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Message: <blockquote data-quote="fishD" data-source="post: 12754112" data-attributes="member: 289272">ThermodynamicsFirst law The <a href="http://en.wikipedia.org/wiki/First_law_of_thermodynamics" target="_blank">first law of thermodynamics</a> may be expressed by several forms of the <a href="http://en.wikipedia.org/wiki/Fundamental_thermodynamic_relation" target="_blank">fundamental thermodynamic relation</a> for a closed system: Increase in internal energy of a system = heat supplied to the system - work done by the system. U = Q - W For a <a href="http://en.wikipedia.org/wiki/Thermodynamic_cycle" target="_blank">thermodynamic cycle</a>, the net <a href="http://en.wikipedia.org/wiki/Heat" target="_blank">heat</a> supplied to the system equals the net <a href="http://en.wikipedia.org/wiki/Work_%28thermodynamics%29" target="_blank">work</a> done by the system. More specifically, the First Law encompasses the following three principles: <ul> <li data-xf-list-type="ul"><a href="http://en.wikipedia.org/wiki/Conservation_of_energy" target="_blank">The law of conservation of energy</a><ul> <li data-xf-list-type="ul">This states that energy can be neither created nor destroyed.  However, energy can change forms, and energy can flow from one place to  another. The total energy of an isolated system remains the same.</li> </ul> </li> <li data-xf-list-type="ul">The flow of <a href="http://en.wikipedia.org/wiki/Heat" target="_blank">heat</a> is a form of energy transfer.<ul> <li data-xf-list-type="ul">(In other words, a quantity of heat that flows from a hot object to a  cold object can be expressed as an amount of energy being transferred  from the hot object to the cold object.)</li> </ul> </li> <li data-xf-list-type="ul">Performing <a href="http://en.wikipedia.org/wiki/Work_%28thermodynamics%29" target="_blank">work</a> is a form of energy transfer.<ul> <li data-xf-list-type="ul">(For example, when a machine lifts a heavy object upwards, some  energy is transferred from the machine to the object. The object  acquires its energy in the form of <a href="http://en.wikipedia.org/wiki/Gravitational_potential_energy" target="_blank">gravitational potential energy</a> in this example.)</li> </ul> </li> </ul>Combining these three principles gives the first law of thermodynamics.Second law The <a href="http://en.wikipedia.org/wiki/Second_law_of_thermodynamics" target="_blank">second law of thermodynamics</a> asserts the existence of a quantity called the <a href="http://en.wikipedia.org/wiki/Entropy" target="_blank">entropy</a> of a system and further states that When two <a href="http://en.wikipedia.org/wiki/Isolated_system" target="_blank">isolated systems</a> in separate but nearby regions of space, each in <a href="http://en.wikipedia.org/wiki/Thermodynamic_equilibrium" target="_blank">thermodynamic equilibrium</a>  in itself (but not necessarily in equilibrium with each other at first)  are at some time allowed to interact, breaking the isolation that  separates the two systems, allowing them to exchange matter or energy,  they will eventually reach a mutual thermodynamic equilibrium. The sum  of the <a href="http://en.wikipedia.org/wiki/Entropy" target="_blank">entropies</a>  of the initial, isolated systems is less than or equal to the entropy  of the final combination of exchanging systems. In the process of  reaching a new thermodynamic equilibrium, total entropy has increased,  or at least has not decreased. It follows that the entropy of an isolated macroscopic system never  decreases. The second law states that spontaneous natural processes  increase entropy overall, or in another formulation that heat can  spontaneously be conducted or radiated only from a higher-temperature  region to a lower-temperature region, but not the other way around. The second law refers to a wide variety of processes, reversible and  irreversible. Its main import is to tell about irreversibility. The prime example of irreversibility is in the transfer of heat by  conduction or radiation. It was known long before the discovery of the  notion of entropy that when two bodies of different temperatures are  connected with each other by purely thermal connection, conductive or  radiative, then heat always flows from the hotter body to the colder  one. This fact is part of the basic idea of heat, and is related also to  the so-called zeroth law, though the textbooks' statements of the  zeroth law are usually reticent about that, because they have been  influenced by Carathéodory's basing his axiomatics on the law of  conservation of energy and trying to make heat seem a theoretically  derivative concept instead of an axiomatically accepted one. Šilahvý  (1997) notes that Carathéodory's approach does not work for the  description of irreversible processes that involve both heat conduction  and conversion of kinetic energy into internal energy by viscosity  (which is another prime example of irreversibility), because "the  mechanical power and the rate of heating are not expressible as  differential forms in the 'external parameters'".