# Which force governs diffusion?

• Homer
In summary, the phenomena of diffusion is governed by the principles of random motion and thermal energy. This is known as diffusion theory, which describes the behavior of particles in populations rather than on an individual level. While it may not accurately predict the behavior of individual particles, it is still a useful tool in understanding diffusion. The driving force behind diffusion is not a specific force, but rather the result of random motion and thermal energy. This can be seen by considering a room with bunched up particles on one side - their random motion will eventually lead to an even distribution in the room due to the constraints of the wall.

#### 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

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.

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

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.

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.

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

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.

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 relevant texts on hand. I also can't seem to find it on the web. Odd - stuff like this is usually easy to find.

Njorl

I'm trying to find it, but I don't seem to have any relevant 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.

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.

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

## 1. What is diffusion?

Diffusion is the process by which particles move from an area of higher concentration to an area of lower concentration. It is a result of random motion of particles and does not require an external force.

## 2. Which force governs diffusion?

The force that governs diffusion is the force of concentration gradient. This force drives particles to move from areas of higher concentration to areas of lower concentration until equilibrium is reached.

## 3. How is diffusion related to temperature?

Diffusion is directly related to temperature. As temperature increases, the kinetic energy of particles also increases, leading to faster random motion and therefore faster diffusion. Conversely, at lower temperatures, diffusion occurs at a slower rate.

## 4. How does the size of particles affect diffusion?

The size of particles does not affect the force that governs diffusion, but it does affect the rate of diffusion. Smaller particles diffuse more quickly than larger particles because they have a higher surface area-to-volume ratio, allowing for more efficient diffusion.

## 5. What are some real-life examples of diffusion?

Diffusion is a fundamental process that occurs in many natural and man-made systems. Some real-life examples of diffusion include the movement of oxygen and carbon dioxide in and out of cells during respiration, the mixing of food dye in water, and the spreading of perfume scent in a room.