Why Do Molecules Move: Exploring the Physics Behind It

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In summary: I'm not sure how that helps.In summary, a single molecule of hydrogen gas at room temperature would be travelling at 6000 feet per second due to its internal vibrations. If you reduced the temperature to 0K, then warmed it up again, the molecule would still move off in a random direction. Head on collision with another with exactly the right circumstances would be necessary to stop its movement.
  • #1
prj45
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I mean, I know that they move faster the more energy there is, but why do they move at all in the first place?

Would a single molecule move if gravity wasn't acting on it and it didn't touch anything else?
 
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  • #2
The simplest answer is inertia. They have energy and momentum. Although collisions result in transfer, there is no loss, if nothing outside is affecting their motion. Also there are significant differences in what is going on depending on the state (solid, liquid, gas).
 
  • #3
I'll take a stab at this. A molecule consists of two or more atoms with all kinds of internal, dynamical "vibrations", thermally induced or otherwise inherently present. This arrangement causes a constant, quasi-cyclic natural shifting of molecular position with respect to the immediate "environment"
Though most readily seen is gases, this occurs in solids as well.
 
  • #4
I've been a little puzzled by this one actually. I was reading up on The Kinetic Theory of Gases and was a little surprised by what I saw. Can anybody tell me this:

Is a molecule of hydrogen gas at a fairly ordinary pressure really traveling at 6000 feet per second?

http://web.lemoyne.edu/~GIUNTA/classicalcs/joule.html

"Therefore, since it is manifest that the pressure will be proportional to the square of the velocity of the particles, we shall have for the velocity of the particles requisite to produce the pressure of 14,831,712 grs. on each side of the cubical vessel, v = (14,831,712/395.938)1/2 x 32 1/6 = 6225 feet per second..."
 
  • #5
Farsight said:
Is a molecule of hydrogen gas at a fairly ordinary pressure really traveling at 6000 feet per second?
That sounds about right for the rms speed of room temperature hydrogen gas.
 
  • #6
So I'm surrounded by air traveling at the speed of a bullet. Hmmn.
 
  • #7
Farsight said:
So I'm surrounded by air traveling at the speed of a bullet. Hmmn.
Yes, but what is the mass of a hydrogen /oxygen / nitrogen etc. molecule compared to a bullet? Compare momentums not velocities.
 
  • #8
Farsight said:
So I'm surrounded by air traveling at the speed of a bullet. Hmmn.

So why's it going at 6000 m/s?

If its all down to internal vibrations, where's the imbalance that causes it to propogate in one particuler direction?
 
  • #9
Farsight said:
So I'm surrounded by air traveling at the speed of a bullet. Hmmn.
Yes, don't you feel it?

[hint: you'd notice if you held your breath and that air suddenly went away...]

edit: ehh, better yet, just squeeze a basketball or soccer ball. What is pushing back at you?
 
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  • #10
prj45 said:
So why's it going at 6000 m/s?

If its all down to internal vibrations, where's the imbalance that causes it to propogate in one particuler direction?
There is no imbalance. Some are going left, some right, some up, some down -- every direction, essentially randomly.

If, however, you release some air into a vacuum, then there would be an imbalance and the air would flow.
 
  • #11
So could I just stop a molecule of hydrogen gas, and if I did would it just sit there, presuming its on its own.
 
  • #12
To completley stop the molecule's movement you would have to reduce it's temperature to 0 K's

Good luck to you.
 
  • #13
prj45 said:
So could I just stop a molecule of hydrogen gas, and if I did would it just sit there, presuming its on its own.
How exactly would you stop it? When it hits something, it bounces.
 
  • #14
JohnSimpson said:
To completley stop the molecule's movement you would have to reduce it's temperature to 0 K's

Good luck to you.
And even at 0K there will still be some movement.
 
  • #15
So, I'm still not getting what causes the locomotion of molecules.

If I reduced temperature to 0K, then warmed it up again, what makes it move off in a particular direction?
 
  • #16
So, I'm still not getting what causes the locomotion of molecules.

If I reduced temperature to 0K, then warmed it up again, what makes it move off in a particular direction?
 
  • #17
prj45 said:
So, I'm still not getting what causes the locomotion of molecules.

