Environmental Engineering Reference
In-Depth Information
Mayer, wrote about forces rather than energies (Helm-
holtz 1847).
Soon afterwards William Thomson, Lord Kelvin (fig.
1.3), identified the Sun as the principal source of kinetic
energies available to humans and wrote about a universal
tendency in nature toward the dissipation of mechanical
energy (Thomson 1853). In 1850 a German theoretical
physicist, Rudolf Julius Emanuel Clausius (1822-1888),
published his first (and still most famous) paper on the
mechanical theory of heat, in which he proved that
the maximum work obtainable from a Carnot cycle
depends solely on the temperatures of the heat reservoirs,
not on the nature of the working substance, and that
there can never be a positive heat flow from a colder to
a hotter body (Clausius 1850).
Clausius continued to refine this fundamental idea. In
1865 he coined the term entropy—from the Greek trop´
(transformation)—to measure the degree of disorder in a
closed system, and formulated the second law of thermo-
dynamics: the energy content of the universe is fixed, but
its distribution is uneven, and thus its conversions seek
uniform distribution, and the entropy of the universe
tends to maximum (Clausius 1867). A system changes
its entropy (DS) in proportion to the amount of heat
(Q) applied at temperature (T, in absolute terms), and
DS is positive when a system's entropy increases:
disordered (high-entropy) form of energy, and the se-
quence is irreversible: diffused heat (and emitted com-
bustion gases) can never be reconstituted as a lump of
coal. Heat thus occupies a unique position in the hier-
archy of energies. All other energies can be completely
converted to it, but the conversion of heat into other
forms of energy can never be complete because only a
portion of the initial input ends up as kinetic energy or
as electricity. The second law of thermodynamics, the
universal tendency toward disorder, can be seen as the
scientific counterpart of the futility cries in Ecclesiastes.
Only at absolute zero (
273
C), in the absence of any
movement, is the entropy nil. This is the third law of
thermodynamics, initially formulated in 1906 as Walther
Nernst's (1864-1941) heat theorem.
The second law is perhaps the grandest of all cosmic
generalizations, yet one of which most nonscientists
remain unaware, as Charles Percy Snow (1905-1980)
eloquently pointed out in his Rede lecture on the two
cultures (Snow 1959). Soon after the precise formulation
of the thermodynamic laws, Josiah Willard Gibbs (1839-
1903), a U.S. physicist, applied their concepts to chemis-
try and introduced the important notion of free energy,
G (Gibbs 1906). Change of this energy (DG)—basically
the maximum amount of work that can be derived from
a reaction proceeding at constant temperature and pres-
sure—is determined by subtracting the product of
temperature and entropy change from the change of en-
thalpy (H, the heat content of a system) energy that
enters a chemical reaction:
DS
¼
Q
T
:
In practical terms this means that in any closed system
(without an external supply of energy) the availability of
useful energy can only decline. A lump of coal is a high-
quality, highly ordered (low-entropy) form of energy; its
combustion will produce heat, a dispersed, low-quality,
DG
¼
DH
TDS
:
Formation of compounds from elements requires
inputs of G: at standard conditions (298 K, 101.3 kPa)
