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Electric field from two point charges and then some

  1. Mar 2, 2009 #1
    1. The problem statement, all variables and given/known data

    A point charge, q1, has a charge of 3.4x10^-9 C. A second point charge, q2, is place 2.1x10^-6 m to the right of q1; it has a charge of -10.2x10^-9C.

    A) What is the magnitude of the field 8 mm to the left of the centre of the two charges?

    B) You could also treat this collection of charges as a dipole added to a point charge. If you think of the charges this way, what would the charge on the point charge be?

    C) What would the field be, due to just that point charge, 8 mm to the left of the charge?

    2. Relevant equations


    3. The attempt at a solution

    A) I figured out part A to be -7.6x10^-3 N/C. I'm a little lost as to what this answer indicates about the direction of the electric field. Also, my answer is using 8x10^-3m for the r in both field calculations. Does it really matter if I consider the whole "from the centre of"?

    B) Confused me completely. I'm not sure what it's asking me to do.

    C) I'm assuming that once B is solved, I plug that value in for Q and will get another answer for the field?
  2. jcsd
  3. Mar 2, 2009 #2


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    Homework Helper

    I don't see how you could possibly get a magnitude without considering the directions!
    Did you add or subtract the two fields? Better begin with a diagram. Take note of whether the 8 mm or the 2.1x10^-6 m is larger when you mark the point where we are calculating the field. Work out the field due to each charge separately and then decide whether you are adding or subtracting. The field direction is away from the positive charge and toward the negative one.
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