1. Not finding help here? Sign up for a free 30min 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!

Time for two charges to meet?

  1. Nov 2, 2007 #1
    1. The problem statement, all variables and given/known data

    I just came up with this seemingly simple problem today, but am having the hardest time solving it:

    Two point charges of +Q and -Q lie a distance R from each other. How long will it take for them to meet?

    2. Relevant equations

    F = (q1*q2)/(4*pi*Eo*r^2) = ma
    a = dv/dt


    3. The attempt at a solution

    So the instantaneous force on both particles will be: (Q^2)/(4*pi*Eo*r^2) towards one another.

    The instantaneous acceleration will be this force divided by the mass of an electron.

    From there you can find instantaneous velocity, but I don't know if that helps any.

    I just can't figure out what it is I need to integrate. Thanks in advance!
     
  2. jcsd
  3. Nov 2, 2007 #2

    Curious3141

    User Avatar
    Homework Helper

    It's tedious, but an almost perfect analogy is with the problem of two identical masses free-falling into each other due to gravity. You can use Kepler's third law of motion to solve that one, or you can solve a second order o.d.e.

    Read all about it here : https://www.physicsforums.com/showthread.php?t=119855 and you should be able to adapt that easily to this problem. Only the constants change.
     
  4. Nov 2, 2007 #3
    This is just what I was looking for. Thanks for the help! Who knew such a seemingly simple problem could be so involved.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?