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: adiabaticity and reversibility



... one of my favorite demonstrations. How many of you
do the Clement-Desormes experiment to measure the ratio of specific
heats in a gas? I always do this as a lecture demonstration when I
teach thermodynamics. It really works very well when one learns to
perform the expansion in a manner which justifies the adiabatic
approximation.

Leigh

Carl Mungan responds:
I've either forgotten (most likely) or never seen this experiment.
can you give a quick rundown ...
I'm teaching thermo this semester for the first time ...
Carl
Dr. Carl E. Mungan

Leigh obliges with this:
See one of the URLs below. The demo we use has a large water bottle,
and I don't try to open and close the valve; I have a large hose into
the bottle which I open and close by clapping my hand over the end.
Speed is important (to the adiabaticity) and both the large bore hose
and the rapid valve action are important in this respect.
Leigh

Leigh can be depended on to add value to his explanations.
In this case we have:

http://ntserv.fys.ku.dk/finnberg/demonstra.htm

Med en "gummibold-håndpumpe" skabes et
* With a hand-pump, over pressure
overtryk x1 i vinballonen. Når ballonen
* a glass flagon to pressure x1.
åbnes til atmosfæren, udvides luften i
* When you release the overpressure
ballonen (næsten) adiabatisk, og dens
* adiabatically, the temperature falls.
temperatur synker. Efter 2-3 sekunder
* After 2 or 3 seconds, reclose
lukkes igen, og trykket stiger langsomt,
* the container and wait for it to
efterhånden som temperaturen af luften i
* regain room temperature.
ballonen nærmer sig omgivelsernes
* Using the new overpressure x2,
temperatur. Med sluttryk x2 har man
* the ratio of specific heats is
g = x1 / (x1 - x2).
* x1 / (x1 - x2)

http://www.chemistry.mcmaster.ca/~allanle/cpcv2.gif

This depiction was composed by Jimmy's Glamorous Mom,
Leah A. Allan, Undergraduate Laboratory Coordinator at
McMaster Regional Centre for Mass Spectrometry,
as a photo of her sibling demonstrates.

http://www.dartmouth.edu/~physics/labs/descriptions/adiabatic.html

Excellent write up as a pdf, indifferent depiction. To quote:
"...despite the algebraic gymnastics..."
(..the relation is straight forward)
and it goes on to describe Ruchardt's method too...

Its clear development using the number of degrees of freedom for a
monatomic divalent and polyvalent gas contrasts with my remembrance
of some of the thermo discussions on this list which featured
many degrees (or was it states?) available to a gas.

http://www.physics.unc.edu/labs/demo/thermo/4B10.60.html

Says it's "Currently unavailable" even when the display card is
presented - with a little prodding!


I'm glad Carl asked! :-)

brian whatcott <inet@intellisys.net>
Altus OK