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Homework Help: Potential Energy of Electric Charges

  1. Jan 26, 2010 #1
    Hello all, my reason for posting is to clarify a topic of electrostatics that I recently covered in physics. I turned in an assignment and my teacher marked an answer wrong and gave a strange explanation of how to solve it. Here is my attempt at the solution.

    1. The problem statement, all variables and given/known data
    A charge of +4nC is moved from infinity to a point 15cm from both a +8nC and +7nC charge (the result is an equilateral triangle). What is the potential energy needed to do this? q1=4nC q2=8nC q3=7nC r=0.15m k=8.99x10^9

    2. Relevant equations

    3. The attempt at a solution

    I first calculated the electric potential at the point 0.15m away from both the 7nC and 8nC charge.
    V=k(q2)/r+k(q3)/r=(8.99x10^9)/0.15 x (0.000000008+0.000000007)=+899 Volts

    I then found the energy needed to place the 4nC charge 0.15m away from the other charges by finding the electric potential energy of a 4nC charge at that point.
    EPE=(q1)(V)=(0.000000004)(899)=3.6x10^(-6) J

    My teacher said something about multiplying the force needed to set the charge in place by the distance it is displaced, which would make sense except that the force is variable and the distance is infinity. Any input is greatly appreciated.
  2. jcsd
  3. Jan 26, 2010 #2


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    Homework Helper

    "What is the potential energy needed" - was this the question really, not work needed instead? You are right, this work can be calculated from the change of electric potential multiplied by the charge.

    Just a note: use the normal form of numbers instead of writing out nine zeros ...

  4. Jan 27, 2010 #3
    No, she definitely wanted a numeric answer. This is actually an AP Physics B class, but our teacher really doesn't know what she is doing. I had to argue almost every question on a kinematics test once until she realized she had the wrong answer key for the problems. She has a PhD is something, but it's definitely not physics.
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