# Why does temperature rise with pressure?

Discussion in 'Physics & Math' started by Magical Realist, Jun 22, 2014.

1. ### Magical RealistValued Senior Member

Messages:
14,087
Seems like the pressure would be holding the atoms stiller, thus lowering the temperature.

to hide all adverts.
3. ### originHeading towards oblivionValued Senior Member

Messages:
11,502
The pressure increase can take many forms but the idea is the same for all of the sources of the pressure increase.
First all of the atoms are in motion in the gas.
Lets look at a piston compressing a gas. As the piston moved towards the bottom of the cylinder the atoms gain the kenetic energy of the moving piston increasing their speed and hence the temperature of the gas through the collisions of these higher KE molecules.

to hide all adverts.
5. ### Magical RealistValued Senior Member

Messages:
14,087
But the pressure on underground rocks creates heat without motion or kinetic energy. Does mere weight create heat energy? And if so, why isn't the water at the bottom of the ocean boiling?

to hide all adverts.
7. ### exchemistValued Senior Member

Messages:
11,155
Pressure does not "hold" anything still. On the contrary, pressure is due to the force exerted on the walls of a container by the molecules hitting it and rebounding off it.

You are presumably speaking about compressing a gas "adiabatically", i.e. without allowing heat exchange with the surroundings. Under these conditions, the volume available to the molecules is reduced, but their total kinetic energy of motion (= heat energy) does not change. So the same amount of energy is concentrated in a smaller volume. This means the temperature must rise.

Kinetically, what happens is the piston moving in to compress the gas transfers extra momentum to the molecules that bounce off its surface, raising their speed and hence the temperature.

8. ### Magical RealistValued Senior Member

Messages:
14,087
Obviously it does or paperweights and anchors wouldn't work.

9. ### exchemistValued Senior Member

Messages:
11,155
Suit yourself.

10. ### sideshowbobSorry, wrong number.Valued Senior Member

Messages:
6,770
Paperweights and anchors work by friction, not pressure.

Pressure is not a thing that can "hold" something else. Pressure and temperature are both caused by the kinetic energy of the molecules.

11. ### Magical RealistValued Senior Member

Messages:
14,087
The increase of friction comes from the pressure the object is exerting on the surface. But this sort of restraint can happen independently of friction. Pushing on a cart to keep it from rolling downhill would be an example of that. The pressure you are exerting on it holds it still.

12. ### sideshowbobSorry, wrong number.Valued Senior Member

Messages:
6,770
As somebody explained, pressure in a gas is caused by the gas molecules bumping against their surroundings. The column of air above your paper exerts a pressure of 14.7 pounds per square inch (at sea level) but it doesn't hold your paper down. On the other hand, a much lighter weight will hold your paper down.

13. ### Magical RealistValued Senior Member

Messages:
14,087
You're hung up on air pressure. Pressure is more general than that. It is the amount of force per unit of area. Without the pressure of the paperweight on the paper and the table, friction wouldn't hold the paper down. Pressure holds it still.

So tell us, why does temperature increase with pressure?

14. ### originHeading towards oblivionValued Senior Member

Messages:
11,502
I told you why an increase in gas pressure increases temperature. For a solid increasing the pressure compresses the material that means that the molecules of the material move. This movement causes friction which increases the temperature. If you increase the pressure the temperature will increase if you maintain the pressure the temperature will decrease as it is transfered to the surroundings. So a material that is under great pressure is not necessarily at a higher temperature.

15. ### sideshowbobSorry, wrong number.Valued Senior Member

Messages:
6,770
Temperature is a measure of how fast the molecules are moving. Pressure is a measure of how hard they are hitting their surroundings - e.g. a container. The faster they move, the harder they hit. Simple.

Pressure isn't outside "holding" anything; it's inside.

16. ### LaymanTotally Internally ReflectedValued Senior Member

Messages:
1,001
Just like when you try to hold a child stiller, they will just start fidgeting a lot more and more. I think it would be due to the electromagnetic force or the resistance of like charges in their electrons. It would be like trying to put the wrong ends of a magnet together. The more pressure you apply, the more resistance you would get.

Water, ice would be an exception to the rule of density and temperature. It is thought that, since water crystallizes, it makes it less dense and take up more space being colder. Then it is just the crystalline structure that makes it take up more space.

17. ### Magical RealistValued Senior Member

Messages:
14,087
So when the barometric pressure is 14 lbs per sq inch, that's a measurement of the pressure objects are exerting from the INSIDE? I thought it was a measure of the external air pressure.

As for pressure in solids, I stand corrected. High pressure in and of itself DOESN'T increase temperature, as in the core of the earth. But increased pressure DOES, being the conversion of the increase of the pressure into more frequent collisions of the molecules resulting in more heat.

18. ### originHeading towards oblivionValued Senior Member

Messages:
11,502
Uh, no. Density is not related to temperature. Lead is a more dense than iron, do you think this has anything to do with their temperature?

19. ### LaymanTotally Internally ReflectedValued Senior Member

Messages:
1,001
OMG, not again. Lead that is at a colder temperature than another piece of lead would be denser. Iron that is at a colder temperature than another piece of iron would be denser. Then since they are different elements they can have different densities and remain at the same temperature.

20. ### sideshowbobSorry, wrong number.Valued Senior Member

Messages:
6,770
That may be the source of the confusion. The temperature inside the container increases the pressure inside the container. There is resistance from the pressure outside the container.

21. ### originHeading towards oblivionValued Senior Member

Messages:
11,502
I misread your post. You are correct on the above quote. But colder ice is more dense that warmer ice. Once you talk about phase changes the brings in a different variable. Anyway, we are talking about pressure and it's effect on temperature. So if you put a piece of lead under 10 tons/sq. in. of pressure it will increase in temperature. If you wait for the temperature to equalize with the surroundings then this higher density lead will be at the same temperature as lead that is less dense and not under pressure.

22. ### arfa branecall me arfValued Senior Member

Messages:
7,652
If increasing the pressure meant the temperature also increased, then if you had a block of ice exerting pressure on say, the surface of a table, a larger block of ice on the same table would mean it had a higher temperature than the smaller block.

So why doesn't that happen? Is it because for the solid block of ice, the pressure at the table surface is larger than the atmospheric pressure? Like in a column of liquid, the pressure increases with depth. A gas in a container doesn't have any pressure gradient.

23. ### exchemistValued Senior Member

Messages:
11,155
Yes the thing one needs to do is distinguish the effect of pressure in compressible fluids (gases), incompressible fluids (liquids) and solids. You initial question makes sense for gases and I took it as such.

It is virtually impossible to compress liquids and solids, so for these, to a first approximation, increasing the pressure does not reduce the volume and therefore there is no temperature increase.

If you increase pressure enough you can affect solids and liquids of course, a well-known example being the melting of ice when it is compressed, e.g. under the blade of a skater. This however is due to the pressure lowering the melting point, not to heating.