Science, Tech, Math › Science Free Energy Definition in Science What Is Free Energy in Chemistry and Physics? Share Flipboard Email Print Free energy is the amount of energy in a system that is available to do work. PM Images, Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated September 12, 2019 The phrase "free energy" has multiple definitions in science: Thermodynamic Free Energy In physics and physical chemistry, free energy refers to the amount of internal energy of a thermodynamic system that is available to perform work. There are different forms of thermodynamic free energy: Gibbs free energy is the energy that may be converted into work in a system that is at constant temperature and pressure. The equation for Gibbs free energy is: G = H – TS where G is Gibbs free energy, H is enthalpy, T is temperature, and S is entropy. Helmholtz free energy is energy that may be converted into work at constant temperature and volume. The equation for Helmholtz free energy is: A = U – TS where A is the Helmholtz free energy, U is the internal energy of the system, T is the absolute temperature (Kelvin) and S is the entropy of the system. Landau free energy describes energy of an open system in which particles and energy may be exchanged with the surroundings. The equation for Landau free energy is: Ω = A - μN = U - TS - μN where N is the number of particles and μ is chemical potential. Variational Free Energy In information theory, variational free energy is a construct used in variational Bayesian methods. Such methods are used to approximate intractable integrals for statistics and machine learning. Other Definitions In environmental science and economics, the phrase "free energy" is sometimes used to refer to renewable resources or any energy that does not require monetary payment. Free energy may also refer to the energy that powers a hypothetical perpetual motion machine. Such a device violates the laws of thermodynamics, so this definition currently refers to a pseudoscience rather than hard science. Sources Baierlein, Ralph.Thermal Physics. Cambridge University Press, 2003, Cambridge, U.K.Mendoza, E.; Clapeyron, E.; Carnot, R., eds. Reflections on the Motive Power of Fire – and other Papers on the Second Law of Thermodynamics. Dover Publications, 1988, Mineola, N.Y.Stoner, Clinton. “Inquiries into the Nature of Free Energy and Entropy in Respect to Biochemical Thermodynamics.” Entropy, vol. 2, no. 3, Sept. 2000, pp. 106–141., doi:10.3390/e2030106.