BasherWo the Science Ninja does it again!
Bruce, that was a killer "lecture", especially about the He/UF6 thing...
I always wondered if there were *another* example of the "speed of sound
in gas" thing to complement the helium experiment virtually everyone has
tried (or at least observed). Breathing UF6 seems just so wrong at so
many levels, but I suppose it stands to reason that the lung physiology
might not "couple" at all with such a heavy and odd element. I suppose
the whole Alveola thing is about Oxygen one direction or the other
(absorb O2, release CO2?) with Nitrogen being pretty benign and CO
wicked. And all the hydrocarbons playing in there somewhere too I
suppose...
I'm feeling an itch to open the encyclopaedia again! Dammit!
A really spectacular (and somewhat dangerous) demo involves what I
would guess is the densest gas of all, uranium hexafloride, with a
mass of 352 gm/mole. Remember that at equal temperature and pressure a
mole of any gas whatsoever occupies a volume of 22.4 liters, so the
grams/mole is proportional to the grams/liter.
The demonstrator breathes in the UF6 and then talks with a very deep
voice, the opposite effect to breathing in helium and talking with a
very high voice. The explanation is that the vocal cavity dimensions
don't change, so resonant wavelengths don't change. The speed of sound
in a gas is similar to the average speed of molecules in the air,
which is v = sqrt(3kT/m), where m is the mass of one molecule, k is
Boltzmann's constant, and T is the absolute temperature. Therefore the
speed of sound is much higher in helium than in air, and much lower in
UF6 than in air. Since the speed of sound = wavelength/period =
wavelength*frequency, the frequency of a resonance in the voice cavity
is high in helium and low in UF6.
In the helium case, it's easy to get rid of the helium and replace it
with air because the helium has lower density than air. But to get rid
of the UF6 the demonstrator has to lean over or stand upside down to
spill the heavy gas out of the lungs. (Note: the radioactivity of
ordinary uranium, which is 99.3% U238, or of "depleted" uranium from
which the U235 has been removed, is quite low. U238 has a very long
half-life of 4.4 billion years, so the number of decays per second in
the UF6 is low. The danger in breathing in UF6 is from lack of oxygen,
not from radioactivity.)
In an atmosphere with approximately constant absolute temperature and
no convection, the density of a particular gas is proportional to e
raised to the power (-mgy/kT), where m is the mass of one molecule in
kilograms, g is 9.8 N/kg (gravitational field strength), y is the
height above the ground in meters, k is Boltzmann's constant in
joules/kelvin, and T is the absolute temperature in kelvins. For
oxygen, at a height y = 7920 meters (roughly the height of Mt.
Everest), (-mgy/kT) = 1, so the density is down by a factor of e to
the -1, which is 0.37. In other words, in this simplified model the
density of oxygen at the top of Everest is about one-third what it is
at sea level. (Note that some climbers have made it to the top
breathing only what air was available there.)
The corresponding "mean heights" for various gases in this model:
oxygen 7920 m
nitrogen 9050 m
CO2 5500 m
helium 63400 m (way above almost all of our atmosphere)
UF6 720 m
Note that the mean heights for oxygen and nitrogen aren't very
different, so one can expect mixing to destroy any significant
separation. On the other hand, one can expect helium to escape from
the Earth, and it does, and one can expect some significant pooling of
UF6 when you pour it out onto the floor.
Bruce
On Tue, Jun 12, 2012 at 12:33 PM, Roger Critchlow<[email protected]> wrote:
Nick --
N2 weighs 28 gm/mole, O2 weighs 32 gm/mole, Ar weighs 40 gm/mole, CO2 weighs
44 gm/mole, and H2O weighs 18 gm/mole.
Why would anyone expect the lighter components of a mixture to fall down
more than the heavier ones? If anything, you'd expect the heavier ones to
concentrate toward the bottom.
And why would anyone expect a mixture to spontaneously separate into pure
components? That happens in real life like where?
As it happens, CO2 is the heaviest normal component and it does pool in
confined spaces often enough that CO2 alarms are available in hardware
stores. Propane, C3H8, weighs 44 gm/mole and is notorious for pooling in
confined spaces and then exploding, often in the bilge of a boat and
spectacularly.
-- rec --
On Tue, Jun 12, 2012 at 10:44 AM, Nicholas Thompson
<[email protected]> wrote:
So, somebody asked me, in my role as a weather nerd, how come the nitrogen
in the atmosphere doesn’t all fall to the bottom on still nights and
suffocate us all. I asked the question of
stupid-answers-to-stupid-questions-asked-by-stupid-people.com and THEY said,
well, there’s just too much going on. N molecules and the O molecules are
just too busy, what with convection and windcurrents, and all, to separate,
even on still nights. Now, that business doesn’t prevent cold molecules of
Nitrogen and Oxygen to separate from warm ones, or wet ones (not sure what
that means) to separate from dry ones. I was hoping that somebody on FRIAM
could give some sort of a clue what kind of a mixture AIR is? It is
suddenly seeming kinda special.
Nicholas S. Thompson
Emeritus Professor of Psychology and Biology
Clark University
http://home.earthlink.net/~nickthompson/naturaldesigns/
http://www.cusf.org
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FRIAM Applied Complexity Group listserv
Meets Fridays 9a-11:30 at cafe at St. John's College
lectures, archives, unsubscribe, maps at http://www.friam.org
============================================================
FRIAM Applied Complexity Group listserv
Meets Fridays 9a-11:30 at cafe at St. John's College
lectures, archives, unsubscribe, maps at http://www.friam.org
============================================================
FRIAM Applied Complexity Group listserv
Meets Fridays 9a-11:30 at cafe at St. John's College
lectures, archives, unsubscribe, maps at http://www.friam.org
