Again with the Force
 

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.

Re: Again with the Force
 

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.

Re: Again with the Force
 

Diffusion.

Re: Again with the Force
 

Law of Entropy?

Re: Again with the Force
 

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?

Re: Again with the Force
 

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.

Re: Again with the Force
 

[ QUOTE ]
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.

[ QUOTE ]
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.

[/ QUOTE ]

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?

Re: Again with the Force
 

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[ QUOTE ]
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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[ QUOTE ]
...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.


[/ QUOTE ]

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.

[ QUOTE ]
[ QUOTE ]
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.

[/ QUOTE ]

The balls would have zero kinetic energy and would not spread out unless you apply a force when you "let them go."


[/ QUOTE ]

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?

[/ QUOTE ]

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.

Re: Again with the Force
 

[ QUOTE ]
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?

Re: Again with the Force
 

[ QUOTE ]
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?

[/ QUOTE ]
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.

Re: Again with the Force
 

[ QUOTE ]
[ QUOTE ]
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?

[/ QUOTE ]
SEE MY FIRST POST MORAN. I OWNED THIS THREAD.


[/ QUOTE ]

Let me quote your first post for you. I'll emphasize some key words:

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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.

[/ QUOTE ]

Now you say there is no force:

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What force is primarily responsible for diffusion?

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No force is responsible for it.... No force is required. The electromagnetic force (or any for that matter) HINDERS and is NOT RESPONSIBLE FOR their movement.


[/ QUOTE ]

So which is it?

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[ QUOTE ]
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.

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Are you saying that this statement, on its own, is wrong?

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Once the [censored] are at room temperature they have kinetic energy, and they fly out in all directions.

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How do they arrive at room temperature? Are you going to refer me to your first post again?

Feel free to disagree, but please do it without calling me names. Judging by the poll responses, lots of people are confused by this question. Spewing venom doesn't help.

Re: Again with the Force
 

You seem confused about what your question actually is. Diffusion occurs because molecules are in constant motion. If your question is "why are molecules in constant motion?" then ask that instead.

[ QUOTE ]
What about the initial kinetic energy of the molecules? It depends on the temperature, but where did it come from

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The same place all the energy in the Universe came from. The universe was created containing a certain amount of energy. That amount will never be raised or lowered.

In a more immediate sense, useful energy on Earth largely comes from the sun. For instance, if you leave wet clothes outside, radiant heat from the sun is absorbed by molecules, increasing their kinetic energy and causing the water to evaporate.

Without the sun, molecules on Earth would all quickly settle into (an extremely cold) thermal equilibrium.

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and how is it maintained?

[/ QUOTE ]

Energy does not need to be maintained. First law of thermodynamics - energy is neither created nor destroyed.

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and then to keep them in motion requires some sort of interaction (force)-- what is it?

[/ QUOTE ]

Newton's first law - a body in motion will remain in motion unless acted on by an external force. Keeping them in motion does not require force.

Probably the source of your confusion is that you are used to the macroscopic world, where collisions between objects lead to dissipation of the kinetic energy of the objects. The energy dissipated is transferred to e.g. the molecules of the objects, as heat, or surrounding air molecules as noise. Collisions between individual molecules do not lead to any loss of their kinetic energy.

Edit: It makes me concerned for the state of science education in America that "No force" is scoring only 14% here, although I can only assume "Weak nuclear force" was someone's idea of a joke.

Re: Again with the Force
 

[ QUOTE ]
[Force or no force...]So which is it?

[/ QUOTE ]
Looking at the diffusion itself, it doesn't require any forces. That's the answer right there, end of thread. But a necessary precondition is that energy is present in the form of a heated gas. But that's what it is, a precondition. It's not relevant to the process of diffusion, any more than nuclear fusion is relevant to the process of lifting your arm. Without nuclear fusion, you couldn't lift your arm, but you'd be crazy to talk about the role of nuclear fusion in arm lifting as the effect is too far removed (sun > plant > carbohydrate > disgestion > muscles > kinetic energy)

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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.

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Are you saying that this statement, on its own, is wrong?

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Yes, I am. No forces are required to keep anything in motion. Forces are only required to change motion.


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Judging by the poll responses, lots of people are confused by this question...Spewing venom doesn't help.

[/ QUOTE ]
Well, you're certainly not helping the issue by framing the question poorly. And I'm joking with the caps and name calling...

