Re: Mass/Energy Conservation

[David Bowman <David_Bowman@GEORGETOWNCOLLEGE.EDU>]

Regarding Doug Craigen's response:

> I did mention sweat (see below).

So you did.  It seems that all this final exam preparation has further
reduced my usually poor literacy skills.

> ...
> > BTW, what's the point of Dave A.'s initial question anyway?
> >
> > > I assume that you are remembering to include drinking water in the
> > > "food" calculation.  There are other mass exchanges as well with
> > > sweating and respiration.  We breath out a lot of water vapor (its been
> > > -30 here in Winnipeg this week, I've seen that one a lot) plus exchange
> > > O2 for CO2.  Hence there is a net loss of weight from breathing.  This
> > > might be an interesting back of the envelope calculation.
> >
> > (Doug, how do you manage to see water *vapor*?  ;-) )
>
> Much as I dislike refering to dictionaries in a physical question...
> Funk and Wagnal's Standard College Dictionary:
> vapor
> -----
> 1. Moisture in the air; especially visible floating moisture, as light
> mist.
> 2. Any light, cloudy substance in the air, as smoke or fumes.
> 3. Any substance in the gaseous state, that under ordinary conditions,
> is usually a liquid or solid.
> 4. A gas below its critical temperature
>
> I suspect that you were focussing on definition 3. above, whereas I
> generally go with 1.  If I mean gas I say gas, if I mean a light mist I
> shorten it to "vapor".

Thanks for the explanation.

Actually, I had 4. in mind, but 3. is nearly equivalent--especially in
the case of water.

But I got confused when you wrote "We breath out a lot of water vapor..."
I thought you meant #3/4 as well.  Things might be different in the
Manitoba tundra, but around here when we can see our breath the moisture
we exhale is in a vapor (#3/4) form while it is in the breathing
passages.  It is only *after* the exhaled breath is chilled by contact
with the air in front of the face that the water vapor (#3/4) in it
condenses to a mist (#1/2) of suspended liquid droplets or frozen ice
crystals--depending on the ambient temperatures involved.  But maybe
after prolonged exposure to the -30 C environmental air where you are,
your mouth and trachea become so chilled that the condensation process
starts while your breath is still in your mouth, so that you actually
breath out the mist itself.

David Bowman
David_Bowman@georgetowncollege.edu