Physicopoeia

φυσικοποιΐα
THINGS I WISH I'D KNOWN WHEN I STARTED TEACHING PHYSICS

The Lennard-Jones Curve

For years, I saw this very characteristic curve in many books but it never seemed to have a name in the books I saw. It seems to be less common in school textbooks these days, which is a shame because it's a great example of a fairly simple idea which is staggeringly fruitful for explaining a huge number of phenomena about the behaviour of materials. For example, you can use it to explain surface tension forces, why many substances obey Hooke's law and also why objects expand when heated.

It basically takes the idea that bonded atoms experience both repulsive and attractive forces that balance at a particular equilibrium separation. The attractive and repulsive forces vary differently with separation which is why they only balance at a very specific separation. Here is an actual plot of the relevant data for a NaCl ionic bond:

 

As you'll find elsewhere on this site, I always prefer to use actual plotted data for this sort of thing rather than the hand-sketchedversions you often see in books.

 

Here is the Excel file for the above graph, and a worksheet which takes you through a lot of the phenomena it explains: