A system possesses energy if it has the ability to do work.
Work shifts energy from one system to another.
- a scalar quantity,
- abstract and cannot always be perceived,
- given meaning through calculation,
- a central concept in science.
Energy can exist in many different forms. All forms of energy are either kinetic or potential. The energy associated with motion is called kinetic energy. The energy associated with position is called potential energy. Potential energy is not "stored energy". Energy can be stored in motion just as well as it can be stored in position. Is kinetic energy "used up energy"?
- kinetic energy — motion
- mechanical energy — motion of macroscopic systems
- wind energy
- wave energy
- sound (sonic, acoustic) energy
- thermal energy-- motion of particles of matter
- geothermal energy
- electrical energy — motion of charges
- household current
- electromagnetic radiation — disturbance of electric and magnetic fields (classical physics) or the motion of photons (quantum physics)
- radio, microwaves, infrared, light, ultraviolet, x-rays, gamma rays
- solar energy
- mechanical energy — motion of macroscopic systems
|type||motion||examples and subtypes|
|motion of macroscopic objects||machines, muscles, projectiles, wind, flowing water, ocean waves, sound,…|
|random motion of microscopic particles of matter (molecules, atoms, ions)||heat, fire, geothermal,…|
|bulk flow of charges (electrons, protons, ions)||household current, AC and DC circuits, lightning,…|
|disturbance propagating through electric and magnetic fields (classical physics) or the motion of photons (modern physics)||radio waves, microwaves, infrared, light, ultraviolet, x-rays, gamma rays|
- potential energy — position
- gravitational potential energy
- roller coaster
- hydroelectric power
- electromagnetic potential energy
- electric potential energy
- magnetic potential energy
- chemical potential energy
- elastic potential energy
- strong nuclear potential energy
- nuclear power
- nuclear weapons
- weak nuclear potential energy
- radioactive decay
- gravitational potential energy
|force field||quantity in field||examples and subtypes|
|gravitational||mass||roller coaster, waterwheel, hydroelectric reservoir,…|
|electromagnetic||charge||electric, magnetic, chemical, elastic,…|
|strong nuclear||color charge||nuclear reactors, nuclear weapons,…|
|weak nuclear||lepton number||radioactive decay,…|
English brewer and scientist James Joule (1818–1889) who determined the mechanical equivalent of heat.
|J =||kg m2||=||N m||⎤
|energy||type||object, phenomena, process, or event|
|3-12||J||gravitational||an apple in a tree|
|60||kJ||heat||making a cup of coffee or tea|
|270||kJ||chemical||an apple in the digestive tract|
|300||kJ||kinetic||car driving at freeway speed|
|1.5||MJ||electrical||television running for four hours|
|2||MJ||chemical||stick of dynamite|
|3.8||MJ||chemical||food for one person for one year|
|4.184||GJ||chemical||ton of TNT|
|63||TJ||nuclear||Hiroshima atomic bomb (Little Boy, 1945)|
|1.8||PJ||kinetic||Chelyabinsk meteor impact (2013)|
|42||PJ||kinetic||Tunguska meteor impact (1908)|
|240||PJ||nuclear||Largest nuclear bomb tested (Tsar Bomba, 1961)|
For those who want some proof that physicists are human, the proof is is the idiocy of all the different units which they use for measuring energy.
