Today, we move energy. 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.
This week, engineers will
meet from all over the world for the National Heat
Transfer Conference here in hot Houston. When I
tell people I'm going to a heat transfer meeting,
they look at me oddly. What's that, they wonder.
Yet these same people live in, and worry about, a
world that eats huge amounts of energy.
The central issue we worry about is how energy
moves from place to place. Ever since Einstein,
we've known that energy and matter are two faces of
the same coin -- the very stuff of which the entire
universe is made. Matter can be seen. We watch it
moving on trucks and trains, in earthquakes, tides,
Energy also swirls around us, but, because it's
invisible, we're less aware of it. Wherever there
are differences in temperature, pressure, voltage
-- energy can flow. So 600 engineers will gather
and tell each other what they know about how
temperature drives energy -- how heat flows. To
help you catch some flavor of the subtleties, I'll
put three riddles before you:
(1) We're supposed to wear white clothes in the
summer because black absorbs more sunshine. Why
then does nature put dark-skinned people on the
equator where the sun pours down energy, and
light-skinned people in the far North?
(2) How did the story that hot water freezes
more quickly than cold get started?
(3) Why does metal seem colder than wood in
First, skin color. While dark skin is better
protected from ultraviolet radiation, it does
absorb more of the energy in visible light than
light skin. But most of the sun's heat comes in
invisible infrared radiation. Dark and light skin
are the same color in that range. Dark skin absorbs
no more heat than light skin does.
Next, the "hot water freezes faster" story: When we
put trays of water in frosty freezers to make ice
cubes, frost insulates them. Hot trays melt the
frost and make better thermal contact with the
cooling coils. But there's more. Some of the hot
water evaporates in the cold dry air. There's less
to freeze, and evaporation speeds cooling. So that
story really can be true.
And what about metal feeling colder than wood? Your
body has to be warmer than the room to shed heat.
Your nerve endings are warmer than either wood or
metal. But metal conducts heat away more quickly
and it holds more heat. Your finger is cooled as it
rapidly drives heat into the metal. That's why
metal, at the same temperature, feels far colder
A large diamond would feel even colder than metal.
But real diamonds are small, and they suck the heat
out of your finger so very fast that they quickly
match your finger's temperature. That's why people
like to say "Diamonds are warm!"
Heat flow has many ways of fooling us. Scale these
ideas up to a huge power plant or down to a laptop
computer and they offer wonderful fun and
challenge. I'd like to say we meet to build a
better quality of life. Well, sure, there's that.
But we really do it for the exquisite pleasure of
facing the questions themselves.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds