Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Force of attraction forumla

  1. Nov 24, 2008 #1
    The force of attraction formula between two charges is


    How does the inverse of r2 fit into the equation? I understand the concept of how distance would need to be the inverse in the function, but why is the distance (r) in the inverse squared? Is this the same principle of why s^-2 is the acceleration formula and time is square in the inverse?

    Last edited: Nov 24, 2008
  2. jcsd
  3. Nov 24, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Coulomb's law is an example of an inverse square law, something quite common in physics. Read about it here: http://hyperphysics.phy-astr.gsu.edu/Hbase/Forces/isq.html" [Broken]
    Last edited by a moderator: May 3, 2017
  4. Nov 24, 2008 #3
    I see! thanks!
  5. Nov 24, 2008 #4


    User Avatar
    Homework Helper

    Note that inverse square law is applies to point or spherical sources. For an infinitely (or very large) long line or cylinder, the ratio of force versus perpendicular distance to the line is 1/r. For an infinitely (or very large) plane, the force is constant (independent of distance).

    Found the link for the other cases at the same site: electrical field

    For the infinite line case, the field strenth is a function of charge "density" over the perpendicular distance "z" to the line ( ... / z).

    For the infinite disc (plane) case, the limit as "R" approaches infinity, the [1 - z/sqrt(z^2 + R^2) ] term approaches [1 - 0], and the field strength is constant, independent of distance
    Last edited by a moderator: May 3, 2017
  6. Nov 26, 2008 #5
    Things get really cool when you start checking out far field proportionalities in systems of multipoles!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook