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Re: isobaric expansion

At 07:05 PM 3/17/00 +0200, Savinainen Antti wrote:
Is there momentarily greater pressure exerted on the piston
for instance in the beginning of the process?

That's an interesting question. Answer: there is not _necessarily_ any
pressure excursion.

The piston must provide a force to balance the gas pressure x area. IF
(big IF) this force comes from the weight of the piston, and IF the
expansion starts suddenly, and IF we don't do anything about it, then yes,
during the startup phase the gas must provide some extra pressure. It must
support the weight of the piston *and* accelerate it.

But on the other hand, we can make the piston very light; the required
force can come from light-but-strong springs and levers and such. Also we
can arrange that the expansion begins very gradually. AFAICT the pressure
excursion can be made arbitrarily small.

We can go even further and make the pressure excursion zero or even
negative by giving the piston an ad-hoc acceleration just at the time that
the heating of the gas begins. We can iterate if necessary to learn to
anticipate the correct amount of acceleration.

Nothing in this world is perfect, so there will never be a _perfect_
isobaric expansion, but there is no reason to believe there are any
fundamental problems worth worrying about.