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Most of the forces in physics obey inverse square law, but why molecular forces don't obey it.. Since molecular forces is also a form of electromagnetic force..

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do read up on van der Waals forces en.wikipedia.org/wiki/Van_der_Waals_force –  anna v Mar 17 '13 at 14:01

2 Answers 2

Molecular forces obbey that law (since it's all electromagnetism), the difference is that in those cases you don't have just two charges, but a more complex set of charges that interact, and maybe those inverse square laws, when superpositioned, don't form another inverse square.

The most simple example of this is an electric dipole: two charges separated by a distance $d$, generally small. When you go far away, it turns out that the electric potential goes as an inverse square, and the force goes as $1/r^3$, instead of $1/r^2$.

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The main reason is the neutrality of molecules, so if you consider it like a point, you will not have an interaction at all. For systems with non-zero summary charge the small dipole, quadrupole, ..., $2^n$-pole fields will be negligible additions compared to Coulomb law on the large distances, but for totally uncharged system summary force or field begins from dipole, since $Q=0$.

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