Again with the Force

[Phil153]

[ QUOTE ]
virtually no physics background who know the answer immediately

[/ QUOTE ]
I have a degree in physics and math, whatever that's worth.

[ QUOTE ]
but I am pretty sure if you had to point to a force which facilitates diffusion, it is the electromagnetic force.

[/ QUOTE ]
There's a bit of a difference between "facilitates" and "primarily responsible for" Is English your first language?

BTW, here is what the OP said:

[ QUOTE ]
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.
What force is primarily responsible for diffusion?

[/ QUOTE ]
The reason they can be detected across the room is because they moved there, ALL ON THEIR OWN. There was no force pushing or pulling them. When they collide with other molecules there is no net force acting on them on average.

Take a real world example. If I throw some dice in craps and they bounce several times before landing, what force would be primarily responsible for them ending up at the other side of the table? The force primarily responsible is my throw, as it's the only thing that gave energy to the dice. The electromagnetic interaction between the dice and table facilitated the final result, but was not primarily responsible for them leaving my hand and ending up somewhere on the table. My throw is responsible for that. Just as the existing kinetic energy of the gas molecules is primarily responsible for smelling perfume across the room.

So I think we're on the same page. The OP just did a horrible job of wording his question and then argued the point which was clearly incorrect. Chips threw a curved ball with the second law, which has nothing to do with anything.

[Matt R.]

[ QUOTE ]
Get a grasp of basic physics, then you can start telling me which of my opinions you find hilarious.

[/ QUOTE ]

What physics courses have you taken again? Oh, right...

[ QUOTE ]
BZZZZZZZZZT!

PV = nRT, ideal gas law. Temperature is proportional to both pressure and volume, so you can increase pressure and decrease volume and temperature is constant. If the gas were at higher temperature, then the temperature would equilibriate with the container. Second Law Of Thermodynamics, you remember him, he came up earlier.


[/ QUOTE ]

Now you're just being retarded. Of course when I'm comparing a canister under high pressure vs. one under low pressure I'm assuming equal volumes. Otherwise my statement is useless. You do know that when you put a gas under high pressure, its temperature increases right?

I really don't understand the points you are trying to make with the rest of your post -- they just don't make any sense in context of the question. First, you are denying that the molecules have a net motion AWAY from each other. Uhh, what? They're in the bottle/canister, and when they are released, they move away from each other. Thus they have a net movement AWAY from each other. Is this difficult or something?

[ QUOTE ]
What is responsible for the increased force is increased frequency of collision between the gas molecules and the container, due to increased gas density. The molecules are not travelling faster. As I said, having higher temperature matter next to lower temperature matter for extended periods is forbidden by the second law

[/ QUOTE ]

I have no idea what point you are trying to make with this. You say there is an increased force (which is electromagnetic), and yet you somehow conclude that it is not responsible at all for the diffusion of the molecules. I never said the molecules were traveling faster either -- just that their net direction is different (it is zero when they are crammed inside the bottle or canister).

And yes, I still find you hilarious.

[Copernicus]

[ QUOTE ]
[ QUOTE ]
Get a grasp of basic physics, then you can start telling me which of my opinions you find hilarious.

[/ QUOTE ]

What physics courses have you taken again? Oh, right...

[ QUOTE ]
BZZZZZZZZZT!

PV = nRT, ideal gas law. Temperature is proportional to both pressure and volume, so you can increase pressure and decrease volume and temperature is constant. If the gas were at higher temperature, then the temperature would equilibriate with the container. Second Law Of Thermodynamics, you remember him, he came up earlier.


[/ QUOTE ]

Now you're just being retarded. Of course when I'm comparing a canister under high pressure vs. one under low pressure I'm assuming equal volumes. Otherwise my statement is useless. You do know that when you put a gas under high pressure, its temperature increases right?

I really don't understand the points you are trying to make with the rest of your post -- they just don't make any sense in context of the question. First, you are denying that the molecules have a net motion AWAY from each other. Uhh, what? They're in the bottle/canister, and when they are released, they move away from each other. Thus they have a net movement AWAY from each other. Is this difficult or something?

[ QUOTE ]
What is responsible for the increased force is increased frequency of collision between the gas molecules and the container, due to increased gas density. The molecules are not travelling faster. As I said, having higher temperature matter next to lower temperature matter for extended periods is forbidden by the second law

[/ QUOTE ]

I have no idea what point you are trying to make with this. You say there is an increased force (which is electromagnetic), and yet you somehow conclude that it is not responsible at all for the diffusion of the molecules. I never said the molecules were traveling faster either -- just that their net direction is different (it is zero when they are crammed inside the bottle or canister).

