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hutchphd
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I believe what is bothering you is the subtlety of what actually happens at a real wall. As @Chestermiller says the internal pressure is defined according to real momentum flux across an imaginary surface deep in the bulk. This is unambiguous and involves only the usual stuff.Swamp Thing said:So the surface analysis, where we think of molecules being pulled back into the gas (leaving less momentum to create pressure on the wall) is just one of those nice heuristic pictures that happen to give useful results? In some sense similar to the popular "waves between two ships" picture of the Casimir force...
Another question: what do we actually mean by pressure of gas in the middle of a container? Is it the momentum per unit time per unit area due to collisions on a hypothetical test surface? In that case, the bulk picture can be linked with the surface picture by introducing a hypothetical test surface which doesn't interact with the molecules, thus leading to a situation like the actual container walls. Maybe... or maybe there is a definition of internal bulk pressure that doesn't involve a special surface?
What happens at a real surface is complicated and depends upon the exact nature of the material. There could still be van der Waals forces and they might be large enough to adsorb the gas! So there will be a surface layer where the density of the the gas will be affected (either greater or less) . But this layer is very thin because the VDW forces are short range. And so the net contribution to the net free energy is de minimus and the net pressure on a real wall corresponds to the internal pressure. It just gets a little messy right at the surface .