Everything about State Function totally explained
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's 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 (for example
temperature,
volume,
pressure). The number of parameters needed to describe the system is the dimension of the
state space of the system (
). For example, a
monatomic gas with a fixed number of particles is a simple case of a two-dimensional system (
). 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
and the volume
as functions of time from time
to
. 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
to time
we calculate
»
It can be seen that the integrand can be expressed as the
exact differential of the function
and that therefore, the integral can be expressed as the difference in the value of
at the end points of the integration. The product
is therefore a
state function of the system.
By way of notation, we'll specify the use of
d to denote an exact differential. In other words, the integral of
will be equal to
. The symbol
δ will be reserved for an inexact differential, which can't be integrated without full knowledge of the path. For example
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
is proportional to the internal energy of an ideal gas, but the work
is the amount of energy transferred as the system performs work. Internal energy is identifiable, it's 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:
Further Information
Get more info on 'State Function'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://state_function.totallyexplained.com">State function Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