If I reduced temperature to 0K, then warmed it up again, what makes it move off in a particular direction?
How do you warm it back up again? The answer to that is the answer to your question...
 
  • #18
russ_watters said:
How exactly would you stop it? When it hits something, it bounces.

Head on collision with another with exactly the right circumstances?
 
  • #19
Head on collision with another with exactly the right circumstances?

Conservation of momentum would simply require the molecule to go at the same speed in the opposite direction.
 
  • #20
To warm it up you'd have to give it energy which would involve hitting it with other particles or shining a light on it. Either way, something will come in, hit the particle and give it energy. The collision will obey the conservation of momentum and so the direction the particle flies off in will be dictated by the particle which warms it up.
 
  • #21
It might help to understand that the molecules (of a piece of matter) have a temperature BECAUSE they have kinetic energy, not the other way around.

The DEFINITION of heat is the measurement of kinetic energy in its atoms.
 
  • #22
Hang on a minute. Kinetic energy doesn't really exist. It's just a term we use when we're talking about masses with motion.

I think prj45 has got a very interesting question here, and I don't think it's been answered. Forgive me if I'm missing the obvious, but I'd like to ask the question again:

I place a piece of dry ice on a piece of cork in a glass bottle, pump out most of the air, and sit it on the workbench. It's rather like an imperfect thermos flask, but it's glass so photons can get through. Now, we understand things like electron energy levels and atomic photon absorption. But how do the CO2 molecules acquire their bullet-like velocities as the dry ice gradually sublimes?

PS: edit, I thought this looked interesting:

http://physicsweb.org/articles/world/16/2/8

Ultracold plasmas and Rydberg gases
Exotic, ultracold states of matter are challenging physicists to draw on expertise from atomic, condensed-matter and plasma physics, and causing a few surprises along the way. The ultracold world has fascinated and surprised scientists since 1911, when Heike Kamerlingh Onnes discovered superconductivity in mercury at 4.2 K. Now physicists routinely achieve temperatures millions of times colder. When atoms are cooled this close to absolute zero, they fall into the lowest possible quantum state with bizarre consequences...
 
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  • #23
Lets examine the experiment with s rough calculation. Let's take a photon with a wavelegnth of 550nm which is the median wavelength in the visible spectrum.

[tex]E = \frac{hc}{\lambda} = \frac{(6.63\times 10^{-34})(3\times10^{8})}{550\times10^{-9}}[/tex]

[tex]E \approx 3.62\times10^{-19} J[/tex]

Now, assuming the CO2 molecule is initially at rest, the photon is totally absorbed by the molecule and all this energy is transferred into kinetic, this gives a velocity of;

[tex]v = \sqrt{\frac{2E}{m}} = \sqrt{\frac{2(3.26\times10^{-19})}{7.31\times10^{-26}}[/tex]

[tex]\mbox{v \approx 2146 m.s^{-1}}[/tex]

And I know this calculation isn't perfect but it does give some idea of the energy required and the speeds we are dealing with.
 
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  • #24
Farsight said:
Hang on a minute. Kinetic energy doesn't really exist. It's just a term we use when we're talking about masses with motion.
That's kinda self contradictory. Kinetic energy exists because we have defined the term to be about "masses with motion". Saying it doesn't exist is saying that "masses with motion" doesn't exist.
But how do the CO2 molecules acquire their bullet-like velocities as the dry ice gradually sublimes?
Exactly how we have already described: through elastic collisions with other molecules.
 
  • #25
So, what would happen if I had a single molecule in a perfect vaccum, and introduced a single photon, and that photon was absorbed by one of the atoms in the molecule?

Would the molecule move?
 
  • #26
prj45 said:
So, what would happen if I had a single molecule in a perfect vaccum, and introduced a single photon, and that photon was absorbed by one of the atoms in the molecule?

Would the molecule move?
I would suggest you read my above post. This concerns an isolated CO2 molecule which absorbs a photon of visible light.
 
  • #27
prj45 said:
So, what would happen if I had a single molecule in a perfect vaccum, and introduced a single photon, and that photon was absorbed by one of the atoms in the molecule?

Would the molecule move?

