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Free energy

Free energy generally suggests kinds of electromagnetic energy that is "free" (i.e. does not cost anything). "Free energy" generally means that a primary energy source is free for consumption. In a technical sense, free energy means an energy source supplied directly by the environment in unlimited quantity (and this can not be depleted). Free energy, in a strict sense, is energy directly transcieved from the environment, and utilized without any artificial aid. This type of energy is somtimes refered to as "radiant energy".

Free energy has a number of consequences. Examining normal physical processes with knowledge of free energy phenomena can lead to interesting insight. During discussions of perpetual motion, the topic of free energy usually encourages serious inquiries.

Thermodynamics

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.

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