# Pressurized gas

1. Apr 22, 2004

### mee

Is the gas in a scuba tank or a butane lighter potential or kinetic energy? Not including gravitational effects.

2. Apr 22, 2004

### chroot

Staff Emeritus
First, a butane lighter is not really just pressurized gas; the butane is a liquid at only a moderate pressure.

In a scuba tank, the stored energy is potential energy. The gas particles have velocities which do not depend upon the pressure, only the temperature.

- Warren

3. Apr 22, 2004

### mee

Then why does air attempt to escape violently from a fully pressurized scuba tank into the atmosphere and not from one that has equalized pressure? I mean, one could power a generator with pressurized air and not with one that has equalized pressure? Sorry if I'm misunderstanding you but Im a novice.

4. Apr 22, 2004

### LURCH

Then it would be kinetic energy. As you release the pressurised air into the gereator, it will make the generator move. At the same time, the pressure inside the tank drops. This is an example of potential energy being converted into kinetic.

5. Apr 23, 2004

### HallsofIvy

Did you understand chroot's answer? He said "In a scuba tank, the stored energy is potential energy." His comment about the velocity of the molecules depending on temperature rather than pressure was irrelevant to that.

6. Apr 23, 2004

### chroot

Staff Emeritus
Halls,

I was just trying to slam home the idea that it's not the kinetic energy of the little gas particles that gives a scuba tank its energy.

- Warren

7. Apr 25, 2004

### ophecleide

Why is it not, at least in part, because of the kinetic energy of the gas molecules? When the gas is released, if it does work equal to the change in potential energy, and no outside energy (such as heat) is added, the gas will be much cooler than when it was compressed, meaning it the speed of the particles has decreased.

8. Apr 26, 2004

### krab

Excellent question! Gas pressurized in a tank contains energy that is available to do mechanical work. If that is your definition of potential energy, then you have it. But such labels do not inform one of the physics. Most of the energy is contained in the kinetic energy of the gas molecules, so you could call it kinetic energy as well. Allowing the gas to expand cools it and uses up some of the molecules' kinetic energy. Here's how that works: Imagine a piston forming one wall of the container. Gas particles bounce off the piston, and their collective effect is a force on the piston. Now allow the piston to be moving. Gas particles hitting the piston will now bounce off with a slightly reduced speed because of the piston motion.

There is also a small component of negative potential energy from the fact that the gas molecules attract each other. It therefore requires energy to separate them. This is the effect that cools the gas on free expansion through a valve. It is a side issue, because even if the gas molecules did not attract each other, you could still recover energy from letting the gas expand.