Re: Again with the Force
 

Hello Devil - The question is phrased in weird way. Diffusion happens because of the Second Law of Thermodynamics.

The Second law of Thermodynamics says that for a closed system the amount of entropy, or disorder, increases with time (here the system is a room full of air and purfume) So diffusion is happening because when you open the bottle you at first have purfume only near the bottle and not in the rest of the room. As the molecules bump around in the air they redistribute themselves so that you end up with a random distribution of purfume molecules in the room eventually. So its the tendancy of a closed system to move from order to randomness that is the mechanism behind diffusion.

The force that you could point to that comes into play when the molecules bounce off each other is the electromagnetic force. When two molecules collide, the electrons which surround the molecules repel each other. Thats whats going on when they are bouncing around and re distributing themselves.

But the physical law that explains why diffusion happens is the Second Law of Thermodynamics.

Re: Again with the Force
 

Can anyone confirm what Chips is saying?

Re: Again with the Force
 

[ QUOTE ]
Can anyone confirm what Chips is saying?

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It's wrong. The Second Law possesses no explanatory power, because it is a statistical law. It says what will happen, but not why it happens.

It's true that the Second Law says that two gases in an enclosed space will, given time, mix with each other as evenly as possible. However the Second Law would still say this even if we didn't know what gases were composed of. To explain why this occurs requires reference to the properties of gases and molecules. For instance, if you're using the Second Law as an "explanation", you will be unable to tell me why a lump of granite doesn't diffuse through the air the way perfume vapors do.

It's a bit like saying that the reason some amount of a gas enclosed in some container will exert x amount of pressure on the sides is because Boyle's Law says it will. That's backwards. Boyle's Law is just a statistical law, derived from the underlying way in which a gas exerts pressure.

Re: Again with the Force
 

Nah I wasn't makin anything up. I've actually taught this stuff at the college level [img]/images/graemlins/smile.gif[/img] The info I gave you is correct.

Re: Again with the Force
 

gotta go with chips here.

Re: Again with the Force
 

Is this a joke thread made to annoy us physics nerds?

[ QUOTE ]
But the physical law that explains why diffusion happens is the Second Law of Thermodynamics.

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ChrisV is right, the Second Law is a statistical law with no explanatory power. The info Chips gave is mostly correct but completely misses the point.

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So its the tendancy of a closed system to move from order to randomness that is the mechanism behind diffusion.

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How is that a "mechanism"? Does matter intrinsically hate order and seek to become disordered? Do atoms give off Orderons when they become too ordered, thus returning to a disordered state? Or maybe when many Orderons get close together they combine to form Disorderons, reducing the number of Orderons?

Re: Again with the Force
 

[ QUOTE ]
gotta go with chips here.

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Morons,

Saying something will happen does not explain why it will happen.

Re: Again with the Force
 

Hey guys, I didn't know you were 2+2ers!

http://users.ox.ac.uk/~orie1381/nerds2.JPG

Re: Again with the Force
 

[ QUOTE ]
I've actually taught this stuff at the college level [img]/images/graemlins/smile.gif[/img]

[/ QUOTE ]

Well, God help the next generation of physicists.

I didn't even take Physics 1, so you're definitely one up on me there. However in this argument, I do possess the advantage of being right.

Re: Again with the Force
 

this is a pretty poorly formed question

Re: Again with the Force
 

[ QUOTE ]
Well, you're certainly not helping the issue by framing the question poorly.

[/ QUOTE ]

Maybe so. oneey13 makes the same claim.

As for your other points, I'll respond to these in a reply to ChrisV.

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And I'm joking with the caps and name calling...

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Ok. Moving on....

Re: Again with the Force
 

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You seem confused about what your question actually is.

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Me: Why do molecules move away from each other?
Internet Voices: Because they are in motion.
Me: [img]/images/graemlins/confused.gif[/img] Ok, so why are they in motion?
Internet Voices: Sorry, that's another question.

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

[/ QUOTE ]

The same place all the energy in the Universe came from. The universe was created containing a certain amount of energy. That amount will never be raised or lowered.

In a more immediate sense, useful energy on Earth largely comes from the sun. For instance, if you leave wet clothes outside, radiant heat from the sun is absorbed by molecules, increasing their kinetic energy and causing the water to evaporate.

Without the sun, molecules on Earth would all quickly settle into (an extremely cold) thermal equilibrium.

