A definition of energy
DOI: 10.1063/PT.3.1198
On reading Lisa Crystal’s review (PHYSICS TODAY, April 2011, page 61 ) of Jennifer Coopersmith’s book Energy, the Subtle Concept: The Discovery of Feynman’s Blocks from Leibniz to Einstein, I remembered having shared Crystal’s desire to learn what energy is—that is, energy without such qualifications as mechanical, chemical, nuclear, solar, and so forth. I finally found the answer, from Max Planck: “The energy of a system is, therefore, sometimes briefly denoted as the faculty to produce external effects.” 1
That definition suggests to me a philosophical answer, which Crystal says is lacking in Coopersmith’s book. I would say that the energy of a system is a measure of its presence in the universe.
References
1. M. Planck, Treatise on Thermodynamics 3rd English ed., A. Ogg, trans., Dover, New York (1945), p. 41.
More about the authors
Mariano Bauer, (bauer@fisica.unam.mx) National Autonomous University of Mexico Mexico City.
