Laws of thermodynamics
From Wikipedia, the free encyclopedia.
| Laws of thermodynamics |
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| Zeroth law of thermodynamics |
| First law of thermodynamics |
| Second law of thermodynamics |
| Third law of thermodynamics |
| Philosophy of thermal and statistical physics |
| Edit |
The laws of Thermodynamics in principle describe the specifics for the transport of heat and work in thermodynamic processes. Since their conception, however, these laws have become some of the most important in all of physics and many other branches of science. They are often associated with concepts far beyond what is directly stated in the wording.
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Zeroth law
- Main article: Zeroth law of thermodynamics
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.
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. 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.
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).
First law
- Main article: First law of thermodynamics
The work exchanged in an adiabatic process depends only on the initial and the final state and not on the details of the process.
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.
Second law
- Main article: Second law of thermodynamics
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. (the so-called Kelvin-Planck Statement)
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.
Third law
- Main article: Third law of thermodynamics
As temperature goes to 0, the entropy of a system approaches a constant.
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.
