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 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 engines.
- Statistical thermodynamics, which concerns the interactions and energy relationships of particles at the microscopic level, with a reliance on quantum theory.
- Chemical thermodynamics, which is heat and energy transfers involving chemical reactions within chemical systems.
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. Measurements in thermodynamics are usually reported on the Kelvin scale 
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.
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.
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.
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.
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).
Joule continued in other experiments, noting the pressure changes caused by electrically-heated gasses, and achieved similar results. This in turn led to a number of other scientists to research in this area, among them the German physicist Rudolf Clausius, who stated in 1850:
- "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 () in the system's internal energy (), is the heat added to the system, and 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 - is dependent on the state of the system itself, and not how that system state was achieved.
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. 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."
What it meant was heat transfers to cooler temperatures, and not the other way around, which led to the concept of entropy. An idealized formula for an increase in entropy when heat is applied reads:
Where is the state of the system or entropy, is the added heat, and is the absolute temperature. This increase in entropy must be at least , but the entropy change itself ( is always greater.
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.
The particular properties of a specific system cannot be calculated from these laws alone. More information is required: so-called thermodynamic equations of state tell us how a particular system will behave under thermodynamic processes. A simple example of such an equation is the Ideal Gas Law that applies to dilute gases.
- David Halliday, "Fundamentals of Physics Extended", John Wiley & Sons, New York, 1997
- Gregory H. Wannier, Statistical Physics, John Wiley & Sons, New York, 1966