[Johann Mahne]

Johann Mahne  asked the Naked Scientists:
   
Hi Chris,

Why do gas molecules speed up when their energy increases?

regards
Johann

What do you think?

[Phractality]

Because that's what energy is. The kinetic energy of a molecule is a measure of how fast it is moving. For a molecule with mass m and velocity v, KE is defined as ½mv². The total energy of the gas includes the sum of all the kinetic energies of the molecules.

[chris]

But why does having a higher temperature mean more kinetic energy?

[Geezer]

But why does having a higher temperature mean more kinetic energy?

Well, see, it's just like when you step on a very hot pavement in your bare feet. That makes you move a lot faster.

[JP]

Temperature is defined to be the average kinetic energy. 

Heat energy tends to flow from higher energy to lower energy systems, so knowing the temperatures of things tells you how heat will flow between them.

[yor_on]

Is energy a expression of motion?

How about Earth, a he* of a lot of 'energy' in a earth, or in any matter. And according to Einstein equal to a motion. I don't really now how far you can take those equalizations? But to me there seems to be a truth in that matter can be translated into motion.

[JP]

Is energy a expression of motion?

Kinetic energy is.  Other forms of energy, for example the energy in an electromagnetic field, aren't.

[Geezer]

Maybe Feynman covers this in one of his interviews too  [] []

[imatfaal]

Carl Sagan definitely does.  Early childhood memory of a mad american scientist talking about everything just jiggling about.  Will search out a link.

[imatfaal]

Carl Sagan definitely does.  Early childhood memory of a mad american scientist talking about everything just jiggling about.  Will search out a link.

[Edit]
Massive climbdown - it was Feynman I saw in my youth not Sagan.  I have remembered that wrongly for years

[/Edit]

[lightarrow]

"Why do gas molecules speed up when their energy increases?"

To find quickly a cool place.  []

[yor_on]

A slightly sardonic 'mad scientist' then

[Geezer]

"Why do gas molecules speed up when their energy increases?"

To find quickly a cool place.  []

Precisely! It's all about entropy.

[Johann Mahne]

My question is exactly what Chris asks.
Thanks Chris.
Why does the higher temperature convert to kinetic energy?

Imagine a container with a perfect vacumn.
The container is heated to a high temperature.
A single gas molecule (N2) at 2 degrees kelvin is dumped into it.
What will happen to the molecule?
Will it remain at the bottom of the container and heat up,or will it start to move?

[JP]

Temperature doesn't convert to kinetic energy.  Temperature is a measure of average kinetic energy in a system.  Temperature isn't going to tell you much useful information about your single N2 molecule because it requires a lot of particles being averaged to be useful.  All it can tell you is that on average, your N2 particle will come into equilibrium with the rest of the box.  But in reality sometimes your N2 particle will be moving faster than average and sometimes slower than average.

If you have a small box full of cold N2 and a large box full of hot gas, and you let the two mix, then the cold N2 will heat up a lot and the hot gas will cool down a tiny bit as their temperatures slowly come into agreement.

[@donjx]

@NakedScientists //why do molecules move faster when they are hot?// Because that's the definition of "hot"

was tweeted by @donjx

[Johann Mahne]

Thanks JP,

That answers my question.

So the force that's behind the movement of gas molecules is convection.
The kinetic energy is a by product of that force of convection.
It's the difference in temperature between gas molecules that causes their movement, and not the temperature as whole.

[JP]

So the force that's behind the movement of gas molecules is convection.
The kinetic energy is a by product of that force of convection.

Technically, you have it backwards.  All molecules are jiggling about due to their kinetic energy.  Temperature is a direct measure of the average kinetic energy of your gas, so it's a measure of how much they jiggle about.  If you have different temperature gases, the way the molecules bounce off each other while jiggling is the cause of convection.  In other words, the jiggling movement of gas molecules due to kinetic energy is what causes convection to happen, not the other way around.

Don't start by thinking of temperature.  Start by thinking of just 2 molecules of gas moving around.  One is jiggling slowly (low kinetic energy) and one quickly (high kinetic energy).  If you wait a long time, they'll collide.   At that collision, the fast-jiggling molecule will slow down and the slow-moving one will speed up.  After many collisions, they're going to have similar kinetic energy.  It's not too hard to compute the exact motion of two molecules, but when you have billions of them, you need to take averages.  That's why you need temperature, which is an average value of kinetic energy.  It tells you the same thing: on average if you have higher temperature (faster moving molecules) mixed in with lower temperature (slower moving particles), they'll eventually come to an equilibrium temperature and all have similar kinetic energies.  Since you have billions of them, not all of them will have the same exact value of kinetic energy.  Some will be jiggling incredibly fast and some incredibly slow, but on average temperature tells you the story of what's going on.

[CZARCAR]

My question is exactly what Chris asks.
Thanks Chris.
Why does the higher temperature convert to kinetic energy?

Imagine a container with a perfect vacumn.
The container is heated to a high temperature.
A single gas molecule (N2) at 2 degrees kelvin is dumped into it.
What will happen to the molecule?
Will it remain at the bottom of the container and heat up,or will it start to move?

i think the hot walls of the contairer will radiate massless photons  thru the vacumn & into the N2 which will cause electrons of the N2 to jump to higher shells? Does the N2 expand? Will enough photons push an electron beyond the outer shell resulting in a free electron & a lighter N2?

[Johann Mahne]

Temperature is a direct measure of the average kinetic energy of your gas,

Does this only apply to gases, or to liquids as well.How about the molecules vibrating in solids?

Are you saying that the single, frozen, molecule will start moving about and eventually bounce off the container walls(assuming it's smooth walled)? Where does the force come from to start the motion?