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Homework Help: Electric field of discrete point charges

  1. Jun 27, 2004 #1
    Five equal negative point charges (-q) are placed symmetrically around a circle of radius R. Calculate the electric field at the center of the circle.

    My Answer:

    Each one will be place 72 degrees from the other one (360/5),
    Each field line is directed toward the center (charges are negative),
    I had;

    E_x = (kq/R^2)[-cos 72 + cos 36 + cos 36 - cos 72]
    = kq/R^2

    E_y = (kq/R^2)[- sin 72 - sin 36 + sin 36 - sin 72]
    = 0 N/C

    My book says that both E_x and E_y are zero, What did I do that was wrong?

  2. jcsd
  3. Jun 27, 2004 #2
    why do you only have 4 charges in your calculations? plus, i don't think you are calculating the distance squared properly, think about it some more

    anyways, i would just make a symmetry argument. hey, works for MIT
  4. Jun 27, 2004 #3
    Remember that the distance between the centre and any point is R. However, the distance in the x and y direction is not R.
  5. Jun 28, 2004 #4
    Thanks! I missed the one on the x-axis. This gives me -kq/R^2 which will cancel the other equal but positive term.

    Parth Dave,
    x- and y- have nothing to do with the problem as I am asked to find E at the center. Thank you for caring to answer me.

    Last edited: Jun 29, 2004
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