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Homework Help: Electrostatic Potential Energy

  1. May 18, 2010 #1
    1. The problem statement, all variables and given/known data

    How much work is required to exchange the positions of q2 and q3?

    q2 q3


    q1 and q2 are separated by a distance of 4.00 cm and q2 and q3 are separated by a distance of 2.00cm

    The charge on q1 is 5.00nC, q2 is -5.00nC and q3 is 10.0nC

    2. Relevant equations

    U=(K(q2)(q3))/(r23) (#1)
    U=qV (#2)

    3. The attempt at a solution

    Since the energy required to hold these charges at their positions is equation 1, my guess to simply found the energy holding the charges and that would be the energy required to move them... Guess I'm wrong... :blushing:

    Any help is appreciated...
  2. jcsd
  3. May 18, 2010 #2
    Can you help us picture the charge configuration? I'm guessing they're all lying on the same line but can you clarify this for me. How are they oriented with respect to eachother?
  4. May 18, 2010 #3
    Work is just the change in potential energy. So just find the difference in the final and initial total potential energies.

    So write down the initial total potential energy and the final total potential energy.
  5. May 18, 2010 #4

    They're positioned just like in my first post: q2 directly over q1 and q3 directly across q2.

    Thx for your input,

    @nickjer: Aren't both potential energies equal? I mean at the end the only difference is that they switched places...
  6. May 18, 2010 #5
    If the concept of moving the charges around in an existing field is confusing you, try moving them to infinity and then back into their new positions and consider the work you're doing.
  7. May 18, 2010 #6
    quebecois22 the distance between charges q1 and q2 will change, as well as q1 and q3. So the total potential energy will change.
  8. May 18, 2010 #7
    I am completely lost...

    I don't know how to take q1 into account... how does it behave on the two charges?
  9. May 18, 2010 #8
    The total potential energy of 3 particles is just [itex]U_{12} + U_{23} + U_{13}[/itex]. You can probably find that in your book.
  10. May 18, 2010 #9
    Ah so the final and initial potential energies will vary according to the distances between them...
    So delta U is the energy required to shift q2 & q3 places?
  11. May 18, 2010 #10
    You want to find the different in total potential energy when you switch q2 and q3. It shouldn't be 0, unless q2=q3 or the distance between q1 and q2 is the same as between q1 and q3.
  12. May 18, 2010 #11
    Yeah it's not so the difference won't yield zero...
    So this difference in potential energies is the energy required to shift those two charges?
  13. May 18, 2010 #12
    Yes, since this is a conservative force.
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