Richard Feynman, 1964
|joule||1||J||1 N m||work|
|erg||0.1||μJ||1 dyne cm||"|
|watt hour||3.6||kJ||3,600 W s||"|
|kilowatt hour||3.6||MJ||1,000 W h||"|
|thermochemical calorie||4.184||J||(by definition)||heat|
|International Table calorie||4.1868||J||(by definition)||"|
|International Table Btu†||1.05505585262||kJ||(by definition)||"|
|cubic meter natural gas||37–39||MJ||(variable)||chemical|
|ton of oil equivalent||41–45||GJ||(variable)||"|
|ton of coal equivalent||29.3||GJ||(approximate)||"|
|ton of TNT||4.184||GJ||(by definition)||"|
Atomic and nuclear units
|atomic mass unit||u||muc2||= 1.492 × 10−10 J|
|electron volt||eV||e||= 1.602 × 10−19 J|
|hartree||Eh||2R∞hc||= 4.35974381 × 10−18 J (exact)|
|inverse meter||m−1||hc||= 1.986 × 10−25 J|
|inverse second||s−1||h||= 6.626 × 10−34 J|
|kelvin||K||k||= 1.381 × 10−23 J|
|kilogram||kg||c2||= 89,875,517,873,681,764 J (exact)|
- ocean currents
- ocean thermal temperature gradients
- fossil fuels
- natural gas
- everything else
- Aristotle of Stagira (384–322 BCE) Greece: first use of the word "energeia" (ἐνέργειά) in the Nicomachean Ethics. Its contemporary meaning has diverged significantly from Aristotle's original meaning. Aristotle's sense of the word is often translated as "activity" or "being at work". Energeia literally means to contain work. (en+ergon). In the Nicomachean Ethics, energeia was contrasted with "hexis" (ἕξις), which meant to "possess" or "to be in the state of". Energeia meant doing. Hexis meant possessing. Aristotle argues that virtue must be an activity, not just a state, to ensure happiness. These are terms of ethical philosophy, not science.
Τοῖς μὲν οὖν λέγουσι τὴν ἀρετὴν ἢ ἀρετήν τινα συνῳδός ἐστιν ὁ λόγος· ταύτης γάρ ἐστιν ἡ κατ᾽ αὐτὴν ἐνέργεια. διαφέρει δὲ ἴσως οὐ μικρὸν ἐν κτήσει ἢ χρήσει τὸ ἄριστον ὑπολαμβάνειν, καὶ ἐν ἕξει ἢ ἐνεργείᾳ. τὴν μὲν γὰρ ἕξιν ἐνδέχεται μηδὲν ἀγαθὸν ἀποτελεῖν ὑπάρχουσαν, οἷον τῷ καθεύδοντι ἢ καὶ ἄλλως πως ἐξηργηκότι, τὴν δ᾽ ἐνέργειαν οὐχ οἷόν τε· πράξει γὰρ ἐξ ἀνάγκης, καὶ εὖ πράξει. ὥσπερ δ᾽ Ὀλυμπίασιν οὐχ οἱ κάλλιστοι καὶ ἰσχυρότατοι στεφανοῦνται ἀλλ᾽ οἱ ἀγωνιζόμενοι (τούτων γάρ τινες νικῶσιν), οὕτω καὶ τῶν ἐν τῷ βίῳ καλῶν κἀγαθῶν οἱ πράττοντες ὀρθῶς ἐπήβολοι γίνονται. With those who identify happiness with virtue or some one virtue our account is in harmony; for to virtue belongs virtuous activity. But it makes, perhaps, no small difference whether we place the chief good in possession or in use, in state of mind or in activity. For the state of mind may exist without producing any good result, as in a man who is asleep or in some other way quite inactive, but the activity cannot; for one who has the activity will of necessity be acting, and acting well. And as in the Olympic Games it is not the most beautiful and the strongest that are crowned but those who compete (for it is some of these that are victorious), so those who act win, and rightly win, the noble and good things in life. Aristotle, ca. 320 BCE
- 1669 Dutch physicist Christiaan Huygens, vis viva or living force is conserved in perfectly elastic collistions
- 1689 German mathematician Gottfried Leibniz defined vis viva as mass times the square of velocity
- 1811 Italian mathematician Joseph Lagrange used calculus to show that a factor of two is involved in the relationship "potential" (potential energy) and "vis viva" (kinetic energy).  As defined via the symbols used by Lagrange, i.e. T as kinetic energy, in his 1788 Analytical Mechanics
- Thomas Young (1773–1829) England: first use of the word energy in the modern sense. His definition is almost the same as our current definition of kinetic energy. He's missing a one-half multiplier out front that makes the units work out. A Course of Lectures on Natural Philosophy and the Mechanical Arts. London: J. Johnson (1807).
- "The term energy may be applied, with great propriety, to the product of the mass or weight of a body, into the square of the number expressing its velocity." Lecture VIII. On Collision.
