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Name: Justin
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Question:
Since Van der Waals and surface tension are really aspects of electrical forces. Does that mean that surface tension and Van der Waals follow inverse square laws?



Replies:
Hi Justin,

I am a little confused by what is meant by 'aspects of electrical forces'. It is true that electrons play a huge role in determining an atom's properties, including its chemical and physical properties -- including surface energy (surface tension) and van der waals forces. However, the basis for each of these properties is very different, and, perhaps not surprisingly, so are the physics around them.

It is a good idea to be a little more specific when defining mathematical relationships like 'inverse square law'. Between what two quantities do you mean? For van der Waals forces, I am guessing you mean the force between two molecules with respect to distance (it turns out it is not a square relationship -- it is highly repulsive when close, weakly attractive at middle distances, and nearly zero at long distances). For surface tension, I am not sure which relationship you mean.

The bottom line is that even if two properties share a broad, qualitative similarity, it is still very possible (even likely) that they are very different.

Hope this helps,

Burr Zimmerman


Not necessarily so. Not all electrical fields (forces) obey an inverse square law. Two "point" charges do, but other interactions of electrons (which form the 'outside' of atoms and molecules) may interact quite differently since effects other than the inverse square law apply. For example: Van der Waals forces are more commonly expressed by the "6-12" potential energy V(r), that is: V(r) = A x [ -B/r^6 + C/r^12] The "attraction" (-1/r^6) is fairly long ranged) and the "repulsion" (+1/^12) increases rapidly as the distance "r" decreases.

Vince Calder



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