Thermodynamics

Thermodynamics (Greek: thermos = heat and dynamis = power) is the physics of heat, work, enthalpy, and entropy changes in relation to the spontaneity of processes. In origins, thermodynamics is the study of engines. Prior to 1698, with the invention of the Savery Engine, horses were used to "power" pulleys, attached to buckets, which lifted water out of flooded salt mines in England. In the years to follow, more variations of steam engines were built; as the Newcomen Engine, and later the Watt Engine. In time, these early engines would eventually be utilized in place of horses. Thus, each engine began to be associated with a certain amount of "horse power" depending upon how many horses it had replaced! The main problem with these first engines was that they were slow and clumsy, converting less than 2% of the input fuel into useful work. In other words, large quantities of coal (or wood) had to be burned to yield only a small fraction of work output. Hence the need for a new science of engine dynamics was born.

The laws of thermodynamics

In Thermodynamics, there are four laws of very general validity, and as such they do not depend on the details of the interactions or the systems being studied. This means they can be applied to systems about which one knows nothing other than the balance of energy and matter transfer with the environment. Examples of this include Einstein's prediction of spontaneous emission around the turn of the 20th century and the current research into the thermodynamics of black holes. Alternative statements that are mathematically equivalent can be given for each law, as follows:

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Zeroth law (Thermodynamic equilibrium):

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If systems A and B are in thermodynamic equilibrium, and systems B and C are in thermodynamic equilibrium, then systems A and C are also in thermodynamic equilibrium.

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When two systems are put in contact with each other, there will be a net exchange of energy and/or matter between them unless they are in thermodynamic equilibrium. Two systems are in thermodynamic equilibrium with each other if they stay the same after being put in contact.

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While this is a fundamental concept of thermodynamics, the need to state it explicitly as a law was not perceived until the first third of the 20th century, long after the first three laws were already widely in use, hence the zero numbering. There is still some discussion about its status.

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Thermodynamic equilibrium includes thermal equilibrium (associated to heat exchange and parameterized by temperature), mechanical equilibrium (associated to work exchange and parameterized generalized forces such as pressure), and chemical equilibrium (associated to matter exchange and parameterized by chemical potential).

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1st Law (Conservation of energy):

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The work exchanged in an adiabatic process depends only on the initial and the final state and not on the details of the process.

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This is equivalent to a statement of the conservation of energy, because no heat flows during an adiabatic process. This means that the only energy flowing into or out of a system during an adiabatic process is work done on or by the system.

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This law is equivalent to dU=dQ+dW , where U is the internal energy of a system, Q is the heat flowing into the system, and W is the work done on the system. The energy received by the system is positive.

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2nd Law (Entropy):

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It is impossible to obtain a process that, operating in cycle, produces no other effect than the subtraction of a positive amount of heat from a reservoir and the production of an equal amount of work. (Kelvin-Planck Statement)

Related Topics:
Kelvin - Planck

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The entropy of a thermally isolated macroscopic system never decreases (see Maxwell's demon), however a microscopic system may exhibit fluctuations of entropy opposite to that dictated by the second law (see Fluctuation Theorem). In fact the mathematical proof of the Fluctuation Theorem from time-reversible dynamics and the Axiom of Causality, constitutes a proof of the Second Law. In a logical sense the Second Law thus ceases to be a "Law" of Physics and instead becomes a theorem which is valid for large systems or long times.

Related Topics:
Entropy - Maxwell's demon - Fluctuation Theorem

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3rd Law

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(Absolute Zero):

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As temperature goes to 0, the entropy of a system approaches a constant.

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It is important to remember that the laws of thermodynamics are only statistical generalizations. That is, they simply describe the tendencies of macroscopic systems. On the quantum level, the laws of thermodynamics often break down. Furthermore, as evidenced by Maxwell's demon, it is theoretically possible to specifically engineer a quantum system to break the laws of thermodynamics. The first law of thermodynamics, however, i.e. the law of conservation, has become the most sound of all laws in science. Its validity has never been disproved.

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