In thermodynamics, energy, from the Greek ενέργειας, meaning "at work", is that which quantifies the effect of "force in action", as well as that which can exist in various forms, such as internal, kinetic, or potential, and that which characterizes the ability of a system to modify the state of its surroundings. [1] In human chemistry, energy can be released or absorbed through the transformations of human chemical bonds. [2] The mathematical connection between the bulk measurements or "state" measurements of energies of human thermodynamic systems, such as a country, to the individual working energy actions of people (human molecules), as mediated through working human bonds, is new area of research. [3] The focused study of energy is called energetics.

Origin of term
The term “energy” stems from the works of Greek philosopher Aristotle, particularly his c. 350 Metaphysics, who used the term enérgeia to mean act or ‘activity’, ‘actuality’, or in a literal sense ‘(a state of) functioning’, deriving from energos "active, working," from en- "at" + ergon "work". [4] Aristotle used the term enérgeia to clarify, in one sense, the definition of “being” as potency (dýnamis) and act (enérgeia).

The modern spelling of the term “energy” came of use in 1599. [5] In the 1728 Cyclopedia - or Universal Dictionary of Arts and Sciences, one of the first general encyclopedias to be produced in English, energy is defined as “an uncommon force, or strength, in a discourse, a sentence, or a word.” [6] In the 1775, energy was defined as “there is in a body in movement an effort of energy which is not at all in a body at rest”. [7]

The modern physics meaning of the term is generally attributed to English physicist and physician Thomas Young who in 1807 used the term energy, based on the Greek word ένεργεια meaning efficacy or effective force, as an abbreviation for the sum of kinetic energy and gravitational potential energy of a mass and the elastic energy of a spring to which the mass may be attached. [8] Scottish mathematical physicist William Thomson stated before an audience, for instance, that “the very name energy, though first used in its present sense by Thomas Young about the beginning of this century, has only come into use practically after the doctrine which defines it had ... been raised from mere formula of mathematical dynamics to the position it now holds of a principle pervading all nature and guiding the investigator in the field of science.” [10]

This pervading principle of nature, Thomson speaks of, are the first and second laws of thermodynamics, established by German physicist Rudolph Clausius between 1850 to 1879. What Clausius did was to extend the basic definition of energy to a many particle system, thus establishing the foundations of the new science of "thermodynamics", using the 1833 work of Irish mathematician William Hamilton, who showed that for a system of particles the sum of the kinetic and potential energy in the system is represented by a set of differential equations known as the Hamilton equations for that system. In short, Clausius showed that the energy U of a system, equates to the sum of the “vis viva” (kinetic energy), symbol T, and the “ergal” (potential energy), symbol J, of the three-dimensional movements of the particles of the system, such that:

U = T + J

and the "energy remains constant during the motion" (conservation of energy). [9]

Mass-energy equivalence
In 1905, with respect to the development of "relativistic thermodynamics", German-born American physicist Albert Einstein showed that energy is proportional, according to the speed of light squared, to matter. [11] Specifically, in his September 27 paper "Does the inertia of a body depend upon its energy-content?", Einstein proposed that the equivalence of mass and energy is a general principle, which is a consequence of the symmetries of space and time. [12] The adjacent photo shows the 3-meter-tall sculpture of Einstein's E = mc² formula at the 2006 Walk of Ideas, Germany.

References
1. Perrot, Pierre. (1998). A to Z of Thermodynamics. New York: Oxford University Press.
2. Thims, Libb. (2007). Human Chemistry (Volume Two), (preview), (ch 13: "Human Chemical Bonding", pgs. 515-560). Morrisville, NC: LuLu.
3. Thims, Libb. (2007). Human Chemistry (Volume One), (preview), (pgs. 90-93: "Enthalpy and bond energies"). Morrisville, NC: LuLu.
4. (a) Libbrecht, Ulrich. (2007). Within the Four Seas: Introduction to Comparative Philosophy, (pg. 233). Peeters Publishers.
(b) Energy (etymology) - Online Etymology Dictionary.
5. Energy (definition) - Merriam-Webster Collegiate Dictionary, 2000, CD-ROM.
6. Chambers, Ephraim. (1728). Cyclopædia, or, An universal dictionary of arts and sciences, (pg. 307). Vol. 1.
7. Diderot, Denis and D’Albert, Jean. (1775). Encyclopédie. Paris.
8. Muller, Ingo. (2007). A History of Thermodynamics - the Doctrine of Energy and Entropy. New York: Springer.
9. Clausius, Rudolf. (1879). The Mechanical Theory of Heat, (2nd ed). London: Macmillan & Co.
10. Thomson, William. (1881). "On the Sources of Energy Available to Man for the Production of Mechanical Effect." BAAS Rep. 51: 513-18 (Quote: pg. 513); PL 2: 433-50.
11. (a) Bodanis, David. (2000). E = mc^2 - a Biography of the World's Most Famous Equation. New York: Berkley Books.
(b) Muller, Ingo. (2007). A History of Thermodynamics - the Doctrine of Energy and Entropy, (ch. 10: Relativistic Thermodynamics, pgs. 289-305). New York: Springer.
12. Einstein, A. (1905), "Ist die Trägheit eines Körpers von seinem Energieinhalt abhängig?", Annalen der Physik 18: 639–643 See also the English translation.