Читать книгу Geochemistry - William M. White - Страница 39

The first law may be stated in various ways:

 Heat and work are equivalent§.

 Energy is conserved in any transformation.

 The change of energy of a system is independent of the path taken.

All are restatements of the law of conservation of energy:

Energy can be neither created nor destroyed.

Mathematically:

(2.22)

Thermodynamics is concerned only with the internal energy of a system. We don't really care whether the system as a whole is in motion, i.e., whether it has kinetic energy (we do care, however, about the internal kinetic energy, or heat). For the most part, we also don't care whether it has potential energy, except to the extent that this influences the state of our system (e.g., pressure in the atmosphere is a function of the altitude, and hence would be of interest to us). In addition, we are almost always concerned only with energy changes, not with the absolute energy of a system. In thermodynamics, ΔU, not U, is the interesting quantity.

Of course, we now understand that it is not energy that is conserved, but rather mass energy. Albert Einstein proposed this important modification of Joule's result in 1905 (Einstein, 1905). Conversion of mass to energy fuels the Sun and the stars and, through radioactive decay, is an important source of energy in the Earth. Radioactive decay and nuclear fusion will be important topics in Chapters 8 and 10. For the geochemical processes we will be interested in the next few chapters, however, we can take the conservation of energy alone to be absolute.

Energy may be transferred between a system and its surroundings in several ways: heat, work, radiation, and advection (i.e., energy associated with mass gained or lost by the system). Whenever possible, we will want to choose our system such that it is closed and we don't have to worry about the latter. In most, but not all, instances of geochemical interest, radiation is not important. Thus in geochemical thermodynamics, heat and work are the forms of energy flow of primary interest.