Encyclopedia of Evolution.pdf - Online Reading Center

The Second Law
A simplified version of the Second Law of Thermodynamics
is that, whenever events occur, the amount of entropy
increases. Entropy can best be understood as disorder, or, as
one of the founders of thermodynamics, chemist J. Willard
Gibbs, described it, “mixedupness.” The natural tendency is
for orderliness to decay into disorder. This occurs because,
for any system, there are far more possible disordered states
than there are ordered states. The process of diffusion allows
the Second Law to produce many of its effects. Diffusion
occurs from the individual movements of atoms or molecules
toward a less ordered, or more uniform, arrangement.
In most events with which humans are familiar, both the
First and Second Laws operate. In nearly every event, energy
changes from one form to another and entropy increases.
When no energy input occurs from outside a system, events
tend to occur in one direction only: They continue until equilibrium
is reached in which energy is uniform throughout the
system (First Law), and maximum disorder has been reached
(Second Law). Consider the following examples:
Diffusion of heat. Heat diffuses from regions of higher
temperature to regions of lower temperature. Warm molecules
move faster (have more kinetic energy) than cold molecules
and can transfer their energy to the cold molecules by
colliding into them. As a result, heat energy diffuses from
regions of high temperature to regions of low temperature.
Diffusion of heat is also called conduction. Convection occurs
when a mass parallel movement of molecules, such as those
in the air, carry heat from a warm region to a cool region. If
conduction and convection go to completion, an equilibrium
of lukewarm molecules will result. This is what happens when
a cup of hot coffee, or a recently dead mammal, cools off to
environmental temperature. (The room actually becomes
slightly warmer from the heat lost by the coffee cup.) The
temperature of an object can increase when heat is conducted
to it from another source that is warmer (see figure). Energy
must be expended, for instance by a refrigerator, to make a
relatively cool place even cooler; the coils in the back of the
refrigerator disperse the heat, from the space inside the refrigerator
and from the machinery, into the air. The First Law
indicates that the total amount of energy is unchanged, and
the Second Law indicates that the energy has reached a maximum
state of disorder: The energy is no longer concentrated
in any one location.
Movement of air. Air moves from regions of high pressure
to regions of low pressure. Gas molecules in air that has
high pressure (high potential energy) flow toward regions of
air that have lower pressure, producing wind. Because wind
involves the parallel movement of many gas molecules, it is
not an example of diffusion. Air movement continues until
an equilibrium is reached in which all regions have equal
pressure. Since warm air has a lower pressure than cool air,
temperature differences can cause air to move. Energy must
be expended, by a fan or a pump, to force air to move in the
absence of pressure differences. Other fluids, such as water,
also move from regions of high to regions of low pressure.
The First Law indicates that the total amount of energy has
remained unchanged, even though it changed from potential
thermodynamics
to kinetic forms; and the Second Law indicates that pressure
has reached maximum uniformity, when equilibrium is
reached.
Diffusion of molecules. Molecules diffuse from locations
in which they are more concentrated toward locations
in which they are less concentrated. For example, a concentrated
mass of sugar molecules is dropped into water. This
is an orderly arrangement of molecules, with all of the sugar
molecules in one place, and the water molecules in another.
Both kinds of molecules move randomly, as a result of kinetic
energy. They become less orderly as the sugar and water
molecules mix together, until an equilibrium arrangement
is reached in which both kinds of molecules have the same
concentration everywhere. The molecules are very unlikely to
Energy and matter change from high energy and highly structured states
to lower energy and disordered states. Heat flows from hot to cold
places, resulting in a uniform temperature; air flows from high to low air
pressure, resulting in a uniform pressure; electricity flows from high to
low voltage, resulting in a uniform voltage; molecules diffuse from high
to low concentration, equalizing their concentration; and big molecules
with much stored energy break down into small molecules with less
stored energy.