Second law of thermodynamics

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In physics, the second law of thermodynamics, in its many forms, is a statement about the quality and direction of energy flow, and it is closely related to the concept of entropy.

This law and its derivatives, such as the law of friction, define the arrow of time: most other physical laws are time-reversal invariant.

General description

The first law of thermodynamics states that one form of energy, e.g. kinetic, potential, electrical energy, thermal,... can be converted into another without loss. The second law states that thermal energy, or heat, is special among the types of energies: all the forms of energy can be converted into heat, but in a way that is not reversible; it is not possible to convert the heat back fully in its original form. In other words, heat is a form of energy of lower quality.

What makes heat so special? According to the kinetic theory, heat is due to the random movement of atoms and molecules, so it looks much like kinetic energy. The difference is that those movements cannot be observed or predicted, while all the other forms of energy are the result of some orderly movement of particles. The second law says that the amount of random movement, i.e. the entropy, can only increase in a closed system, i.e. that we cannot put this randomness in order without some external influence. (Some systems, for example living cells, spontaneously become structured when they receive energy from the outside - see dissipative structures).

The following example illustrates the law. When a stone falls on earth, its kinetic energy is converted into heat, i.e. it becomes random movements of earth particles. The second law says that this random movement will never become ordered again. For example, the random movement will never become synchronized to throw the stone back in the air: the heat energy will not revert to the original kinetic energy.

Yet, there is one thing predictable about heat: it flows from hot to cold bodies. This can be used to convert some heat into mechanical energy, using a Carnot heat engine. The cycle stops when both bodies reach the same temperature: it can be shown that the amount of random movements has not decreased in the process.

The second law of thermodynamics is important to engineers because it provides a way to determine the quality, as well as the amount of degradation of energy during a process. It is also used to determine the theoretical upper limits for the performance of many commonly used engineering systems like refrigerators, internal combustion engines, and chemical reactors.

Any device that violates the second law of thermodynamics would be called a perpetual motion machine of the second kind. One example of this would be a device that can do work such as pumping water, simply by taking energy from the air.

History and recent developments

The first theory on the conversion of heat into mechanical work is due to Sadi Carnot in 1824. He was the first to realize correctly that the efficiency of the process depends on the difference of temperature between the hot and cold bodies.

Recognizing the significance of James Prescott Joule's work on the conservation of energy, Rudolf Clausius was the first to formulate the second law in 1850, in this form: heat does not spontaneously flow from cold to hot bodies. While common knowledge now, this was contrary to the caloric theory of heat in vogue at the time, which considered heat as a liquid. From there he was able to infer the law of Sadi Carnot and the definition of entropy (1865).

Established in the 19th century, the Kelvin-Planck statement of the second law of thermodynamics says, "It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work." This was shown to be equivalent to the statement of Clausius.

The second law of thermodynamics is essentially a macroscopic law about non-reversibility. Boltzmann had first investigated the link with microscopic reversibility. He has given explanation by means of Statistical mechanics, first for diluted gases in his H-theorem. He did not derive the second law of thermodynamics from mechanics alone, but also from the probability arguments. His idea was to use the coarse graining, grouping of microstates into macrostates, and then to make a statement about what is most probable to happen for a macrostate - for some microstates the entropy will decrease, but this happens with low probability.

A key assumption in his approach is that particles are not correlated before a collision, and are correlated afterwards. This is the key element that introduces the arrow of time.

The Ergodic hypothesis is also important for the Boltzmann approach. It says that, over long periods of time, the time spent in some region of the phase space of microstates with the same energy is proportional to the volume of this region, i.e. that all accessible microstates are equally probable over long period of time. Equivalently, it says that time average and average over the statistical ensemble are the same.

In 1871, James Clerk Maxwell proposed a thought experiment that challenged the second law. It is now called Maxwell's demon and is an example of the importance of observability in discussing the second law (see the article for details).

In Quantum mechanics, the ergodicity approach can also be used. However, there is an alternative explanation, which involves Quantum collapse - it is a straightforward result that quantum measurement increases entropy of the ensemble. Thus, second law of thermodynamics is intimately related to quantum measurement theory and quantum collapse - and none of them is completely understood.

The relation between the second law of thermodynamics and microscopic reversibility has been demonstrated theoretically in 1993 in the form of the Fluctuation theorem, and later observed experimentally. This has important applications in nanotechnology.

Other

Flanders and Swann produced a particularly insightful explanation of thermodynamics, which was popular in the 1950's and 1960's. Their work, entitled First and Second Law is unique in having been set to music.


The Second Law is exhibited (coarsely) by a box of electrical cables. Cables added from time to time tangle, inside the 'closed system' (cables in a box) by adding and then removing cables. The best way to untangle them is to start by taking the cables out of the box and placing them stretched out. The cables in a closed system (the box) will never untangle, but giving them some extra space starts the process of untangling (by going outside the closed system).

Evolution, creationism and the second law

Creationists often use the second law of thermodynamics to argue against biological evolution. However, the argument itself is often misconstrued. The apparent originator of the argument, Henry M. Morris, has claimed that the second law suggests a tendency to go from order to disorder and that biological evolution lacks the means to overcome this tendency.


Anti-creationists have sometimes misconstrued the argument, confusing “tendency” to mean “invariability.” Thus there have been such responses as pointing to local decreases in entropy such as crystallization, where there is a localized increase in order (at the expense of a net increase in entropy for the universe as a whole). This does not attack the actual creationist position since they have argued tendency, not invariability.


Another response based on the misunderstanding of the creationist position is the “open system argument.” Put simply, the second law says that entropy cannot decrease in an isolated system. What the open system argument points out is that the earth is not an isolated system because the sun's energy constantly bathes the planet. Therefore entropy can decrease (and “order” can increase) for evolution to work. The problem is that this response, like the one before, does not refute the actual creationist position. Morris and others claim that certain criteria are needed for order (at least the sort of order evolution requires to increase in organisms) to increase, and that evolution does not meet all of these criteria. These criteria are not part of the second law itself, but such creationists believe that we have good empirical basis for accepting them. An open system and available energy is part of the criteria, but there are others. Generally throwing raw energy into a system does not produce order but instead destroys the order already there (e.g. an atomic bomb introduced and detonated into a system can release a very large amount of energy with very little net increase in order).


Nonetheless, the majority of scientists believe that evolution meets any and all criteria needed for organisms to increase in biological complexity, and that the second law poses no insurmountable problem to biological evolution.

Quotes including the second law

  • "Nothing in life is certain except death, taxes and the second law of thermodynamics. All three are processes in which useful or accessible forms of some quantity, such as energy or money, are transformed into useless, inaccessible forms of the same quantity. That is not to say that these three processes don't have fringe benefits: taxes pay for roads and schools; the second law of thermodynamics drives cars, computers and metabolism; and death, at the very least, opens up tenured faculty positions"---Seth Lloyd, writing in Nature 430, 971 (26 August 2004); doi:10.1038/430971a
  • "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. And if your theory contradicts the facts, well, sometimes these experimentalists make mistakes. 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"---Arthur Eddington
  • "A good many times I have been present at gatherings of people who, by the standards of the traditional culture, are thought highly educated and who have with considerable gusto been expressing their incredulity at the illiteracy of scientists. Once or twice I have been provoked and have asked the company how many of them could describe the Second Law of Thermodynamics. The response was cold: it was also negative."---C.P._Snow Rede Lecture in 1959 entitled "The Two Cultures and the Scientific Revolution".

See Also

General

"Evolution violates the second law"

"Evolution does not violate the second law"

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