Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation. It interrelates macroscopic variables, such as temperature, volume and pressure, which describe physical properties of material bodies and radiation, which in this science are called thermodynamic systems.
Historically, thermodynamics developed out of a desire to increase the efficiency of early steam engines, particularly through the work of French physicist Nicolas Léonard Sadi Carnot (1824) who believed that the efficiency of heat engines was the key that could help France win the Napoleonic Wars. Scottish physicist Lord Kelvin was the first to formulate a concise definition of thermodynamics in 1854
Thermo-dynamics is the subject of the relation of heat to forces acting between contiguous parts of bodies, and the relation of heat to electrical agency.
Initially, the thermodynamics of heat engines concerned mainly the thermal properties of their 'working materials', such as steam. This concern was then linked to the study of energy transfers in chemical processes, for example to the investigation, published in 1840, of the heats of chemical reactions by Germain Hess, which was not originally explicitly concerned with the relation between energy exchanges by heat and work. Chemical thermodynamics studies the role of entropy in chemical reactions. Also, statistical thermodynamics, or statistical mechanics, gave explanations of macroscopic thermodynamics by statistical predictions of the collective motion of particles based on the mechanics of their microscopic behavior.
Thermodynamics describes how systems change when they interact with one another or with their surroundings. This can be applied to a wide variety of topics in science and engineering, such as engines, phase transitions, chemical reactions, transport phenomena, and even black holes. The results of thermodynamics are essential for other fields of physics and for chemistry, chemical engineering, aerospace engineering, mechanical engineering, cell biology, biomedical engineering, materials science, and are useful for other fields such as economics.
Many of the empirical facts of thermodynamics are comprehended in its four laws. The first law specifies that energy can be exchanged between physical systems as heat and thermodynamic work. The second law concerns a quantity called entropy, that expresses limitations, arising from what is known as irreversibility, on the amount of thermodynamic work that can be delivered to an external system by a thermodynamic process. Many writers offer various axiomatic formulations of thermodynamics, as if it were a completed subject, but non-equilibrium processes continue to make difficulties for it.
Thermodynamics is built on the study of energy transfers that can be strictly resolved into two distinct components, heat and work, specified by macroscopic variables. Though thermodynamics originated in the study of cyclic non-equilibrium processes such as the working of heat engines, study of the subject gradually revealed that the notion of heat is inextricably tied to the notion of thermodynamic equilibrium. Thermodynamics is well understood and validated for systems in thermodynamic equilibrium, but as the systems of interest become further and further from thermodynamic equilibrium, their thermodynamical study becomes more and more difficult. Systems in thermodynamic equilibrium have very well experimentally reproducible behaviour, and as interest moves further towards non-equilibrium systems, experimental reproducibility becomes more difficult. The present article takes a gradual approach to the subject, starting with a focus on cyclic processes and thermodynamic equilibrium, and then gradually beginning to further consider non-equilibrium systems.
Classical thermodynamics describes macroscopic properties of bodies. This means that "any reference to [the] atomic constitution [of matter] is foreign to classical thermodynamics" in its macroscopic sense, as distinct from classical statistical thermodynamics. Here the word 'classical' is being used in a special sense, not the same as the sense 'pre-quantum' that contrasts with 'quantum'; here the word 'classical' means 'long established in physics', including both pre-quantum and quantum physics. The term 'statistical mechanics' will be clarified a little later in this article, but here it means simply that the processes in a body are considered in terms of the atomic constitution of matter.
Nevertheless, Fowler and Guggenheim observe that classical statistical thermodynamics provides a perspective from which to view the nature of classical macroscopic thermodynamics, as follows: for classical macroscopic thermodynamics and classical statistical mechanics to apply to a process in a body, it is necessary that the atomic mechanisms of the process fall into just two classes: those so fast that, in the time frame of the process or phenomenon of interest, the atomic states effectively visit all of their accessible range, and those so slow that their effects can be neglected in the time frame of the experiment or phenomenon of interest. "When intermediate rates are present, [classical] thermodynamics and [classical] statistical mechanics cannot be applied." This theme of separation of time scales of atomic processes recurs throughout the subject. This is a correlate of the division of transfers of energy into those as heat and those as work. This is another way of saying that non-equilibrium processes can be very much harder to study than processes that stay near equilibrium.
Basic for thermodynamics are the concepts of system and surroundings.
There are two fundamental kinds of entity in thermodynamics, states of a system, and processes of a system. This allows two fundamental approaches to thermodynamic reasoning, that in terms of states of a system, and that in terms of cyclic processes of a system.
A thermodynamic system can be defined in terms of its states. In this way, a thermodynamic system is a macroscopic physical object, explicitly specified in terms of macroscopic physical and chemical variables which describe its macroscopic properties. The macroscopic state variables of thermodynamics have been recognized in the course of empirical work in physics and chemistry.
A thermodynamic system can also be defined in terms of the processes which it can undergo. Of particular interest are cyclic processes. This was the way of the founders of thermodynamics in the first three quarters of the nineteenth century.
The surroundings of a thermodynamic system are other thermodynamic systems that can interact with it. An example of a thermodynamic surrounding is a heat bath, which is considered to be held at a prescribed temperature, regardless of the interactions it might have with the system.
The macroscopic variables of a thermodynamic system can under some conditions be related to one another through equations of state. They express the constitutive peculiarities of the material of the system. Classical thermodynamics is characterized by its study of materials that have equations of state that express relations between mechanical variables and temperature that are reached much more rapidly than any changes in the surroundings. A classical material can usually be described by a function that makes pressure dependent on volume and temperature, the resulting pressure being established much more rapidly than any imposed change of volume or temperature.
Thermodynamic facts can often be explained by viewing macroscopic objects as assemblies of very many microscopic or atomic objects that obey Hamiltonian dynamics. The microscopic or atomic objects exist in species, the objects of each species being all alike. Because of this likeness, statistical methods can be used to account for the macroscopic properties of the thermodynamic system in terms of the properties of the microscopic species. Such explanation is called statistical thermodynamics; also often it is also referred to by the term 'statistical mechanics', though this term can have a wider meaning, referring to 'microscopic objects', such as economic quantities, that do not obey Hamiltonian dynamics.