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

New Trends in Physics Teaching IV
The absorption of ‘hot’ solar radiation by the green leaves of plants (photosynthesis) may be
regarded as a first step in a long, roundabout path which eventually leads to the release of the
energy to space as invisible low temperature radiation. In this first step, the spontaneous process
of decay of biological material is reversed. Left to itself, and deprived of sunlight, biomatter
decays, absorbs oxygen, produces carbon dioxide and water and emits heat to its surroundings;
photosynthesis reverses the process. Using sunlight, biomatter is synthesized from carbon dioxide
and water with the emission of oxygen.
A second step on this roundabout path may be the utilization (i.e., burning) of fossil fuels to
generate (e.g., by means of thermal power plants) mechanical and electrical energy. Having been
buried under massive layers of rock for many millions of years, huge masses of biomatter have
been prevented from decaying. In a power plant, the energy degradation which might have been
dissipated in the process of decay is harnessed to reverse important processes (such as the dissipation
of electrical and mechanical energy) and hence upgrade energy. The overall degradation is
thus reduced.
Further steps may include (i) the application of the dissipation of the electrical energy to drive
in reverse processes which are important for human purposes and (ii) the use of biomatter as
food, making possible human and animal life in its various manifestations.
The driving of the natural water cycle provides another example of a roundabout path of
great importance. In condensing and then falling from the clouds as rain or snow, water transforms
huge quantities of mechanical energy into heat. It is lifted back to high altitude by the
reverse process of evaporation which is driven by sunlight. Condensing yet again to form clouds,
water may once more fall back to Earth and so on. Under appropriate conditions, the precipitation
may accumulate in the hills,form rivers and then, instead of dissipating the rest of its mechanical
energy by flowing to the sea, it may be harnessed to drive hydroelectric power stations.
A further example of a potentially useful roundabout path lies in the driving backwards of
spontaneous temperature balancing processes in the atmosphere, producing further temperature
differences. The balancing of those differences through the movement of air as wind may then be
used to reverse other spontaneous processes useful for human affairs.
These few examples may suffice to indicate how, in the end, these and similar events may be
explained by the possibility that spontaneous processes, when running down, may pass along
roundabout paths and drive in reverse other spontaneous processes of importance to mankind.
By the overall process of energy degradation, the absorption of visible sunlight and the emission
of invisible infra-red radiation, many processes important for life on Earth are driven through
cycles; running spontaneously in one direction, they are harnessed to run up in the other direction.
Energetically speaking, we may say that energy degradation may cause energy to be upgraded.
visible sunlight
photosynthesis
Figure 1. One of the life cycles on Earth driven by the dissipation of sunlight (after Bent [5]).
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