atom preserves its identity during a chemical reaction but loses it during a
nuclear reaction. Atoms may also possess electric and magnetic dipole-
moment energieswhen subjected to external electric and magnetic fields
due to the twisting of the magnetic dipoles produced by the small electric
currents associated with the orbiting electrons.
The forms of energy already discussed, which constitute the total energy
of a system, can be containedor storedin a system, and thus can be viewed
as the staticforms of energy. The forms of energy not stored in a system
can be viewed as the dynamicforms of energy or as energy interactions.
The dynamic forms of energy are recognized at the system boundary as they
cross it, and they represent the energy gained or lost by a system during a
process. The only two forms of energy interactions associated with a closed
system are heat transferand work.An energy interaction is heat transfer if
its driving force is a temperature difference. Otherwise it is work, as
explained in the next section. A control volume can also exchange energy
via mass transfer since any time mass is transferred into or out of a system,
the energy content of the mass is also transferred with it.
In daily life, we frequently refer to the sensible and latent forms of inter-
nal energy as heat, and we talk about heat content of bodies. In thermody-
namics, however, we usually refer to those forms of energy as thermal
energyto prevent any confusion with heat transfer.
Distinction should be made between the macroscopic kinetic energy of an
object as a whole and the microscopic kinetic energies of its molecules that
constitute the sensible internal energy of the object (Fig. 2–7). The kinetic
energy of an object is an organizedform of energy associated with the
orderly motion of all molecules in one direction in a straight path or around
an axis. In contrast, the kinetic energies of the molecules are completely
randomand highly disorganized. As you will see in later chapters, the orga-
nized energy is much more valuable than the disorganized energy, and a
major application area of thermodynamics is the conversion of disorganized
energy (heat) into organized energy (work). You will also see that the orga-
nized energy can be converted to disorganized energy completely, but only a
fraction of disorganized energy can be converted to organized energy by
specially built devices called heat engines(like car engines and power
plants). A similar argument can be given for the macroscopic potential
energy of an object as a whole and the microscopic potential energies of the
molecules.More on Nuclear Energy
The best known fission reaction involves the split of the uranium atom (the
U-235 isotope) into other elements and is commonly used to generate elec-
tricity in nuclear power plants (440 of them in 2004, generating 363,000
MW worldwide), to power nuclear submarines and aircraft carriers, and
even to power spacecraft as well as building nuclear bombs.
The percentage of electricity produced by nuclear power is 78 percent in
France, 25 percent in Japan, 28 percent in Germany, and 20 percent in the
United States. The first nuclear chain reaction was achieved by Enrico
Fermi in 1942, and the first large-scale nuclear reactors were built in
1944 for the purpose of producing material for nuclear weapons. When a56 | Thermodynamics
WaterDamMacroscopic kinetic energy
(turns the wheel)Microscopic kinetic
energy of molecules
(does not turn the wheel)FIGURE 2–7
The macroscopickinetic energy is an
organized form of energy and is much
more useful than the disorganized
microscopickinetic energies of the
molecules.