| Megajoules (MJ) | Electronvolts (eV) |
|---|---|
| 1 Megajoule | 6.24150907446 × 1024 eV |
| 2 Megajoules | 1.24830181489 × 1025 eV |
| 3 Megajoules | 1.87245272234 × 1025 eV |
| 4 Megajoules | 2.49660362978 × 1025 eV |
| 5 Megajoules | 3.12075453723 × 1025 eV |
| 10 Megajoules | 6.24150907446 × 1025 eV |
| 20 Megajoules | 1.24830181489 × 1026 eV |
| 25 Megajoules | 1.56037726862 × 1026 eV |
| 50 Megajoules | 3.12075453723 × 1026 eV |
| 100 Megajoules | 6.24150907446 × 1026 eV |
| Reference | Megajoules (MJ) | Electronvolts (eV) |
|---|---|---|
| One food calorie (kcal) | 0.004184 MJ | 2.61145 × 1022 eV |
| An AA alkaline battery | 0.01 MJ | 6.24151 × 1022 eV |
| Daily adult food intake | 8.368 MJ | 5.22289 × 1025 eV |
| One unit on an electricity bill | 3.6 MJ | 2.24694 × 1025 eV |
| A lightning strike | 1000 MJ | 6.24151 × 1027 eV |
The megajoule is a unit of energy equal to one million joules. Its symbol is MJ. It is the scale at which energy stops being a laboratory quantity and becomes something with visible physical consequences.
A moving vehicle is the easiest anchor. A family car of 1500 kilograms travelling at 100 kilometres per hour carries about 0.58 megajoules of kinetic energy, and all of it must go somewhere in a collision. Crash structures are designed around exactly this figure, which is why impact energy rather than speed is the quantity written into vehicle safety standards.
Explosives are rated here too. A stick of dynamite releases roughly one megajoule, and a kilogram of TNT about 4.2. Because the TNT equivalent is defined rather than measured, the megajoule provides the bridge between chemical explosives and other energy releases, allowing a meteor impact or an industrial accident to be described on the same scale as a demolition charge.
Daily human energy intake is around ten megajoules, which is the same figure as 2400 kilocalories seen from the other side. Physical work is comparable: a day of manual labour above resting metabolism costs a few megajoules, and a long cycling stage costs perhaps fifteen.
Fuel energy densities live at this scale per unit mass. Petrol holds about 46 megajoules per kilogram, wood around 16, and a modern lithium battery about 0.9. That last comparison, a factor of fifty, is the single number that explains why electric aircraft remain difficult while electric cars do not.
Industrial and military systems use the unit for stored and delivered energy. Railgun and electromagnetic launcher performance is quoted in megajoules at the muzzle, flywheel storage in megajoules, and the energy delivered by a large capacitor bank in a fusion experiment likewise.
Comparing storage technologies in this unit is instructive because the gaps are so wide. A kilogram of petrol holds about 46 megajoules, a kilogram of dry wood about 16, and a kilogram of lithium-ion battery only about 0.9 — some fifty times less than the fuel. That single ratio explains most of the engineering difficulty of electric aviation and much of the weight of an electric car. Working the other way, a kilogram of uranium-235 fissioned completely releases about 80 million megajoules, and a kilogram of matter converted entirely to energy would give 90 billion. The unit spans that whole range without changing prefix more than a few times.
One megajoule equals one million joules, about 0.278 kilowatt-hours, or about 239 kilocalories.
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.