Internal Energy Definition

Internal Energy Definition

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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.


  • Crawford, F. H. (1963). Heat, Thermodynamics, and Statistical Physics, Rupert Hart-Davis, London, Harcourt, Brace & World, Inc.
  • Lewis, Gilbert Newton; Randall, Merle: Revised by Pitzer, Kenneth S. & Brewer, Leo (1961). Thermodynamics (2nd ed.). New York, NY USA: McGraw-Hill Book Co. ISBN 0-07-113809-9.


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