1. The density of air is determined (pressure & temperature being equal) by
the average of the molecular masses of its components. Nitrogen has a
"weight" of 28 grams per mole and Oxygen has 32 grams/mole. While not as
"light" as hydrogen (2 g/mole) or helium (4 g/mole), water vapor (18 g/mole)
is considerably "lighter" than nitrogen or oxygen and thus has a tendency to
float to the ceiling, making the air there more humid. To make the air on
upper floors more humid, it is not necessary to move all of the air, just
the water vapor.
2. Hot air is less dense than cold air *of the same pressure*. Recall
pressure is related to momentum transfer. Since the molecules of hot air
have more kinetic energy and therefore more momentum per particle (on
average) than the atoms of cold air, one can produce the same pressure with
fewer molecules. Fewer molecules = less mass of gas, so a volume hot air has
less mass than the equivalent volume of cold air. Less mass translates to
less density (volumes being the same), so hot air rises.
3. Yes, sort of. If you make a "density column" of several different
immiscible liquids in a graduate cylinder (such as oil & water, etc.), there
will be sharp boundaries between the different liquids. This sharpness is
largely due to surface tension effects. Try the same thing with miscible
liquids such as water & dyed corn syrup, and the boundary between the two
fluids will not be distinct. Left to themselves, the two fluids will mix.
However, the liquid at the bottom will still have a greater density than the
liquid at the top. Eventually an "equilibrium density gradient" will be
achieved. The situation with gases will be similar. There won't be any sharp
boundaries between gases. Since the energy per molecule is large compared to
gravitational P.E., you'll need a very tall container to detect the gradient
at all. That, or unusual conditions such as experienced by a friend of mine
who was in a Fermilab tunnel right after a magnet quench that released both
liquid helium & liquid nitrogen. There was fog at the ceiling due to the He
gas, and fog near the floor due to the cold nitrogen, but clear air in the
middle.
Vickie Frohne
-----Original Message-----
From: cliff parker [mailto:cparker@CHARTER.NET]
Sent: Sunday, December 01, 2002 7:43 PM
To: PHYS-L@lists.nau.edu
Subject: HVAC questions
I was speaking with my brother who is a mechanical engineer working w=
ith HVAC design in large buildings. As we discussed some of the phys=
ics involved with his work he told me that one of the factors that mu=
st be considered when designing ventilation systems is that humid air=
tends to rise to the ceiling. As I pondered why this may be a few q=
uestions came to mind that I am having difficulty answering on my own=
. The first comes directly from the conversation with my brother. =
=20
1) Why should humid air in a building tend to rise to the ceiling? T=
he thought that comes to my mind is that the source of humidity is ge=
nerally people breathing hot air. Hot air rises and therefor the hum=
idity goes along for the ride. However, I am not satisfied with this=
answer because it would seem that the air breathed from people would=
mix with the room air too quickly to cause much convection movement.=
Is there more that I am missing. It seems like there must be.
2) Why is hot air less dense than cool air? I basically understand b=
ut my ideas need a little polishing. In a balloon the hotter air wou=
ld collide with the balloon surface with more kinetic energy and ther=
efore stretch the balloon to a larger size. Larger size and same # o=
f particles =3D less density. Less density =3D greater buoyant lifti=
ng force than gravitational force. However when hot air is unconfine=
d in the atmosphere what acts as the surface of the balloon?
3) If the air I am breathing right now was trapped into a well insula=
ted tightly sealed container and was allowed to sit on a shelf for a =
period of time would that various molecules settle into layers. Heav=
y molecules on the bottom and light on the top?
Any answers pointing me in a useful direction would be appreciated.
Cliff Parker
Never express yourself more clearly than you can think. Niels Bohr=
=20
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