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

Energy degradation
empty and in which their parents have to pay more for electrical energy and gas as each month
goes by do not exactly exclude the possibility of ‘energy consumption’. In order to avoid the
discrepancy between what the pupil perceives as reality and the scientific description of reality,
it must be stressed that the words ‘consumption’ and ‘conservation’ as applied to energy do not
imply a contradiction when used in discussions on energy. They express complementary aspects
of the same experience.
ENERGY CONSUMPTION AS DEGRADATION OF ENERGY
From the physical point of view, this procedure is fully justified. It reflects the fact that these
two aspects correspond to the physical concepts of ‘energy’ and ‘entropy’. These are as closely
related to each other as are the corresponding common-sense concepts of ‘energy’ and ‘energy
consump tion’.
To talk about ‘energy consumption’ is in accord with the use of the word ‘consumption’ in
other fields. For instance, in a household, water is used up (permanently) for washing, cleaning,
flushing the water closet etc. But, obviously, the water is not annihilated; it is conserved quantitatively.
This may even appear in the fee charged for the disposal of the spoiled (waste) water and
sometimes calculated from the quantity of water discharged into the sewer. Water drained off
into the sewage system is used up in the sense that it is impossible to re-use it as it is. Similarly,
a motor-car is ‘used up’ because, in the course of time, it depreciates - it gets more and more
defective and finally ends its life at the scrap yard.
Such examples show that the words ‘to conserve’ or ‘to use up’ are not normally applied in
the sense of a destruction of quantity but of a change in quality. Because of the impossibility
(or, at best, ‘limited possibility’) of re-using the above mentioned things for their original purpose,
the change in quality is a loss of value, a degradation.
As the following examples show, the expressions ‘energy consumption’, ‘energy loss’, and
‘to use up energy’ may be applied in the same sense.
Example I Hot water, exposed to cooler surroundings, cools to the temperature of those
surroundings. This process involves a degradation of energy in that the energy of the water,
originally supplied, perhaps, by an immersion heater, has disappeared. It has been transferred
irrevocably to the surroundings. There, it is good for nothing.
Example 2 The water of a waterfall, falling through a height, may be used to drive turbines
and, by means of a generator, produce electrical energy. Without turbines, the potential energy
of the water would have been transformed into thermal energy and shared between the water
and the surroundings. The energy has been degraded.’
Example 3 A motor-car uses up the chemical energy contained in petrol. Except during
acceleration, this energy is used to overcome air and rolling resistance, and it is dissipated. It is
degraded because it may not be re-used.
These few examples may suffice to show:
Energetic processes are always accompanied by a degradation of energy.
This finally transforms the energy into a form of which no use can be made. (1)
1. This example shows how difficult it may be to detect energy conservation in the phenomena observed. Here the effects of the
turning turbines which are, in turn, responsible for other obvious effects are much more striking than the almost imperceptible
thermal effect.
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