| Hectojoules (hJ) | Electronvolts (eV) |
|---|---|
| 1 Hectojoule | 6.24150907446 × 1020 eV |
| 2 Hectojoules | 1.24830181489 × 1021 eV |
| 3 Hectojoules | 1.87245272234 × 1021 eV |
| 4 Hectojoules | 2.49660362978 × 1021 eV |
| 5 Hectojoules | 3.12075453723 × 1021 eV |
| 10 Hectojoules | 6.24150907446 × 1021 eV |
| 20 Hectojoules | 1.24830181489 × 1022 eV |
| 25 Hectojoules | 1.56037726862 × 1022 eV |
| 50 Hectojoules | 3.12075453723 × 1022 eV |
| 100 Hectojoules | 6.24150907446 × 1022 eV |
| Reference | Hectojoules (hJ) | Electronvolts (eV) |
|---|---|---|
| One food calorie (kcal) | 41.84 hJ | 2.61145 × 1022 eV |
| An AA alkaline battery | 100 hJ | 6.24151 × 1022 eV |
| Daily adult food intake | 83680 hJ | 5.22289 × 1025 eV |
| One unit on an electricity bill | 36000 hJ | 2.24694 × 1025 eV |
| A lightning strike | 10000000 hJ | 6.24151 × 1027 eV |
The hectojoule is a unit of energy equal to one hundred joules. Its symbol is hJ. Like the decajoule below it, it is a properly formed metric unit that hardly anybody writes, because a three-digit number of joules is already short enough and because the kilojoule waits only one decimal place further up.
The quantity is squarely human. Lifting a ten-kilogram case by one metre takes about 98 joules, so almost exactly one hectojoule. Climbing a single stair step costs an adult roughly 120 joules, and a hundred-watt lamp burns one hectojoule every second. A resting adult body releases about one hectojoule of heat per second simply by staying alive, which makes the unit a fair measure of one second of ordinary existence.
Several familiar quantities land here without carrying the name. A studio flash is rated between one and ten hectojoules, though the trade calls them watt-seconds. A defibrillator delivers two hectojoules in a shock. Structural steel is required to absorb at least 27 joules in a notched-bar impact test, and tough grades absorb well over one hectojoule, which is the difference between a plate that bends and one that shatters.
The prefix itself is far from dead, but it survives attached to other units. The hectopascal is the unit of atmospheric pressure in every weather forecast on earth, chosen precisely because one hectopascal equals one millibar exactly and forecasters could adopt SI without changing a single number on their charts. The hectare measures land almost everywhere, and the hectolitre is the trading unit for wine, beer and milk.
With the joule, none of those conditions were met. There was no legacy unit for the hectojoule to match, no established range of values it made tidier, and no trade that needed it. Engineering had already fixed on kilo as the first step up, so the hectojoule was left describing a range that ordinary decimal notation handles perfectly well on its own.
Reading one is therefore a matter of a single multiplication. A hundred hectojoules is ten kilojoules; four hectojoules is 400 joules. The joule form is what modern specifications, laboratory reports and datasheets will use, so converting on sight keeps a figure consistent with everything printed around it.
One hectojoule equals 100 joules, about 23.9 calories, or about 0.0278 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.