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''This article discusses a variety of meanings of '''free energy''' in
thermodynamics; see
Free energy (disambiguation) for other possible uses.''
In thermodynamics, '''free energy''' is a measure of the amount of work that can be extracted from a system. In this sense, it measures not the energy content of the system, but the "useful energy" content.
In different situations, ''free energy'' is related to
internal energy in different ways. For instance, in chemistry one is usually concerned with fluid systems at constant temperature, undergoing chemical reaction in closed containers (at constant volume) or in open containers (at constant pressure). In the first case one would use the ''
Helmholtz free energy'' as a measure of available work, and in the second one would use the ''
Gibbs free energy''. Physicists carry out more abstract analysis of systems which are not necessarily fluid. In that context, the ''
Helmholtz free energy'' is more common, and it is also directly related to the
partition function of a
canonical ensemble in
statistical mechanics. In the isothermal-isobaric ensemble, the partition function is related to the Gibbs free energy. For this reason, there is some confusion in terminology, which we disentangle below.
The usage in physics and chemistry is as follows:
* In
physics, ''free energy'' denotes the
thermodynamic potential ''F''=''U''-''TS'', also called
Helmholtz free energy. In chemistry, this quantity is called the
Helmholtz function or the
work content and is denoted ''A'' after the
German word ''Arbeit'', meaning ''work''.
* In
chemistry, ''free energy'' denotes the
thermodynamic potential ''F''=''U''-''TS''+''PV'', also called the
Gibbs function. In physics, this quantity is called the
Gibbs free energy and denoted ''G''. Note that it is common usage in chemistry to denote
internal energy by ''E'' instead of ''U''
* Recently a
compromise notation has become common, using ''A'' for the
Helmholtz function, ''G'' for the
Gibbs function, and avoiding ''F'' entirely. The functions are then referred to as the ''
Helmholtz free energy'' and ''
Gibbs free energy''.
In another definition, '''free energy''' refers to free
electrons in air, water, earth, and in space. Scientists attempt to harness these free electrons for practical electricity needs. Successful attempts are solar cells, hydroelectric plants, wind power, geothermal energy, and nuclear power.
'''Free energy''' also means zero cost ($0) electricity or fuel (such as solar electricity) taken from nature, rather than from an electric company. No electric bill is paid.
*
"FREE ENERGY" AS SEEN ON BRITISH T.V.
de:Freie Energie
fr:nergie libre
it:Energia libera
pl:Energia swobodna
sl:Prosta energija
