Today, I learn from a water glass as I come out of
sleep. The University of Houston's College of
Engineering presents this series about the machines
that make our civilization run, and the people
whose ingenuity created them.
I was half waking, half
sleeping this morning -- still in bed, worrying
about troubles abroad in the land. I found myself
staring at the glass on my nightstand. The water
level was, of course, dropping, but it drops far
too slowly for anyone to see movement. In a week, I
suppose, the water might evaporate away entirely.
Just above the water surface is a mixture of air
and water vapor that happens to be 38 molecules of
air for each molecule of water. As long as they
stay in that proportion, the vapor above and the
liquid below are in equilibrium, and no evaporation
can take place. But nature has means for clearing
out the water vapor, so evaporation can continue.
Since the molecules stay in constant motion,
bouncing off one another, the vapor in the air near
the surface gradually stirs itself outward. More
water then evaporates from the surface to sustain
the equilibrium.
All the air around us contains water. The more it
contains, the slower the outward diffusion of
evaporated molecules. In a humid climate, water
will stay in the glass for many days. Out in the
Sahara, it might be gone overnight.
So consider the extreme case: Imagine that you
could magically find a way to remove every water
molecule as it left the surface. Think about
evaporation, totally unimpeded by that milling
gaggle of molecules. I did the calculation, and
found that, if you could instantly clean away every
new water vapor molecule, the level in your bedside
glass would begin falling at the astonishing rate
of four inches per second. And, even if
you could do that, nature would then call other
thermal forces into play to slow evaporation.
All this helps me keep my equanimity in these
too-often-troubled times. As we read the headlines,
we get the impression that our world is about to
fall into some abyss of chaos. So come back to that
bedside glass: The water in it seemingly wants to
whoosh away in seconds; but nature does not like
catastrophic completions of its processes.
Iron would quickly rust away in the open air, but
nature intervenes. Rust itself turns into a coating
that protects steel girders or iron grillwork for
centuries. The murderous disease of Ebola kills its
hosts so quickly that it limits its own spread.
And terrorism: It may seem rampant today, but it
creates such odium, and calls up such resistance,
that it's self-limiting in exactly the same way as
rust, evaporation, or Ebola.
Catastrophe does occur, of course. Disease and
terrorism do kill. Now and then a structural member
rusts out, and a building falls. The cat might jump
up on my nightstand and knock my water glass to the
floor. But our direst expectations are very seldom
met, for the remarkable reason that nature is
ultimately our friend. And, as long as we don't
systematically mistreat her, she looks after us far
better than we might first think.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.
(Theme music)
What I have done here is to provide illustrations of
the thermodynamic principle of Le Chatelier and
Braun, which says that all spontaneous processes call
up resistance to their own completion. (See Episode 1564.)
The principle is especially well described in
Epstein, P.S., Textbook of Thermodynamics.
New York: John Wiley & Sons, Inc., 1937,
Chapter 21; and in Callen, H.G., Thermodynamics
and an Introduction to Thermostatistics. New
York: John Wiley & Sons, 1985, Sections 8.4 and
8.5.
photo by John Lienhard
The Engines of Our Ingenuity is
Copyright © 1988-2002 by John H.
Lienhard.
