jcsd
  3. Jan 21, 2010 #2


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    Science Advisor
    Homework Helper
    Gold Member

    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.
  4. Jan 22, 2010 #3
    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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