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Homework Help: Potential of a dipole at P?

  1. May 18, 2010 #1
    I know that the potential at any point P is the sum of the two potentials of a dipole. But we make two assumptions to get an equation:

    1 - the distance from the dipole to P is very large relative to the dipole distance (d)
    2 - we also make the assumption that r2 - r1 = dcosθ

    r2 and r1 is the distance from the dipole charges to P - how do we get cosine?

    I don't understand the second assumption. Any help will be nice
  2. jcsd
  3. May 18, 2010 #2
    You need to explain to us what r2 and r1 is in this context. I don't have a picture in front of me so I have no idea where those vectors are pointing to.
  4. May 18, 2010 #3


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    Staff Emeritus
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    1. Draw the picture - label r1, r2 and θ.

    2. See if you can't find a right triangle in there which you can use ... preferably one which has a side equal to r1-r2. Make any approximations that work in the limit that r1 >> d.

    3. Use this triangle to derive the above relation.
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