Science, Tech, Math › Science Internal Energy Definition Share Flipboard Email Print Internal energy is a measure of the energy of a closed system. seksan Mongkhonkhamsao / 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 December 08, 2019 In chemistry and physics, internal energy (U) is defined as the total energy of a closed system.Internal energy is the sum of potential energy of the system and the system's kinetic energy. The change in internal energy (ΔU) of a reaction is equal to the heat gained or lost (enthalpy change) in a reaction when the reaction is run at constant pressure. Internal Energy of an Ideal Gas The internal energy of an ideal gas is a good approximation of a real-world system. In such as system, the particles in an ideal gas are considered to be point objects that have completely elastic collisions with each other. The real behavior of the monatomic gases (e.g., helium, argon) mirrors this model. In an ideal gas, internal energy is proportional to the number of particles of moles of a gas and its temperature: U = cnT Here, U is internal energy, c is the heat capacity at constant volume, n is the number of moles, and T is the temperature. Sources Crawford, F. H. Heat, Thermodynamics, and Statistical Physics. Rupert Hart-Davis, London, Harcourt, Brace & World, Inc., 1963.Lewis, Gilbert Newton, and Merle Randall. Thermodynaics, revised by Kenneth S. Pitzer and Leo Brewer, 2nd ed., McGraw-Hill Book Co., 1961.