Basic Concepts of Thermodynamics
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Basic Concepts of Thermodynamics
Combustion is one of the most complex subjects that involve primarily such disciplines as physics, chemistry, thermodynamics and fluid mechanics. Thermodynamics enables us to calculate the energies of system changes in composition. As such it enables us to determine, for example, the temperature and pressure changes when a system undergoes a chemical transformation. It will be seen that thermodynamics can also be used to tell us what the composition change will be when a system undergoes a reaction. It is not used however to determine rates of chemical transition. That is the subject of chemical kinetics. The subject of thermodynamics is only concerned with beginning and end thermodynamic states for a system, with no concern for the process path between them. Therefore it will be important to recall the necessary basic concepts of thermodynamics in this course. The purpose of this chapter is to make students life easier, by providing review of the main basic concepts of thermodynamics and necessary definitions. Similarly, in several chapters below the necessary concepts of fluid mechanics will be given, some of them usually missed in standard courses of fluid mechanics.
1.1 The Entropy Thermodynamic physical quantities are those, which describe macroscopic states of system. They include some, which have both a thermodynamic and a purely mechanical meaning, such as energy, volume, density, etc. There are also, however, quantities of another kind, which appear as a result of purely statistical laws and have no meaning when applied to non-macroscopic systems, for example, entropy. In what follows we shall define a number of relations between thermodynamic quantities. When thermodynamic quantities are discussed, the negligibly small fluctuations to which they are subject are usually of no interest, so that, we shall entirely ignore such fluctuations, regarding the thermodynamic quantities as varying only with the macroscopic state of the system. M.A. Liberman, Introduction to Physics and Chemistry of Combustion, DOI: 10.1007/978-3-540-78759-4_1, Ó Springer-Verlag Berlin Heidelberg 2008
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1 Basic Concepts of Thermodynamics
Entropy is one of the main thermodynamic functions. To introduce meaning of the entropy let us consider a closed system for a period of time long compared to its relaxation time, assuming that the system is in complete statistical equilibrium. Let us divide the system into a large number of macroscopic parts, which we call subsystems. Let w ¼ wðEn Þ be the distribution function for the subsystem. The probability that the subsystem has energy between within the interval between E and E þ dE is the product of w ¼ wðEn Þ on the number of quantum states with energies in this interval. Let ðEÞ be the number of quantum states with energies less than or equal to E. Then the required number of states with energy between E and E þ dE can be written as dðEÞ dE dE, and the energy probability distribution is WðEÞ ¼
dðEÞ wðEÞ; dE
(1:1:1)
which must satisfy the normal
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