<a href="http://en.wikipedia.org/wiki/Laws_of_thermodynamics#cite_note-9" target="_blank">[10]</a> The second law tells also about kinds of irreversibility other than  heat transfer, and the notion of entropy is needed to provide that wider  scope of the law. According to the second law of thermodynamics, in a reversible heat transfer, an element of heat transferred, δQ, is the product of the temperature (T), both of the system and of the sources or destination of the heat, with the increment (dS) of the system's conjugate variable, its <a href="http://en.wikipedia.org/wiki/Entropy" target="_blank">entropy</a> (S) <img src="http://upload.wikimedia.org/wikipedia/en/math/0/6/d/06d4d643bc8b4821d724e0a8e2274bee.png" alt="" class="fr-fic fr-dii fr-draggable " style="" /><a href="http://en.wikipedia.org/wiki/Laws_of_thermodynamics#cite_note-Guggenheim_1985-0" target="_blank">[1]</a> The second law defines <a href="http://en.wikipedia.org/wiki/Entropy" target="_blank">entropy</a>,  which may be viewed not only as a macroscopic variable of classical  thermodynamics, but may also be viewed as a measure of deficiency of  physical information about the microscopic details of the motion and  configuration of the system, given only predictable experimental  reproducibility of bulk or <a href="http://en.wikipedia.org/wiki/Macroscopic" target="_blank">macroscopic</a>  behavior as specified by macroscopic variables that allow the  distinction to be made between heat and work. More exactly, the law  asserts that for two given macroscopically specified states of a system,  there is a quantity called the difference of entropy between them. The  entropy difference tells how much additional microscopic physical  information is needed to specify one of the macroscopically specified  states, given the macroscopic specification of the other, which is often  a conveniently chosen reference state. It is often convenient to  presuppose the reference state and not to explicitly state it. A final  condition of a natural process always contains microscopically  specifiable effects which are not fully and exactly predictable from the  macroscopic specification of the initial condition of the process. This  is why entropy increases in natural processes. The entropy increase  tells how much extra microscopic information is needed to tell the final  macroscopically specified state from the initial macroscopically  specified state.<a href="http://en.wikipedia.org/wiki/Laws_of_thermodynamics#cite_note-10" target="_blank">http://en.wikipedia.org/wiki/Laws_of_thermodynamics#cite_note-10</a><a href="http://en.wikipedia.org/wiki/Laws_of_thermodynamics#cite_note-10" target="_blank">http://en.wikipedia.org/wiki/Laws_of_thermodynamics#cite_note-10</a>Third law The <a href="http://en.wikipedia.org/wiki/Third_law_of_thermodynamics" target="_blank">third law of thermodynamics</a> is sometimes stated as follows: The <a href="http://en.wikipedia.org/wiki/Entropy" target="_blank">entropy</a> of a perfect <a href="http://en.wikipedia.org/wiki/Crystal" target="_blank">crystal</a> at <a href="http://en.wikipedia.org/wiki/Absolute_zero" target="_blank">absolute zero</a> is exactly equal to zero. At zero temperature the system must be in a state with the minimum  thermal energy. This statement holds true if the perfect crystal has  only one <a href="http://en.wikipedia.org/wiki/Microstate_%28statistical_mechanics%29" target="_blank">state with minimum energy</a>. Entropy is related to the number of possible microstates according to S = kBln(Ω), where S is the entropy of the system, kB Boltzmann's constant, and Ω the number of microstates (e.g. possible configurations of atoms). At absolute zero there is only 1 microstate possible (Ω=1) and ln(1) = 0. A more general form of the third law that applies to systems such as <a href="http://en.wikipedia.org/wiki/Glass" target="_blank">glasses</a> that may have more than one minimum energy state: The entropy of a system approaches a constant value as the temperature approaches zero. The constant value (not necessarily zero) is called the <a href="http://en.wikipedia.org/wiki/Residual_entropy" target="_blank">residual entropy</a> of the system.<a href="http://en.wikipedia.org/wiki/Laws_of_thermodynamics#cite_note-10" target="_blank">http://en.wikipedia.org/wiki/Laws_of_thermodynamics#cite_note-10</a><a href="http://en.wikipedia.org/wiki/Laws_of_thermodynamics#cite_note-10" target="_blank"></a><a href="http://en.wikipedia.org/wiki/Laws_of_thermodynamics#cite_note-10" target="_blank"></a></blockquote>

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