In article , Richard J Kinch
says...

The case of venting it into vacuum is the same as venting it to
atmosphere.

I disagree. Nothing to push against, no force, no work, no heat. Just
eternal expansion.

Look, you seem to be a very thoughtful individual, who maybe has not
had a great deal of formal training on the subject. I've had a little,
and there are others here who have had a lot. So making bald
statements like the one above simply won't fly. An introductory
book on thermodynamics will show you that the two cases (connecting
a charged reservoir of compressed gas to two different second
reservoirs - one at zero pressure absolute, and the other at 15psi
absolute - are handled exactly the same way mathmatically.

And that the two cases where the second reservoir is about the
same size as the first, or very much larger, are also part and
parcel of the same behavior.

The rules don't require any piston to push against, when connecting
two compressed gas bottles, each at different pressures or sizes.
The gas law explains what happens when the valve is opened, and
a number of posters here have pointed this out. Replying
"but it's just not *so*" may be emotionally satisfying but really
does not have a great deal of predictive power, nor does it
enlighten you about the subject at hand.

Thermodynamics does indeed explain how things like ideal gasses
behave, what entropy really is (and not just the popular
definition one finds in the media) and how one can predict exactly
how efficient a given process or engine is going to be, and what
the source of the various losses are. It arose partly because
at the time, there were real world engines being manufactured
and folks wanted a theoretical basis for their behavior - which
had been wanting. It also arose to explain certain non-classical
behaviors and was a glimpse into the idea that the world really
was not explainable with newtonian physics alone - that one
could not simply model things with little levers and gears and
so on, and *still*get*the*right*answer*.

Your attempts to explain the behavior of ideal gasses without
using thermodynamics is leading you to the same point that our
predecessors reached: the wrong answer.

Charging a compressed gas bottle from a compressor, or from
another, higher pressure bottle, will yeild the same result: the
bottle that is filling will warm up. Thermodynamics says this
happens in both cases, and further that it happens for the
exact same reason. And the experiement has been done time and
time again over the years since the theory was written down,
and each and every time the experiment has agreed with the
theory.

Jim

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