Discussion

A system possesses energy if it has the ability to do work.

Work shifts energy from one system to another.

Energy is …

• 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
    • machines
    • wind energy
    • wave energy
    • sound (sonic, acoustic) energy
  • thermal energy-- motion of particles of matter
    • geothermal energy
  • electrical energy — motion of charges
    • household current
    • lightning
  • 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
• potential energy — position
  • gravitational potential energy
    • roller coaster
    • waterwheel
    • 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

Another scheme (economic)

• solar
  • sunshine
  • wind
  • ocean currents
  • ocean thermal temperature gradients
  • biomass
    • food
    • wood/charcoal
    • dung
  • fossil fuels
    • coal
    • petroleum
    • natural gas
• everything else
  • geothermal
  • tidal
  • nuclear

units

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.

Energy Units Discussed in This Book
unit	joule equivalent			other equivalent		reference
joule	1	J			N m	work
erg	0.1	μJ			dyne cm	"
foot pound	1.35582	J	(approximate)			"
watt second	1	J				power
watt hour	3.6	kJ			3600 W s	"
kilowatt hour	3.6	MJ			1000 W h	"
thermochemical calorie	4.184	J	(by definition)			heat
International Table calorie	4.1868	J	(by definition)			"
kilocalorie*					1000 calorie	"
thermochemical Btu**	1.054350	kJ	(approximate)			"
International Table Btu**	1.05505585262	kJ	(by definition)			"
therm					100,000 Btu	"
quad***	1.055	EJ	(approximate)		1015 Btu	"
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)			"

*	The kilocalorie is also known as the kilogram calorie, dietetic calorie, food calorie, and Calorie (with an uppercase "C").
**	Btu is the abbreviation for "British thermal unit".
***	Quad is the shortened form of "quadrillion Btu".

Energy Equivalents from Atomic and Nuclear Physics
unit	symbol	joule	equivalent
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)

Where:	c	=	speed of light in a vacuum	k	=	Boltzmann constant
	e	=	elementary charge	mu	=	atomic mass unit
	h	=	Planck constant	R∞	=	Rydberg constant

Historical Notes

• Aristotle a.k.a. Αριστοτέλης (384–322 BCE) Greece (Macedonia–Athens): 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 more "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.
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• 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." page 59
• 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.

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.

Rankine, W.J. Macquorn. On the Phrase "Potential Energy," and on the Definitions of Physical Quantities. Proceedings of the Philosophical Society of Glasgow. Vol. 6, No. 3 (23 January 1867): 186–190.

• Lord Kelvin (1824-1907) added the adjective "kinetic"