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-   -   Which force governs diffusion (http://www.physicsforums.com/showthread.php?t=487)

Homer Mar26-03 01:55 PM

Which force governs diffusion
 
Hello,
In a biological course in the university, I asked my professor Which force governs diffusion and He didn't know.
Note that i don't mean Fick's first or second law, Einstein's law of diffusion, the Brownian Motion and not even the The Second Law of Thermodynamics, simply what "compels" particles to move and thus to balance concentrations.
For example, falling objects towards Earth is governed by the universal law of Gravitation
Any ideas?
Thanks

Mentat Mar26-03 03:33 PM

I would think it's the fact that all things follow the path of least resistance. I suppose that's not governed by one of the "forces", but is rather a principle that allows the "forces" to work. IOW, the "forces" (or curvatures in spacetime (please, everyone, don't side-track the thread just debate this point)) only influence an object's actions because that object is inclined to follow the path of least resistance.

Loren Booda Mar26-03 04:09 PM

Try inertia, electromagnetism, thermal energy, statistical mechanics and quantum uncertainty. One cannot easily ignore the theories/laws you referred to, though.

Monique Mar26-03 06:09 PM

I would agree with Mentat, the main force of diffusion is that molecules move, they bounce against eachother and transfer energy between them. An over-symplistic model: think of a large square area w/ horizontally moving walls. You put some tennisballs in one corner, the bouncing wall will set the tennisballs in motion, after which they bounce and bounce until all the forces are distributed evenly and the balls have 'diffused' over an area and are homogeneously spread out.

LBooda, could you explain what you mean by statistical mechanics and how quantum uncertainty facilitates diffusion?

Loren Booda Mar26-03 11:08 PM

Statistical mechanics is a classical method which involves both qualitative and quantitative assessment of many similar particles interacting thermally. Atoms numbering on the order of Avogadro may be modelled in a closed system (say a box with sides of length x) toward the distribution of states (say quantum numbers). Although every possible state of a particle has an equal probability, the constraint of thermal equilibrium requires a very well defined overall statistical distribution of states in relation to, say, temperature. You may have heard of the Boltzmann distribution, which gives the population of classical particles (most atoms and molecules) as a function of energy or temperature.

Einstein showed in 1905 that Brownian motion, a process much like diffusion, is a quantum mechanical process. Basically, each particle in suspension has an uncertainty in position and velocity associated with itself that manifests as random motion. This movement overcomes the bonds present in the liquid, and diffusion procedes.

Another God Mar27-03 05:52 AM

Quote:

Originally posted by Loren Booda
Try inertia, electromagnetism, thermal energy, statistical mechanics and quantum uncertainty. One cannot easily ignore the theories/laws you referred to, though.
I have always thought of it simply as : Anything above absolute vibrates. (thermal energy) If particles such as atoms and molecules are close to each other, they will in due course vibrate into each other, and so 'bounce' off away from each other (electromagnetism causes this). In this way, over time, they will tend to bounce away from each other, causing an eventual tendency for them to keep pushing outwards. The less close the molecules are to each other, the less they will push away from each other.

This is also related to 2nd Law of Thermodynamics. Diffusion, is the system tending towards entropy.

Mike H Mar27-03 01:52 PM

To paraphrase my former stat mech prof, "It's just a simple case of understanding chemical potential and knowledge of what diffusive equilibrium is!"

Heh.

Monique Mar27-03 07:14 PM

Quote:

Originally posted by Loren Booda
Statistical mechanics is a classical method which involves both qualitative and quantitative assessment of many similar particles interacting thermally. Atoms numbering on the order of Avogadro may be modelled in a closed system (say a box with sides of length x) toward the distribution of states (say quantum numbers). Although every possible state of a particle has an equal probability, the constraint of thermal equilibrium requires a very well defined overall statistical distribution of states in relation to, say, temperature. You may have heard of the Boltzmann distribution, which gives the population of classical particles (most atoms and molecules) as a function of energy or temperature.

Einstein showed in 1905 that Brownian motion, a process much like diffusion, is a quantum mechanical process. Basically, each particle in suspension has an uncertainty in position and velocity associated with itself that manifests as random motion. This movement overcomes the bonds present in the liquid, and diffusion procedes.

Thanks for that great explanation :)

Homer Mar28-03 01:31 PM

Hey,
Thanks for the responses.
I have a follow-up question then.
If random walk and Thermal energy do indeed govern diffusion, why is it that each gas tend to equalize its partial concentration only with regard to itself.
E.g. If we intermix two systems of oxygen and helium, each gas will achive equilibrium only with its own kind (so to speak).
Thanks.

Loren Booda Mar29-03 09:28 AM

Partial pressure arises from the ideal gas law p=nkT, where n is the sum of the number of molecules of the constituent gases. Thus p is pressure of the contituent gas "c," where pc=nckT.

russ_watters Mar29-03 12:44 PM

Quote:

why is it that each gas tend to equalize its partial concentration only with regard to itself.
Quote:

Partial pressure arises from the ideal gas law p=nkT
Thats not how it works, but not WHY. Homer, if each component gas equalizes with itself, then they also all equalize with each other. This is part of diffusion and the same concepts apply.

You can look at each gas by itself or all together. The rules are the same. Its just easier to look at each one individually.


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