94 The Poetry of Physics and The Physics of Poetry
transfer their kinetic energy or hear through collisions with the molecules
of the converting fluid, which in turn transfer their kinetic energy or heat
through molecular collisions to the cooler body. The third common
process of heat transfer referred to as radiation involves the emission of
electromagnetic radiation in the infrared region of the spectrum by the
molecules of the hotter of the two bodies as a result of their thermal
motion and the absorption of this radiation by the molecules of the cooler
body.
In all processes involving the transfer of heat from one body to
another it is always found that the total energy is conserved. This is the
first law of thermodynamics. In terms of the atomic description of
matter, the conservation of energy is a result of the fact that the
molecular collisions conserve kinetic energy. In processes such as
friction where mechanical energy is converted into heat energy
conservation of energy holds for the same reason. In this process the
average motion of all the individual molecules is reduced and the energy
that is lost by reduction of the overall kinetic energy of the body is
completely randomized into random thermal motion. Thus the process
whereby mechanical energy is converted into thermal energy conserves
energy in accordance with the first law of thermodynamics, which is a
statement of the conservation of energy.
Friction also increases the disorder of the universe in accordance with
the second law of thermodynamics, which states that any spontaneous
change in a physics system increases its entropy or disorder. Entropy is a
quantitative measure of the disorder of a system whose exact definition
need not concern us here. We will use the term synonymously with
disorder. The natural tendency of the entropy or disorder of physical
systems to increase spontaneously results from the fact that states, in
which the order increases are highly unlikely to occur from a
probabilistic point of view. The probabilities become exceedingly small
because of the enormous number of molecules composing any given
system. The best way of illustrating this concept is to consider a pile of
papers place outdoors on a windy day. As soon as the winds gust, the
orderly pile of paper will become a random collection of papers scattered
all about. If, on the other hand, I started with a random scattering of
paper I could not expect the wind to blow the papers into a nice neat pile.
This illustrates the natural tendency of physical processes to increase the
disorder or entropy of the universe.