Today, we chase a will-o-the-wisp. 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.
Energy is pure
delight, wrote William Blake. But for me the
purest delight of energy is its elusiveness. Ask
the dictionary what energy is. It says it's "the
capacity to do work." Then it tells you work is a
transference of energy. The technical literature
takes you around the same circle. For energy is a
primary concept, like force or space. We cannot
define it. We can only compare experience and then
try to agree upon that experience.
The binding tissue around energy is a physical
principle: the Conservation of Energy or the
First Law of Thermodynamics. We know forms
of energy can be swapped. We work at rubbing our
cold hands together and they grow warm. I run
electricity through a light bulb. The electricity
is transformed. It leaves as light and heat.
We take those equivalences for granted. But we had
no light bulbs three centuries ago: no electricity,
no steam engines. In the late 18th century we
decided heat must be an indestructible material
called caloric. When a carriage wheel
heated its axle, we thought the rubbing was
releasing caloric from the wood. It never occurred
to anyone that energy was being transmuted from one
form to another.
But the new 18th-century steam engines undermined
that thinking. Steam power plants burned coal,
boiled water, and let the steam's energy do work.
For a while, people tried to compare steam engines
with water wheels. Water flows through a water
wheel, does work, and degrades gravitational
energy. Shouldn't steam engines work the same way,
with caloric flowing through and being conserved
the same way water is? No one saw that heat was not
being conserved but was being transformed into
work.
In the 1790s Count Rumford, an American expatriate
working in the Bavarian court, took an interest in
artillery. Rumford noticed that when cannon barrels
were reamed they got much hotter if the bit was
blunt. Instead of cutting, there was a lot of
frictional rubbing. Rumford found he could keep
generating heat as long as he worked at turning the
bit. So much for releasing caloric (or heat) from
the surface! The supply of surface caloric in the
material would have to be inexhaustible to do that.
Rumford did another, very different, cannon
experiment. He fired cannons with and without
cannonballs. The cannons without cannonballs were
far hotter after they'd been fired. Why? The reason
was, the explosion did work accelerating the
cannonball. The ball carried energy away. When
there was no cannonball, the energy couldn't change
form, and it simply heated the cannon.
So, piece by piece, a concept took on flesh and
blood. Energy is pure delight just because its form
keeps changing. But that concept is subtle and
difficult. A hundred and fifty years ago, we didn't
have it sorted out. And today our dictionaries
still don't know what to do with energy -- elusive,
changing, and ever-present.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.
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