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Topic: What cause gas pressure?  (Read 53006 times)

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Offline Winga

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What cause gas pressure?
« on: August 04, 2004, 03:05:26 AM »
Is it the repulsion force between gas molecules?

Offline gregpawin

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Re:What cause gas pressure?
« Reply #1 on: August 04, 2004, 05:07:00 AM »
Presure is merely force over a defined area.  Gas pressure is just the collective force of gas molecules running into a container wall.  Since gas molecules have such small masses, these gas molecules are moving at very fast speeds.

The reason why these things do bounce off the walls and one another is because molecules and atoms have clouds of electrons around them that repel each other, positive repels positive and negative repels negative.  So these things didn't have these repulsive electrostatic forces, they'd just run right through each other.  So you're right.
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Offline Winga

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Re:What cause gas pressure?
« Reply #2 on: August 04, 2004, 06:51:12 AM »
One more thing, pressure increases as temperature increases (keep volume as constant).
So, how temperature affects the electron clouds of the molecules?

Offline jdurg

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Re:What cause gas pressure?
« Reply #3 on: August 04, 2004, 04:29:48 PM »
As temperature increase, the motion of the atoms increases as well.  (Hence when you are boiling water, the heat causes the water molecules to move around faster and faster until they have so much energy from the heat that they become a gas).  The higher energy/movement of the molecules is also what causes density to decrease as temperature increases.  In the case of the gas, the gas molecules are moving around at a faster rate and with more energy, so the force they exert on each other and the walls of the container increases as well.  
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Offline Winga

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Re:What cause gas pressure?
« Reply #4 on: August 04, 2004, 09:51:13 PM »
As temperature increase, the motion of the atoms increases as well.  (Hence when you are boiling water, the heat causes the water molecules to move around faster and faster until they have so much energy from the heat that they become a gas).  The higher energy/movement of the molecules is also what causes density to decrease as temperature increases.  In the case of the gas, the gas molecules are moving around at a faster rate and with more energy, so the force they exert on each other and the walls of the container increases as well.  
Yes, I know, and I also want to know the increasing movement of the molecules is due to the larger repulsion between them which is caused by the increasing temperature or not.

Thus, higher the temperature, larger the repulsion, faster the movement of molecules.
Is it?

Offline gregpawin

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Re:What cause gas pressure?
« Reply #5 on: August 05, 2004, 05:29:44 AM »
The point is that the change in temperature increases the kinetic energy of the gas, making them faster and run into the walls of the container more often giving the impression of even pressure.

The whole point of talking about electron clouds is to be able to treat gases as hard objects, things that bounce off each other for the most part.  However, in terms of gas pressure, as temp goes up, gases move faster and *Ignore me, I am impatient* into walls more often creating pressure.
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Offline Winga

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Re:What cause gas pressure?
« Reply #6 on: August 05, 2004, 06:34:00 AM »
The point is that the change in temperature increases the kinetic energy of the gas, making them faster and run into the walls of the container more often giving the impression of even pressure.

The whole point of talking about electron clouds is to be able to treat gases as hard objects, things that bounce off each other for the most part.  However, in terms of gas pressure, as temp goes up, gases move faster and *Ignore me, I am impatient* into walls more often creating pressure.

For instance, wind, the moving air molecules, when it hits the wall at particular area, pressure is also generated (faster the movement of molecules, higher the pressure).
But the temperature of those (wind) molecules can be cool, right?

Furthermore, we assume charge particles as gas molecules. If we compress the charge particles (like charge) from large volume to smaller volume in a container, it will increase the potential energy of the particles just like we compress the spring. Thus, if we compress the charge particles harder, when they are released, they will move faster because of higher kinetic energy which due to higher potential energy.
   
From my point of view, regarding the gas molecules as charge particles, in a constant volume, when temperature increases, the electron clouds of gas molecules are being affected (but don't know how). This effect will increase the "charge" (nucleus-nucleus / electron-electron repulsions) of the molecules that produced higher kinetic energy and the molecules move faster.

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Re:What cause gas pressure?
« Reply #7 on: August 05, 2004, 06:54:08 AM »
What i know is that by increasing temperature the energy of gas paricles increases.

E=3/2RT [For monoatomic gases]
and this results in overall increase in energy by which transational energy increases so motion of gas particles increases.

I wonder how change in temp. will affect the electron clouds,perhaps due to increase in random motion of the gas molecules there will be more repulsion between the electron clouds.

