[TheForge] More on fire piston physics

[Bruce Freeman]

So I just spent half a day with Andy.  Do you think I
got a word in edgewise to explain why Gay-Lussac's law
is irrelevant?

Anyway, Gay-Lussac's, Charles' and Boyle's laws are
all encapsulated in the ideal gas law.  The ideal gas
law applies (to IDEAL gases) but if we limit our
attention then these other laws apply as well.  In the
case of Boyle's law, if we (gently) compress a gas,
its absolute pressure will increase the same multiple
that its volume decreases.  This is WITHOUT a
temperature change.  Hence, neither Boyle's law nor
the ideal gas law explain how a fire piston work.

Invoking Charles' or Gay-Lussac's laws is missing the
point.  Those are included in the ideal gas law.  If
pressure, volume AND temperature change, the ideal gas
law will apply, but Gay-Lussac's, Charles' and Boyle's
laws WILL NOT.

So we're back to conservation of energy.  Energy (as
work) in equals energy (as heat) out.  The temperature
change will be the heat energy divided by the average
heat capacity of the system.  

Of course, determining the average heat capacity is
damn near impossible.  Mass that's insulated from the
heat will be irrelevant.  With good design, using
inherently insulating materials (like wood), much of
the heat will go into the tinder to raise its
temperature.

Bruce
NJ

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