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Homework Help: Calculate the component of the electric field

  1. Dec 21, 2008 #1
    http://img367.imageshack.us/img367/5459/69671206ju9.th.jpg [Broken]

    See image above. Point charges A, of +2.0nC, and B, of –3.0 nC, are 200mm apart in a vacuum, as shown by the figure. The point P is 120mm from A and 160mm from B.

    (i) Calculate the component of the electric field at P in the direction AP.
    (ii) Calculate the component of the electric field at P in the direction PB.

    The markscheme uses E= kQ/(r^2)

    where k = 1/4*pi*epsilon

    However, for part (i), uses only the 2nC charge, while uses only the -3nC charge for part ii - can someone please explain why this is the case.

    Thanks
     
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Dec 21, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    In what direction is the field at P due to A? Due to B?
     
  4. Dec 21, 2008 #3
    ahh yes, i understand now, thanks
     
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