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'''Thermodynamics''' is the study of the effects of work, heat, and energy on a system. Thermodynamics is only concerned with so-called ''macroscopic'' observations, which are observations on large numbers of particles.<ref>http://www.grc.nasa.gov/WWW/K-12/airplane/thermo.html</ref> Measurements in thermodynamics are usually reported on the [[International System of Units#Kelvin|Kelvin scale]] <ref>David Halliday, "Fundamentals of Physics Extended", John Wiley & Sons, New York, 1997 </ref>

'''Thermodynamics''' (from Greek: ''θερμός'' "theromos"; ''δύναμης'' "dynamis") is a part of the physical sciences involved in the study of the effects of work, heat, and energy on a system. The study of thermodynamics is central to the subjects of physics and chemistry, as well as important to the processes within biology and geology. 

==Thermodynamics and statistical mechanics==

Thermodynamics includes several sub-disciplines within its framework:

*''Classical thermodynamics'', which concerns the transfer of energy and work in normal systems with no consideration of the interactions of particles at the microscopic level.  Classical thermodynamics makes ''no'' explicit reference to the constituent particles of a system. It consists of a number of "empirical" laws, which are derived purely from observations on thermodynamical systems, such as vessels of gas, or [[steam engine]]s.

For systems consisting of many particles (such as a [[gas]] consisting of many [[molecule]]s), it is possible to describe and predict the properties of the system accurately, even though it is virtually impossible to know what individual particles do. In essence, because of the large number of particles, one can apply [[statistics]] to learn about the "average" behavior of the system. The branch of [[physics]] known as [[statistical mechanics]] does just this.

Thermodynamics has an emphasis on a beginning or initial state of a system, and an end or final state of a system, with the system being all of the interacting components on this energy path.

"Classical" thermodynamics, on the other hand, predates this field, and makes ''no'' explicit reference to the constituent particles of a system. It consists of a number of "empirical" laws, which are derived purely from observations on thermodynamical systems, such as vessels of gas, or [[steam engine]]s. Well-known "laws" of thermodynamics are:<ref>Mark W. Zemansky, ''Heat and Thermodynamics'', McGraw-Hill, New York, 1957</ref>

About 120 A.D. Heron of Alexandria created the first reaction turbine, essentially a copper sphere with two bent nozzles mounted opposite each other; the sphere itself was mounted above a fire which heated water within the sphere, causing the sphere to rapidly rotate via the steam escaping from the nozzles. Simply a curious novelty at the time, Heron's sphere would cause speculation on the nature of heat and heat transfer, and spark some investigation into using heat transfer to accomplish meaningful work.

;The Zeroth Law of Thermodynamics

In 1789 Antoine Lavoisier demonstrated the law of conversion of mass, when he observed that heat flowed from a warm body to a cold one.  He proposed that heat was an element (he called this element ''caloric''), and speculated that it was a type of fluid surrounding an atom, and seemingly confirmed his theory when he removed oxygen from mercuric oxide. 

:allows us to define the concept of [[temperature]], by stating that "''Two systems in thermal [[equilibrium]] with a third one are in thermal equilibrium with each other''". This law is called "zeroth" because it was only formulated after the three others, but is actually more fundamental, and hence deserves a lower number.

 

;[[The First Law of Thermodynamics]]

Prior to that, the conversion of heat to work was being accomplished on the industrial level.  By the end of the 17th century, Thomas Savory invented the first practical steam-operated machine, a pump used to draw up well water.  This in turn led to the first piston engine, invented by Thomas Newcomen in 1712, and based on a refinement of Savory's work, which in turn was further revised by James Watt by the end of the 18th century.

:states that [[energy]] is conserved and that [[heat]] and [[work]] are transfers of energy.

 

; [[Second Law of Thermodynamics|The Second Law of Thermodynamics]]

The downfall of Lavoisier's caloric theory happened at the arsenal in Munich, Germany. The Bavarian minister of war was a British expatriate, Sir Benjamin Thompson, and he observed that work was being converted into heat by observing the boring of a cannon.  If caloric theory was correct, he reasoned, no more heat would be made once all of the caloric was removed from the cannon at the atomic level, yet his observations on this procedure - including a cannon bored while under water - demonstrated that work can be converted into heat, like the steam engines of his time converting heat into work.

:states (in one of its various formulations) that [[entropy]] in an isolated system cannot decrease, and that ''irreversible processes'' can only make it increase.<ref>Gregory H. Wannier, ''Statistical Physics'', John Wiley & Sons, New York, 1966</ref> An equivalent formulation states that heat cannot spontaneously flow from a cooler body to a hotter body.

 

;[[The Third Law of Thermodynamics]]

James Joule in 1849 made a precise determination of the mechanical equivalent of heat into work.  His stirring of water in a pot (work input with a mechanical stirring rod, caused by the motion of a 1-kg weight falling 42.4 cm) caused a temperature increase (heat output); his homemade, yet very-precise thermometers recorded a conversion factor of 0.241 calories of heat energy (now called ''one joule'').

:also known as ''Nernst's Law'', states that it is not possible to bring any system to the [[absolute zero]] of temperature in a finite number of operations. Also stated as follows: The entropy of a perfect crystal at absolute zero is zero.

 

==[[First Law of Thermodynamics]]==

:''"In any process, energy can be changed from one form to another, but it is never created or destroyed."''

What Clausius discovered was the first law of thermodynamics, which states that energy is conserved, and heat and work are transfers of energy.  The formula can be summed up as this:

 

Where the change (<math>\Delta</math>) in the system's internal energy (<math>E</math>), <math>q</math> is the heat added to the system, and <math>w</math> is the work done.  In thermodynamics a system is defined as having isolation from the remaining universe, and one with defined boundaries.  This would include a cup filled with hot tea, a human body, or an engine cylinder.  The internal energy of such a system - defined as <math>E</math> - is dependent on the state of the system itself, and not how that system state was achieved.

 

==[[Second Law of Thermodynamics]]==

James Watt's steam engine drew heat from a specific source and converted some of it to useful work; the remainder of the heat was transferred to a cooler reservoir.  In 1824 a French engineer, N.L. Sadi Carnot, proposed the Carnot cycle, consisting of two isothermal processes (involving a constant temperature) and two adiabatic processes (no heat is gained or lost).  The result was, in theory, the most efficient-working heat engine cycle of any kind, one involving processes that must be reversible and involve no change in entropy.  What was discovered in practice was the second law of thermodynamics, which states (in one of its various formulations) that [[entropy]] in an isolated system cannot decrease, and that ''irreversible processes'' can only make it increase.<ref>Gregory H. Wannier, ''Statistical Physics'', John Wiley & Sons, New York, 1966</ref> An equivalent formulation states that heat cannot spontaneously flow from a cooler body to a hotter body. Clausius stated that ''"It is impossible for a self-acting machine, unaided by external agency, to convey heat from a body at one temperature to another body at a higher temperature."''

 

 

==[[The Third Law of Thermodynamics]]==

Also known as ''Nernst's Law'', states that it is not possible to bring any system to the [[absolute zero]] of temperature in a finite number of operations. Also stated as follows: The entropy of a perfect crystal at absolute zero is zero.

These laws tell us to what constraints ''any'' system is subject. For example, it allows us to calculate the maximum possible efficiency of an [[engine]] once we know the temperature at which it operates.

These laws tell us to what constraints ''any'' system is subject. For example, it allows us to calculate the maximum possible efficiency of an [[engine]] once we know the temperature at which it operates.