Again with the Force

[Andrew Karpinski]

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

[ChrisV]

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

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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.

[oneeye13]

this is a pretty poorly formed question

[uDevil]

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

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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....

[uDevil]

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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

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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?

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


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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?

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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.

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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.

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

[uDevil]

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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:

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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.

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

[Copernicus]

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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.

[ChrisV]

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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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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.

[Matt R.]

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.

[ChrisV]

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

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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.

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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.

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