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Homework Help: Coulomb's Law and electric fields

  1. Jan 21, 2009 #1
    1. The problem statement, all variables and given/known data

    three point charges are placed at the following points on the x-axis +2 uC at x=0, -3uC at x=40 cm, -5 uC at x=120 cm. find the force (a) on the -3 uC charge, (b) on the -5 uC charge

    ans. (a) -0.55N ; (b) 0.15 N

    uC=nano coulomb

    2. Relevant equations

    Fm=qvB ?



    3. The attempt at a solution

    I cannot seem to solve such a simple problem, i need sufficient help. I have the idea that it has something to do with minusing each Fe charge by eachother but im uncertain please need help with a solution
  2. jcsd
  3. Jan 21, 2009 #2
    You'll use the 3rd equation, and you will sum the forces. For instance, for the -3 nC charge, you will have to figure out the force created by the +2 nC Charge, then for the -5 nC charge. Once you get those two numbers, just add them together.
  4. Jan 21, 2009 #3
    but wait, try it for yourself, apparently you dont get the given answer of -0.55 N, what is the answer u thus get?
  5. Jan 21, 2009 #4
    Well, I didn't get -0.55 N exactly, but I did get -0.5479 N. Which is close enough. I came to this solution saying the uC is actually micro-coulombs. Usually nano-coulombs is denoted nC, and if you don't want to go into LaTex, microcoulombs is uC. This is because u is close enough to the Greek Letter mu.

    A few more hints on this problem.... Draw a free-body diagram. Then think about what "should" be happening, like which direction a force should go.
  6. Jan 22, 2009 #5
    got it
    now im trying part b
    thanks man
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