Being concerned only with bulk matter and energy, classical thermodynamics is independent of theories of their microscopic nature; its axioms are sturdily empirical, and from them theorems are derived with mathematical rigor. Classical thermodynamics is basic to engineering, parts of geology, metallurgy, and physical chemistry.
Building on earlier studies of the thermodynamic functions temperature and heat, Sadi Carnot pioneered the science by his investigations of the cyclic heat engine in 1824, and in 1850 Clausius stated the first two laws. Thermodynamics was first developed by Joshua Gibbs, Hermann von Helmholtz, William Thomson (Lord Kelvin), and James Clerk Maxwell.
In thermodynamics, a system is any defined collection of matter: a closed system is one that cannot exchange matter with its surroundings; an isolated system can exchange neither matter nor energy. The state of a system is specified by determining all its properties such as pressure, volume, etc. A system in stable equilibrium is said to be in an equilibrium state, and has an equation of state (e.g., the general gas law) relating its properties. (See also phase equilibria.) A process is a change from one state A to another B, the path being specified by all the intermediate states. A state function is a property or function of properties which depends only on the state and not on the path by which the state was reached; a differential dX of a function X (not necessarily a state function) is termed a perfect differential if it can be integrated between two states to give a value XAB (= integral from A to B of dX) which is independent of the path from A to B. If this holds for all A and B, X must be a state function.
Laws of thermodynamics
Historically scientists first derived three laws called the first, second and third laws of thermodynamics, each having different formulations. Then an even more fundamental law was recognized. It has been labelled the "zeroth" law of thermodynamics.
If a hot and a cold object are brought into contact, they finally reach the same temperature. The hot object emits more heat energy than it receives and the cold object has a net absorption of heat. Both objects absorb and emit energy continually, although in unequal quantities, and the exchange process continues until the temperatures equalize. Each object is then absorbing and emitting equal amounts of heat and the objects are said to be in "thermal equilibrium". The zeroth law states that, if two objects are each in thermal equilibrium with a third object, then they are in thermal equilibrium with each other. This underlies the concept of temperature.
Cooling substancesThe third law of thermodynamics states that it is impossible to cool any substance to absolute zero. This zero of temperature would occur for example in a gas whose pressure was zero. All its molecules would have stopped moving and possess zero energy, so that extracting further energy and achieving corresponding cooling would be impossible. A substance becomes progressively more difficult to cool as its temperature approaches absolute zero (-273.16C)
From the statement of the second law, a heat transfer process naturally proceeds "downhill" - from a hotter to a cooler object. There must be some property or parameter of the system that is a measure of its internal state (its order or disorder), and which has different values at the start and the end of a possible process (one allowed by the first law). This parameter is termed "entropy", and the second law maintains that the entropy of an isolated system can only remain constant or increase.
Careful observation of machines shows that they consume more energy than they convert to useful work. Even if no energy is wasted in friction or lost by necessity, as in a radiator, the available mechanical energy is less than that supplied by the heat source. The entropy of the system is a reflection of its inaccessible energy, and the second law says that it cannot decrease. Heat is a random motion of atoms and when the energy is degraded towards the inaccessible energy pool, these atoms assume a more disorderly state - and entropy is a measure of this disorder.
Under the constraints imposed by the laws of thermodynamics it is possible for a system to undergo a series of changes of its state (in terms of pressure, volume and temperature). In some cases the series ends with a return to the initial state, useful work having been done during the series.
Heat cycles and efficiencyThe sequence of changes of the system is called a heat cycle and the theoretical maximum efficiency for such a "heat engine" would be obtained from following the so-called Carnot cycle which is named after the Frenchman Nicholas Carnot (1796-1832). If it were possible to construct a machine operating in cycles, would generate more energy in the form of work than was supplied to it in the form of work than the dream of the perpetual motion machine would be possible. The first law states the impossibility of achieving this result and the second law denies the possibility of even merely converting all the heat to an exactly equivalent amount of mechanical work.
Quantum statistical thermodynamicsQuantum statistical thermodynamics, based on quantum mechanics, arose in the 20th century. It treats a system as an assembly of particles in quantum states. The entropy is given by S = k log P, where k is the Boltzmann constant and P the statistical probability of the state of the system. Thus entropy is a measure of the disorder of the system.
Related category• HEAT AND THERMODYNAMICS
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