# Why Do Molecules Move?

#### prj45

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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#### mathman

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).

#### pallidin

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.

#### Farsight

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 travelling 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..."

#### Doc Al

Mentor
Farsight said:
Is a molecule of hydrogen gas at a fairly ordinary pressure really travelling at 6000 feet per second?
That sounds about right for the rms speed of room temperature hydrogen gas.

#### Farsight

So I'm surrounded by air travelling at the speed of a bullet. Hmmn.

#### Hootenanny

Staff Emeritus
Gold Member
Farsight said:
So I'm surrounded by air travelling 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.

#### prj45

Farsight said:
So I'm surrounded by air travelling 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?

#### russ_watters

Mentor
Farsight said:
So I'm surrounded by air travelling 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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#### russ_watters

Mentor
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.

#### prj45

So could I just stop a molecule of hydrogen gas, and if I did would it just sit there, presuming its on its own.

#### JohnSimpson

To completley stop the molecule's movement you would have to reduce it's temperature to 0 K's

Good luck to you.

#### russ_watters

Mentor
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.

#### Hootenanny

Staff Emeritus
Gold Member
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.

#### prj45

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?

#### prj45

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?

#### russ_watters

Mentor
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...

#### prj45

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?

#### mathman

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.

#### AlphaNumeric

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.

#### DaveC426913

Gold Member
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.

#### Farsight

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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#### Hootenanny

Staff Emeritus
Gold Member
Lets examine the experiment with s rough calculation. Lets take a photon with a wavelegnth of 550nm which is the median wavelength in the visible spectrum.

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

$$E \approx 3.62\times10^{-19} J$$

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

$$v = \sqrt{\frac{2E}{m}} = \sqrt{\frac{2(3.26\times10^{-19})}{7.31\times10^{-26}}$$

$$\mbox{v \approx 2146 m.s^{-1}}$$

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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#### russ_watters

Mentor
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.

#### prj45

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?