Laws of thermodynamics
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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 other branches of science connected to thermodynamics. They are often associated with concepts far beyond what is directly stated in the wording.
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[edit] History
The first principle of thermodynamics to be established was the Second Law, as formulated by Sadi Carnot in 1824. By 1860, as found in the works of those as Rudolf Clausius and William Thomson, there were two established "principles" of thermodynamics, the first principle and the second principle. As the years passed, these principles turned into "laws". By 1873, for example, thermodynamicist Willard Gibbs, in his “Graphical Methods in the Thermodynamics of Fluids”, clearly stated that there were two absolute laws of thermodynamics, a first law and a second law. Presently, there are a total of four laws. Over the last 80 years or so, occasionally, various writers have suggested added Laws, all of which are far from unanimously accepted.
[edit] Zeroth law
| If two thermodynamic systems are in thermal equilibrium with a third, they are also in thermal equilibrium with each other. |
When two systems are put in contact with each other, there will be a net exchange of energy between them unless they are in thermal 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. The Zeroth Law asserts that thermal equilibrium, viewed as a binary relation, is an equivalence relation.
[edit] First law
| In any process, the total energy of the universe remains constant. |
More simply, the First Law states that energy cannot be created or destroyed; rather, the amount of energy lost in a process cannot be greater than the amount of energy gained.
This is the statement of conservation of energy for a thermodynamic system. It refers to the two ways that a closed system transfers energy to and from its surroundings - by the process of heating (or cooling) and the process of mechanical work. The rate of gain or loss in the stored energy of a system is determined by the rates of these two processes. In open systems, the flow of matter is another energy transfer mechanism, and extra terms must be included in the expression of the first law.
The First Law clarifies the nature of energy. It is a stored quantity which is independent of any particular process path, i.e., it is independent of the system history. If a system undergoes a thermodynamic cycle, whether it becomes warmer, cooler, larger, or smaller, then it will have the same amount of energy each time it returns to a particular state. Mathematically speaking, energy is a state function and infinitesimal changes in the energy are exact differentials.
All laws of thermodynamics but the First are statistical and simply describe the tendencies of macroscopic systems. For microscopic systems with few particles, the variations in the parameters become larger than the parameters themselves, and the assumptions of thermodynamics become meaningless. The First Law, i.e. the law of conservation, has become the most secure of all basic laws of science. At present, it is unquestioned.
[edit] Second law
| There is no process that, operating in a 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. |
This version is the so-called Kelvin-Planck Statement. In a simple manner, the Second Law states that energy systems have a tendency to increase their entropy (heat transformation content) rather than decrease it.
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.
Stephen Hawking described this using time as an entropy base. For example, when time moves in a forward direction and one, say, breaks a cup of coffee on the floor, no matter what happens, in our universe, one will never see the cup reform. Cups are breaking all the time, but never reforming. Since the Big Bang, the entropy of the universe has been on the rise, and so the Second Law states that this process will continue to increase.
[edit] Third law
| As temperature approaches absolute zero, the entropy of a system approaches a constant. |
The Third Law deals with the fact that there is an absolute constant in the universe known as absolute zero. Derived from the Gibbs free energy equation, where ΔG = ΔH - TΔS (where ΔG is the change in free energy, ΔH is the change in enthalpy (or total heat), T is Temperature and ΔS is the change in entropy (or unusable heat), as the temperature reaches 0 or a very low value, ΔS naturally will also approach 0 or a very small value.
Put another way, if one imagines atoms flying around in a box, hitting each other randomly, all the time, one can imagine a lot of chaos. Then, imagine what would happen if the temperature begins to decrease. The atoms slow down, hit each other less frequently, begin to settle as gravity has more effect on them; the chaos decreases.
[edit] Combined law
Aside from the established four basic laws of thermodynamics described above, there is also the combined law of thermodynamics. The combined law of thermodynamics is essentially the 1st and 2nd law subsumed into a single concise mathematical statement as shown below:
Here, E is energy, T is temperature, S is entropy, P is pressure, and V is volume.
[edit] Tentative fourth laws or principles
In the late 19th century, thermodynamicist Ludwig Boltzmann argued that the fundamental object of contention in the life-struggle in the evolution of the organic world is 'available energy'. Since then, over the years, various thermodynamic researchers have come forward to ascribed to or to postulate potential fourth laws of thermodynamics; in some cases, there are even fifth or sixth laws of thermodynamics supposed. The majority of these tentative fourth law statements are attempts to apply thermodynamics to evolution. Most fourth law statements, however, are speculative and far from agreed upon.
The most common proposed Fourth Law is the Onsager reciprocal relations. Another example is the maximum power principle as put forward initially by biologist Alfred Lotka in his 1922 article Contributions to the Energetics of Evolution. Most variations of hypothetical fourth laws (or principles) have to do with the environmental sciences, biological evolution, or galactic phenomena.
