[ChrisV]

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It's not a force that causes diffusion

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Hmmm. I don't see any votes for "no force".

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You do now. I voted "no force".

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There may not be a net force, but there surely are forces acting.

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Why?

Well, I mean, of course there are. But they aren't necessary for diffusion. I would think diffusion would still occur in the absence of gravitational or electromagnetic forces.

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...it's the kinetic energy of the molecules.

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You can explain it terms of energy. But there should be an equivalent explanation in terms of force.


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What? Why?

If the molecules are already moving (which they are, extremely fast) then no force is necessary to get them to move across the room. Newton's first law and all that. The air molecules are merely an obstacle. The process would be faster in a vacuum.

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It's like having two different colors of super elastic balls and letting them go at two ends of a gymnasium. Eventually they spread out to fill the thing evenly.

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The balls would have zero kinetic energy and would not spread out unless you apply a force when you "let them go."


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But molecules never have zero kinetic energy, that's the whole point.

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If you keep track of what happens to an individual ball after letting it go, you'll find that it's kinetic energy is not constant. It changes as it interacts with other balls and the gymnasium walls. What is the nature of that interaction?

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Well, with molecules, it's ultimately electromagnetic, I guess. But as I said, the perfume molecules would spread faster in a vacuum, so that force is in no sense responsible for diffusion, but rather in opposition to it.