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Homework Help: Calculating Electric Field (what am i doing wrong?)

  1. Sep 10, 2005 #1
    I believe this may be a problem with the website where I do my homework. Please check my solution and see if there is a mistake or if I am missing something. The following is the arrangement of point charges in the problem:

    8.26 micro C ---2.93 cm---> P1 ---1.59 cm ---> 4.94 micro C ---2.73 cm--->-1.23 micro C

    What is the electric field at point P1? Coulomb's constant is 8.98755 Nm^2/C^2.

    I convert all distances to meters which gives:
    r1 = 2.93 cm = .0293 m
    r2 = 1.59 cm = .0159 m
    r3 = 1.59 cm + 2.73 cm = 4.32 cm = .0432 m

    Since all points are on the X-axis only, I do not have to worry about any Y-axis components. I perform the following steps:

    E1 = (8.98755e+9 Nm^2/C^2)(8.26e-6 C)/.0293 m^2 = +86474115.0159 N/C
    E2 = (8.98755e+9 Nm^2/C^2)(4.94e-6 C)/.0159 m^2 = -175620018.987 N/C
    E3 = (8.98755e+9 Nm^2/C^2)(1.23e-6 C)/.0432m^2 = +5923507.4267 N/C

    E1-E2+E3 = -83222396.5444 N/C

    My homework website says the answer is wrong. We do not need to worry a obout significant figures in the answer. What errors can you see, or what am I missing.
  2. jcsd
  3. Sep 10, 2005 #2
    What units is it asking for the answer in? I'm doing similar homework and it asks for the answer in kN/C not N/C.
  4. Sep 10, 2005 #3
    Answer should be in N/C
  5. Sep 10, 2005 #4
    OK, the next thing I'd check is the direction of your electric fields. Remember that for negative charge points, the fields go toward the points and for positive charge points, the fields go away from the points.
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