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: Two Point Charges Net Electric Field

  1. Feb 1, 2010 #1
    1. The problem statement, all variables and given/known data

    Two point charges lie on the x axis. A charge of -2.5 µC is at the origin, and a charge of +8.5 µC is at x = 10.0 cm.

    What is the net electrical field at points x = +4 and at x = -4?

    2. Relevant equations

    E = kq/r^2

    3. The attempt at a solution

    For x = +4...

    E= 9x10^9(8.5x10^-6)/.06^2 = 21250000
    E= 9x10^9(-2.5x10^-6)/.04^2 = -14062500

    Net E = 7187500 (which is wrong)

    How do I solve this? Thanks.
  2. jcsd
  3. Feb 1, 2010 #2
    use absolute values for charges and then add/subtract them up qualitatively.

    the -2.5uC will make the E field LARGER at point x=+4 so you would add it instead of subtract.
  4. Feb 1, 2010 #3
    That didn't work.
  5. Feb 1, 2010 #4
    I'm pretty sure it should work. What's the correct answer for both positions?
  6. Feb 1, 2010 #5
    It doesn't give me the right answer until I submit the right answer. I entered 35312500 for x = +4 and it says that is incorrect. (I submit the answers online)
  7. Feb 1, 2010 #6
    the answer is 3.5*10^7 after sig figs.
  8. Feb 1, 2010 #7
    It says that is wrong too :yuck:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook