The is an expression of the universal law of increasing Entropy . In simple terms, it is an expression of the fact that over time, differences in temperature, pressure, and density tend to even out in a physical system that is isolated from the outside world. Entropy is a measure of how far along this evening-out process has progressed. The most common enunciation of second law of thermodynamics is essentially due to Rudolf Clausius :
There are many versions of the second law, but they all have the same effect, which is to explain the phenomenon of Irreversibility in nature.
There are many statements of the second law which use different terms, but are all equivalent. 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 system with a mass of more than a few Picograms , the second law is true to within a few parts in a million.1
There are many ways of stating the second law of thermodynamics, but all are equivalent in the sense that each form of the second law logically implies every other form . Thus, the theorems of thermodynamics can be proved using any form of the second law.
The formulation of the second law that refers to entropy directly is due to Rudolf Clausius :
:
Thus, the system can either stay the same, or undergo some physical process that increases entropy. Processes that decrease total entropy of an isolated system do not occur. If a system is at equilibrium, by definition no spontaneous processes occur, and therefore the system is at maximum entropy.
Also due to Clausius is the simplest formulation of the second law, the heat formulation:
:
Informally, "Heat doesn't flow from cold to hot", which is obviously true from everyday experience. Note that from the mathematical definition of Entropy , a process in which heat flows from cold to hot has decreasing entropy, so the entropy formulation of the second law directly implies the heat formulation.
A third formulation of the second law, the heat engine formulation, by Lord Kelvin , is:
:
That is, it is impossible to extract energy by heat from a high-temperature energy source and then convert all of the energy into work. At least some of the energy must be passed on to heat a low-temperature energy sink. Thus, a heat engine with 100% efficiency is thermodynamically impossible.
Thermodynamics is a theory of macroscopic systems at equilibrium and therefore the second law applies only to macroscopic systems with well-defined temperatures. No violation of the second law of thermodynamics has ever been observed in a macroscopic system. But on scales of a few atoms, the second law does not apply; for example, in a system of two molecules, it is possible for the slower-moving ("cold") molecule to transfer energy to the faster-moving ("hot") molecule. Such tiny systems are outside the domain of thermodynamics, but they can be investigated using Statistical Mechanics . 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.2
The is an axiom of thermodynamics concerning heat, entropy, and the direction in which thermodynamic processes can occur. For example, the second law implies that heat does not spontaneously flow from a cold material to a hot material, but it allows heat to flow from a hot material to a cold material. Roughly speaking, the second law says that in an isolated system, concentrated energy disperses over time, and consequently less concentrated energy is available to do useful work. Energy dispersal also means that differences in temperature, pressure, and density even out. Again roughly speaking, thermodynamic Entropy is a measure of energy dispersal, and so the second law is closely connected with the concept of entropy.
In a general sense, the 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 heat occurs, in the form of entropy, when work is done.3 Pressure differences, density differences, and particularly temperature differences, all tend to equalize if given the opportunity. This means that an Isolated System will eventually come to have a uniform temperature. A Heat Engine is a mechanical device that provides useful work from the difference in temperature of two bodies:
During the 19th century, the second law was synthesized, essentially, by studying the dynamics of the Carnot Heat Engine in coordination with James Joule's Mechanical Equivalent Of Heat experiments. Since any thermodynamic engine requires such a temperature difference, it follows that no useful work can be derived from an Isolated System in equilibrium; there must always be an external energy source and a cold sink. By definition, would have to
violate the second law to function.
See Also: History of entropy
The first theory on the conversion of heat into mechanical work is due to Nicolas Léonard Sadi Carnot in 1824. He was the first to realize correctly that the efficiency of this conversion depends on the difference of temperature between an engine and its environment.
Recognizing the significance of of heat popular 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 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 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.
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According to the
First Law Of Thermodynamics , the change ''dU'' in the internal energy of the sub-system is the sum of the heat ''δq'' added to the sub-system, ''less'' any work ''δw'' done ''by'' the sub-system, ''plus'' any net chemical energy entering the sub-system ''d ∑μ
iRN
i'', so that:
:
where μ
iR are the
Chemical Potential s of chemical species in the external surroundings.
Now the heat leaving the reservoir and entering the sub-system is
:
where we have first used the definition of entropy in classical thermodynamics (alternatively, the definition of temperature in statistical thermodynamics); and then the Second Law inequality from above.
