Re: HVAC questions

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

cliff parker wrote:

> 2) Why is hot air less dense than cool air?

P = (N/V) kT

Higher temperature, same pressure, same mass per
particle ==> less mass per volume.

> In a
> balloon the hotter air would collide with the balloon surface with
> more kinetic energy and therefore stretch the balloon to a larger
> size.
OK.

> Larger size and same # of particles = less density.  Less density =
> greater buoyant lifting force than gravitational force.

OK.

> However when hot air is unconfined in the atmosphere what acts as the
> surface of the balloon?

Key idea:  Air is a fluid.  It has a pressure everywhere.
Pressure is not something that happens just at surfaces.

Each parcel of fluid pushes on the neighboring parcels
of fluid.

For more on this, see
  http://www.monmouth.com/~jsd/how/htm/airfoils.html#sec-fluid

> 1) Why should humid air in a building tend to rise to the ceiling?
> The thought that comes to my mind is that the source of humidity is
> generally people breathing hot air. Hot air rises and therefor the
> humidity goes along for the ride.

That's not it.

> However, I am not satisfied with this answer

Good intution.

Again: P = (N/V) kT

Same pressure, same temperature ==> same number
per volume.  Then less mass per particle ==>
less mass per volume.

The H2O molecule is a lot less massive than N2.

Humidity rises for the same reason a helium
balloon rises.

This is a significant driving force for weather.

> 3) If the air I am breathing right now was trapped into a well
> insulated tightly sealed container and was allowed to sit on a shelf
> for a period of time would that various molecules settle into layers.
>  Heavy molecules on the bottom and light on the top?

In equilibrium there would be a very slight
concentration gradient.  There would be a contest
between mobility under the influence of the
gravitational force (trying to separate things)
and diffusion (trying to undo the concentration
gradient).

Some guy named Einstein had something to say
about this:  diffusion constant = mobility times
absolute temperature.  This is really worth
remembering.

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.)

This posting is the position of the writer, not that of Brown,
Einstein, Smoluchowski, or Nernst.

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