| Electronvolts (eV) | Centijoules (cJ) |
|---|---|
| 1 Electronvolt | 1.602176634 × 10-17 cJ |
| 2 Electronvolts | 3.204353268 × 10-17 cJ |
| 3 Electronvolts | 4.806529902 × 10-17 cJ |
| 4 Electronvolts | 6.408706536 × 10-17 cJ |
| 5 Electronvolts | 8.01088317 × 10-17 cJ |
| 10 Electronvolts | 1.602176634 × 10-16 cJ |
| 20 Electronvolts | 3.204353268 × 10-16 cJ |
| 25 Electronvolts | 4.005441585 × 10-16 cJ |
| 50 Electronvolts | 8.01088317 × 10-16 cJ |
| 100 Electronvolts | 1.602176634 × 10-15 cJ |
| Reference | Electronvolts (eV) | Centijoules (cJ) |
|---|---|---|
| One food calorie (kcal) | 2.61145 × 1022 eV | 418400 cJ |
| An AA alkaline battery | 6.24151 × 1022 eV | 1000000 cJ |
| Daily adult food intake | 5.22289 × 1025 eV | 836800000 cJ |
| One unit on an electricity bill | 2.24694 × 1025 eV | 360000000 cJ |
| A lightning strike | 6.24151 × 1027 eV | 1 × 1011 cJ |
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 centijoule is a unit of energy equal to one hundredth of a joule, or 0.01 joules. Its symbol is cJ. It is a perfectly valid member of the metric system, formed the same way as the centimetre and the centilitre, but it is one of the least written units in the whole of the International System, and understanding why says something useful about how prefixes actually get chosen.
The quantity itself is easy to picture. A small coin of about two grams falling half a metre lands with roughly one centijoule. Four keystrokes on a keyboard cost about the same, as does compressing a soft spring by a few millimetres. It is the energy of small deliberate gestures: enough to hear, not enough to feel as force.
The reason the unit is rare is that engineering settled on prefixes that step by a factor of a thousand. Milli, micro, nano and their upward counterparts kilo, mega and giga cover every scale with three orders of magnitude between neighbours, which keeps written values between one and a thousand without ever needing centi or deci. Ten millijoules is the form almost everyone writes, and it means exactly the same thing as one centijoule.
Centi has survived elsewhere because human dimensions happen to suit it. The centimetre matches the width of a finger, the centilitre matches a measure of drink, and the centigram matches a dose. Where a hundredth lands on a convenient human quantity the prefix persists; where it does not, as with the joule, it quietly disappears from practice without ever being withdrawn from the standard.
A few technical fields do keep it. Viscosity is still quoted in centipoise and centistokes across the lubricant and process industries, radiotherapy doses are prescribed in centigray rather than in grays, and soil moisture tension is read in centibars. These are cases where a hundredth of the base unit lands on the working range, exactly as with the centimetre, and the habit has outlived several attempts to modernise it.
When a centijoule does appear, usually in older text, in translated documentation or in a teaching example about prefixes, the safe move is to convert it at once. Multiply by ten to read it as millijoules, or divide by a hundred to read it as joules. No information is lost either way, and the resulting figure will match everything else on the page.
One centijoule equals 0.01 joules, about 0.00239 calories, or about 0.00000278 watt-hours.