And yes, I still find you hilarious.

[/ QUOTE ]


[ QUOTE ]
I am no fluid dynamics or statistical mechanics expert

[/ QUOTE ]

And apparently no physics maven either. You are wrong, chris is right

[Matt R.]

English is my first language. Given the context of the question, I believe it is reasonable to assume that the OP was questioning "What force (if any) is responsible for molecules diffusing into open space." If you theoretically take away the electronmagnetic force, it will take MUCH MUCH longer for a gas to expand into the space. There would be nothing pushing them apart. Thus, given the speed at which a gas under high pressure expands into open space, I think it is reasonable to assume the OP is wondering what force makes the process of diffusion faster/easier.

[ QUOTE ]
When they collide with other molecules there is no net force acting on them on average.

[/ QUOTE ]

I think this may be where we disagree. Take a molecule inside a pressurized canister. It has no NET force acting on it, because it is being bombared equally in every direction by other molecules or the walls of the container. Now, if the container busts, the molecules on the outside of the "system" are only being hit by molecules that were further inside the container. Thus they have a net force AWAY from the other molecules. The ones further in are still being hit from all directions and they tend to stay put. But over time these distances spread out as the molecules diffuse, and they reach equilibrium again in the larger space.

Basically, I think as the molecules initially diffuse when they are released from a pressurized container there is definitely a net force acting on some of the molecules causing them to spread out.

[Matt R.]

[ QUOTE ]
And apparently no physics maven either. You are wrong, chris is right

[/ QUOTE ]

If you pointed out specifically where I was wrong, I may take you more seriously.

I may not be a physics maven, but I am majoring in physics. And given how I have done in my classes thus far (including a class that deals with this subject matter), I think I am probably closer to a physics maven then Chris is.

[Copernicus]

[ QUOTE ]
[ QUOTE ]
And apparently no physics maven either. You are wrong, chris is right

[/ QUOTE ]

If you pointed out specifically where I was wrong, I may take you more seriously.

I may not be a physics maven, but I am majoring in physics. And given how I have done in my classes thus far (including a class that deals with this subject matter), I think I am probably closer to a physics maven then Chris is.

[/ QUOTE ]

Your last post clarifies either your lack of understanding or possibly a semantic difference in the use of "cause"

[ QUOTE ]
Basically, I think as the molecules initially diffuse when they are released from a pressurized container there is definitely a net force acting on some of the molecules causing them to spread out

[/ QUOTE ]

There is no net force acting on them to "cause" them to spread out. The link above explains what is happening quite well, despite not "satisfying" the poster:

It is very often to here the term "diffusion", although I know that diffusion is caused by the different conc. gradient between substances, I don't understand the phenomemon in terms of forces. Is there any force which causes diffusion? Also, why diffusion can make the substances evenly distributed?

Answer:
Answer from Dr. K. Y. Wong of the Physics Departmemt: This is a very interesting question which intrigued great physicists such as Maxwell, Boltzmann and Liouville. Even modern day physicists who work on chaos find inspiration by posing this question. It depends on which level of the problem you are considering: [1] If you just consider a box of gas molecules, each with different initial position and momentum confined to a small space in the beginning, it will soon distribute themselves throughout the entire box. No forces are needed. The major reason is just probability. If you divide the space into two boxes, and you are free to put each of 100 molecules into one of the two boxes, what is the probability that all molecules stay in one box? The answer is 2^{-100}, which is about 1 over 1 followed by 30 zeros! Now what is the probability that half of the molecules stay in one box, and the rest stay in the other? The answer is that there are much more combinations to achieve this. We have to multiply 2^{-100} by the number of combinations of dividing 100 indistinguishable objects into 2 groups, which gives a multiplicative factor of 100!/50!50!. The final result is 0.08. This simple example shows that it is much easier to distribute molecules uniformly in space. So long as the molecules move, they will eventually diffuse throughout the entire space. Non force is needed. (This is closely related to the modern ideas of entropy.) [2] However, after all, why do molecules move with different speed? If, instead, all molecules move with the same speed, they will march like an army everywhere in space, and there should be no diffusion. The answer is that molecules exert forces on one another, and these forces transfer momenta among them. It does not matter what kind of force is in action. Even if the molecules behave like billiard balls which exert forces only on collision, this is already sufficient. Of course, in real life, molecules interact via various kinds of forces which you may learn from your physical chemistry. In summary, no forces cause diffusion directly; there are no explicit outward forces pushing the molecules from the region of high concentration outwards. Rather, forces (of any nature) are needed to cause momentum transfer molecules, which in turn give rise to the phenomenon of diffusion.

to clarify the last sentence: the system reached a state of pressurization by the addition of energy to the system. Once the system is in that state, and the molecules have that energy, there is no additional force required, electromagnetic or otherwise, for diffusion to occur

I aced my PDE course...it took 20 years for me to understand it. Grades arent evidence of understanding.