It therefore follows that any net work ''δw'' done by the sub-system must obey
:
It is useful to separate the work ''δw'' done by the subsystem into the ''useful'' work ''δw
u'' that can be done ''by'' the sub-system, over and beyond the work ''p
R dV'' done merely by the sub-system expanding against the surrounding external pressure, giving the following relation for the useful work that can be done:
:
It is convenient to define the right-hand-side as the exact derivative of a thermodynamic potential, called the ''availability'' or
Exergy ''X'' of the subsystem,
:
The Second Law therefore implies that for any process which can be considered as divided simply into a subsystem, and an unlimited temperature and pressure reservoir with which it is in contact,
:
i.e. the change in the subsystem's exergy plus the useful work done ''by'' the subsystem (or, the change in the subsystem's exergy less any work, additional to that done by the pressure reservoir, done ''on'' the system) must be less than or equal to zero.
When no useful work is being extracted from the sub-system, it follows that
:
with the
Exergy ''X'' reaching a minimum at equilibrium, when ''dX=0''.
If no chemical species can enter or leave the sub-system, then the term ''∑ μ
iR N
i'' can be ignored. If furthermore the temperature of the sub-system is such that ''T'' is always equal to ''T
R'', then this gives:
:
If the volume ''V'' is constrained to be constant, then
:
where ''A'' is the thermodynamic potential called
Helmholtz Free Energy , ''A=U-TS''. Under constant volume conditions therefore, ''dA ≤ 0'' if a process is to go forward; and ''dA=0'' is the condition for equilibrium.
Alternatively, if the sub-system pressure ''p'' is constrained to be equal to the external reservoir pressure ''p
R'', then
:
where ''G'' is the
Gibbs Free Energy , ''G=U-TS+PV''. Therefore under constant pressure conditions ''dG ≤ 0'' if a process is to go forwards; and ''dG=0'' is the condition for equilibrium.
In sum, if a proper ''infinite-reservoir-like'' reference state is chosen as the system surroundings in the real world, then the Second Law predicts a decrease in ''X'' for an irreversible process and no change for a reversible process.
:
is equivalent to
This expression together with the associated reference state permits a
Design Engineer working at the macroscopic scale (above the
Thermodynamic Limit ) to utilize the Second Law without directly measuring or considering entropy change in a total isolated system. (''Also, see
Process Engineer ''). Those changes have already been considered by the assumption that the system under consideration can reach equilibrium with the reference state without altering the reference state. An efficiency for a process or collection of processes that compares it to the reversible ideal may also be found (''See
Second Law Efficiency ''.)
This approach to the Second Law is widely utilized in
Engineering practice,
Environmental Accounting ,
Systems Ecology , and other disciplines.
Owing to the somewhat ambiguous nature of the formulation of the second law, i.e. the postulate that the quantity
Heat divided by
Temperature increases in spontaneous natural processes, it has occasionally been subject to criticism as well as attempts to dispute or disprove it. Clausius himself even noted the abstract nature of the second law. In his 1862 memoir, for example, after mathematically stating the second law by saying that integral of the differential of a quantity of heat divided by temperature must be greater than or equal to zero for every cyclical process which is in any way possible:
:
Clausius then states:
This statement, curiously, from one perspective, may be said to be equally valid to this very day. That is, owing to ambiguous nature of
Heat and
Temperature themselves, as they relate to sciences such as
Chemistry and
Particle Physics , entropy is still an ambiguous quantity. As such, scientists are forever trying to find loop-holes in the second law.
Before 1850,
Heat was regarded as an indestructible particle of matter. This was called the “material hypothesis”, as based principally on the views of Isaac Newton. It was on these views, partially, that in 1824 Sadi Carnot formulated the initial version of the second law. It soon was realized, however, that if the heat particle was conserved, and as such not changed in the cycle of an engine, that it would be possible to send the heat particle cyclically through the working fluid of the engine and use it to push the piston and then return the particle, unchanged, to its original state. In this manner
Perpetual Motion could be created and used as an unlimited energy source. Thus, historically, people have always been attempting to create a perpetual motion machine so to disprove the second law.
In 1871,
James Clerk Maxwell proposed a
Thought Experiment , now called
Maxwell's Demon , which challenged the second law. This experiment reveals the importance of observability in discussing the second law. In other words, it requires a certain amount of energy to collect the information necessary for the demon to "know" the whereabouts of all the particles in the system. This energy requirement thus negates the challenge to the second law. Moreover, to reconcile this apparent paradox from another perspective, one may resort to a use of
Information Entropy , although this is considered questionable by some.
