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Homework Help: Calculating the electric field

  1. Oct 5, 2007 #1
    1. The problem statement, all variables and given/known data

    What are the strength and direction of the electric field at the position indicated by the dot in the figure?


    2. Relevant equations

    Edipole = ~ [1/(4πε0)] * [2p/r3 ]
    on the axis of an electric dipole

    Edipole = ~ [-1/(4πε0)] * [p/r3 ]
    in the plane perpendicular to an electric dipole

    3. The attempt at a solution

    Which equation should I use??

    Last edited: Oct 5, 2007
  2. jcsd
  3. Oct 5, 2007 #2


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    Science Advisor

    Neither. Your equations only apply if the two charges in the dipole are much closer together than the distance r from the dipole.
  4. Oct 6, 2007 #3
    what formula would i use?
  5. Oct 6, 2007 #4
    my textbook uses these forumulas...so i tried the problem using them

    so here is my process:

    p = qs


    1/(4πε0) = 9*109


    plugging everything into the second equation {[-1/(4πε0)] * [p/r3 ]}, i get -7200, but this is incorrect
  6. Oct 6, 2007 #5


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    Homework Helper

    Why not just use coulomb's law, and superimpose the two electric fields?
  7. Oct 6, 2007 #6
    good idea..this was my original approach, which didn't work for some reason


    using the two charges:


    and their respective distances:

    r2=0.01250.5 (square root)

    and doing all calculations, and then adding the two charges (3600 and -720) gives me 2880, which is incorrect...
  8. Oct 7, 2007 #7
    lol so yea....umm...any help?
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