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Electrostatics, ONE TEENSY little problemo! please HELPPPPP thanks

  1. Mar 2, 2008 #1
    [SOLVED] electrostatics, ONE TEENSY little problemo! please HELPPPPP thanks!!!!

    1. The problem statement, all variables and given/known data

    Two charges of q1 = 1.3 µC and q2 = -2.6 µC are d = 0.60 m apart at two vertices of an equilateral triangle as in Figure P16.56.



    (d) What is the work required to move a 3.2 µC charge from infinity to point P?


    2. Relevant equations

    kq1q2 / r = Ue

    3. The attempt at a solution

    i tried 0 and used 19,478.33 V as voltage, but i dont really understand how to do this problem, like you dont need to tell me the answer, i just want to be able to understand how to find something when the distance is infinity.
     
  2. jcsd
  3. Mar 2, 2008 #2

    Doc Al

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    How does the electric potential energy change as the particle goes from infinity to its final location? (Consider the charges in pairs.)
     
  4. Mar 2, 2008 #3
    i don't think i understand what you are saying?
     
  5. Mar 2, 2008 #4

    Doc Al

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    What's the electric potential energy of the system when the 3.2 µC charge is in its initial position? (Just consider the charge pairs that include that 3.2 µC charge, plug into your formula, and add up the energies.)

    Then figure the electric potential energy when the charge is at infinity. (Same formula.)

    The work required will equal the change in potential energy.
     
  6. Mar 2, 2008 #5
    okay so do i do kq1q2 / r?
     
  7. Mar 2, 2008 #6

    Doc Al

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    Yes. That's the potential energy between two charges separated by a distance r.
     
  8. Mar 2, 2008 #7
    are the q's the two charges in the problem of 3.2e-6?
     
  9. Mar 2, 2008 #8

    Doc Al

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    Call the charges q1, q2, and q3. You'll have PE between: q1 & q2, q1 & q3, and q2 & q3. (Note that the PE between q1 & q2 doesn't change, so you can skip that one.)

    So you have two pairs that you need to use to calculate the total electric PE.
     
  10. Mar 2, 2008 #9
    for q1 & q3
     
    Last edited: Mar 2, 2008
  11. Mar 2, 2008 #10
    for q2 & q3


    IS THAT RIGHT?
     
    Last edited: Mar 2, 2008
  12. Mar 2, 2008 #11

    Doc Al

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    Looks pretty good to me. (Don't forget the units.) Add them up to get a total.

    What's the PE when q3 is at infinity?
     
  13. Mar 2, 2008 #12
    so i just add up: q1andq3 + q2andq3
     
    Last edited: Mar 2, 2008
  14. Mar 2, 2008 #13
    or do i need to add q1andq2 to that also?
     
  15. Mar 2, 2008 #14

    Doc Al

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    Yep. (Don't forget units.)
     
  16. Mar 2, 2008 #15
    okay thanks. so then how would i find the work to move the charge from infinity to point P?
     
  17. Mar 2, 2008 #16

    Doc Al

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    You could add the PE from q1&q2 to get the total. But since all we care about is the change in PE, that part will not count anyway. (q3 is the only charge that's being moved.)
     
  18. Mar 2, 2008 #17

    Doc Al

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    Find the PE when q3 is at infinity. The work needed will equal the change in PE.
     
  19. Mar 2, 2008 #18
    but i don't get what the R for teh equation would be for q3
     
    Last edited: Mar 2, 2008
  20. Mar 2, 2008 #19

    Doc Al

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    Hint: q3 is infinitely far away. So what happens to the PE?
     
  21. Mar 2, 2008 #20
    the PE becomes 0?
     
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