Offline Winga

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Re:What cause gas pressure?
« Reply #8 on: August 05, 2004, 09:01:06 AM »
e.g. if temperature increases that causing the electron(s) jump to the higher state (another orbital), more details of this mechanism will be found out.

From textbook, it just said higher the temperature, higher the kinetic energy!
BUT WHY?
« Last Edit: August 05, 2004, 09:25:28 AM by Winga »

Offline gregpawin

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Re:What cause gas pressure?
« Reply #9 on: August 05, 2004, 10:24:43 AM »
If you get such a thing as wind, or a pressure gradient, PV=nRT no longer works.  We're assuming the gas is in a static condition.  Also, yes pressure is related to temperature but not exclusively so.

Gas molecules/atoms in "normal" conditions neutral with the electrons canceling out the charge of the nucleus.  If they were charged, gosh... I can't imagine, gases would fly off into space or something and a little wind could cause all electric circuits to blow.

Temperature is merely a macroscopic property that manifests as a result of collective microscopic events.  By the kinetic gas theory we can say that the temperature is the average kinetic energy of the gas.  Energy can be in the form of electrical, kinetic, or due to the prescence of other fields, ie gravity.  However, gases in average non-ultra high temperatures are always in the ground state electrically.  The only energy that ever changes is its mechanical energy which is in the form of translational and vibrational energy.  If there is just one atom comprising of the gas molecule, then there is no possible vibration mode, so there's only translational energy, therefore it would be safe to say that we can relate the gas's average speed to its energy.
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Offline Winga

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Re:What cause gas pressure?
« Reply #10 on: August 05, 2004, 11:25:23 AM »
Using an iron bar as example, increasing the temperature will also increase the speed of vibrational motion of the iron atoms.

Before increasing the temperature, iron atoms are attracted together because the attractive force between the nuclei and the sea of electrons is greater than that of nucleus-nucleus repulsion plus electron-electron repulsion.

When temperature increases, the atoms oscillate. Once they oscillate, the atomic radii increase. In this case, the nucleus-nucleus repulsion plus electron-electron repulsion are greater than the nuclei-electrons attraction, is it?

Therefore, increasing temperature causes the greater repulsion between atoms (?).


By the way, I am not really meant that the molecules carrying charges.
« Last Edit: August 05, 2004, 11:29:00 AM by Winga »

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Re:What cause gas pressure?
« Reply #11 on: August 05, 2004, 11:34:34 AM »
I think temperature does not effect the atomic radii.It does increase its kinetic energy.

Offline movies

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Re:What cause gas pressure?
« Reply #12 on: August 05, 2004, 12:17:56 PM »
I agree with everything greg has written so far, but I'm going to try to add something to clarify the problem a bit:

In the gas phase the atoms are so far apart that the electrostatic repulsions are probably negligeable (see Coulomb's law).  If two atoms did get close enough to make these repulsions significant, the result would just be a standard collision and the two particles would wander off in different directions.  That type of collision, however, would have no effect on the pressure in the vessel, since there is no force exerted on the walls of the vessel.

greg is the expert on this though, so I would listen to him.

Offline jdurg

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Re:What cause gas pressure?
« Reply #13 on: August 05, 2004, 12:31:29 PM »
I think temperature does not effect the atomic radii.It does increase its kinetic energy.

Actually, I think it does.  When you are increasing temperature you are adding energy to the atom/molecule.  As the energy is absorbed, the electrons wind up having higher energy as well, so they will move up to a higher energy level.  Since those higher energy levels are further away from the nucleus, the atomic radius of those atoms will increase.  If you apply enough energy, the electrons will have so much energy that they will leave the atoms altogether.  This is how a plasma is formed.  (All the atoms in the middle of the sun have no electrons.  It's simply a very high temperature plasma).  
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Offline gregpawin

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Re:What cause gas pressure?
« Reply #14 on: August 05, 2004, 05:48:37 PM »
The kind of heating up I think you're talking about, not the kind of temperature that creates plasmas, does not affect the electronic structure. In a static environment, you can think of everything as electrostatic interactions, however, temperature changes I think you're talking about is at the scale of vibrations.  Boiling off of molecules from the bulk is then the interplay of mechanical energy and electrostatics where we're trying to see if there's enough mechanical energy in the current geometry (lets say at the surface) to break free of the electrostatic attractions of the bulk.  
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