Internal energy

Figure 1. A glass of water is deceivingly calm; internally it is a mass of high speed particles and strong chemical and nuclear bonds.[1]

Internal energy [math](U)[/math] is the microscopic energy contained in a substance, given by the random, disordered kinetic energy of the molecules. In addition it includes the potential energy between these molecules, and the nuclear energy contained in the atoms of these molecules.[2] Internal energy and thermal energy are very similar in a basic thermodynamic context. However, they differ becuase internal energy encompasses far more than just the average kinetic energy of molecules. This distinction is important because the potential energies between molecules and atoms is important for understanding phase changes, chemical reactions, nuclear reactions, and many more microscopic phenomena.

All objects in space exhibit macroscopic and microscopic energy. Although they are very similar in concept, the main distinction is that microscopic energy cannot be seen. For example, a glass of water on a table has no apparent macroscopic energy, but on the microscopic scale it is a mass of high speed molecules traveling at hundreds of meters per second.[3]

The First law of thermodynamics states that the internal energy can be changed in a system by doing work on it, adding/removing heat from it, or a combination of the two. If the system is isolated it is forbidden from interactions with its surroundings, meaning that the internal energy cannot change.

For Further Reading


  1. flyupmike, Pixabay [Online], Available:[Accsessed: July 13, 2018]
  2. Randall Knight, Physics for Scientists and Engineers, 3rd Ed. New York: Pearson, 2013, Ch. 17, p. 470.
  3. Hyperphysics, Internal energy [Online], Available: