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: Coloumbs Law when have 3 charges at unknown distances

  1. May 13, 2016 #1
    1. The problem statement, all variables and given/known data
    I have a question that I can't figure out.

    Question is
    "Two charges are fixed in location: charge q1 = +8e is located at the origin and charge q2 = -2e is located on the x-axis at x = L. At what point (other than infinitely away) can a proton (a unit positive charge e) be placed so that it has net zero force acting on it"

    2. Relevant equations
    F = (kq1q2)/r^2

    3. The attempt at a solution
    I have got that q1 is (8e x 1.602 x 10^-19C) = 1.282 x 10^-18C
    and q 2 is (-2e x 1.602 x 10^-19C) = 3.204 x 10^-19C
    k = 8.988 x 10^9Nm^2C^-2
    Am assuming that a unit positive charge is 1C (although did originally think it could possibly be 1.602 x 10^-19C, the size of one proton, would that be correct?)

    When pumping in these values into the formula, and rearranging the formula so F = 0N, this will obviously give a distance of 0 which would be incorrect, also not sure how the added proton fits in anywhere. Have spent a few hours trying to find another way but am a bit stuck. Thanks in advance for any help =)
  2. jcsd
  3. May 13, 2016 #2
    one should make out a force diagram and see /calculate the resultant of the two forces and make it zero.
  4. May 13, 2016 #3
    Thanks for your reply. How would I go about that if I dont have distances?
  5. May 13, 2016 #4
    Try and feel it :-
    if you put a +ve unit charge IN BETWEEN 8e and -2e, then it the 8e would cause repulsion and -2e would cause attraction, plot it and you would get that this is not possible.

    if you put a +ve unit charge on the line joining the two charges ( but not in b/w them ) then it is possible that you find two points on which the net force =0
  6. May 13, 2016 #5
    Arrrr yes! That would work perfectly, thanks for your time in helping =)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted