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Science nerds of Lemmy: Is there an upper limit on the volume of a "confined gas"?
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this post was submitted on 29 Sep 2025
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The understanding of a "confined" gas I got from high school physics class is one in a sealed rigid container, so both the volume and mass of the gas was constant. I did a lot of solving PV=nRT a lot, and there was a lot of "What is the pressure of x kilograms of this gas in a y liter container at z celsius?" Convert kilos of that gas to mols, convert celsius to kelvin, plug and chug. Or we'd calculate things like, you have a pneumatic cylinder with this much gas in it, the cylinder's dimensions are this by that, there's a weight on the piston, increasing the temperature by this much, how far will it lift the piston? So, maybe a more robust way of stating it is we're holding all factors constant except one independent variable and one dependent variable.
It occurs to me that we only did that kind of thing to dimensions of human machinery. That kind of math can reasonably model the physics of, say, a steam engine. But you can scale the apparatus up to the point that other factors become significant, I suppose is the breakdown here.
Earth's atmosphere is held to the planet by gravity but the force of pressure can resist gravity; heat up the atmosphere and it expands up into space. Gravity works almost like a balloon in that way, heat the contents of a balloon and it expands. Except gravity causes a pressure gradient throughout the gas in a way an elastic balloon doesn't. When we're talking about planetary atmosphere scale, we get into different areas of it being heated unevenly, and it taking significant time for air to move around and equalize pressures and densities...
When I initially asked the question, I imagined encasing the earth in a rigid shell at Armstrong's line, which I don't think would have much of an effect on the behavior of atmospheric pressure. I was trained to model the gas in a sealed bottle as one parcel of fluid that behaves as one thing, the pressure everywhere in a 1L bottle of gas is the same, but by the time you get to the size of a planet's atmosphere you have to take gravity and sloshing and such into account.
TL; DR: There's a gulf between 150 level college physics class and aviation meteorology.