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# Re: [Phys-l] T dS versus dQ

And I thought the rapid compression to ignite the tissue in the cylinder was to make it "adiabatic". So even in a well insulated app. a slow depression of the piston would not ignite the tissue. This explains the clouds of soot* emitted by a slowly moving truck and why they have sooo many gear ratios?

* Each particle is coated with extremely carcinogenic benzo-pyrenes.

bc educated?

p.s. The tissue igniting app. was a make and take during a recent So. Cal. Physics Techs. Assoc. meeting. This, T dS, may have been explained, but my deafness precluded knowing.

On 2010, Jan 13, , at 08:53, John Mallinckrodt wrote:

Jeffrey Schnick wrote:

John M: I took Bob's words to mean that the final temperature
resulting from a fast compression would be the same as the final
temperature resulting from a slow compression (not the same as the
initial temperature). I disagree with Bob on this point in that
the greater the speed of the pistion the greater the local pressure
at the face of the piston. The relative velocity between the
piston and the gas molecules is greater when the piston is closing
in on them and hence the momentum transfer to the piston is greater
with each collision and you have a greater collision rate; hence
the greater pressure. Thus the piston does more work on the gas
when the piston is moving faster so the final internal energy of
the thermaly insulated gas must be greater and the final
temperature of the gas must be greater.

Right. I prefer, however, to think about the work done in terms of
force (and displacement of the piston) rather than pressure (and
volume change of the gas), because the pressure in the gas is ill-
defined. Indeed, the pressure that you want to use in this
calculation can only be determined finding the force applied to the
gas by the piston and then dividing by the piston area. Note also
that one can at least imagine constructing a compression procedure
during which that "pressure" is sometimes close to zero even when the
piston is moving toward the gas. For instance, give a big push on
the piston, then pull it back very quickly, and then immediately
begin "compressing the gas" again. The point is that the big inward
push and retreat creates a bit of a void that the piston moves
through during the initial phase of the subsequent compression. It
is a minor triumph of thermodynamics that we know that the net work
done on the gas will, in ANY event be larger than that done in a
reversible (isentropic in this case) compression precisely BECAUSE of
the irreversible nature of the process.

John Mallinckrodt
Cal Poly Pomona
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