| Decajoules (daJ) | Electronvolts (eV) |
|---|---|
| 1 Decajoule | 6.24150907446 × 1019 eV |
| 2 Decajoules | 1.24830181489 × 1020 eV |
| 3 Decajoules | 1.87245272234 × 1020 eV |
| 4 Decajoules | 2.49660362978 × 1020 eV |
| 5 Decajoules | 3.12075453723 × 1020 eV |
| 10 Decajoules | 6.24150907446 × 1020 eV |
| 20 Decajoules | 1.24830181489 × 1021 eV |
| 25 Decajoules | 1.56037726862 × 1021 eV |
| 50 Decajoules | 3.12075453723 × 1021 eV |
| 100 Decajoules | 6.24150907446 × 1021 eV |
| Reference | Decajoules (daJ) | Electronvolts (eV) |
|---|---|---|
| One food calorie (kcal) | 418.4 daJ | 2.61145 × 1022 eV |
| An AA alkaline battery | 1000 daJ | 6.24151 × 1022 eV |
| Daily adult food intake | 836800 daJ | 5.22289 × 1025 eV |
| One unit on an electricity bill | 360000 daJ | 2.24694 × 1025 eV |
| A lightning strike | 100000000 daJ | 6.24151 × 1027 eV |
The decajoule is a unit of energy equal to ten joules. Its symbol is daJ, and the spelling dekajoule with a k is also accepted. It is the least written of all the joule multiples, not because the quantity is obscure but because deca is the least used prefix in the entire metric system, and understanding that is more useful than memorising the unit itself.
The quantity is one of the easiest in physics to picture. Lifting a one-kilogram bag of sugar by one metre takes 9.81 joules, so almost exactly one decajoule. A heart beats about a joule per stroke, so ten beats. A rifle pellet from an unlicensed air rifle in Britain carries up to 16 joules, or a little over 1.6 decajoules. These are quantities the body understands directly as effort.
Deca survives in ordinary life in only a handful of places, and almost all of them involve mass. In Austria, Poland, Hungary and the Czech lands, delicatessen counters still sell by the dekagram, so a customer asks for twenty deka of ham rather than two hundred grams. The decanewton is used for rope and cable ratings and, in France, for towing capacities, because it happens to be close to the weight of a kilogram.
Elsewhere the prefix simply lost. Engineering standardised on steps of a thousand, which places kilo directly above the base unit and leaves nothing for deca to do that a two-digit number does not already do better. Writing 40 joules is shorter than writing 4 decajoules, and it sorts correctly in a table alongside 400 joules and 4 kilojoules without any mental arithmetic.
The scale itself is worth knowing even under another name, because it is the scale of single human actions. Throwing a ball, closing a heavy door, lifting a full kettle onto a hob, drawing a bow: each of these costs a few tens of joules. Impact tests on packaging and protective equipment are specified here too, and so are the energy limits written into rules for airguns and industrial fastening tools.
When a decajoule appears in a document, converting it takes one step in either direction. Multiply by ten for joules, divide by a hundred for kilojoules. Both forms are exact, and the joule form is the one that will match every other figure in a modern specification.
One decajoule equals 10 joules, about 2.39 calories, or about 0.00278 watt-hours.
The electronvolt is a unit of energy equal to the work done on a single electron moved through a potential difference of one volt. Its symbol is eV. Since the 2019 redefinition of the SI it has an exact value, 1.602176634 times ten to the power minus nineteen joules, because the elementary charge itself is now a defined constant.
It exists because the joule is hopelessly large for the atomic world. A single visible photon carries between about 1.6 and 3.3 electronvolts, and expressing that in joules requires nineteen leading zeros. Working in electronvolts lets a physicist state the energy of a chemical bond, an atomic transition or a subatomic collision as a number between one and a few, which is what a unit is supposed to do.
The scale organises whole disciplines. Chemistry lives at a few electronvolts, the energy of bonds and of the visible and ultraviolet photons that break them. X-rays occupy kiloelectronvolts. Nuclear physics works in megaelectronvolts, the scale of binding energies and radioactive decay. Particle physics uses gigaelectronvolts, where the proton's rest mass is 0.938, and teraelectronvolts at the Large Hadron Collider, whose protons carry 6.8 each.
Mass is quoted in the same unit through the mass-energy relation. A particle's rest mass is given in electronvolts divided by the speed of light squared, and physicists routinely drop the divisor and simply say that the electron is 511 kiloelectronvolts. The Higgs boson is 125 gigaelectronvolts on the same convention.
Semiconductor engineering depends on it. The band gap that determines what a material does electrically and optically is an energy in electronvolts: about 1.1 for silicon, 3.4 for gallium nitride, and this single number decides whether a device is a solar cell, a transistor or a blue light-emitting diode.
Astronomy inherits the convention for high-energy photons, so gamma-ray telescopes report observations in gigaelectronvolts and teraelectronvolts rather than in wavelengths.
Particle accelerators are named after the energies they reach, which is why their names track the prefixes. The Cockcroft-Walton machine that first split a nucleus in 1932 worked at a few hundred kiloelectronvolts; the Bevatron of the 1950s reached billions and found the antiproton; and the Large Hadron Collider brings protons to 6.8 teraelectronvolts each. Cosmic rays put all of this in perspective by arriving with energies up to 10 to the twentieth electronvolts, tens of millions of times more than any machine can produce, though only a handful of such particles strike the Earth in a century over a square kilometre.
One electronvolt equals about 1.602 times ten to the power minus nineteen joules, or roughly 3.83 times ten to the power minus twenty calories.