A couple of things to consider:
1] The molecule will have an initial amount of vibrational energy. Bonds vibrate laterally, radially and rotationally.
2] The molecule will have an initial amount of kinetic energy.
Neither of those two can you eliminate in your experiment.

Now, given the above, and given that your one photon was absorbed by your molecule, the answer is yes, its kinetic energy would change. Yes, it would move.
 
  • #28
DaveC426913 said:
1] The molecule will have an initial amount of vibrational energy. Bonds vibrate laterally, radially and rotationally.
2] The molecule will have an initial amount of kinetic energy.
Neither of those two can you eliminate in your experiment.

You could turn the temperature down...
 
  • #29
Thanks for your input guys.

I guess what I don't get, is what is happening when a photon meets an molecule. Or let's keep it simple and say an electron. Which is related to the fact that I can only guess at what a photon is, and what an electron is.
 
  • #30
masudr said:
You could turn the temperature down...
Even at zero kelvin there would still be some movement.
 
  • #31
Farsight said:
Thanks for your input guys.

I guess what I don't get, is what is happening when a photon meets an molecule. Or let's keep it simple and say an electron. Which is related to the fact that I can only guess at what a photon is, and what an electron is.
If you do not want to go into things too deeply,you can just think of a photon as a particle, it carried kinetic energy and momentum, just think of it as a moving particle (photon) colliding with a stationary particle.
 
  • #32
Yup, thanks everyone, v interesting.

I'll not ask what started all this movement off in the first place... :p
 
  • #33
Hootenanny said:
If you do not want to go into things too deeply,you can just think of a photon as a particle...
I do want to get into it more deeply, Hooteney. Particles are giving me a big problem at the moment. I just can't see a photon as a particle any more. It seems more like an action - I don't know, a punch, a whiplash, a shout, a ripple with no pinpoint location and any size you like. And an electron seems like some kind of standing wave onion ring ripple. I'm somewhat driven by the search for concepts here.
 
  • #34
Farsight said:
I do want to get into it more deeply, Hooteney. Particles are giving me a big problem at the moment. I just can't see a photon as a particle any more. It seems more like an action - I don't know, a punch, a whiplash, a shout, a ripple with no pinpoint location and any size you like. And an electron seems like some kind of standing wave onion ring ripple. I'm somewhat driven by the search for concepts here.

In all your postings, you seem to have ignored one other possibility - that the concept that you are using such as "particle" may be the one at fault!

It is very possible that the property of "particle" may have no relevancy at such a scale. So trying to force something like a photon or an electron to have a particle property is like trying to force a square object through a round hole. When it doesn't fit, you blame the square object. Why not also blame the round hole? You are trying to force two incompatible concept together that may or may not work.

Electrons can, under classical conditions, be thought of as classical particles. People working in particle accelerators use this all the time to excellent accuracy. Photons can be thought of as clumps of energy. That model has worked everywhere. So let's leave it at that, rather than trying to dress it with properties that it wasn't meant to have in the first place.

Zz.
 
  • #35
Farsight said:
I guess what I don't get, is what is happening when a photon meets an molecule. Or let's keep it simple and say an electron. Which is related to the fact that I can only guess at what a photon is, and what an electron is.
Well, it depends on the energy (wavelength/frequency) of the photon. First of all, molecules are composition particles, and they are three dimensional. They can translate, rotate and vibrate at different characteristic frequencies (corresponding to different energy states). In addition, some (many) molecules have charge asymmetries leading to a dipole moment.

The interaction of the photon depends upon whether or not that photon matches anyone of many characteristic frequencies. In some cases, a photon may just simply pass right pass or 'through' the molecule with little interaction. On the other hand, if not absorbed in some excitation process, the photon will be scattered - redirected.

Photons and electrons exhibit both particle and wave behavior. Particle in the sense of being 'localized' or 'quanitized', and wave in the sense of collective behavior, e.g. interference.

How do know this? By experiment and observation.

We observe the Universe/Nature in which we exist. We use mathematics and physics to try to describe, explain and understand what we observe. Sometimes we can accurately describe the behavior we observe, and others time not. In the end, the Universe/Nature is what it is, and the challenge is to understand it, and even appreciate it, in the limited time of our existence. :smile:
 

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