[Phys-L] Re: A "simple" physics problem?

[rlamont <rlamont@POSTOFFICE.PROVIDENCE.EDU>]

Again, The N2 and O2 have only an insignificant effect on the
process. Rephrase your problem. At 40 C the amount of water vapor
necessary for condensation in a I m^3 closed container is 0.0512
kg. With this amount of moisture, if a droplet forms its
evaporation rate will equal its condensation rate. Now, evacuate
the container and put in 90 percent of this - 0.0461 kg. If a
droplet should form, there are not enough water molecules in the
container to provide a condensation rate that matches the
evaporation rate. So a droplet cannot survive. Now lower the
temperature. At some point the evaporation rate from any cooler
droplet that may form will be low enough so that it matches the
condensation rate with the number of vapor molecules available.
Droplets can now form (if the proper condensation nuclei are
present) and sustain themselves.

Now put the N2 and O2 back and repeat the experiment. Nothing
changes! The other gases have nothing to do with the evaporation
and condensation of the water. Therefore, the N2 and O2 do not
"hold" moisture. They are irrelevant to the process. All that
matters are the temperature and the number of kilograms of water
per cubic meter of air present.

Bob at PC


> -----Original Message-----
> From: Forum for Physics Educators [mailto:PHYS-
> L@list1.ucc.nau.edu] On Behalf Of Folkerts, Timothy J
> Sent: Friday, April 08, 2005 12:56 PM
> To: PHYS-L@LISTS.NAU.EDU
> Subject: Re: A "simple" physics problem?
>
> OK,  for the "air doesn't hold water" discussion, let me ask a
questions.
> Suppose you have a 1 m^3 space filled with ~80% N2 and 20% O2
at
> 40C and 90% relative humidity.  The mixture is then cooled to
5C, at
> which point the water starts to condense and falls out of the
space.
> Which of the following would people be comfortable saying?
>
> a)  the 1 m^3 of space has fewer water molecules in it than it
did
> before.
> b)  the 1 m^3 of space is holding less water than it did
before.
> c)  the 1 m^3 of atmosphere is holding less water than it did
before.
> d)  the 1 m^3 of air is holding less water than it did before.
>
> Or you would phrase the question completely differently?  Or
answer it
> differently?
>
>
> Tim Folkerts
>
> P.S  What's it the Mississippi River flow puzzle?  I don't
think I'm
> familiar with that one.
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