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Homework Help: Coulomb's Force

  1. Jul 26, 2012 #1
    1. The problem statement, all variables and given/known data
    4 identical charges each equal to Q are placed at the 4 vertices of a regular triangular pyramid of each side equal to 'a'. Find the net electrostatic force on any one charge.

    2. Relevant equations

    F = kQ^2/a^2

    3. The attempt at a solution

    find the force due to each of the charges, resolve it into x- and y- components and hence add them..but this way i could not get the correct answer.
  2. jcsd
  3. Jul 26, 2012 #2
    A problem I used to have is visualizing how the forces would interact. Where I believe you are going wrong is you are trying to make a 3d problem a 2d problem. When dealing with an regular triangular pyramid you'll find that due to symmetry and your charges all being the same most components cancel out.

    You should have three forces F1=F2=F3.

    You should be using the interior angle, the angle that the edge makes with the base plane. ( most people forget that its a 3d shape and find the angle between the edges.

    Hint: If you imagine the pyramid on your desk the combined forces on the top vertex should be pointing straight up out of your desk.

    Hope this helps.

    Last edited: Jul 26, 2012
  4. Jul 26, 2012 #3


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    Welcome to PF.

    Can you show the work you did, so people can see how far you got or where you might have gone wrong? Just saying, basically, "I tried this approach but it didn't work" doesn't really give a good basis for others to provide help. (For all we know, your answer is correct and the "correct" answer is wrong. Or maybe your answer is pretty close to the correct answer, and a reasonable grader would actually give you full credit for your solution. Without seeing your work and your solution, we have no way to tell.)
  5. Jul 27, 2012 #4
    Being new, I responded before reading the rules (which states that you must show your work before anyone can help), my apologies.
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