Again with the Force

[uDevil]

A perfume bottle is opened. A short time later, perfume molecules can be detected across the room. The perfume molecules have diffused from a region of high concentration to one of low concentration.

[Phil153]

Whichever force gave them kinetic energy in the first place; probably the Sun would be the answer on Earth. This is caused by a combination of gravity and strong and weak nuclear forces.

[Andrew Karpinski]

Diffusion.

[OneForTheMel]

Law of Entropy?

[uDevil]

For those who say gravity: how can an attractive force account for the movement of molecules away from each other?

For those who say some other force: shouldn't that force be equivalent to one (or a combination) of the forces listed? If so, which one(s)? If not, why not?

[Phil153]

This is gay. It's not a force that causes diffusion...it's the kinetic energy of the molecules. 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.

[uDevil]

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This is gay.

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This means what? Uninteresting? Pointeless? Maybe so.

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

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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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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." 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?

[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.

[uDevil]

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I voted "no force".

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That may be the best answer. But I'm not sure yet.

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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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Good point. Given molecules initially in motion, I agree.

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If the molecules are already moving (which they are, extremely fast) then no force is necessary to get them to move across the room.

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Already agreed. (The average molecular velocity depends on the temperature and the mass of the molecules (roughly 440 m/s for O2 and N2 at room temperature-- faster than the speed of sound!).
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But molecules never have zero kinetic energy, that's the whole point.

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Yes. As noted above, the velocity depends on the temperature.

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Well, with molecules, it's ultimately electromagnetic, I guess.

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The interaction between balls and between balls and walls is also (mostly) electromagnetic.

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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.

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Not so fast. [img]/images/graemlins/smile.gif[/img]

What about the initial kinetic energy of the molecules? It depends on the temperature, but where did it come from, and how is it maintained? To get molecules in motion in the first place and then to keep them in motion requires some sort of interaction (force)-- what is it?

[Phil153]

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What about the initial kinetic energy of the molecules? It depends on the temperature, but where did it come from, and how is it maintained?

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SEE MY FIRST POST MORAN. I OWNED THIS THREAD.

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To get molecules in motion in the first place and then to keep them in motion requires some sort of interaction (force)

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NO. Here's your initial question:

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What force is primarily responsible for diffusion?

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No force is responsible for it. Once the [censored] are at room temperature they have kinetic energy, and they fly out in all directions. No force is required. The electromagnetic force (or any for that matter) HINDERS and is NOT RESPONSIBLE FOR their movement.

As for where they got kinetic energy, see my post first post in this thread.