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

New Trends in Physics Teaching IV
We, the teachers of physics, must help our students to understand - we must put them into a
position in which they can interpret what they read or what they are told about energy in the
press, by the oil companies, by the trade unions (to see coal miners leading an anti-nuclear lobby
is surely worthy of sceptical comment) and all the other pressure groups.
As teachers, we must have access to reliable information about energy in the global context,
and about the effect of that knowledge on the approaches we might adopt in our teaching. That
is the purpose of this first half of the part on Energy. However, the part is largely concerned with
the impersonal and a few comments on the energy needs of human beings might be welcome.
The energy unit which is most familiar to the men and women in the street is the kilowatt-hour
of the electricity bill. Although it is, of course, 3600 kilojoules, its very familiarity makes it an
attractive unit when one wishes to relate energy to a human context.
An average requirement of energy for a human being is around 10 000 kJ per day. That is just
under 3 kWh. And the rate of supply is just over 0.1 kW.
If I switch a 100 W electric lamp on for 24 hours, that lamp converts energy at a rate of 0.1
kW; and, in 24 hours, wil use 8640 kJ or 2.4 kWh.
So you and I live by converting energy at about the same rate that a 100 W lamp converts
energy.
We derive this energy for living from our food. Any biology text will supply details of the
energy which is available to us from various foodstuffs. However, those textbooks usually omit
to point out that the foods themselves require energy for their production and that this too has
to be paid for. Consider a loaf of bread - a staple item in so many lands. According to P. Chapman
[ 11 a typical loaf of about 1.4 kg provides its consumer with about 14 000 kJ or nearly 4 kWh.
To make that loaf from the wheat (allowing for the farmer’s, the miller’s, the baker’s and the
shopkeeper’s usage of energy) requires 20000 kJ (5.5 kWh).
The field of wheat from which the bread was derived may be thought of as a solar farm consuming
solar energy and storing it up in the grain. We human beings then extract about 3.5 kWh
(12 500 kJ) from each kilogram of the harvested crop. If we were to burn the crop we should
get three times as much. But not in so useful a form. The energy would have been degraded.
The energy we extract from the loaf of bread has, through the process of photosynthesis,
been derived from solar radiation. Measurements from space craft confirm that the Earth receives
energy from the Sun at a rate of about 1.4 kW per square metre. Taking the Earth’s diametric
plane as of area 1.23 X lOI4 m2, the total input of energy to the Earth is 17.2 X 1OI6 W. Since,
over a long period of years the mean temperature of the Earth has remained very close to its
present value (280 K), we must conclude that just as much energy is radiated back into space as
is received. Figure 1 gives details of the intervening energy flows.
Of course, Stefan’s Law describes the process of radiating the energy back into space.
and
Dividing Eq. 2 by Eq. 1
E = kT4 Eq. 1
AE = k4T3AT Eq. 2
AA E = 4r AT
or Eq. 3
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