The second law of thermodynamics (page 324) The entropy
of a closed system always increases, or at best stays the same:
∆S≥0.The third law of thermodynamics (page 339) The entropy of
a system approaches zero as its temperature approaches abso-
lute zero.From a modern point of view, only the first law deserves to be called
a fundamental law of physics. Once Boltmann discovered the mi-
croscopic nature of entropy, the zeroth and second laws could be
understood as statements about probability: a system containing a
large number of particles is overwhelmingly likely to do a certain
thing, simply because the number of possible ways to do it is ex-
tremely large compared to the other possibilities. The third law
is also now understood to be a consequence of more basic physical
principles, but to explain the third law, it’s not sufficient simply to
know that matter is made of atoms: we also need to understand the
quantum-mechanical nature of those atoms, discussed in chapter 13.
Historically, however, the laws of thermodynamics were discovered
in the eighteenth century, when the atomic theory of matter was
generally considered to be a hypothesis that couldn’t be tested ex-
perimentally. Ideally, with the publication of Boltzmann’s work on
entropy in 1877, the zeroth and second laws would have been imme-
diately demoted from the status of physical laws, and likewise the
development of quantum mechanics in the 1920’s would have done
the same for the third law.5.5 More about heat engines
So far, the only heat engine we’ve discussed in any detail has been a
fictitious Carnot engine, with a monoatomic ideal gas as its working
gas. As a more realistic example, figure m shows one full cycle
of a cylinder in a standard gas-burning automobile engine. This
four-stroke cycle is called the Otto cycle, after its inventor, German
engineer Nikolaus Otto. The Otto cycle is more complicated than a
Carnot cycle, in a number of ways:- The working gas is physically pumped in and out of the cylin-
der through valves, rather than being sealed and reused indef-
initely as in the Carnot engine. - The cylinders are not perfectly insulated from the engine block,
so heat energy is lost from each cylinder by conduction. This
makes the engine less efficient that a Carnot engine, because
heat is being discharged at a temperature that is not as cool
as the environment.
340 Chapter 5 Thermodynamics