what is the 2nd law of thermodynamics?

Answer 1

It says that the total entropy of an isolated system is either increasing or constant.

Entropy is a measure of disorder. I find it easiest to understand in information-theory terms: it's related to the amount of information you need to describe the system in all details.

Suppose you have two gas containers, one with the temperature 0 K (-273 degrees Celsius), the other with some higher temperature, say 400 K. In order to describe the complete state of the system you need this information:
- The location of each "hot" molecule
- The velocity of each "hot" molecule
- The location of each "cold" molecule

You don't need the velocity of the "cold" molecules since they have temperature 0 and are therefore known not to move, i.e. velocity is zero.

Now you mix them and the mix gets the temperature of 200 K. Now you need information about the velocities of all the molecules.

More generally, if you mix a hot and a cold gas or liquid the entropy rises.

So the second law says that it will never be possible for a lukewarm gas or liquid spontaneously to divide itself into a hot and a cold fraction. It can be done artificially (e.g. in an airco) but this requires free energy that comes from somewhere: the coal burnt at the power plant that feeds your airco gains entropy, so the total entropy stays the same, or rises.

Decrease in entropy can also happen naturally, e.g. when a plant grown. Again, the free energy comes from somewhere, in this case from the sun.

Answer 2

One of the statements of the 2nd law of thermodynamics as i know it is 'entropy of the universe is constantly increasing'
eg: A spontanoeus process like the cooling of a hot object

Answer 3

The second law in simple language means that you cannot obtain more milk out of a cow without feeding the cow.

And the mass of milk is always less than the mass of the food fed.
The reason is that some of the food mass is lost to the surounding.
The Same rule applies to the Universe.

"it is impossible to obtain any thing with out paying a price"
this is the golden rule of thermodynamics.

Answer 4

The second law of thermodynamics is a axiom of nature regarding the directional flow of heat in relation to work and which accounts for the phenomenon of irreversibility in thermodynamic systems.

Answer 5

there r many forms in which one can describe the 2nd law. one way of saying is entropy of the universe is always increasing.
or all the systems in the universe are moving towards max. entropy. / entropy of a system alwayz increases.

Answer 6

On the average, things in the universe are getting more and more disorderly, and there's no way to avoid it.

There are exceptions though: living things actually *increase* order in the universe. But I said ON THE AVERAGE.

Answer 7

Energy is conserved, in otherwords, there is no such thing as getting more energy out of a system than you put into it.

Answer 8

U can find that in any text books

Answer 9

LOG ON TO :-

http://en.wikipedia.org/wiki/Second_law_of_thermodynamics

You will find more information about 2nd law of thermodynamics on this uniform resource locator ( called as URL ).......................

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Laws of thermodynamics

Second law of thermodynamics


The second law of thermodynamics is a axiom of nature regarding the directional flow of heat in relation to work and which accounts for the phenomenon of irreversibility in thermodynamic systems. The most common enunciation of second law of thermodynamics is:

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

This is equivalent to this scientific statement:

The entropy of an isolated system not at equilibrium will tend to increase over time, approaching a maximum value.

The second law is most applicable to macroscopic systems. For example, when one part of an isolated system interacts with another part, energy will tend to distribute equally among the accessible energy states of the system. As a result, given time, the system will reach a state of thermodynamic equilibrium, at which point the entropy will have a maximum value and the deviation of free energy will be greater than or equal to zero.


--------------------General description----------------------------------


In a general sense, the Second Law says that the differences between systems in contact with each other tend to even out. 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 thermodynamic engine is an engine that provides useful work from the difference in temperature of two bodies. 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 energy fed from the outside. The Second Law is often invoked as the reason why perpetual motion machines cannot exist.

The Second Law can be stated in various succinct ways, including:

It is impossible to produce work in the surroundings using a cyclic process connected to a single heat reservoir (Kelvin, 1851).
It is impossible to carry out a cyclic process using an engine connected to two heat reservoirs that will have as its only effect the transfer of a quantity of heat from the low-temperature reservoir to the high-temperature reservoir (Clausius, 1854).
If thermodynamic work is to be done at a finite rate, free energy must be expended.[1]
A mathematical statement of the Second Law is:

on average.
where

S is the entropy and t is time.

Note that this equation does not need to be true, but the Second Law asserts that it is usually true. Entropy can decrease, but this simply tends not to happen as often. A common misconception is that the Second Law means that entropy never decreases. In fact, the Second Law asserts only a statistical tendency, hence it is only highly unlikely that entropy will decrease in a closed system at any given instant.


An important and revealing idealised special case is to consider applying the Second Law to the scenario of an isolated system (called the total system or universe), made up of two parts: a sub-system of interest, and the sub-system's surroundings. These surroundings are imagined to be so large that they can be considered as an unlimited heat reservoir at temperature TR and pressure PR — so that no matter how much heat is transferred to (or from) the sub-system, the temperature of the surroundings will remain TR; and no matter how much the volume of the sub-system expands (or contracts), the pressure of the surroundings will remain PR.

Whatever changes dS and dSR occur in the entropies of the sub-system and the surroundings individually, according to the Second Law the entropy Stot of the isolated total system must increase:


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 ∑μiRNi, so that:


where μiR are the chemical potentials 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 done δw done by the subsystem into the useful work δwu that can be done by the sub-system, over and beyond the work PR 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.


------------------Special cases: Gibbs and Helmholtz free energies

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 ∑ μiR Ni can be ignored. If furthermore the temperature of the sub-system is such that T is always equal to TR, 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 PR, 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.


--------------------Application------------------------------------

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.


----------------Complex systems and the Second Law----------------

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 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).[citation needed]


-------------------------------------History--------------------------------------

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 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 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 Second Law is a law about macroscopic irreversibility. 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.

A paradox arises when the second law is considered in the context of a cyclical Big Bang because a cyclical Big Bang, repeating itself, would necessarily violate the second law. Some researchers suggest that this is not a paradox because the universe is not a closed system.

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, now called Maxwell's demon, that challenged the Second Law. This experiment reveals the importance of observability in discussing the Second Law. To reconcile this apparent paradox, one must resort to a questionable use of information entropy.

The second law is generally only appicable to systems with a large number of molecules. When a small number of molecules are considered, a non-trivial possibility exists that heat will transfer from a colder region to a warmer region. In general, even for systems with a large number of molecules, there is a tiny, but non-zero probability that heat will transfer from a colder region to a warmer region.

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, the Second Law is intimately related to quantum measurement theory and quantum collapse - and none of them is completely understood.


---------------------------------Miscellany-------------------------------------

Flanders and Swann produced a setting of a statement of the Second Law of Thermodynamics to music, called "First and Second Law".
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).
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).