Thermodynamics

56 | ThermodynamicsMicroscopic kineticenergy of molecules(does not turn the wheel)WaterDamMacroscopic kinetic energy(turns the wheel)FIGURE 2–7The macroscopic kinetic energy is anorganized form of energy and is muchmore useful than the disorganizedmicroscopic kinetic energies of themolecules.atom preserves its identity during a chemical reaction but loses it during anuclear reaction. Atoms may also possess electric and magnetic dipolemomentenergies when subjected to external electric and magnetic fieldsdue to the twisting of the magnetic dipoles produced by the small electriccurrents associated with the orbiting electrons.The forms of energy already discussed, which constitute the total energyof a system, can be contained or stored in a system, and thus can be viewedas the static forms of energy. The forms of energy not stored in a systemcan be viewed as the dynamic forms of energy or as energy interactions.The dynamic forms of energy are recognized at the system boundary as theycross it, and they represent the energy gained or lost by a system during aprocess. The only two forms of energy interactions associated with a closedsystem are heat transfer and work. An energy interaction is heat transfer ifits driving force is a temperature difference. Otherwise it is work, asexplained in the next section. A control volume can also exchange energyvia 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 internalenergy as heat, and we talk about heat content of bodies. In thermodynamics,however, we usually refer to those forms of energy as thermalenergy to prevent any confusion with heat transfer.Distinction should be made between the macroscopic kinetic energy of anobject as a whole and the microscopic kinetic energies of its molecules thatconstitute the sensible internal energy of the object (Fig. 2–7). The kineticenergy of an object is an organized form of energy associated with theorderly motion of all molecules in one direction in a straight path or aroundan axis. In contrast, the kinetic energies of the molecules are completelyrandom and highly disorganized. As you will see in later chapters, the organizedenergy is much more valuable than the disorganized energy, and amajor application area of thermodynamics is the conversion of disorganizedenergy (heat) into organized energy (work). You will also see that the organizedenergy can be converted to disorganized energy completely, but only afraction of disorganized energy can be converted to organized energy byspecially built devices called heat engines (like car engines and powerplants). A similar argument can be given for the macroscopic potentialenergy of an object as a whole and the microscopic potential energies of themolecules.More on Nuclear EnergyThe best known fission reaction involves the split of the uranium atom (theU-235 isotope) into other elements and is commonly used to generate electricityin nuclear power plants (440 of them in 2004, generating 363,000MW worldwide), to power nuclear submarines and aircraft carriers, andeven to power spacecraft as well as building nuclear bombs.The percentage of electricity produced by nuclear power is 78 percent inFrance, 25 percent in Japan, 28 percent in Germany, and 20 percent in theUnited States. The first nuclear chain reaction was achieved by EnricoFermi in 1942, and the first large-scale nuclear reactors were built in1944 for the purpose of producing material for nuclear weapons. When a