1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Homework Helper

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


    User Avatar
    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:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook