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free energy

 Definition/Summary Free energy is energy available for doing work. Free energy as an independent concept does not really make any sense unless conditions are specified under which the work is to be done, for example: Helmholtz free energy (or Helmholtz energy) $A\ =\ U\ - TS$ is the energy available for doing work, at constant volume and temperature. Gibbs free energy (or Gibbs energy) $G\ =\ H\ -\ TS\ =\ A\ +\ PV\ =\ U\ +\ PV\ -\ TS$ is the energy available for doing work, at constant pressure and temperature. The melting point of a material is the temperature at which the Gibbs free energies of the solid and liquid forms are equal.

 Equations Helmholtz free energy (internal energy minus absolute temperature times entropy): $$A\ =\ U\ - T\,S$$ Gibbs free energy (enthalpy minus absolute temperature times entropy): $$G\ =\ H\ - T\,S\ =\ U\ +\ P\,V\ -\ T\,S$$

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 Breakdown Physics > Statistical & Thermal >> Heat & Heat Transfer