New trends in physics teaching, v.4; The ... - unesdoc - Unesco

New Trends in Physics Teaching IV
Entropy and information
R. U. SEXL AND A. PFLUG
WHO IS AFRAID OF ENTROPY?
Among the many quantities used in physics, such as energy, momentum, electrical charge or
temperature, there is one which has often been considered to be somewhat mysterious and hard
to understand: entropy. It is interesting to ask why entropy acquired this reputation.
The first reason is that entropy - unlike energy, electrical currents or velocities - plays no
important role in everyday life. Second, in contrast to energy or momentum, entropy is not a
conserved quantity which would remain constant in all possible physical processes. On the
contrary, entropy generally increases in the course of time. Finally there is no piece of experimental
equipment which could be used to measure the entropy of a body directly. The entropy
is a ‘theoretical concept’ which can be calculated only with the help of a specific heat according
to the well-known relation
Here dS denotes the increase in entropy of a body when the amount of heat dQ is supplied
at the temperature T, and c is the corresponding specific heat. [ 11
What are the motivations for this strange definition of entropy? In (Eq. 1) a quantity has been
defined which is capable of describing the direction of all thermodynamic processes. In this
respect entropy is the characteristic quantity of thermodynamics, since only in this sub-field of
physics are the two directions of time not equivalent.
The considerations which follow aim to suggest an approach to the concept of entropy which
wil lead to an intuitive understanding of this physical quantity, comparable to the ‘feeling’ one
usually has for charge, temperature or energy. In many cases our ‘feelings’ about these quantities
can be used to derive qualitative statements about the behaviour of a physical system even without
details and exact calculations.
A suitable approach starts from one of the most important scientific concepts of our century,
the concept of information. It was the discovery of Claude Shannon and Norbert Wiener that
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