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Direction of a dipole moment

  1. Nov 1, 2009 #1
    1. The problem statement, all variables and given/known data

    Can any one explain why the direction of the dipole moment of an electric dipole is always taken as "from -q to +q" but not "from +q to -q"? In fact when we draw the electric lines of force we are only drawing in such a way that they start from +q and terminate at -q.Then why this contradiction? What is the correct explanation for this convention?

    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Nov 1, 2009 #2

    Doc Al

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    It's just a convention.
    What does that have to do with the definition of dipole moment? There's no contradiction.
     
  4. Nov 1, 2009 #3

    tiny-tim

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    But why is the convention for a dipole the opposite to the other convention? :confused:
     
  5. Nov 1, 2009 #4

    Doc Al

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    Ah... now I understand the question. The "natural" definition of dipole moment (the first moment of the charge distribution) is:

    [tex]\vec{p} = q_1\vec{r}_1 + q_2\vec{r}_2[/tex]

    That will give the direction of the dipole moment as minus to plus.
     
  6. Nov 1, 2009 #5

    rl.bhat

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    When you keep a dipole in an electric field, it acquires the stable equilibrium position with positive charge toward the electric field. Potential energy for a dipole is given by
    U = - p.E
    It has minimum value = -pE at the stable equilibrium position. It is possible only when p is parallel to E, i.e. p is from -q to +q.
     
  7. Nov 1, 2009 #6

    Doc Al

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    But if you defined the dipole moment with the opposite convention, U = p.E. And the minimum value would be when p is anti-parallel to E. The physics wouldn't change. (Not that I'm suggesting one flout convention. :wink:)
     
  8. Nov 2, 2009 #7
    Will you please elaborate this point?
     
  9. Nov 2, 2009 #8

    Doc Al

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    I'll try. Let q1 = +q and q2 = -q, then:

    [tex]\vec{p} = q_1\vec{r}_1 + q_2\vec{r}_2 = q\vec{r}_1 - q\vec{r}_2 = q(\vec{r}_1 - \vec{r}_2)[/tex]

    The vectors r1 and r2 are the position vectors of +q and -q. Thus the vector r1 - r2 points from -q to +q.
     
  10. Nov 3, 2009 #9
    Thank you. Now it is clear.
     
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