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: Finding electric force between two particles

  1. Sep 11, 2010 #1
    1. The problem statement, all variables and given/known data
    Two particles with electric charges Q and −3Q are separated by a distance of 2.5 m.
    If Q = 4.9 C, what is the electric force between the two particles?

    2. Relevant equations

    Coloumb's equation: F=k*q1*q2/r^2

    3. The attempt at a solution
    I just tried to plug in the variables into the above equation.
    so I got F= (9e9)*(4.9)*(3*4.9)/((2.5/2)^2)
    F= 4.15e11
    Apparently, that is not the correct answer, but I don't know what I did wrong!
    I also tried to divide that number by 2 to get F= 2.07e11 which was also not right.

    What am I doing wrong?
  2. jcsd
  3. Sep 11, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    If the separation between the charges is r = 2.5 m, then r^2 = 2.5^2, not (2.5/2)^2. I am not sure what all those divisions by 2 are meant to accomplish.
  4. Sep 11, 2010 #3


    User Avatar
    Homework Helper
    Gold Member

    Why are you dividing the distance by 2? Consider your signs, too.

    [edit: beaten by kuruman. not happy]
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook