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Electric dipole moment problem

  1. Sep 9, 2015 #1
    1. The problem statement, all variables and given/known data
    find the electric dipole moment of the system shown in figure
    pic.png

    2. Relevant equations
    In the simple case of two point charges, one with charge +q and the other one with charge −q, the electric dipole moment p is:

    where d is the displacement vector pointing from the negative charge to the positive charge.
    3. The attempt at a solution
    I thought dipole moment would be zero because there is no dipole in the system.+2q and -q can not make a dipole because for a dipole we need two equal and opposite charges which is not the case here.+2q and +q can not make a dipole either.and +q and +q they don't fulfil the criteria so if no dipole "dipole moment "should be zero.other confusion that i am facing is whether this system is in equilibrium .or the charges can move due to repulsion/attraction.
     
  2. jcsd
  3. Sep 9, 2015 #2

    ehild

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    The charges are supposed to be fixed, they can not move.
    You can consider the array as if there were two -q charges at the bottom left corner, that is there are two dipoles. The dipole of the array is the resultant.
    A charge distribution can be characterized by its net charge and momenta. It can have a net dipole momentum, of quadrupole momentum and so on.


    dipolethree.JPG
     
  4. Sep 9, 2015 #3
    Thanks.My answer came out to be qa√2
     
  5. Sep 9, 2015 #4

    ehild

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    That is the magnitude of the dipole moment. But it is a vector. Write it as a vector either with its components, or give the direction.
     
  6. Sep 9, 2015 #5
    it would be in between the two vectors at 45 degrees
     
  7. Sep 9, 2015 #6

    ehild

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    It is correct.
     
  8. Sep 9, 2015 #7
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