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Homework Help: Two point charges

  1. Sep 21, 2010 #1
    1. The problem statement, all variables and given/known data
    A charge of 0.611 nC is placed at the origin. Another charge of 0.383 nC is placed at x1 = 8.1 cm on the x-axis.
    At which point on the x-axis does this potential have a minimum?

    2. Relevant equations
    U=(kq1q2)/r


    3. The attempt at a solution
    I really have no idea how to even start this problem. I've been thinking about it for a couple hours now. Any help would be appreciated.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
    Last edited: Sep 21, 2010
  2. jcsd
  3. Sep 21, 2010 #2

    rl.bhat

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    Homework Helper

    Post the complete question.
     
  4. Sep 21, 2010 #3
    I edited the post. My apologies.
     
  5. Sep 21, 2010 #4
    And also once I messed up enough the hint was that
    "A necessary condition for the potential to have a minimum is that its derivative is 0."

    But the derivative of what? The only thing I can think of is the V(r)=kQ/r
     
  6. Sep 21, 2010 #5

    rl.bhat

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    Homework Helper

    Potential at a point distance x from q1 is given by

    V = k*q1/x + k*q2/(d-x) where d is the distance between the charges.

    To find the minimum potential between the charge, find dV/dx and equate it to zero.
    And find x.
     
  7. Sep 21, 2010 #6
    Thanks a lot! It was kind of a messy derivative but I am so thankful you helped me out. :)
     
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