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: How to graph potential energy for 2 atoms in diatomic molecule w/ 3 unknowns

  1. Oct 24, 2011 #1
    1. The problem statement, all variables and given/known data

    The potential energy of two atoms in a diatomic molecule is approximated by

    U(r)=a/r^12 - b/r^6,

    where r is the spacing between the atoms and a and b are positive constants.

    Find the force F(r) on one atom as a function of r.

    Make two graphs one of U(r) versus r and one of F(r) versus r.

    2. Relevant equations

    F(r) = -(dU/dr)

    3. The attempt at a solution

    I know F(r) = -(dU/dr).

    So, F(r) = (12a/r^13) - (6b/r^7)

    And then I get stuck. I don't know how to make a plot of U(r) vs r and F(r) vs r when I don't know a and b.

    I want to plug values of r=[0.1, 0.2, 0.3, ...] into the F(r) equation, but I have no idea what to do about a and b.

    This question has been asked before on this forum and I have looked at the hints, but I am still completely lost.

    Could someone please get me pointed in the right direction.

    Thanks.
     
  2. jcsd
  3. Oct 24, 2011 #2
    So I figured out that U(r) = a/r^12 - b/r^6 is a simplified version of the Lennard-Jones potential and I know what the graph looks like now, but I still don't understand how you get it. I am so lost. Could someone please explain. Thanks.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook