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Homework Help: Electric fields question

  1. Sep 19, 2008 #1
    "Three charges are placed at 100-m intervals along a horizontal line: a charge of –3.0 C
    on the left, 2.0 C in the middle, and 1.0 C on the right. What is the electric field E on
    the horizontal line halfway between the –3.0 C and 2.0 C charges?"

    When I do the calculation E = E1 - E2 - E3 with E1 the electric field produced on the point from the -3.0 C charge, I get 3.2*10^6 N/C:

    E = k[(3.0 C / (50 m)^2) - (2.0 C / (50 m)^2) - (1.0 C / (150 m)^2)] = 3.2*10^6 N/C.

    However, the answer key says it is -1.8*10^7 N/C. How do you get this?
  2. jcsd
  3. Sep 19, 2008 #2


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    Homework Helper

    This term should have a minus sign due to the value of the -3C charge.
  4. Sep 19, 2008 #3


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    Science Advisor
    Homework Helper

    Hi jaejoon89! :smile:

    Hint: if a test charge is positive, say, will each of the three charges move it to the left or the right? :wink:
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