Thermodynamics in its original form (now known as classical thermodynamics)
is a theory which is based on a set of postulates about how macroscopic
matter behaves. This theory was developed in the 19th century, before the
atomic nature of matter was accepted, and it makes no reference to atoms. The
postulates (the most important of which are energy conservation and the impossibility
of complete conversion of heat to useful work) can t be derived within
the context of classical, macroscopic physics, but if one accepts them, a very
powerful theory results, with predictions fully in agreement with experiment.
When at the end of the 19th century it nally became clear that matter was
composed of atoms, the physicist Ludwig Boltzmann showed that the postulates
of classical thermodynamics emerged naturally from consideration of the
microscopic atomic motion. The key was to give up trying to track the atoms individually
and instead take a statistical, probabilistic approach, averaging over
the behavior of a large number of atoms. Thus, the very successful postulates of
classical thermodynamics were given a rm physical foundation. The science of
statistical mechanics begun by Boltzmann encompasses everything in classical
thermodynamics, but can do more also. When combined with quantum mechanics
in the 20th century, it became possible to explain essentially all observed
properties of macroscopic matter in terms of atomic-level physics, including esoteric
states of matter found in neutron stars, superfluids, superconductors, etc.
Statistical physics is also currently making important contributions in biology,
for example helping to unravel some of the complexities of how proteins fold.
Even though statistical mechanics (or statistical thermodynamics) is in a
sense \more fundamental" than classical thermodynamics, to analyze practical
problems we usually take the macroscopic approach. For example, to carry out
a thermodynamic analysis of an aircraft engine, its more convenient to think
of the gas passing through the engine as a continuum fluid with some speci ed
properties rather than to consider it to be a collection of molecules. But we
do use statistical thermodynamics even here to calculate what the appropriate
property values (such as the heat capacity) of the gas should be.
The two central concepts of thermodynamics are energy and entropy. Most
other concepts we use in thermodynamics, for example temperature and pressure,
may actually be de ned in terms of energy and entropy. Both energy
and entropy are properties of physical systems, but they have very di erent
characteristics. Energy is conserved: it can neither be produced nor destroyed,
although it is possible to change its form or move it around. Entropy has a
di erent character: it can t be destroyed, but it s easy to produce more entropy
(and almost everything that happens actually does). Like energy, entropy too
can appear in di erent forms and be moved around.
A clear understanding of these two properties and the transformations they
undergo in physical processes is the key to mastering thermodynamics and learning
to use it con dently to solve practical problems. Much of this book is focused
on developing a clear picture of energy and entropy, explaining their origins in
the microscopic behavior of matter, and developing e ective methods to analyze
complicated practical processes1 by carefully tracking what happens to energy.