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Electrostatics question

  1. Dec 13, 2003 #1
    Wow, it's been a long time since I've done any physics, but my sister asked me for help on a question and I am stumped.

    The problem is:
    What I remember is since the two charges have different signs, the third charge must be placed outside of the other two. It cannot lie in-between the other two.

    Other than that... I'm drawing a blank. Any help / helpful hints would be much appreciated!
  2. jcsd
  3. Dec 14, 2003 #2
    Great! That's correct physical thinking, in my opinion. Another idea is that, as a consequence of symmetry, the desired position must lie on the line connecting the 2 charges. Now, since the force decreases with distance, the desired position must be further away from the big charge than fom the small charge. To solve, remember the 'inverse square law'...
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