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Use Coulomb's Law?

  1. Jun 22, 2009 #1
    1. The problem statement, all variables and given/known data
    Consider the two parallel electric dipoles shown.


    + (Q) + (Q) | (Distance=d/2)
    ------------------------------------------x------
    - (-Q) - (-Q) | (Distance=d/2)
    |-------Distance=X-----------|

    a) Do the two dipoles attract or repel from each other
    b) Let x>>d. Find an approximate expression for the force between the two dipoles. The expression is in terms of Q,d, and x.
    C) sketch the graph of F vs. x.
    2. Relevant equations
    Coulombs's law
    F=k(q1q2/r2

    3. The attempt at a solution
    I an completely new to Coulomb's law and dipoles so i don't know how to start.

    a) The answer is they repel but i dont get why. I know the top/bottom two charges will repel but wont they attract each other diagonally?
    b)Have not attempted. Don't know how to start.
    c) Stuck on b. didnt get to this yet
     
  2. jcsd
  3. Jun 22, 2009 #2

    diazona

    User Avatar
    Homework Helper

    I'm guessing your diagram was supposed to look like this?
    Code (Text):

       + (Q)                        + (Q)
       |                            |      (Distance=d/2)
     --x----------------------------x-----------
       |                            |      (Distance=d/2)
       - (-Q)                       - (-Q)
       |-------Distance=x-----------|
    Forget about the fact that they're dipoles. Just think about it as a set of four charges at the corners of a rectangle, and figure out the force on each charge. You don't need the exact magnitude and direction (at least not for part a), but you can easily figure out whether there's more force acting to the left or the right on any particular charge.
     
    Last edited: Jun 22, 2009
  4. Jun 22, 2009 #3
    yes, it should look like that. thanks.

    i dont know how to find the forces.
    would it be like this? assuming the Q's are: Q1(top left), Q2(top right), -Q3(bottom left), -Q4(bottom right)

    FQ1=[k(Q1Q2)/x2]+[k(Q1-Q3)/d2]+[k(Q1-Q4)/(x2+d2)]?

    and i do this for Q2, -Q3, -Q4?
     
  5. Jun 22, 2009 #4

    ideasrule

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    Homework Helper

    No equations needed for part a. Just think: the charge at the upper-left is repelled by one of the charges on the right and attracted by another. Which force is greater? Ditto for the charge at the lower-left.
     
    Last edited: Jun 22, 2009
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