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Homework Help: Determin the magnitude of the net electric field of three point charges

  1. Dec 6, 2011 #1
    1. The problem statement, all variables and given/known data

    There are three point charges, q1 has a magnitude of 3.00mC (the sign is not known) and is 5.0cm away from P and 13.0 cm away from q2. P is 12.0cm away from q2 and the three points form a triangle. The net electric field at point P is entirely in the negative y-direction. The charge for q2 is not known.

    Determine the magnitude of the net electric field.

    2. Relevant equations

    3. The attempt at a solution
    I think you have to use the equation above and then add the 3 values together, but I don't know how to calculate E when I'm only given the value for q1 and none of the other points.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Dec 6, 2011 #2
    If you know the net electric field at p is only in one direction, the sum of the contributions in all other directions at point p must be zero. Try writing the total E-field in different directions at point p (sum the contributions from each charge) and set them equal to zero for all the cases but the y-dir. My guess is you will have a solvable system of equations from which you can get the charge values and the y-dir field at p.
  4. Dec 7, 2011 #3
    For part a) I had to: Sketch the correct force configuration and work out the signs for the
    charges q1 and q2. Do I use my answer for a) in this part?
  5. Dec 7, 2011 #4
    is the third charge at P?? Nothing is mentioned about the third charge? What three points form a triangle? If the problem is referring to point P, charge1 and charge 2 all this says is that they are not along a line. Where is charge 3?
  6. Dec 7, 2011 #5
    The question had a picture and P, q1 and q2 formed a triangle. Does the question only use q1 and q2? I'm not too sure what P is.
  7. Dec 7, 2011 #6
    The problem says three charges but only talks about two charges and point P. Electric field is defined as a force per unit charge but the problem isn't clear to me. Is there a unit charge at point P that we're supposed to assume? See if something is missing.
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