The Second Law of Thermodynamics
In thermodynamics, the second law of thermodynamics (also known as the law of entropy increase) is a fundamental principle of thermodynamics that states that the total entropy of a thermodynamic system can never decrease over time, despite any energy transfer or work performed on the system. Entropy is a measure of the disorder, or randomness, within a system, and may be thought of as a measure of how much a system's energy is spread out, or lost, in the form of heat. As stated by the second law of thermodynamics, any process that occurs spontaneously will inevitably lead to a system in a more disordered state, unless processes that reduce entropy occur. The second law of thermodynamics is the basis of the concept of "entropy" in thermodynamics. The second law of thermodynamics is an expression of the impossibility of creating and/or destroying energy in an isolated system. In other words, it is impossible for a system to produce energy out of "thin air". A system receives energy only by its surroundings, and energy can be transferred in only one direction, from surroundings to system. It can be argued that the second law of thermodynamics is a statement about the statistical nature of energy and its transfer. The second law of thermodynamics is also notable for its ubiquity in everyday life, as it is the basis for the entropy laws of thermodynamics, which govern all processes, large and small, that occur in nature. All of these processes share certain characteristics in common, and all of them are limited by the second law of thermodynamics. These characteristics include (1) the production of heat (energy) and work on a local scale, (2) the dissipation of mechanical energy and the transfer of energy to and from a system, and (3) the transformation of energy into less useful forms. In the 19th century, the French physicist Sadi Carnot laid the foundations of thermodynamics, and the second law of thermodynamics was formulated by the 20th-century German physicist Rudolf Clausius, who was the first to study the notion of entropy. In 1850, William Thomson, later Lord Kelvin, was the first to propose the modern form of the second law, although his original proposal was not the final statement of the second law that is known today. In 1854, William Rankine stated the second law of thermodynamics in its final form. The second law was formulated in 1857 by James Clerk Maxwell in response to the theoretical work of Rudolf Clausius. It was Maxwell's work that is credited with establishing thermodynamic equilibrium as the "first law of thermodynamics". The second law of thermodynamics can be understood from three complementary perspectives: (1) the logical basis of thermodynamics (entropy and the impossibility of certain processes); (2) the physical basis of thermodynamics, and (3) the thermodynamic implications of the second law. The first and third perspectives are particularly emphasized by the leading contemporary textbook, "Thermodynamics and Statistical Mechanics", by Michael R. Douglas and Peter R. Holland. The second law of thermodynamics is a particular statement of the law of conservation of energy. In that sense, it can be stated as follows: In the above statement, the first law is the law of conservation of energy, and the second law is the law of conservation of energy for a thermodynamic system. The second law of thermodynamics can be stated in a number of equivalent ways, depending on the context and the field of view of the statements. In the 19th century, the second law was expressed in terms of a closed system. The first law of thermodynamics states that energy is constant in an isolated system, and it is the balancing of the second law of thermodynamics with the first law of thermodynamics that is responsible for the constancy of the energy of an isolated system. The second law is also expressed as the impossibility of creating and/or destroying energy. The second law of thermodynamics can be stated in terms of entropy. The entropy of a thermodynamic system can be defined as the number of thermodynamic degrees of freedom per particle in the system. Entropy is a property of the system, but not a property of the particles of the system. Entropy is a function of state; that is, it depends only on the internal state of the system, and not on the history of the system. In particular, the entropy of a perfect crystal at absolute zero is zero. The entropy of a system can be increased or decreased, but the entropy of the universe as a whole cannot decrease. The second law of thermodynamics is the change in entropy of a system, in response to an applied work input, divided by the total change in entropy in the universe, over all time. In other words, it is the change in entropy of the universe divided by the change in entropy of the system.