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Angle of electric field line heading to infinity

  1. Feb 11, 2016 #1
    1. The problem statement, all variables and given/known data
    Charges 2q and -q are located on the x axis at x=0 and x=a respectively.

    (a) Find the point on the x axis where the electric field is zero, and make a rough sketch of some field lines.

    (b) You should find that some of the field lines that start on the 2q charge end up on the -q charge, while others head off to infinity. Consider the field lines that form the cutoff between these two cases. At what angle (with respect to the x axis) do these lines leave the 2q charge?

    2. Relevant equations
    E = 1/(4*pi*e0) q^2/r^2

    3. The attempt at a solution

    I found the solution for part (a) to be x = a(2 + sqrt(2)), but part (b) has me stumped and I'm not really sure how to approach it. Any pointers in the right direction would be greatly appreciated.
  2. jcsd
  3. Feb 11, 2016 #2
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