See perpetual motion or renewable energy for discussion of those topics.
In thermodynamics, the term free energy denotes either of two related concepts of importance. They express the total amount of energy which is used up or released during a chemical reaction. Both attempt to capture that part of the total energy of a system which is available for "useful work" and is hence not stored in "useless random thermal motion". As a system undergoes changes, its free energy will decrease.
When a system of molecules undergoes change, whether chemical reaction or changes in physical states such as phase changes, there are two tendencies driving the changes:
If
E represents the energy,
T the temperature, and
S the entropy, these two tendencies can be combined by stating that the expression
- E - TS, the Helmholtz function
tends to decrease. Strictly, this is only true in situations where the volume is constant, as in sealed containers. If the pressure is constant, as in open containers, the
enthalpy H = E + PV (where
P represents the pressure and
V represents the volume) replaces the energy, and thus the quantity that must be minimized is
- H - TS = E + PV - TS, the Gibbs function.
Physicists have tended to use the term
free energy and the symbol
F for the Helmholtz function, using
G to represent the Gibbs function; chemists have preferred to denote the Helmholtz function by
A [from the
German word
Arbeit(=work)] and call it the
work content, reserving the term
free energy and the symbol
F for the Gibbs function. 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.
