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Homework Help: Coulombs Law, 4 seperate charges.

  1. Sep 16, 2009 #1
    1. The problem statement, all variables and given/known data
    the particles have charges q1 = -q2 = 296 nC and q3 = -q4 = 198 nC, and distance between each particle is 5.5cm.

    2. Relevant equations
    Find the net electrostatic force on particle 3
    ::Answer is required in vector form <x,y,z>::

    3. The attempt at a solution
    Using Coulombs law to find the fnet of each particle.
    eq1 = [(9e9) * 1.98e-7 * 2.96e-7] / .055^2 = force on particle 3 by q2 in x and y direction AND q1 in y direction
    eq2 = [(9e9) * 1.98e-7 * 1.98e-7] / .055^2 = force on particle 3 by q4 in x direction

    Then adding the forces together, applying the fact that q 1 and q3 repel each other (negative force)

    getting <eq1+eq2, eq2-eq1>
  2. jcsd
  3. Sep 16, 2009 #2


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    The net electrostatic force in q3 is the sum of three forces, one from each of the other particles. First draw these three forces in your diagram. Second, calculate their magnitudes. Third, add them as vectors, all the x-components together (which you must find) and all the y-components together (which you must also find).
  4. Sep 17, 2009 #3
    That is what I tried to do in my solution, but it came out being wrong. I believe that I am confused on how to find the forces for each x and y component separately.
  5. Sep 17, 2009 #4


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    Draw three arrows with their tails on q3 in the appropriate directions. Break each arrow into x and y components assuming that q3 is at the origin of your axes. When you are ready, show us what you have done so that we can correct it if necessary.
  6. Sep 17, 2009 #5
    I figured it out.

    For anyone wanting to know, to find the x and y components of q2 on q3, you need to take the force and multiply it by cos45 (for x) and sin45(for y) before adding the vectors.

    Thank you kuruman
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