In article ,
says...
Ned Simmons wrote:

Richard J Kinch wrote:
1. ANY EXPANSION OF COMPRESSED AIR NECESSARILY DOES WORK.

This is key to my misunderstanding. In everything I've read on the previous
(original) thread, this wasn't explained, only asserted. So, I have to ask:

Why does any expansion of compressed air necessarily do work?
Grant

Your intuition is right, expansion of a gas doesn't
necessarily do work, but the entropy of the gas increases
as it expands.

As I've said before, the "lossy" character of regulators is
easy to demonstrate without resorting to thermodynamics.
However, understanding the energy balance of the loss is
impossible without thermo, and entropy is central to the
explanation.

I'm not looking for a demonstration, Ned. A simple explanation would do.
If you find it "impossible" to explain then that's one thing, but could
you at least try? I'm very frustrated with people who simply assert complex
facts without trying to explain them. They do teach thermodynamics to college
freshmen, and I don't usually fail to understand concepts from freshman physics.
Grant



Unfortunately the demonstration is much easier than the
explanation. I just looked at Resnick and Halliday (my
freshman physics text) and it devotes about 40 pages to a
good quick introduction to thermo. I took a full semester
thermo course as a sophomore or junior MechE student around
1972 and haven't thought about it much since. This is my
long winded way of saying that even if I felt qualified,
I'd be reluctant to try to explain one of the most abstract
concepts in the classical ME curriculum here.

That said, if you're willing to spend some time digging for
material you find readable, here are a few of the concepts
that are relevant. Please don't take any of this as a
rigorous presentation, and don't waste time with web sites
that use entropy to prove the existence (or non-existence)
of God. g

The first law is the conservation of energy, pretty
straightforward and intuitive to most of us. However, many
processes can be imagined that, although they do not
violate the first law, do not proceed naturally. For
example a hot chunk of metal will not jump up in the air
and drop in temperature to compensate for its increased
potential and kinetic energy.

The second law predicts which processes *will* occur
naturally, and entropy is the property that quantifies the
2nd law. Any process that occurs on its own will move in a
direction such that net entropy increases. Entropy is *not*
conserved, and to my mind, is not at all intutitive.

Entropy can also be thought of as an indicator of how much
of the energy in a system is available to do work. As
entropy rises, energy available for work falls. Relevant to
this thread, the entropy of a gas increases as it expands,
reducing available work.

That's as far as I'm sticking my neck out-hopefully it'll
give you a place to start.

Ned Simmons