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Homework Help: Finding the electrical

  1. Jan 15, 2012 #1
    1. The problem statement, all variables and given/known data

    The electric potential difference due to a point charge is 4.8 V at a distance of 4.2 cm from the charge. What will be the electric potential energy of the system if a second charge of +6.0 μC is placed at that location?

    2. Relevant equations

    V = (EQ)/q
    V = kq/r
    ΔV = W/q

    3. The attempt at a solution

    I've tried all three equations but none of them give me the right answer.

    I try to find the charge of the first point

    step 1. 4.8 = kx/.042

    step 2. .042(4.8) = kx

    step 3 (.042(4.8))/k = x

    step 4 = 2.24 * 10^-11 (that seems like a ridiculously small number, most charges are 10^-6. Adding 6*10^-6 will do nothing to the number, so I must be doing something wrong. I just can't figure out what. Even if I do get the right number and I add the charge I can't figure out what equation to use next.
  2. jcsd
  3. Jan 15, 2012 #2
    Write down formula of potential energy and potential side by side,
    then you'll know a much simpler method, and btw your calculation are fine, it doesn't mater how small a charge is

    smallest possible free charge is e = 1.6 * 10-19
  4. Jan 15, 2012 #3


    User Avatar

    Staff: Mentor

    Make sure that you know the difference between the potential at a location and potential energy of the system as a whole. The problem statement suggests that the second charge is being brought to the location where the potential due to the first charge is 4.8V, 4.2 cm from the first charge; it's not being added to the first charge.
    Last edited: Jan 15, 2012
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