# Concepts

Zeroth law of thermodynamics. The zeroth law of thermodynamics states that if two separate thermodynamic systems are each in thermal equilibrium with a third, then all three are in thermal equilibrium with each other.

Two systems are said to be in the relation of thermal equilibrium if they are linked by a wall permeable only to heat, and do not change over time.[1] As a convenience of language, systems are sometimes also said to be in a relation of thermal equilibrium if they are not linked so as to be able to transfer heat to each other, but would not do so if they were connected by a wall permeable only to heat. Viscosity. Etymology The word "viscosity" is derived from the Latin "viscum", meaning "anything sticky, birdlime made from mistletoe, mistletoe".

A viscous glue called birdlime was made from mistletoe berries and was used for lime-twigs to catch birds.[2] Definition Dynamic (shear) viscosity Laminar shear of fluid between two plates. Third law of thermodynamics. The third law of thermodynamics is sometimes stated as follows, regarding the properties of systems in equilibrium at absolute zero temperature: The entropy of a perfect crystal, at absolute zero kelvin, is exactly equal to zero.

The Nernst-Simon statement of the third law of thermodynamics is in regard to thermodynamic processes, and whether it is possible to achieve absolute zero in practice: The entropy change associated with any condensed system undergoing a reversible isothermal process approaches zero as temperature approaches 0 K, where condensed system refers to liquids and solids. A simpler formulation of the Nernst-Simon statement might be: Thermodynamic system. Thermodynamic state. For thermodynamics, a thermodynamic state of a system is fully identified by values of a suitable set of parameters known as state variables, state parameters or thermodynamic variables.

Once such a set of values of thermodynamic variables has been specified for a system, the values of all thermodynamic properties of the system are uniquely determined. Thermodynamics sets up an idealized formalism that can be summarized by a system of postulates of thermodynamics. Thermodynamic states are amongst the fundamental or primitive objects or notions of the formalism, in which their existence is formally postulated, rather than being derived or constructed from other concepts.[1][2]

Thermodynamic process. A thermodynamic process is the energetic development of a thermodynamic system proceeding from an initial state to a final state.

Paths through the space of thermodynamic variables are often specified by holding certain thermodynamic variables constant. A state function is a thermodynamic variable which depends only on the current state of the system, not the path taken to reach that state. Conversely a process function does depend on the path. Overview Thermodynamic potential. Description and interpretation Five common thermodynamic potentials are:[1] These five common potentials are all energy potentials, but there are also entropy potentials.

Thermodynamic equilibrium. In thermodynamics, a thermodynamic system is in thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, radiative equilibrium, and chemical equilibrium.

Equilibrium means a state of balance. In a state of thermodynamic equilibrium, there are no net flows of matter or of energy, no phase changes, and no unbalanced potentials (or driving forces), within the system. A system that is in thermodynamic equilibrium experiences no changes when it is isolated from its surroundings. In non-equilibrium systems there are net flows of matter or energy, or phase changes are occurring; if such changes can be triggered to occur in a system in which they are not already occurring, it is said to be in a metastable equilibrium. Temperature. A map of global long term monthly average surface air temperatures in Mollweide projection.

Statistical ensemble (mathematical physics) In mathematical physics, especially as introduced into statistical mechanics and thermodynamics by J.

State function. In contrast, mechanical work and heat are process quantities because their values depend on the specific transition (or path) between two equilibrium states.

History It is likely that the term “functions of state” was used in a loose sense during the 1850s and 60s by those such as Rudolf Clausius, William Rankine, Peter Tait, William Thomson, and it is clear that by the 1870s the term had acquired a use of its own. In 1873, for example, Willard Gibbs, in his paper “Graphical Methods in the Thermodynamics of Fluids”, states: “The quantities V, B, T, U, and S are determined when the state of the body is given, and it may be permitted to call them functions of the state of the body.”

Overview ). ). Spontaneous process. A spontaneous process is the time-evolution of a system in which it releases free energy (usually as heat) and moves to a lower, more thermodynamically stable energy state.[1][2] The sign convention of changes in free energy follows the general convention for thermodynamic measurements, in which a release of free energy from the system corresponds to a negative change in free energy, but a positive change for the surroundings. Depending on the nature of the process, the free energy is determined differently. For example, the Gibbs free energy is used when considering processes that occur under constant pressure and temperature conditions whereas the Helmholtz free energy is used when considering processes that occur under constant volume and temperature conditions.

A spontaneous process is capable of proceeding in a given direction, as written or described, without needing to be driven by an outside source of energy. Second law of thermodynamics. The second law of thermodynamics states that every process occurring in nature proceeds in the sense in which the sum of the entropies of all bodies taking part in the process is increased. In the limit, i.e. for reversible processes, the sum of the entropies remains unchanged.[1][2][3] The second law is an empirical finding that has been accepted as an axiom of thermodynamic theory. Statistical thermodynamics, classical or quantum, explains the law. The second law has been expressed in many ways. Its first formulation is credited to the French scientist Sadi Carnot in 1824 (see Timeline of thermodynamics). Introduction Reversible process (thermodynamics) In thermodynamics, a reversible process -- or reversible cycle if the process is cyclic -- is a process that can be "reversed" by means of infinitesimal changes in some property of the system without entropy production (i.e. dissipation of energy).[1] Due to these infinitesimal changes, the system is in thermodynamic equilibrium throughout the entire process.

Since it would take an infinite amount of time for the reversible process to finish, perfectly reversible processes are impossible. However, if the system undergoing the changes responds much faster than the applied change, the deviation from reversibility may be negligible. In a reversible cycle, the system and its surroundings will be exactly the same after each cycle.[2] In thermodynamics, processes can be carried out in one of two ways: reversibly or irreversibly.

Reversibility in thermodynamics refers to performing a reaction continuously at equilibrium. Pressure. Pressure as exerted by particle collisions inside a closed container. Pressure (symbol: p or P) is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled gage pressure)[a][not in citation given] is the pressure relative to the local atmospheric or ambient pressure. Definition Partition function (statistical mechanics) The canonical partition function is where the "inverse temperature", β, is conventionally defined as with kB denoting Boltzmann's constant. The exponential factor exp(−βEs) is known as the Boltzmann factor. Non-equilibrium thermodynamics. Internal energy. For practical considerations in thermodynamics and engineering it is rarely necessary or convenient to consider all energies belonging to the total intrinsic energy of a sample system, such as the energy given by the equivalence of mass.

Typically, descriptions only include components relevant to the system under study. Ideal gas law. Heat. Thermodynamic free energy. First law of thermodynamics. History. Equipartition theorem. Thermal motion of an α-helicalpeptide. The jittery motion is random and complex, and the energy of any particular atom can fluctuate wildly. Equation of state. Overview The most prominent use of an equation of state is to correlate densities of gases and liquids to temperatures and pressures. One of the simplest equations of state for this purpose is the ideal gas law, which is roughly accurate for weakly polar gases at low pressures and moderate temperatures.

However, this equation becomes increasingly inaccurate at higher pressures and lower temperatures, and fails to predict condensation from a gas to a liquid. Therefore, a number of more accurate equations of state have been developed for gases and liquids. Entropy.

Enthalpy. Conjugate variables (thermodynamics) Boltzmann constant.