State function


State function

In thermodynamics, a state function, "state quantity", or a "function of state", is a property of a system that depends only on the current state of the system, not on the way in which the system got to that state. A state function describes the equilibrium state of a system. For example, internal energy, enthalpy and entropy are "state quantities" because they describe quantitatively an equilibrium state of thermodynamic systems. At the same time, mechanical work and heat are process quantities because they describe quantitatively the "transition" between equilibrium states of thermodynamic systems.

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, P, 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

A thermodynamic system is described by a number of thermodynamic parameters (e.g. temperature, volume, pressure). The number of parameters needed to describe the system is the dimension of the state space of the system (D). For example, a monatomic gas with a fixed number of particles is a simple case of a two-dimensional system (D=2). In this example, any system is uniquely specified by two parameters, such as pressure and volume, or perhaps pressure and temperature. These choices are equivalent. They are simply different coordinate systems in the two-dimensional thermodynamic state space. An analogous statement holds for higher dimensional spaces.

When a system changes state continuously, it traces out a "path" in the state space. The path can be specified by noting the values of the state parameters as the system traces out the path, perhaps as a function of time, or some other external variable. For example, we might have the pressure P(t) and the volume V(t) as functions of time from time t_0 to t_1. This will specify a path in our two dimensional state space example. We can now form all sorts of functions of time which we may integrate over the path. For example if we wish to calculate the work done by the system from time t_0 to time t_1 we calculate

:W(t_0,t_1)=int_{ extrm{state 0^{ extrm{state 1P,dV=int_{t_0}^{t_1}P(t)frac{dV(t)}{dt},dt

It is clear that in order to calculate the work W in the above integral, we will have to know the functions P(t) and V(t) at each time t, over the entire path. A state function is a function of the parameters of the system which only depends upon the parameters' values at the endpoints of the path. For example, suppose we wish to calculate the work plus the integral of VdP over the path. We would have:

:Phi(t_0,t_1)=int_{t_0}^{t_1}Pfrac{dV}{dt},dt+int_{t_0}^{t_1}Vfrac{dP}{dt},dt= int_{t_0}^{t_1} frac{d(PV)}{dt},dt=P(t_1)V(t_1)-P(t_0)V(t_0)

It can be seen that the integrand can be expressed as the exact differential of the function P(t)V(t) and that therefore, the integral can be expressed as the difference in the value of P(t)V(t) at the end points of the integration. The product PV is therefore a state function of the system.

By way of notation, we will specify the use of "d" to denote an exact differential. In other words, the integral of dPhi will be equal to Phi(t_1)-Phi(t_0). The symbol "δ" will be reserved for an inexact differential, which cannot be integrated without full knowledge of the path. For example delta W=PdV will be used to denote an infinitesimal increment of work.

It is best to think of state functions as quantities or properties of a thermodynamic system, while non-state functions represent a process during which the state functions change. For example, the state function PV is proportional to the internal energy of an ideal gas, but the work W is the amount of energy transferred as the system performs work. Internal energy is identifiable, it is a particular form of energy. Work is the amount of energy that has changed its form or location.

Examples

The following are a few examples of state functions:
* Enthalpy
* Entropy
* Helmholtz free energy
* Temperature
* Gibbs free energy
* Fugacity
* Density
* Internal Energy

ee also

*Markov property


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • state function — būsenos funkcija statusas T sritis Standartizacija ir metrologija apibrėžtis Termodinaminė funkcija, kurios vertė priklauso tik nuo sistemos pradinės ir galinės būsenos. atitikmenys: angl. state function vok. Zustandsfunktion, f rus. функция… …   Penkiakalbis aiškinamasis metrologijos terminų žodynas

  • state function — būsenos funkcija statusas T sritis chemija apibrėžtis Termodinaminė funkcija, kurios vertė priklauso tik nuo sistemos pradinės ir galinės būsenos. atitikmenys: angl. state function rus. функция состояния …   Chemijos terminų aiškinamasis žodynas

  • state function — būsenos funkcija statusas T sritis fizika atitikmenys: angl. state function vok. Zustandsfunktion, f rus. функция состояния, f pranc. fonction d’état, f …   Fizikos terminų žodynas

  • Characteristic state function — The characteristic state function in statistical mechanics refers to a particular relationship between the partition function of an ensemble. In particular, if the partition function P satisfies P = exp( − βQ) or P = exp( + βQ) in which Q is a… …   Wikipedia

  • Configuration state function — In quantum chemistry, a configuration state function (CSF), is a symmetry adapted linear combination of Slater determinants. A CSF must not be confused with a configuration. Contents 1 Definition 2 From Configurations to Configuration State… …   Wikipedia

  • a segment by state function — veiklos segmentas pagal valstybės funkciją statusas Aprobuotas sritis buhalterinė apskaita ir finansinė atskaitomybė apibrėžtis Veiklos dalis, apimanti vienos valstybės funkcijos, nustatytos Lietuvos Respublikos valstybės ir savivaldybių biudžetų …   Lithuanian dictionary (lietuvių žodynas)

  • State — Состояние (англ. State) шаблон проектирования. Используется в тех случаях, когда во время выполнения программы объект должен менять свое поведение в зависимости от своего состояния. Паттерн состоит из 3 блоков: Widget класс, объекты которого… …   Википедия

  • State (polity) — This article is about the general concept of the state. For the subjects of international law, see Sovereign state. For other uses, see State (disambiguation). The frontispiece of Thomas Hobbes Leviathan A state is an organised political… …   Wikipedia

  • State actor — A state actor is a term used in United States civil rights law to describe a person who is acting on behalf of a governmental body, and is therefore subject to regulation under the United States bill of rights including the First, Fifth and… …   Wikipedia

  • State diagram — State diagrams is a diagram used in the field of computer science, representing the behavior of a system, which is composed of a finite number of states. There are many forms of state diagrams, which differ slightly and have different semantics.… …   Wikipedia


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.