| Electronvolts (eV) | Decijoules (dJ) |
|---|---|
| 1 Electronvolt | 1.602176634 × 10-18 dJ |
| 2 Electronvolts | 3.204353268 × 10-18 dJ |
| 3 Electronvolts | 4.806529902 × 10-18 dJ |
| 4 Electronvolts | 6.408706536 × 10-18 dJ |
| 5 Electronvolts | 8.01088317 × 10-18 dJ |
| 10 Electronvolts | 1.602176634 × 10-17 dJ |
| 20 Electronvolts | 3.204353268 × 10-17 dJ |
| 25 Electronvolts | 4.005441585 × 10-17 dJ |
| 50 Electronvolts | 8.01088317 × 10-17 dJ |
| 100 Electronvolts | 1.602176634 × 10-16 dJ |
| Reference | Electronvolts (eV) | Decijoules (dJ) |
|---|---|---|
| One food calorie (kcal) | 2.61145 × 1022 eV | 41840 dJ |
| An AA alkaline battery | 6.24151 × 1022 eV | 100000 dJ |
| Daily adult food intake | 5.22289 × 1025 eV | 83680000 dJ |
| One unit on an electricity bill | 2.24694 × 1025 eV | 36000000 dJ |
| A lightning strike | 6.24151 × 1027 eV | 1 × 1010 dJ |
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.
The decijoule is a unit of energy equal to one tenth of a joule, or 0.1 joules. Its symbol is dJ. It is formed exactly as the decimetre and the decilitre are, and it is a legitimate part of the metric system, but it is written far less often than either of them because the quantity it names is usually reported as a decimal fraction of a joule instead.
The scale is easy to feel. An apple of a hundred grams falling ten centimetres arrives with about a decijoule. A spring-loaded mousetrap stores a few, a strong finger flick delivers roughly one, and a small pebble tossed underarm carries several. This is the first region on the way up from the millijoule where a person can sense the energy directly as a push rather than as a click.
Product safety uses the range even when it does not use the name. Toy projectile rules are written in joules and tenths of a joule, air-powered replicas are limited to about one joule in several countries, and impact tests on small plastic specimens report results in tenths. The figures are given as 0.3 J or 0.5 J rather than as 3 dJ or 5 dJ, but the quantity being controlled is precisely what the decijoule names.
Deci has survived in other units for reasons of convenience rather than logic. The decilitre is a standard kitchen measure across Scandinavia, Switzerland and much of central Europe, and the cubic decimetre is exactly the litre, which is why the prefix is quietly built into a unit almost everyone uses. The decigram appears in pharmacy and in precious-metal trading, and the decibel exists because the bel itself proved too coarse a step for describing sound.
With the joule, none of those pressures applied. Engineering had already fixed on prefixes stepping by a thousand, and the decimal point does the rest of the work: 0.4 joules is no harder to write than 4 decijoules and fits the same table as 40 joules or 4 kilojoules without a change of unit. A prefix earns its place by shortening what people write, and here it does not.
Encountering the unit is therefore mostly a matter of older literature, of translated technical documents and of teaching material that walks through the whole prefix ladder. The conversion is trivial in either direction: multiply by a hundred for millijoules, divide by ten for joules, and the value drops straight into whatever notation surrounds it.
One decijoule equals 0.1 joules, about 0.0239 calories, or about 0.0000278 watt-hours.