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Homework Help: Griffiths page 150

  1. Feb 24, 2008 #1
    [SOLVED] Griffiths page 150

    1. The problem statement, all variables and given/known data
    Please stop reading unless you have Griffith's E and M book.

    On this page, Griffith's start talking about "pure" and "physical" dipoles. Can someone define what these terms mean?

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Feb 24, 2008 #2


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    Isn't it explained pretty clearly in the same page?

    Where exactly are you having difficulty?
    Last edited: Feb 24, 2008
  4. Feb 24, 2008 #3

    D H

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    A physical dipole comprises a pair of equal but opposite charges [itex]q[/itex] separated by a vector [itex]2a\hat r[/itex]. The dipole moment is [itex]2aq\hat r[/itex]. By decreasing the separation distance but increasing the charge you can keep the dipole moment constant. A pure dipole has a zero separation distance but a non-zero dipole moment. Such a thing is not physically realizable.
  5. Feb 25, 2008 #4


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    What G means in Ex. 3.8 is that the point charges example is one physical configuration that has a dominant dipole moment. He seems to define a "pure" dipole as a configuration that has ONLY a dipole moment, and no higher moments. As he says, that point charges model is only a "pure" dipole in the limit -->0. The sphere with with cos charge distribution is a pure dipole because its potential for r>R is pure dipole.
    None of this is too clear in G because he does not discuss higher dipole moments in good detail. Some things are clearer in more advanced texts.
  6. Mar 18, 2008 #5
  7. Mar 18, 2008 #6
    DH and pam explained nce...

    You may try to get a copy of Corson and Lorrain for more rigorous treatment of higher terms.
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