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Homework Help: Calculating the Potential Energy of a 3-point charge config.

  1. Feb 19, 2017 #1
    1. The problem statement, all variables and given/known data
    Charge q1= 8.5 nC is located at the coordinate system origin (0,0), while charge q2= -4.46 nC is located at (a,0), where a= 1.5 m. The point P has coordinates (a,b), where b=0.95 m. A third charge q3= 16.5 nC will b added later. It is a 3 part question, first asking for the Electric Potential at point P, then asking for the work done to bring q3 from infinity to point P, and then finding the total potential energy of the final configuration of three charges.

    2. Relevant equations
    U=q*V= -W
    Utotal= U1 + U1,2 + U1-2,3

    3. The attempt at a solution
    The question is in three part. The first part I found the electric potential at point P, which is 0.83314271 V. The second part I found the work (W) it took to move q3 to point P, which is 1.3746*10-8 J. I understand that what we are doing is finding the work it takes to move the 3 charges to the point and the the Potential Energy of the first charge is 0 J.

    I know the answer, which is -28.9 J, but I don't know how to get there.
    Last edited: Feb 19, 2017
  2. jcsd
  3. Feb 19, 2017 #2


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    Staff: Mentor

    Hi DaveedL,

    Welcome to Physics Forums!

    Can you please provide the complete problem statement? The Problem statement section of your post doesn't say anything about what is to be found or calculated. Is q2 really exactly one nC?

    You should show details of any calculations that you've done. If something looks "off" with any answers you quote we have no way to tell what went wrong if you don't show your work. For example, you say that you found a potential of about 0.83V at point P, but looking at just the potential there due to the first charge alone it should be greater than 40 V. Can't fix what we can't see.
  4. Feb 19, 2017 #3

    Just updated it! Sorry about that. But yes the V at Point P to my knowledge should be the combination of V from q1 and V from q2.
  5. Feb 19, 2017 #4


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    Staff: Mentor


    But the problem statement is still incomplete. Nowhere in your problem statement do you explain what is to be calculated. Helpers should be able to work the entire problem from the information given in the problem statement.
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