The second law allows a distinguished temperature scale, which defines an absolute, thermodynamic temperature, independent of the properties of any particular reference thermometric body. For a body in thermal equilibrium with another, there are indefinitely many empirical temperature scales, in general respectively depending on the properties of a particular reference thermometric body. The zeroth law of thermodynamics in its usual short statement allows recognition that two bodies in a relation of thermal equilibrium have the same temperature, especially that a test body has the same temperature as a reference thermometric body. This is because a general process for this case may include work being done on the system by its surroundings, which must have frictional or viscous effects inside the system, and because heat transfer actually occurs only irreversibly, driven by a finite temperature difference. For an actually possible infinitesimal process without exchange of matter with the surroundings, the second law requires that the increment in system entropy be greater than that: d S > δ Q T (closed system, actually possible, irreversible process). ĭifferent notations are used for infinitesimal amounts of heat ( δ) and infinitesimal amounts of entropy ( d) because entropy is a function of state, while heat, like work, is not. In a fictive reversible process, an infinitesimal increment in the entropy ( d S) of a system is defined to result from an infinitesimal transfer of heat ( δ Q) to a closed system divided by the common temperature ( T) of the system and the surroundings which supply the heat: d S = δ Q T (closed system, idealized fictive reversible process). Its modern definition is in terms of entropy. For example, heat always flows spontaneously from hotter to colder bodies, and never the reverse, unless external work is performed on the system. It asserts that a natural process runs only in one sense, and is not reversible. The second law is concerned with the direction of natural processes. The first law of thermodynamics provides the basic definition of internal energy, associated with all thermodynamic systems, and states the rule of conservation of energy. Its first formulation is credited to the French scientist Sadi Carnot in 1824, who showed that there is an upper limit to the efficiency of conversion of heat to work in a heat engine. The second law has been expressed in many ways. Statistical thermodynamics, classical or quantum, explains the microscopic origin of the law. Historically, the second law was an empirical finding that was accepted as an axiom of thermodynamic theory. Derivation for systems described by the canonical ensemble.Derivation of the entropy change for reversible processes.Statement for a system that has a known expression of its internal energy as a function of its extensive state variables.Relation between Kelvins statement and Plancks proposition.Equivalence of the Clausius and the Kelvin statements.The increase in entropy accounts for the irreversibility of natural processes, and the asymmetry between future and past. It can remain constant in ideal cases where the system is in a steady state (equilibrium) or undergoing a reversible process. The second law of thermodynamics states that the total entropy of an isolated system can only increase over time.
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