Environmental Engineering Reference
In-Depth Information
together are so much larger then those that hold molecules together, nuclear reactions are much more
energetic than molecular ones. Nevertheless, we can consider both molecules and atomic nuclei
to possess energies of formation that must be taken into account in expressing the conservation of
energy for material bodies that experience chemical or nuclear changes in composition.
3.2.4
Electric and Magnetic Energy
Molecules that possess a magnetic or electric dipole moment can store energy when they are in
the presence of a magnetic or electric field, in the form of magnetic or electric polarization of the
material. This energy is associated with the interaction of the molecular dipoles of the material
body with the external electric charges and currents that give rise to the applied electric or magnetic
field. Since capacitors and inductors are common components of electronic and electrical circuits,
this form of energy is important to their functioning.
3.2.5
Total Energy
The various forms of energy that can be possessed by a material body, as described above, can be
added together to define a total energy, to which we give the symbol
E
,
E
≡
KE
+
PE
+
U
+
E
chem
+
E
nuc
+
E
el
+
E
mag
(3.1)
It is very seldom that more than just a few of these forms are significant in any practical process
for which there are changes in the total energy. There are many examples. In a gasoline engine, the
combustion of the fuel-air mixture involves
U
and
E
chem
; in a steam and gas turbine, only
KE
and
U
change; in a nuclear power plant fuel rod,
U
and
E
nuc
are involved; and in a magnetic cryogenic
refrigerator,
U
and
E
mag
are important. Nevertheless, the manner in which the various forms of
energy enter into the laws of thermodynamics is expressed through the total energy
E
, a result of
very great generality and consequence.
3.3
WORK AND HEAT INTERACTIONS
Thermodynamics deals with the interaction of a thermodynamic material system and its environ-
ment.
1
It is through such interactions that we are able to generate mechanical power or other useful
effects in the environment. There are two quite different but important modes of interaction of a
system with its environment, called the work interaction and the heat interaction. Each of these is
a process in which, over time, the system and its environment undergo physical and/or chemical
changes related to the kind of interaction taking place, either work or heat (or both simultaneously).
As we shall see below, work and heat interactions are distinguishable from each other by the
character of the changes in the system and the environment. Both are quantifiable in terms of the
interaction, being expressed in energy units. Neither is a form of energy, but only a transaction
1
In thermodynamics, the environment of a thermodynamic system is that part of its material surroundings
with which the system interacts.
