Second law of thermodynamics

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The second law of thermodynamics states that whenever energy is transformed from one form to another form, or matter moves freely, entropy increases.

Over time, differences in temperature, pressure, and density tend to even out in a horizontal plane. Due to the force of gravity, density and pressure do not even out in a vertical plane. In contrast, in a body in thermodynamic equilibrium, temperature evens out in a vertical plane.

Entropy is a measure of spread or dispersal of matter and energy to everywhere they have access.

The most common wording for the second law of thermodynamics is essentially due to Rudolf Clausius:

It is impossible to construct a device which produces no other effect than transfer of heat from lower temperature body to higher temperature body

In layman's terms: everything tries to maintain the same temperature over time.

There are many statements of the second law which use different terms, but are all equal. Another statement by Clausius is:

Heat cannot of itself pass from a colder to a hotter body.

An equivalent statement by Lord Kelvin is:

A transformation whose only final result is to convert heat, extracted from a source at constant temperature, into work, is impossible.

The second law is only applicable to macroscopic systems. The second law is actually a statement about the probable behavior of an isolated system. As larger and larger systems are considered, the probability of the second law being practically true becomes more and more certain. For any isolated system with a mass of more than a few picograms, the second law is true to within a few parts in a million.[1]

A simple stylized diagram of a heat pump's vapor-compression refrigeration cycle: 1) condenser, 2) expansion valve, 3) evaporator, 4) compressor.

Overview[change | change source]

In a general sense, the second law says that temperature differences between systems in contact with each other tend to even out and that work can be obtained from these non-equilibrium differences, but that loss of thermal energy occurs, when work is done and entropy increases.[2] Pressure, density and temperature differences in an isolated system, all tend to equalize if given the opportunity; density and pressure, but not temperature, are affected by gravity. A heat engine is a mechanical device that provides useful work from the difference in temperature of two bodies.

Quotes[change | change source]

The law that entropy always increases, holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the universe is in disagreement with Maxwell's equations — then so much the worse for Maxwell's equations. If it is found to be contradicted by observation — well, these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.
--Sir Arthur Stanley Eddington, The Nature of the Physical World (1927)
The tendency for entropy to increase in isolated systems is expressed in the second law of thermodynamics -- perhaps the most pessimistic and amoral formulation in all human thought.
--Greg Hill and Kerry Thornley, Principia Discordia (1965)
There are almost as many formulations of the second law as there have been discussions of it.
--Philosopher / Physicist P.W. Bridgman, (1941)

Miscellany[change | change source]

  • Flanders and Swann produced a setting of a statement of the Second Law of Thermodynamics to music, called "First and Second Law".
  • The economist Nicholas Georgescu-Roegen showed the significance of the Entropy Law in the field of economics (see his work The Entropy Law and the Economic Process (1971), Harvard University Press).

References[change | change source]

  1. Landau, L.D.; Lifshitz, E.M. (1996). Statistical Physics Part 1. Butterworth Heinemann. ISBN 0-7506-3372-7.
  2. Mendoza, E. (1988). Reflections on the Motive Power of Fire – and other Papers on the Second Law of Thermodynamics by E. Clapeyron and R. Clausius. New York: Dover Publications. ISBN 0-486-44641-7.

Further reading[change | change source]

  • Goldstein, Martin, and Inge F., 1993. The Refrigerator and the Universe. Harvard Univ. Press. A gentle introduction, a bit less technical than this entry.
  • Maxwell's demon 2 : entropy, classical and quantum information, computing. Edited by Harvey S. Leff and Andrew F. Rex. Bristol; Philadelphia : Institute of Physics, 2003

Other websites[change | change source]