Re: HVAC questions

["John S. Denker" <jsd@MONMOUTH.COM>]

cliff parker wrote:

> Pressure equals force per unit area.  Seems like a
> surface is an necessity.

1) Dimensional analysis doesn't always tell the
whole story.  Dimensionally, pressure is a force
per unit area.  It's also an energy per unit
volume.

2) Physically, each parcel of fluid has a boundary.
The bdy isn't tangible, but it's there nevertheless.
The fluid applies outward force across each element
of area on the boundary, opposed (more or less) by
the force due to the pressure of neighboring parcels.

> ... that each air molecule collides with one or another of its
> neighbors 10,000,000,000 times per second.  How is something like this
> determined?

Work it out:  you know thermal energy per particle.
You know the mass.
Find the velocity.
Divide into the mean free path.

If you don't know the mean free path by heart,
you know the number density (particles per
volume) and you know a pretty good estimate
for the cross section, so you can find the
MFP.

> Would unconfined helium rise just as well as confined helium?

Initially, yes.  See below.

> > At ordinary temperatures, you would need a
> >container many miles high in order to see
> >much fractionation of the atmosphere.  (And
> >you would need to suppress convection, etc.)
>
> Doesn't this last statement conflict with the previous idea that the less
> massive H2O rises to the ceiling of a room (much less than many miles
> high).

Timescales.

Feynman defined equilibrium as when all the
fast things have happened but the slow things
have not.  That's humorous, but also profound.

If you let loose a great gush of helium, it will
go up and collect on the ceiling.  On a longer
timescale, it will diffuse down into the rest
of the room.  On a yet longer timescale, it
will diffuse out of the room through cracks.

This posting is the position of the writer, not that of
Etienne Montgolfier, Joseph Montgolfier, or Jaques Charles.

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.