| Electronvolts (eV) | Kilocalories (kcal) |
|---|---|
| 1 Electronvolt | 3.82929405832 × 10-23 kcal |
| 2 Electronvolts | 7.65858811663 × 10-23 kcal |
| 3 Electronvolts | 1.1487882175 × 10-22 kcal |
| 4 Electronvolts | 1.53171762333 × 10-22 kcal |
| 5 Electronvolts | 1.91464702916 × 10-22 kcal |
| 10 Electronvolts | 3.82929405832 × 10-22 kcal |
| 20 Electronvolts | 7.65858811663 × 10-22 kcal |
| 25 Electronvolts | 9.57323514579 × 10-22 kcal |
| 50 Electronvolts | 1.91464702916 × 10-21 kcal |
| 100 Electronvolts | 3.82929405832 × 10-21 kcal |
| Reference | Electronvolts (eV) | Kilocalories (kcal) |
|---|---|---|
| One food calorie (kcal) | 2.61145 × 1022 eV | 1 kcal |
| An AA alkaline battery | 6.24151 × 1022 eV | 2.39006 kcal |
| Daily adult food intake | 5.22289 × 1025 eV | 2000 kcal |
| One unit on an electricity bill | 2.24694 × 1025 eV | 860.421 kcal |
| A lightning strike | 6.24151 × 1027 eV | 239006 kcal |
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 kilocalorie is a unit of energy equal to 1000 calories, or exactly 4184 joules. Its symbol is kcal. It is the unit in which food energy is counted almost everywhere in the world, whether or not the label actually says so.
The naming is a mess that nobody has been willing to fix. In the United States and much of the English-speaking world, food labels print the word calorie when they mean kilocalorie, sometimes capitalising it as Calorie to mark the difference, a distinction that survives no spoken conversation and few printed ones. Most of Europe avoids the trap by printing kcal explicitly, usually alongside kilojoules.
The figures on a label are not measured by burning the food. They come from the Atwater system, devised in the late nineteenth century, which assigns fixed energy values to the macronutrients: four kilocalories per gram of protein and carbohydrate, nine per gram of fat, and seven per gram of alcohol. The values are averages that ignore how much energy the body actually extracts, which varies with cooking, processing and the individual.
Human requirements sit at a convenient scale. An adult needs roughly 2000 to 2500 kilocalories a day depending on size and activity, a walk of an hour costs around 250, and a kilogram of body fat stores about 7700. That last figure is where the common advice to run a deficit of 500 kilocalories a day to lose half a kilogram a week comes from, though the body adjusts its expenditure and real weight loss is slower than the arithmetic suggests.
Sports science and clinical nutrition use the unit precisely, in indirect calorimetry and in the calculation of basal metabolic rate. Animal feed, hospital nutrition and food manufacturing all specify energy density in kilocalories per hundred grams.
Older chemistry and engineering literature uses the kilocalorie for reaction enthalpies and fuel heating values, where it competes with the kilojoule and requires care in reading.
Exercise figures in the unit are routinely overstated, and the reason is arithmetic rather than dishonesty. A machine that reports 300 kilocalories for half an hour usually counts gross energy expenditure, including the amount the body would have burned sitting still, so the extra attributable to the exercise is smaller. The three macronutrients themselves differ sharply: fat supplies about 9 kilocalories per gram against 4 for protein and carbohydrate, and alcohol falls between at 7, which is why a small change in the fat content of a food moves its figure so much. Understanding those two points explains most of the confusion the unit causes in everyday life.
One kilocalorie equals 1000 calories, 4184 joules, or about 1.162 watt-hours.