In the statistical approach the entropy of an isolated (constant energy) system is k B log Ω, where k B is Boltzmann's constant and the function log stands for the natural (base e) logarithm. The quantum statistical point of view, too, will be reviewed in the present article. Boltzmann's definition of entropy was furthered by John von Neumann to a quantum statistical definition. In 1877 Ludwig Boltzmann gave a definition of entropy in the context of the kinetic gas theory, a branch of physics that developed into statistical thermodynamics. The second law states that the entropy of an isolated system increases in spontaneous (natural) processes leading from one state to another, whereas the first law states that the internal energy of the system is conserved. In this approach, entropy is the amount of heat (per degree kelvin) gained or lost by a thermodynamic system that makes a transition from one state to another. The "engineering" manner-by an engine-of introducing entropy will be discussed below. Carnot's work foreshadowed the second law of thermodynamics. The traditional way of introducing entropy is by means of a Carnot engine, an abstract engine conceived of by Sadi Carnot in 1824 as an idealization of a steam engine. The state variable "entropy" was introduced by Rudolf Clausius in 1865, see the inset for his text, when he gave a mathematical formulation of the second law of thermodynamics. I have deliberately constructed the word entropy to resemble as much as possible the word energy, since both quantities to be named by these words are so closely related in their physical meaning that a certain similarity in their names seems appropriate to me. As I deem it better to derive the names of such quantities - that are so important for science - from the antique languages, so that they can be used without modification in all modern languages, I propose to call the quantity S the entropy of the body, after the Greek word for transformation, ἡ τροπή. Miller, Computational Methods of Neutron Transport, American Nuclear Society, 1993, ISBN: 2-4.Translation: Searching for a descriptive name for S, one could - like it is said of the quantity U that it is the heat and work content of the body - say of the quantity S that it is the transformation content of the body. Hetrick, Dynamics of Nuclear Reactors, American Nuclear Society, 1993, ISBN: 3-2. Neuhold, Introductory Nuclear Reactor Dynamics, American Nuclear Society, 1985, ISBN: 9-4. Bezella, Introductory Nuclear Reactor Statics, American Nuclear Society, Revised edition (1989), 1989, ISBN: 3-2. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993. Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.Physics of Nuclear Kinetics. Addison-Wesley Pub. Nuclear and Particle Physics. Clarendon Press 1 edition, 1991, ISBN: 978-0198520467 Nuclear Reactor Engineering: Reactor Systems Engineering, Springer 4th edition, 1994, ISBN: 978-0412985317 Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 8-1. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983). The absolute value of specific entropy is unknown is not a problem, however, because it is the change in specific entropy (∆s) and not the absolute value that is important in practical problems. For example, the specific entropy of water or steam is given using the reference that the specific entropy of water is zero at 0.01☌ and normal atmospheric pressure, where s = 0.00 kJ/kg. Normally, the entropy of a substance is given for some reference value. In general, specific entropy is a property of a substance, like pressure, temperature, and volume, but it cannot be measured directly. Because entropy tells so much about the usefulness of an amount of heat transferred in performing work, the steam tables include values of specific entropy (s = S/m) as part of the information tabulated. M = mass (kg) T-s diagram of Rankine CycleĮntropy quantifies the energy of a substance that is no longer available to perform useful work. It equals the total entropy (S) divided by the total mass (m). The specific entropy (s) of a substance is its entropy per unit mass. Engineers use the specific entropy in thermodynamic analysis more than the entropy itself. The entropy can be made into an intensive or specific variable by dividing by the mass.
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