The Physics
Opus in profectus


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

Work shifts energy from one system to another.

Energy is…

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

Types of kinetic energy (classified by type of object)
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

Types of potential energy (classified by type of field)
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 = Nm =  kg m2
Approximate energy of selected events
energy type object, phenomena, process, or event
650 nJ kinetic falling snowflake
85 µJ kinetic falling raindrop
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
390 kJ kinetic car driving at freeway speed
1.4 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, by definition
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)
21 EJ electrical one year of operation of Google (2017)
37 EJ electrical one year of Bitcoin mining (2017)


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

Energy Units Discussed in This Book * 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".
unit joule equivalent equivalent reference
joule 1 J   1 N m work
erg 0.1 μJ   1 dyne cm "
foot pound 1.35582 J (approximate)   "
watt second 1 J     power
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)   "
kilocalorie*       1,000 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)   "

Atomic and nuclear units

Energy equivalents from atomic and nuclear physics c = speed of light in a vacuum, k = Boltzmann constant, e = elementary charge, mu = atomic mass unit, h = Planck constant, R = Rydberg constant
unit symbol joule equivalent
atomic mass unit u muc2  = 1.492 × 10−10 J
electronvolt eV e  = 1.602 × 10−19 J
hartree Eh 2Rhc  = 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)


Another scheme

Historical Notes

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