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Homework Help: Electric Potential Energy with Three Postive Charges

  1. Jun 7, 2012 #1
    1. The problem statement, all variables and given/known data
    You have 3 charges, all +2.5 μC arranged on a field at positions (-3.0 cm, 0), (3.0 cm, 0), and (0, 3.0 cm). I have already calculated the stored electric potential energy to be 3.85 J. The question is: If the third charge (0, 3.0 cm) is released from rest, what is its change in electric potential energy?

    2. Relevant equations

    3. The attempt at a solution
    I am thinking that since the charge is released, it will be pushed away into inifinity and therefore the change in electric potential energy will be the negative value of the electric potential energy stored when it is at rest, but I am admittedly having a hard time thinking about potential and kinetic energy in terms of charge. I am definitely looking for more of a theoretical explanation than an explanation of the steps of the math. I am capable of doing the math, but I'm not sure where to start.
    Thanks so much!
    Last edited: Jun 7, 2012
  2. jcsd
  3. Jun 8, 2012 #2
    Electrical potential energy for a charge at a given point is defined as the negative of work done in bringing the charge from infinity to that given point. So, once released, it does get pushed back to infinity where the potential energy is zero. So your reasoning is correct that the change is

    [tex]\Delta U = U_{final} - U_{initial}[/tex]


    [tex]\Delta U = -U_{initial}[/tex]
  4. Jun 8, 2012 #3
    Awesome, thank you so much, Infinitum!
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