# Which force governs diffusion?

#### Homer

[SOLVED] 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.
Any ideas?
Thanks

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#### russ_watters

Mentor
The measure of kinetic energy in particles is temperature (and I sure hope your prof knew that). If you're asking why heat makes particles move, I guess the answer is it just does.

BTW, this thread would work better in a different forum - like the physics one.

#### Homer

Hey,
Thanks
Note that The particels do not move randomaly as a whole, rather they somehow equalize their differences in its concentration levels.

#### russ_watters

Mentor
Note that The particels do not move randomaly as a whole, rather they somehow equalize their differences in its concentration levels.
I'm not clear on what you mean. Particle motion IS random, thereby equalizing differences. Its just a probibility function. If particles move randomly, it is very unlikely you will find them bunched together.

#### Homer

I'll explain my problem (so to speak). If their movement is random, they wouldn't necessarily equalize their differences in the concentration levels.
Since, that is always the case (equalization of partial concentration) I take it that there is a force that drives them to that end.
Furthermore, we know of the The Second Law of Thermodynamics. But the second law just explain what should happen and not how it's done. For example, If i hold a ball up in the air and then let go, in order to fulfil the second law's requirement the ball has to be on the floor but it's the gravitational force that drives the ball toward that end.
I'm looking for the Force begine the phenomena.

Thanks.

#### Njorl

Diffusion theory is not quite accurate. It is descriptive, and generally useful, but not realistic. If you study it closely, you will see that it describes some small number of particles moving with infinite velocity.

According to diffusion theory:

Assume a room with an airtight dividing wall. Oxygen is on one side of the wall, nitrogen on the other. When the wall drops, there is some small concentration of oxygen at the far end of the nitrogen side in less time than it would take light to get there (and the converse is true for nitrogen in the oxygen side). This is not possible.

The theory is not sound when considering individual particles. It is useful in describing behaviors of particles in populations large enough so that random behavior is generalized.

Njorl

#### russ_watters

Mentor
Assume a room with an airtight dividing wall. Oxygen is on one side of the wall, nitrogen on the other. When the wall drops, there is some small concentration of oxygen at the far end of the nitrogen side in less time than it would take light to get there (and the converse is true for nitrogen in the oxygen side). This is not possible.
I don't understand. Why does it say that? I was under the impression that diffusion theory took into account velocity - ie, higher velocity means faster diffusion. It does NOT say its an instantaneous process.

I'll explain my problem (so to speak). If their movement is random, they wouldn't necessarily equalize their differences in the concentration levels.
Why not? To have an unequal distribution is not random. To be random you must have an equal distribution.
I'm looking for the Force begine the phenomena.
There is no driving force other than the forces associated with the random motion of the particles.

Think about it this way. If you have some particles all bunched up on one side of a room, all bouncing around, which direction are the particles more likely to go when they bounce? Toward the area of high concentration or away from it? Random motion, so they are as likely to go either way and hence not change the concentration, right? Wrong. Its a trick question. Since the wall is closer on the side where they are bunched up, they can't move very far in that direction. Their random motion is only random insofar as it can't be exactly predicted on a particle by particle basis. Since the wall is in the way, the particles will on average move away from it - its the only direction they can move. Because of this they will eventually be roughly evenly distributed in the room.

#### Njorl

Originally posted by russ_watters
I don't understand. Why does it say that? I was under the impression that diffusion theory took into account velocity - ie, higher velocity means faster diffusion. It does NOT say its an instantaneous process.

There is a factor for the temperatures of the gases, but at an infinitesmal time after the opening, the diffusion formula predicts at least an infinitesmal concentration at all points in space. It is not a bad theory. It describes what it is intended to describe. It is just not intended to describe the behavior of individual particles.

I'm trying to find it, but I don't seem to have any relevent texts on hand. I also can't seem to find it on the web. Odd - stuff like this is usually easy to find.

Njorl

#### russ_watters

Mentor
I'm trying to find it, but I don't seem to have any relevent texts on hand. I also can't seem to find it on the web.
Don't bother. I see what you mean now. In any probability function you will have a handful of particles predicted to break the laws of physics. The probability function may work well in a macro view, but not for individual particles. Thanks for the clarification.

#### Homer

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.

#### Njorl

Originally posted by Homer
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.
I don't understand. Can you give an example?

Njorl

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