Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: DW/DW Summary



Jim,
Nice summary, it should help to sort out the discussion.

1) The First Law is dE=dbarQ + dbarW -- can I please write DQ and DW
--(sorry, Dan, I have trouble with W&Q -- I wish ASCII had a dbar)


From now on in my posts on this thread DW and DQ mean dbarW and dBarQ

2) DQ and DW are BOTH _work_!!!


I agree, with qualifiers. You convinced me in one of the previous
incarnations of this point; but only for the situation of examining the
heating process in microscopic detail; i.e. understanding heating at the
microscopic level as being work done at the microscopic level in an
unorganized fashion. At the macroscopic level, which is what I believe to
be the point of "standard" thermodynamics, I'm not so sure I'm ready to
yield the point.

3) DQ is not necessarily connected to temperature difference or to
microscopic activity. (The Joule experiments give rise to a
DQ but have no
external dT and are clearly macroscopic.)


I disagree here, for the moment. This is like Leigh's paddling in the canoe;
and I want to say the Joule paddle wheels do work on the system; so I
attribute the dE here to DW not DQ; I'm still defering DQ to those energy
transfers due to temperature differences (excluding radiative and convective
processes for purposes of this discussion). I agree that this DW is clearly
microscopic.

4) For an _irreversible_ process, it is difficult to see a difference
between DQ and DW
And for the First Law mox nix.


I agree its difficult, I'm so used to doing reversible thermodynamics; I'm
entering into a vast realm of ignorance for me when I talk about
irreversible thermodynamics, much less non-equilibrium thermodynamics.

But, just because it is difficult to see, doesn't mean there isn't a
difference.

5) I am having trouble thinking of a variety of reversible
processes so
let's go with the usual adiabatic cylinder on a hot plate and a
frictionless piston:

OK

5a) If sand is dribbled on the piston, that contribution
would be to DW
_NOT_ because it is a mechanical or macroscopic process, but
because it
does not give rise to a change in the entropy. (See my web page for a
preliminary sketch of this calculation)

I'm tempted to say that the two statements are synonomous in the context of
the example. so I agree and disagree here. How's that for waffling.

Or let's say that it is DW because it is not DQ; because no energy transfer
occurred due to a temperature difference.

5b) If the knob on the hot plate is slowly turned. the plate _does_
contribute to the entropy so we call that contribution DQ.
The molecules
of the plate surface do _work_ via collisions, but that work is in DQ.


Or to paraphrase, Here we have DQ because that energy transfer is due to a
temperature difference. (To be sure, at the microscopic level this energy
transfer is due to collisions at the interface of our system and its
environment; aber, das macht nichts)

6) If the process is _irreversible_, usually one can not
clearly see what
DW or DQ is. If I smash the piston with a 4 ton rock for example.


I'm going to say that it is DW here. Assuming the 4 ton rock, the piston
and the fluid (the system) were all at the same temperature to begin with;
or assuming the whole process occurs so quickly that a neglible amount of DQ
occurs.

7) The only way to calculate DQ for the irreversible process
is to choose a
reversible process with the same end states as the irreversible one,
determine the end point entropy of that process and then
calculate DQ=TdS.

I think I share Bob's objection here.

8) In some cases dS might be determined from a carefully
chosen DQ of the
reversible process, but that would be the same DQ for the irreversible
process and DW would be nil.

Again I share Bob's objection.


Now, Joel, Bob, do we agree with at least the above???


oops. But we have clarified points of disagreement for further discussion,
perhaps.

NB I have not used the evil four-lettered word above.


Yes, we have wiped out evil in this discussion. :-)

Joel