See Also: Entropy (arrow of time)
The second law is a law about macroscopic irreversibility, and so may appear to violate the principle of
T-symmetry .
Boltzmann first investigated the link with microscopic reversibility. In his
H-theorem he gave an explanation, by means of
Statistical Mechanics , for dilute gases in the zero density limit where the
Ideal Gas equation of state holds. He derived the second law of thermodynamics not from mechanics alone, but also from the probability arguments. His idea was to write an equation of motion for the probability that a single particle has a particular position and momentum at a particular time. One of the terms in this equation accounts for how the single particle distribution changes through collisions of pairs of particles. This rate depends of the probability of pairs of particles. Boltzmann introduced the assumption of
Molecular Chaos to reduce this pair probability to a product of single particle probabilities. From the resulting
Boltzmann Equation he derived his famous
H-theorem , which implies that on average the entropy of an ideal gas can only increase.
The assumption of molecular chaos in fact violates time reversal symmetry. It assumes that particle momenta are uncorrelated ''before'' collisions. If you replace this assumption with "anti-molecular chaos," namely that particle momenta are uncorrelated ''after'' collision, then you can derive an anti-Boltzmann equation and an anti-H-Theorem which implies entropy decreases on average. Thus we see that in reality Boltzmann did not succeed in solving
Loschmidt's Paradox . The molecular chaos assumption is the key element that introduces the
Arrow Of Time .
See Also: Entropy and life
The second law of thermodynamics has been proven mathematically for thermodynamic systems, where entropy is defined in terms of
Heat divided by the
Absolute Temperature . The second law is often applied to other situations, such as the complexity of
Life , or orderliness.
4 Some, however, object to this application, on possibly
Philosophical or
Theological grounds, reasoning that thermodynamics does not apply to the process of life. In sciences such as
Biology and
Biochemistry , however, the application of thermodynamics is well-established, e.g.
Biological Thermodynamics . The general viewpoint on this subject is summarized well by biological thermodynamicist Donald Haynie; as he states: "Any theory claiming to describe how organisms originate and continue to exist by natural causes must be compatible with the first and second laws of thermodynamics."
5
In
Statistical Thermodynamics , which uses
Probability Theory to calculate
Thermodynamic Variable s, such as entropy, the second law only holds for
Ensemble Average s and the probability for single systems to violate it increases with decreasing size. The
Fluctuation Theorem describes this behaviour.
It is occasionally claimed that the second law is incompatible with autonomous
Self-organisation , or even the coming into existence of complex systems. The entry
Self-organisation explains how this claim is a misconception.
In fact, as hot systems cool down in accordance with the second law, it is not unusual for them to undergo
Spontaneous Symmetry Breaking , i.e. for structure to spontaneously appear as the temperature drops below a critical threshold. Complex structures, such as
Bénard Cells , also spontaneously appear where there is a steady flow of energy from a high temperature input source to a low temperature external sink. It is conjectured that such systems tend to evolve into complex, structured, critically unstable "
Edge Of Chaos " arrangements, which very nearly maximise the rate of energy degradation (the rate of entropy production).
6
Furthermore, the concept of entropy in thermodynamics is not identical to the common notion of "disorder". For example, a thermodynamically closed system of certain solutions will eventually transform from a cloudy liquid to a clear solution containing large "orderly" crystals. Most people would characterize the former state as having "more disorder" than the latter state. However, in a purely thermodynamic sense, the entropy has increased in this system, not decreased. The units of measure of entropy in thermodynamics are "units of energy per unit of temperature". Whether a human perceives one state of a system as "more orderly" than another has no bearing on the calculation of this quantity. The common notion that entropy in thermodynamics is equivalent to a popular conception of "disorder" has caused many non-physicists to completely misinterpret what the second law of thermodynamics is really about.
:::::::::--Sir
Arthur Stanley Eddington , ''The Nature of the Physical World'' (1927)
:::::::::::::--
Greg Hill and
Kerry Thornley , ''
Principia Discordia '' (1965)
:::::::::::::::::--Philosopher / Physicist
P.W. Bridgman , (1941)
- 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).
- Creationist Duane Gish incorrectly used the Second Law of Thermodynamics to argue that evolution was impossible.
