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Entropy is a mathematically-defined thermodynamic quantity that helps to account for the flow of energy through a thermodynamic process. Entropy was originally defined for a thermodynamically reversible process as where the uniform temperature ( T ) of a closed system is divided into an incremental reversible transfer of heat energy into that system ( d Q ). The above definition is sometimes called the macroscopic definition of entropy because it can be used without regard to any microscopic picture of the contents of a system. In thermodynamics, entropy has been found to be more generally useful and it has several other reformulations. Entropy was discovered when it was noticed via mathematics to be a quantity that behaves as a function of state .
Enthalpy is a measure of the total energy of a thermodynamic system . It includes the internal energy , which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure . Enthalpy is a thermodynamic potential . It is a state function and an extensive quantity. The unit of measurement for enthalpy in the International System of Units (SI) is the joule , but other historical, conventional units are still in use, such as the British thermal unit and the calorie . The enthalpy is the preferred expression of system energy changes in many chemical, biological, and physical measurements, because it simplifies certain descriptions of energy transfer .
The four laws of thermodynamics define fundamental physical quantities ( temperature , energy , and entropy ) that characterize thermodynamic systems . The laws describe how these quantities behave under various circumstances, and forbid certain phenomena (such as perpetual motion ). The four laws of thermodynamics are: [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 6 ]
In thermodynamics , a quasistatic process is a thermodynamic process that happens infinitely slowly. No real process is quasistatic, but such processes can be approximated by performing them very slowly. [ edit ] Quasistatic and reversibility
In the philosophy of thermal and statistical physics , Maxwell's demon is a thought experiment created by the physicist James Clerk Maxwell to "show that the Second Law of Thermodynamics has only a statistical certainty". [ 1 ] It demonstrates Maxwell 's point by hypothetically describing how to violate the Second Law: a container of gas molecules at equilibrium, is divided into two parts by an insulated wall, with a door that can be opened and closed by what came to be called "Maxwell's demon". The demon opens the door to allow only the faster than average molecules to flow through to a favored side of the chamber, and only the slower than average molecules to the other side, causing the favored side to gradually heat up while the other side cools down, thus decreasing entropy . [ edit ] Origin and history of the idea
The Carnot cycle is a theoretical thermodynamic cycle proposed by Nicolas Léonard Sadi Carnot in 1823 and expanded by in the 1830s and 40s. It can be shown that it is the most efficient cycle for converting a given amount of thermal energy into work, or conversely, creating a temperature difference (e.g. refrigeration) by doing a given amount of work. Every single thermodynamic system exists in a particular state. When a system is taken through a series of different states and finally returned to its initial state, a thermodynamic cycle is said to have occurred.