properties of gas and effects on expansion work

[Anonymous No.11260380]

How do different gases effect what meaningful work can be done by the expansion of that gas?

As an example, imagine a compressed air gun, or potato gun. It's a tank of compressed air which opens and expands, firing a projectile.

Assuming all other variables are constant, like initial pressure, mass of projectile, length; how does the type of gas affect the energy delivered to the projectile?

Would a tank of helium, or propane, launch a projectile at the same velocity as normal air? I would instinctively assume that 100 PSI (or whatever) of a compressed gas has the same potential to do expansion work, regardless of the type of gas, but is that true? Surely intermolecular forces in the gas would have an effect. Would a denser gas expand more slowly, resulting in a slower projectile, or would the more massive gas particles deliver more energy to the projectile over the same distance?

[Anonymous No.11260381]

As someone studying thermodynamics myself, you've awakened my interest too now. bump

[Anonymous No.11260389]

>>11260380
The boundary work performed by an expanding gas on its surroundings is given by
[eqn] W_B=\int_{1}^{2}P\text{ d}V [/eqn]
where P is pressure, V is volume of whatever contains the gas, and 1 and 2 are initial and final states of the gas. The most important thing to realize about this equation is that work performed by the gas depends on the path it takes from 1 to 2. So, the work done by a gas subjected to some kind of heat or impetus absolutely depends on how that gas behaves (if it acts like an ideal gas, for example), and also on the mechanical and thermal constraints of its container (like, is the gas permitted to expand at constant pressure, constant temperature, if it is insulated, etc).
>Would a tank of helium, or propane, launch a projectile at the same velocity as normal air?
Do they travel the same path from compressed state to atmospheric state? Meaning, are the starting and final states the same for each gas? They yes. But I don't think you can manage that with two really different things like helium and propane.

In short, no.

[Anonymous No.11260395]

>>11260389
>Meaning, are the starting and final states the same for each gas?
I phrased that bad. The path is just as important as the states 1 and 2.

[Anonymous No.11260415]

>>11260389
I would imagine that it wouldn't be the same, because I mean even two instances of the same gas back to back wouldn't be the same.

What effects would be significant in modelling the behavior? Handwave away heat loss to the environment, things like that.
With no math to back it up, I could believe that more bulky atoms or molecules would require more energy to compress, and therefore would release that energy more readily when the valve is opened. Just as like a steric effect. I'd figure there would be less potential to do work from a canister of compressed hydrogen than a container of compressed argon, all other things equal.

[Anonymous No.11260438]

>>11260415
>I'd figure there would be less potential to do work from a canister of compressed hydrogen than a container of compressed argon, all other things equal.
doesn't make sense now that I think about it. A hard to compress spring with 50 pounds on it, and an easy to compress spring with 50 pounds on it, are both pushing back with 50 pounds, one is just shorter than the other.
In the same way, a hard to compress gas at 100 PSI may have fewer particles than an easy to compress gas at 100 PSI, but it still exerts the same pressure on the container.