The Physics
Opus in profectus


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

Energy is transferred or transformed whenever work is done..

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 nuclear reactors,
nuclear weapons, …
weak nuclear flavor radioactive decay,
the first step in solar fusion, …



English brewer and scientist James Joule (1818–1889) who determined the mechanical equivalent of heat.

J = N m =  kg m2

Approximate energy of selected events (smallest to largest)
energy (J) type device, event, phenomenon, process
650  × 10−9 kinetic falling snowflake
85  × 10−6 kinetic falling raindrop
1   one joule
3–12 gravitational an apple in a tree
51 kinetic most energetic cosmic ray
60  × 103 heat making a cup of coffee or tea
270  × 103 chemical an apple in the digestive tract
390  × 103 kinetic car driving at freeway speed
1.4  × 106 electrical television running for four hours
2  × 106 chemical stick of dynamite
3.8  × 109 chemical food for one person for one year
4.184  × 109 chemical ton of TNT, by definition
63  × 1012 nuclear Hiroshima atomic bomb (Little Boy, 1945)
1.8  × 1015 kinetic Chelyabinsk meteor impact (2013)
42  × 1015 kinetic Tunguska meteor impact (1908)
240  × 1015 nuclear Largest nuclear bomb tested (Tsar Bomba, 1961)
21  × 1018 electrical one year of operation of Google (2017)
37  × 1018 electrical one year of Bitcoin mining (2017)
1.21  × 1034 em radiation one year output of the Sun
2  × 1047 em radiation most energetic gamma ray burst (2022)


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, 1965 (paid link)

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". § Exact by definition.
unit joule
in this
joule 1 J 1 N m work
erg 0.1 μJ 1 dyne cm "
foot pound force 1.355 818… J "
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§ heat
Inter­national Table calorie 4.1868 J§ "
kilocalorie* 1 000 calorie "
thermo­chemical Btu 1.054 350… kJ "
therm 105.4350… MJ 100 000 Btu "
Inter­national Table Btu 1.055 055 852 62 kJ§ "
quad 1.055 055 852 62 EJ§ 1015 Btu "
cubic meter natural gas 37~39 MJ chemical
ton of oil equivalent 41~45 GJ "
ton of coal equivalent ~29.3 GJ "
ton of TNT 4.184 GJ§ "

Atomic and nuclear units

Energy equivalents from atomic and nuclear physics Note: All values are exact by definition or calculation except for the atomic mass unit which is accurate to the number of digits displayed. Symbols: 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 = 1.492 418 086 × 10−10 J
electronvolt (eV) e = 1.602 176 634 × 10−19 J
hartree (Eh) 2Rhc = 4.359 743 81 × 10−18 J
inverse meter (m−1) hc = 1.986 445 857 × 10−25 J
inverse second (s−1) h = 6.626 070 15 × 10−34 J
kelvin (K) k = 1.380 649 × 10−23 J
kilogram (kg) c2 = 89 875 517 873 681 764 J


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