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Homework Help: Voltage and point charge

  1. Aug 19, 2008 #1
    1. The problem statement, all variables and given/known data
    There is an equilateral triangle with one point charge at each vertex. The point charges have charges of +Q, +2Q, and -4Q. The length of one side of the triangle is L. Determine an expression in simplest form for the voltage at a point halfway between the +Q and +2Q point charges.

    2. Relevant equations

    3. The attempt at a solution
    Going off of Vnet=V1+V2+V3:
    Vnet= (kQ/r) + (k2Q/r) - (k4Q/r)
    I think once substituting the values for L in place of r, you get:
    Vnet= (kQ/(L/2)) + (k2Q/(L/2)) - (k4Q/Lcos30)
    Am I heading in the right direction?
  2. jcsd
  3. Aug 19, 2008 #2


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    Yes, that looks good.
  4. Aug 19, 2008 #3
    I think I made a mistake, I think it should be:
    Vnet= (kQ/(L/2)) + (k2Q/(L/2)) - (k4Q/(L/cos 30))

    This can then be simplified:

    Am I on the right track?
  5. Aug 19, 2008 #4


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    The distance from the far vertex is L(cos 30). I think you were right the first time.
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