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Author Topic: why do molecules move faster when they are hot?  (Read 28886 times)

Johann Mahne

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why do molecules move faster when they are hot?
« on: 09/05/2011 21:01:01 »
Johann Mahne  asked the Naked Scientists:
   
Hi Chris,

Why do gas molecules speed up when their energy increases?

regards
Johann

What do you think?
« Last Edit: 09/05/2011 21:01:01 by _system »


 

Offline Phractality

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why do molecules move faster when they are hot?
« Reply #1 on: 09/05/2011 21:14:25 »
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.
 

Offline chris

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why do molecules move faster when they are hot?
« Reply #2 on: 09/05/2011 22:36:16 »
But why does having a higher temperature mean more kinetic energy?
 

Offline Geezer

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why do molecules move faster when they are hot?
« Reply #3 on: 09/05/2011 23:40:26 »

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.
 

Offline JP

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why do molecules move faster when they are hot?
« Reply #4 on: 10/05/2011 05:34:27 »
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.
 

Offline yor_on

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why do molecules move faster when they are hot?
« Reply #5 on: 10/05/2011 12:47:57 »
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.
 

Offline JP

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why do molecules move faster when they are hot?
« Reply #6 on: 10/05/2011 13:00:08 »
Is energy a expression of motion?

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

Offline Geezer

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why do molecules move faster when they are hot?
« Reply #7 on: 10/05/2011 16:47:23 »
Maybe Feynman covers this in one of his interviews too  ;D ;D
 

Offline imatfaal

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why do molecules move faster when they are hot?
« Reply #8 on: 10/05/2011 17:30:40 »
Carl Sagan definitely does.  Early childhood memory of a mad american scientist talking about everything just jiggling about.  Will search out a link.
 

Offline imatfaal

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why do molecules move faster when they are hot?
« Reply #9 on: 10/05/2011 17:35:55 »
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]

 

Offline lightarrow

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why do molecules move faster when they are hot?
« Reply #10 on: 10/05/2011 17:41:21 »
"Why do gas molecules speed up when their energy increases?"

To find quickly a cool place.  ;D
 

Offline yor_on

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why do molecules move faster when they are hot?
« Reply #11 on: 10/05/2011 18:03:06 »
A slightly sardonic 'mad scientist' then :)

 

Offline Geezer

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why do molecules move faster when they are hot?
« Reply #12 on: 10/05/2011 18:48:51 »
"Why do gas molecules speed up when their energy increases?"

To find quickly a cool place.  ;D

Precisely! It's all about entropy.
 

Johann Mahne

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why do molecules move faster when they are hot?
« Reply #13 on: 11/05/2011 10:33:16 »
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?

 

Offline JP

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why do molecules move faster when they are hot?
« Reply #14 on: 11/05/2011 11:11:51 »
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.
« Last Edit: 11/05/2011 11:15:44 by JP »
 

@donjx

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« Reply #15 on: 11/05/2011 17:03:29 »
@NakedScientists //why do molecules move faster when they are hot?// Because that's the definition of "hot"

was tweeted by @donjx
 

Johann Mahne

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why do molecules move faster when they are hot?
« Reply #16 on: 12/05/2011 07:15:59 »
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.


 

Offline JP

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why do molecules move faster when they are hot?
« Reply #17 on: 12/05/2011 10:20:21 »
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.
 

Offline CZARCAR

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why do molecules move faster when they are hot?
« Reply #18 on: 12/05/2011 22:14:08 »
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

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why do molecules move faster when they are hot?
« Reply #19 on: 13/05/2011 13:05:32 »
Quote
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?
 

Offline yor_on

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why do molecules move faster when they are hot?
« Reply #20 on: 14/05/2011 05:07:32 »
Now you're ever so happily moving into Terra incognito :) From how to define the 'energy' in solids and gas, in fact everything that is made out of particles/matter, too what 'energy' really, and I do mean really, is?

That one is interesting indeed. A photon is probably the closest definition of 'pure energy' as it is mass less, intrinsically time less, as well as not really being 'here' as it has no discernible 'size'.

Then you can also define 'energy' as a outcome of interactions where some of that 'energy' involved is gained in a transformation, by some particle or other, as 'usable energy', and some is 'lost' to SpaceTime as 'unusable energy'.

And in that motto 'energy' becomes a description of interactions, not existing on its own. In QM you have the quantum foam existing everywhere though, and that might also be used as a description.

 

Offline JP

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why do molecules move faster when they are hot?
« Reply #21 on: 14/05/2011 08:36:51 »
Quote
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?
Gases are the easiest to think about since the gas molecules bounce about pretty freely in space.  In liquids and solids they're more confined, but the basic idea still holds: things vibrate faster when hot and if you bring a hot object in contact with a cold object, the collisions between vibrating particles will transfer heat energy until they're all vibrating at roughly the same rate.

Quote
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?
In reality, nothing can ever be frozen.  No matter how much energy you take out of that molecule, it will still be vibrating slightly.  This is just a result of how things behave on tiny scales (quantum mechanics).  If the container walls are hot and the molecule is cooled down to almost motionless, it will take a long time, but eventually it will hit a wall and then it will gain energy--it will move faster, hit another wall, gain more energy, etc. 

Also, as CZARCAR said objects emit photons just because of their vibrations.  The hotter they are, the more photons they tend to emit--this is why really hot iron glows red, for example.  If you heat up the walls of your box, it will be emitting photons and some of these will hit your gas molecule and give it energy. 
 

Offline CZARCAR

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why do molecules move faster when they are hot?
« Reply #22 on: 14/05/2011 12:43:58 »
4get the N2 & use He to simplify. Cold He is placed in hot vacumn chamber, photons hit & electrons jump to higher shells. The He expands spatially? Bombarded with enough photons, will the electrons break free from the outermost shell & result in a freed electron & the He become + charged as a result of losing 1 electron?
 

Offline JP

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why do molecules move faster when they are hot?
« Reply #23 on: 15/05/2011 03:36:10 »
4get the N2 & use He to simplify. Cold He is placed in hot vacumn chamber, photons hit & electrons jump to higher shells. The He expands spatially? Bombarded with enough photons, will the electrons break free from the outermost shell & result in a freed electron & the He become + charged as a result of losing 1 electron?

Yeah, this will happen eventually.
 

Offline CZARCAR

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why do molecules move faster when they are hot?
« Reply #24 on: 15/05/2011 21:51:20 »
4get the N2 & use He to simplify. Cold He is placed in hot vacumn chamber, photons hit & electrons jump to higher shells. The He expands spatially? Bombarded with enough photons, will the electrons break free from the outermost shell & result in a freed electron & the He become + charged as a result of losing 1 electron?

Yeah, this will happen eventually.
has it been proven experimentally or theoretically?   just curious
 

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why do molecules move faster when they are hot?
« Reply #24 on: 15/05/2011 21:51:20 »

 

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