Environmental Engineering Reference
In-Depth Information
1.2 Approaches to Understanding: Concepts,
Variables, Units
Energy and power are the key variables of energetics,
the first measured in joules J, the other in watts W. The
nouns energy and power are often used to express attri-
butes that have little to do with their scientifically defined
meanings. As a result, it is not surprising that incorrect
understanding and improper use of these key terms are
so widespread. This is not a new frustration: the term en-
ergy became practically indistinguishable from power or
force centuries ago. In 1748, David Hume complained
in An Enquiry Concerning Human Understanding (sec.
VII, 49), ''There are no ideas, which occur in meta-
physics, more obscure and uncertain, than those of
power, force, energy or necessary connexion, of which it
is every moment necessary for us to treat in all our
disquisitions.''
And in 1842 the seventh edition of Encyclopaedia Bri-
tannica offered only a brief entry, which described en-
ergy as ''the power, virtue, or efficacy of a thing. It is
also used figuratively, to denote emphasis in speech.'' Po-
tential energy represents yet another semantic problem:
potential commonly signifies a possibility or latency, but
in a strict physical sense, potential energy represents a
change in the spatial setting of a mass or its configuration.
Gravitational potential energy (the product of the ele-
vated mass, its mean height above ground, and the grav-
itational constant) is the consequence of a changed
position in the Earth's gravitational field. Springs that
have been tensioned by winding exemplify the practical
use of elastic potential energy: it is stored through the
springs' deformation and released as useful work when
the coil unwinds.
At the outset of the twenty-first century the scientifi-
cally specific terms energy and power continue to be
used imprecisely even by scientists and engineers (for in-
stance, in such ingrained phrases as ''total consumption
of energy,'' although energy cannot be consumed, only
transformed).
There is no difficulty defining power simply as the rate
of flow of energy:
W
¼
J/s
:
One can provide an equally elegant derivative for energy
as the integral of power
E
¼
ð
P dt
;
but this formulation does not make for an intuitive un-
derstanding of the phenomenon.
Richard Feynman (1918-1988) summed up the prob-
lem in his famous Lectures on Physics: ''It is important to
realize that in physics today, we have no knowledge of
what energy is. We do not have a picture that energy
comes in little blobs of a definite amount. It is not that
way'' (1963, 4-2). I prefer Rose's (1986, 5) solution,
which acknowledges the difficulty by being appropriately
evasive: Energy ''is an abstract concept invented by phys-
ical scientists in the nineteenth century to describe quan-
titatively a wide variety of natural phenomena.'' The
leading choice of science textbooks and encyclopedias is
''the capacity for doing work,'' a definition that is too
reductionist in its inevitable mechanical connotation and
that fills the intended space only when work is not under-
stood in the everyday sense of invested labor but as the
generalized physical act that produces a change of config-
uration in a system in opposition to any force that resists
such a change (Maxwell 1872). Defining energy as the
