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

New Trends in Physics Teaching IV
of transformations, all of which take it from the same initial state to the same final state (thermodynamic
equilibrium states). Variations in the internal energy of the system do not depend on
the path followed in the transformation considered.
The first law of thermodynamics provides a general application of this conclusion to any
system exchanging only heat and work with the surroundings. As the internal energy is a function
of state, for any given equilibrium state there wil be clearly defined macroscopic variables; with
this can be associated a system describing a succession of states of equilibrium, and we thus have
a reversible transformation. Here it should be pointed out that such a transformation does not
exist in nature and must be regarded as the limit of a real transformation which is infinitely slow
and subject to a negligible dissipation of energy.
CONCLUSION
In conclusion, and perhaps popularizing somewhat, I shall attempt to introduce the concept of
entropy. Entropy is interpreted as a measure of the disorder of a system (more commonly
referred to as the degradation of the system’s energy). It seems reasonable to say that a reduction
of this disorder is accompanied by a decrease in the entropy and that the entropy is zero at
absolute zero temperature (all thermal agitation having then disappeared).
Some attempts are currently being made to popularize the concept of entropy, and it is
applied in this connection to the universe itself, the latter being regarded as an isolated system.
It may then be considered that the universe, which is the site of irreversible real transformations,
does not pass through the same state twice.
We may therefore visualize entropy as continually increasing and attaining its maximum value,
thus leading to a universe in which the temperature would be the same at every point, which
would preclude any form of life (an eventuality known to be very remote).
The second law of thermodynamics provides us with a better understanding of this aspect in
everyday life. In the same way as, in order to put a hydraulic machine into operation there must
be a difference in levels, a thermal machine can operate only if there is a temperature difference,
that is, two sources of heat at different temperatures.
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