Again with the Force

[uDevil]

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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?

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SEE MY FIRST POST MORAN. I OWNED THIS THREAD.


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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.

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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.


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So which is it?

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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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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.

[ChrisV]

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.

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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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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.

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.

[Phil153]

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

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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.

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

[Chips_]

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.

[Andrew Karpinski]

Can anyone confirm what Chips is saying?

[ChrisV]

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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.

[Chips_]

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.

[daryn]

gotta go with chips here.

[Phil153]

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

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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?

[HLMencken]

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gotta go with chips here.

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Morons,

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