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

New Trends in Physics Teaching IV
This statement contains the qualitative content of the second law of thermodynamics. According
to Brillouin [3, p.3221, ‘the inanimate world, governed by physics and chemistry, obeys a
natural law of degradation, of loss of value. This law sums up the essentials of thermodynamics,
but the notion of value remains linked to. . . energy. Physics has not been able to (and probably
cannot) separate these two entities.’
THE QUALITATIVE CONCEPT OF ENERGY DEGRADATION
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In order to compare different processes with respect to their different degrees of degradation, the
concept itself must be made more precise. This requires us to free the valuation of energetic
processes from their subjective attributes. This cannot be done without introducing a certain
artificiality in explanation which must be accepted as the price we pay for a gain in .precision.
As may be concluded from the examples given above, degradation consists (objectively) of a
kind of irrevocable change; one which cannot be undone. The cooling of the hot water in a cold
room is no more reversible than is the transformation of the kinetic energy of a motor-car into
thermal energy of its surr0undings.l
To object that the mechanical energy of the motor-car could be recovered by the consumption
of additional chemical energy of petrol and that the cooled water can be re-heated is invalid. For,
in addition to restoring the original states, there is an increase in the thermal energy of the
surroundings and an equal decrease in the chemical energy of the petrol in the first case and a
further input of electrical energy in the second case.
A process is accompanied by a degradation of energy if it cannot be reversed
without causing an additional change in its surroundings. (2)
Such irreversible, energy-degrading processes which run spontaneously in a certain, natural
direction we shall call ‘spontaneous processes’.
Extending this consideration to composite processes, proposition (2) even holds in situations
where a part of a whole process consists of a process running opposite to its natural direction
(e.g. the objection already referred to).
Reversing the ‘cooling of water’, i.e. the ‘heating of water’ combined with the ‘consumption
of electrical energy’ considered as a whole, is a process which runs down without causing any
additional change in the surroundings and, therefore, gives rise of an energy degradation.
Thus although the spontaneous process ‘cooling of water’ is reversed and therefore accompanied
by an upgrading of energy, it may take place because simultaneously, another process, ‘the consumption
of electrical energy’ is running down providing for an even larger degradation. Overall,
a small degradation results.
Or, more generally:
The energy degradation due to a spontaneous process a is greater than that due to a
second spontaneous process /3 if a can drive /3 backwards (i.e. in the opposite direction
to the natural one). This implies that the degradation of energy associated with a
exceeds the upgrading of energy due to the reversed process 0, (3)
1. In the following pages, the processes which are fully described as ‘cooling of a hot system by transferring energy to the
sunoundings’ and ‘braking a moving system by transferring energy to the surroundings’ are sometimes abbreviated to ‘cooling’
and ‘braking’.
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