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Find the net force on charge

  1. Apr 25, 2012 #1
    1. The problem statement, all variables and given/known data
    13 equal charges are placed in the corners of a regular 13-sided polygon. What is the force on a test chrage at the center?


    2. Relevant equations
    principle of superposition
    symmetry

    3. The attempt at a solution
    I am not sure if the net force is 0 or not. Had the polygon been 12-sided the symmetry would be obvious, but I'm not sure in this case. Can anyone explain if the net force is 0 or not?
     
  2. jcsd
  3. Apr 25, 2012 #2

    tiny-tim

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    hi aaaa202! :smile:
    try it first for an equilateral triangle (3 equal charges) …

    is that 0 ?​

    then use the same method for 13 (you'll need a formula for summing a trig series)
     
  4. Apr 25, 2012 #3
    Did the calculations for a triangle as you said, where i fixed the coordinates such that one of the charges sat on the y-axis. From there it was easy to see that Fres = 0, and I think that this must hold for any rotation of the coordinate system. After all I think the length of a vector is more or less defined to be preserved on a rotation in the euclidean coordinate system. However, I'm not sure, so can you confirm this? :/
    And can you perhaps come up with a symmetry argument that makes it easy to see, that rotation of the coordinate system should not change the lFresl? :)
     
  5. Apr 25, 2012 #4

    tiny-tim

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    yes!

    presumably you did cos0 + cos2π/3 + cos4π/3 = 1 - 1/2 - 1/2 = 0 ?

    ok you now need to prove cos0 + cos2π/13 + cos4π/13 + … cos24π/13 = 0 :smile:
     
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