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and how is it maintained?

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Energy does not need to be maintained. First law of thermodynamics - energy is neither created nor destroyed.


[/ QUOTE ]

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Energy can be lost from a system depending on how the system is defined. Your cooling earth is an example. As you say, the sun helps to maintain the earth's temperature.

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and then to keep them in motion requires some sort of interaction (force)-- what is it?

[/ QUOTE ]

Newton's first law - a body in motion will remain in motion unless acted on by an external force. Keeping them in motion does not require force.

Probably the source of your confusion is that you are used to the macroscopic world, where collisions between objects lead to dissipation of the kinetic energy of the objects. The energy dissipated is transferred to e.g. the molecules of the objects, as heat, or surrounding air molecules as noise. Collisions between individual molecules do not lead to any loss of their kinetic energy.

[/ QUOTE ]

Total kinetic energy is conserved, but the kinetic energy of molecules in a system can change-- objects can be heated and cooled. Intermolecular (primarily electromagnetic) interactions are one mechanism of energy transfer. Radiant heat (again electromagnetic) as you mention, is another.

So I claim that molecules are in constant motion mostly because of electromagnetic forces, since gravitational effects are far weaker and nuclear forces act over too short a range. Objections?

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Edit: It makes me concerned for the state of science education in America that "No force" is scoring only 14% here, although I can only assume "Weak nuclear force" was someone's idea of a joke.

[/ QUOTE ]

You can add India to your list of concerns, as one of my real life tormentors is a graduate of an IIT.

Re: Again with the Force
 

[ QUOTE ]
this is a pretty poorly formed question

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I've been conditioned never to ask questions, so I'm out of practice. However, I'm not the first (and surely not last) to ask this question:

[ QUOTE ]
ate: 8/12/1999 3:27:32 PM
Question:
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.

[/ QUOTE ]

The linked page tries to answer the question, but I wasn't completely happy with it.

Re: Again with the Force
 

[ QUOTE ]
gotta go with chips here.

[/ QUOTE ] bad bet.

Chips makes the common mistake of equating "entropy" with the macroscopic notions of "disorder" or "randomness". It is the amount of energy in a system that is not available to perform work.

Re: Again with the Force
 

[ QUOTE ]
Total kinetic energy is conserved, but the kinetic energy of molecules in a system can change-- objects can be heated and cooled. Intermolecular (primarily electromagnetic) interactions are one mechanism of energy transfer. Radiant heat (again electromagnetic) as you mention, is another.

So I claim that molecules are in constant motion mostly because of electromagnetic forces, since gravitational effects are far weaker and nuclear forces act over too short a range. Objections?

[/ 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.

Both of those processes are natural processes. They need reasons NOT to happen.

Re: Again with the Force
 

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. 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.

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. 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. 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?

As a side note, I think it is hilarious how Maxwell, Boltzmann, and Liouville found this queston fascinating, and yet there are people in SMP with virtually no physics background who know the answer immediately and think the answer is obvious. My intellect is not this powerful, and I am no fluid dynamics or statistical mechanics expert, but I am pretty sure if you had to point to a force which facilitates diffusion, it is the electromagnetic force.

Re: Again with the Force
 

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In diffusion, the molecules undergo a NET movement away from each other

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http://tinypic.com/8whxf6.jpg

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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.

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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.

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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. 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?

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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 ]
As a side note, I think it is hilarious how Maxwell, Boltzmann, and Liouville found this queston fascinating, and yet there are people in SMP with virtually no physics background who know the answer immediately and think the answer is obvious. My intellect is not this powerful, and I am no fluid dynamics or statistical mechanics expert, but I am pretty sure if you had to point to a force which facilitates diffusion, it is the electromagnetic force.

[/ QUOTE ]

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

Re: Again with the Force
 

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

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

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

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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.

Re: Again with the Force
 

[ 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.

Re: Again with the Force
 

[ 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

Re: Again with the Force
 

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.

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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.

Re: Again with the Force
 

[ 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.

Re: Again with the Force
 

[ 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.

Re: Again with the Force
 

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

Re: Again with the Force
 

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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.

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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:


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Diffusion occurs as a result of the Second Law of Thermodynamics...

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

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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.

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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.

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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.

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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.


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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.

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Yes.

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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?

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

Re: Again with the Force
 

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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.

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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.

Re: Again with the Force
 

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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.

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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?

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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.

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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.