Modernist-Cuisine-Vol.-1-Small

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Like engineers and the public at
large, we use “heat” throughout the
book to refer to thermal energy—
that is, a form of internal energy
that affects the temperature of an
object or substance. The strict
scientific definition of heat,
however, is different: heat is
energy in transit from bits of
matter at a higher temperature to
other bits of matter at a lower
temperature. In the language of
thermodynamics, heat is actually
a process, not a property.
Just as bumper cars jostle one another at
varying angles and speeds, molecules
collide and transfer some of the energy of
their motion.
THE NATURE OF
HEAT AND TEMPER ATURE
Energy is a fundamental attribute of every physical
system in the universeso fundamental that it
practically eludes our capacity to define it. Standard
physics textbooks define energy as “the
capacity of a system to do work.” But the concept
of work is also maddeningly abstract. An informal
approach might define energy as “the ability to
make things happen.” That definition is more
useful for our purposes because it is easier to
recognize what energy does than what energy is.
The actions of energy are central to a cook’s
concerns. Energy heats food, and energy cools it;
energy transforms flavors, textures, and colors. To
cook is to transform food by putting energy into it,
and to eat is to get energy out of food by transforming
it in a different way.
Energy takes many different forms, and it
moves in a variety of ways. In cooking, the most
common movement of energy is heat. Although
technical dictionaries define heat as a transfer of
energy (see note at left), from a cook’s point of
view it is much more useful to think of heat as
a form of internal energy, one that always flows
from a substance at a higher temperature to
another at a lower temperature. To understand
heat, we thus need a sense of what internal energy
and temperature are.
Internal energy is the sum of lots of different
kinds of energy stored in a chunk of matter (which
can be as small as a single atom or as big as you
care to define it). In a hot baked potato, for
example, there is internal energy in the chemical
bonds of the starch molecules, in the steam
trapped under the skin, and even in the nuclear
forces that hold the atoms together. But a lot of the
internal energyand much of what we think of as
heatis stored in the continuous, random movements
and fleeting collisions of the potato’s
countless molecules.
Even though the potato may look solid, those
molecules are indeed always moving; the motion
is simply too small to see without special instruments.
The discovery that the microscopic particles
of all substancessolid, liquid, and gasjostle
constantly was one of the notable achievements
of 19th-century physics. That insight led directly
to some of the theoretical breakthroughs made by
Albert Einstein in the 20th century.
Think of molecules in a solid as behaving like
bumper cars in a carnival ride. When two lurching
cars collide, they transfer momentum and energy
to one another. The faster car slows down, and the
slower car speeds up.
In a gas such as air, the molecules zip around
and bump their neighbors in all directions. In
solids, the particles are typically bound to one
another, so their movements are more constrained.
Still, they rattle back and forth, bouncing
off one another like bumper cars connected with
rubber bands.
If you were to measure the speed of each
bumper car at a single moment, you would find
that some are completely still (or nearly so), some
are moving quite fast, and the speeds of the rest
are distributed between those two extremes. The
same is true of molecular motion. The faster the
particles within a substance are moving, the
greater the internal energy of the substance. But
even in superhot plasma like the surface of the
sun, some particles remain stationary at any given
moment. Amazing, but true.
We cannot perceive the different speeds of all
these particles without sophisticated tools. What
we actually experienceand what matters when
cookingis the average speed of all the molecules.
There is a simple and familiar measure
related to that average speed: temperature.
When Thermal Worlds Collide
Take a steak out of the refrigerator. Throw it on
a hot pan. As every cook knows, the cold steak will
cool the pan, and the steamy skillet will heat the
steak. At the surface where the two meet, the
molecules in the pan bang into the molecules in
the steak, with predictable consequences. On
average, the particles in the pan are moving faster
THE HISTORY OF
Defining Temperature
than those in the steak. Just as a fast-moving
bumper car donates some of its momentum to
a slower-moving car when the two bang together,
each fast-moving molecule in the pan decelerates
when it hits a slower molecule in the steakand
the slower molecule speeds up.
Thus we arrive at one of the fundamental laws
of heat transfer: thermal energy flows in only one
direction, from hotter (faster-motion, highertemperature)
matter to colder (slower-motion,
lower-temperature) matter.
Think about where the heat flowing from metal
to meat comes from in the first place. Are chefs
somehow defying the laws of physics, creating
heat where none existed? No. The heat comes
We don’t normally think of temperature as a measure of speed. But that is essentially
what temperature is. To be precise, it is a quantity proportional to the square of the
average speed of molecules in a given substance as they wiggle in random directions.
Working independently, the 19th-century physicists James Clerk Maxwell and
Ludwig Boltzmann worked out the math that connects the speed of particles in a gas to
the temperature of the gas. Maxwell and Boltzmann were early believers in the existence
of atoms and molecules, and their work on energy distributions still serves as a
foundation of statistical mechanics. But their ideas were controversial in their time, and
the controversy drove Boltzmann to despair. He committed suicide in 1906.
Fraction of molecules
Speed (mph)
200 400 600 800 1,000 1,200 1,400
Gas temperature: 0 °C / 32 °F
100 °C / 212 °F
200 °C / 390 °F
400 °C / 750 °F
100 200 300 400 500 600
Speed (m/s)
Molecules inside a bottle of oxygen gas that is at equilibrium at 0 °C / 32 °F jostle at a wide
range of speeds (diagram at right and brown curve in chart above); 400 m/s (1,440 kph /
900 mph) is the most common. At higher temperatures, such as 100 °C / 212 °F (violet curve),
200 °C / 390 °F (blue curve), and 400 °C / 750 °F (green curve), the average speed of the
molecules is greater, but the distribution of speeds is broader.
The random movement of atoms
and molecules in a solid, liquid, or
gas is called Brownian motion. It is
named after the British botanist
Robert Brown, who was one of the
first scientists to describe it.
264 VOLUME 1 · HISTORY AND FUNDAMENTALS
HEAT AND E NERGY 265