- "Hence is derived the idea conveyed by the term living or ascending force; for since the height to which a body will rise perpendicularly, is as the square of its velocity, it will preserve a tendency to rise to a height which is as the square of its velocity whatever may be the path into which it is directed, provided that it meet with no abrupt angle, or that it rebound at each angle in a new direction without losing any velocity. The same idea is somewhat more concisely expressed by the term energy, which indicates the tendency of a body to ascend or to penetrate to a certain distance, in opposition to a retarding force." Lecture V. On Confined Motion.
- William Rankine (1820–1872) Scotland: first mention of "potential energy" as distinguished from "actual energy". Since kinetic energy was the first form identified, he attached a modifier to the form of energy he discovered. Thus the unfortunate notion that kinetic energy is actual energy and potential energy is energy that has the potential to be actual energy. Energy is energy. No form of energy is any more or less "actual" than any other. The unfortunate terminology is due to Aristotle who applied the dichotomous terms potentiality and actuality to several disciplines — motion (Physics, Physica, Τα Φυσικη), causality (Metaphysics, Metaphysica, Τα Μετά Τα Φυσικά), ethics (Nicomachean Ethics, Ethica Nicomachea, Ηθικά Νικομάχεια), and physiology (On the Soul, De Anima, Περὶ Ψυχῆς). Philosophy is not science (although there is such a thing as philiosophy of science). Science should always strive to describe reality as close as possible when it relies on words to do so. Grand philosophical modes of organization may be used to some extent, but they shouldn't override the physical reality. On the general law of the transformation of energy. William John Macquorn Rankine. Philosophical Magazine Series 4. Vol. 5 No. 30 (1853): 106–117.
- "Actual, or Sensible Energy, is a measurable, transmissible, and transformable condition, whose presence causes a substance to tend to change its state in one or more respects. By the occurrence of such changes, actual energy disappears, and is replaced by Potential or Latent Energy; which is measured by the product of a change of state into the resistance against which that change is made. (The vis viva of matter in motion, thermometric heat, radiant heat, light, chemical action, and electric currents, are forms of actual energy; amongst those of potential energy are the mechanical powers of gravitation, elasticity, chemical affinity, statical electricity, and magnetism). The law of the Conservation of Energy is already known—viz., that the sum of all the energies of the universe, actual and potential, is unchangeable. The object of the present paper is to investigate the law according to which all transformations of energy, between the actual and potential forms, take place."
- William Thomson, Lord Kelvin (1824–1907) Ireland–Scotland replaced "actual" with "kinetic", but no one has dared replace "potential" with something better, so we're stuck with it. Kelvin originally proposed the terms dynamical and statical. On a universal tendency in nature to the dissipation of mechanical energy. William Thomson. Philosophical Magazine Series 4. Vol. 4 No. 25 (1852): 304–306.
- "The object of the present communication is to call attention to the remarkable consequences which follow from Carnot's proposition, established as it is on a new foundation, in the dynamical theory of heat; that there is an absolute waste of mechanical energy available to man, when heat is allowed to pass from one body to another at a lower temperature, by any means not fulfilling his criterion of a "perfect thermo dynamic engine". As it is most certain that Creative Power alone can either call into existence or annihilate mechanical energy, the "waste" referred to cannot be annihilation, but must be some transformation of energy. To explain the nature of this transformation, it is convenient, in the first place, to divide stores of mechanical energy into two classes — statical and dynamical. A quantity of weights at a height, ready to descend and do work when wanted, an electrified body, a quantity of fuel, contain stores of mechanical energy of the statical kind. Masses of matter in motion, a volume of space through which undulations of light or radiant heat are passing a body having thermal motions among its particles (that is not infinitely cold), contain stores of mechanical energy of the dynamical kind."
- The term "kinetic energy" first appeared in an article by Kelvin (then William Thomson) and Peter Guthrie Tait in the magazine Good Words — a magazine with a largely religious readership. Thomson incorporated several quotations from the King James Version of the Bible. Energy. William Thomson and Peter Guthrie Tait. Good Words (October 1862): 601–607.