Geoscience Reference
In-Depth Information
The laws of thermodynamics
KEY CONCEPTS
The basic principles of energy are embodied in the laws of thermodynamics. These were initially developed in 1843
by Prescott Joule, an English physicist, to explain processes seen in steam engines. Since then it has been appreciated
that they have far wider significance, and they now represent basic precepts of science. The first two laws of
thermodynamics state that:
•
Energy can be transformed but not destroyed.
•
Heat can never pass spontaneously from a colder to a hotter body; a temperature change can never occur
spontaneously in a body at uniform temperature.
The first law therefore defines the conservation of energy. The second law leads to the principle that energy transfers
are a result of inequalities in energy distribution and that energy is always transferred from areas of high energy
status to areas of low energy status, that is, down the energy gradient.
The third law of thermodynamics is less easy to explain. In very general terms, it says that systems tend towards
equilibrium, that is, a random distribution of energy over time.
changes. We cannot, therefore, describe the processes
operating throughout the entire global system in any
detail. We can, however, try to identify the dominant
transfers operating at a global scale and indicate, within
this general pattern, the roles played by the various
subsystems.
We have already noted that the balance between
incoming and outgoing radiation is such that marked
disparities occur between the energy status of different
parts of the globe. The most obvious effect of this is the
range in temperature we find when travelling from pole
to equator, a range of 30
C to 60
C, depending upon the
time of year and the hemisphere. In simple terms, it is
these differences that drive the global energy circulation.
In order to achieve equilibrium, energy is transferred
from the warmer to the cooler parts of the globe. If
someone turned off the sun these transfers would result
eventually in a more or less uniform distribution of energy
across the globe; the fact that the sun continues to supply
this unequal distribution of energy, however, maintains
the imbalance and makes the attempt to achieve unifor-
mity a losing battle. On the other hand, if the battle
were not fought, the fact that the equatorial areas are
constantly gaining more energy than they lose, while the
polar areas are losing more than they gain, would result
in a massive accumulation of heat in the lower latitudes
and indescribable cold in higher latitudes.
Thus there is a net poleward transfer of energy, and
this transfer maintains the existing pattern of energy
distribution; it feeds the higher latitudes and drains the
lower latitudes (
Figure 2.16
).
This transfer is brought
Energy and work
Transfer and conversion of energy are associated with the
performance of work. The sun performs work in heating
Earth through its provision of radiant energy. A river uses
kinetic energy to perform the work of moving bed load.
The weathering of rocks or the decomposition of plant
debris involves work carried out largely by chemical
energy. Indeed, it is the work done in these ways that
characterizes the myriad processes operating in the
environment.
When this work is carried out, therefore, energy is
transferred from one body to another, and in some cases
it is also converted from one form to another. In the
process, the total energy content remains the same, it is
changed only in form. When a river or glacier cuts a
valley, the energy it uses is not destroyed but transferred
or converted to other forms - some to heat energy, some
to potential energy, some remaining as kinetic energy.
When a plant grows, it takes in energy from the sun, from
the air and from the soil and stores it. The energy is not
lost, it is conserved but may be transferred and
transformed in the process.
Every feature and every part of the globe is at some stage
or another involved in energy transfers and transfor-
mations, and, as conditions change, so the nature of the
transfers and conversions operating at any one place also