[ChrisV]

That text above in bold is a perfect explanation of my position. Rather than posting any more, I just direct you to that.

[uDevil]

[ QUOTE ]
Where the [censored] do you people come up with this [censored]? Chips has had the best response by far, and I believe uDevil is correct as well.

[/ QUOTE ]

I'm not taking a fixed position. I actually think ChrisV is correct, though there are a couple of loose ends that bother me.

Chips is not the only one who has tried to explain this to me on the basis of entropy. In fact, this is the explanation given on Wikipedia:


[ QUOTE ]
Diffusion occurs as a result of the Second Law of Thermodynamics...

[/ QUOTE ]

However, I don't like this explaination for the reasons ChrisV gave.

[ QUOTE ]
In diffusion, the molecules undergo a NET movement away from each other (we're not talking Brownian motion here -- these processes have no net direction). If you want to point to a force causing the molecules to spread out, I would think it would have to be the electromagnetic force as this is what mediates the interaction between the particles.

[/ QUOTE ]

This was one of my first thoughts too, but it's wrong. There is no net force. Diffusion occurs in all directions. An individual molecule may experience a net force, but if you add up all of the forces that have pushed all of the individual molecules to their final positions you get zero.

[ QUOTE ]
Think of a canister under high pressure. A gas under high pressure is at a higher internal temperature than one under low pressure, and thus the particles have a higher kinetic energy.

[/ QUOTE ]

I don't know what you mean by internal temperature. The temperature is independant of the pressure. The kinetic energy of the molecules doesn't depend on the pressure.


[ QUOTE ]
High pressure inside a canister of course exerts a lot of force on the interior of the walls. The electromagnetic force is what is responsible for this.

[/ QUOTE ]

Yes.

[ QUOTE ]
As the molecules try to "spread out" inside this canister, do you really think they are not exerting a force on each other, facilitating diffusion?

[/ QUOTE ]

As ChrisV pointed out, this actually tends to slow diffusion.

[uDevil]

[ QUOTE ]
Yes, kinetic energy of individual molecules is changed, constantly, by electromagnetic forces. Thermal radiation is all the time being emitted and absorbed. However none of this is necessary for diffusion to occur. If you imagine a thought experiment with a container with impermeable walls, filled with two types of molecules which do not interact with each other in any way, moving randomly in different directions, then if they start bunched together at different ends of the container, they will eventually end up evenly mixed. As I've noted already, molecules in motion don't require any intervention to keep moving. So while electromagnetic interaction is happening constantly, it's not in any way responsible for either continued molecular motion or the process of diffusion.

[/ QUOTE ]

Except for the parts I've emphasized, I'm ok with this. The situation seems to me to be:

1) I am allowed to apply electromagnetic forces to molecules.

2) By doing so, I can increase or decrease the kinetic energy (temperature) and hence, the rate of diffusion of those molecules. If I could cool them to absolute zero, I could even turn diffusion off, for all practical purposes.

3) Despite this, I cannot claim that electromagnetic forces cause diffusion.

[uDevil]

[ QUOTE ]
Take a real world example. If I throw some dice in craps and they bounce several times before landing, what force would be primarily responsible for them ending up at the other side of the table? The force primarily responsible is my throw, as it's the only thing that gave energy to the dice. The electromagnetic interaction between the dice and table facilitated the final result, but was not primarily responsible for them leaving my hand and ending up somewhere on the table. My throw is responsible for that. Just as the existing kinetic energy of the gas molecules is primarily responsible for smelling perfume across the room.

[/ QUOTE ]

The reason I used the words "primarily responsible for" is that I recognize that more than one force may act at the same time.

Here, you say "The force primarily responsible is my throw, as it's the only thing that gave energy to the dice." What is the nature of the force associated with your throw?

[ QUOTE ]
So I think we're on the same page. The OP just did a horrible job of wording his question and then argued the point which was clearly incorrect.

[/ QUOTE ]

You are focused on who is right and who is wrong. I'm looking for understanding, not just the answer. I'd rather not let my ego get in the way, but your tone makes that difficult.