| Microjoules (µJ) | Megajoules (MJ) |
|---|---|
| 1 Microjoule | 1 × 10-12 MJ |
| 2 Microjoules | 2 × 10-12 MJ |
| 3 Microjoules | 3 × 10-12 MJ |
| 4 Microjoules | 4 × 10-12 MJ |
| 5 Microjoules | 5 × 10-12 MJ |
| 10 Microjoules | 1 × 10-11 MJ |
| 20 Microjoules | 2 × 10-11 MJ |
| 25 Microjoules | 2.5 × 10-11 MJ |
| 50 Microjoules | 5 × 10-11 MJ |
| 100 Microjoules | 1 × 10-10 MJ |
| Reference | Microjoules (µJ) | Megajoules (MJ) |
|---|---|---|
| One food calorie (kcal) | 4.184 × 109 µJ | 0.004184 MJ |
| An AA alkaline battery | 1 × 1010 µJ | 0.01 MJ |
| Daily adult food intake | 8.368 × 1012 µJ | 8.368 MJ |
| One unit on an electricity bill | 3.6 × 1012 µJ | 3.6 MJ |
| A lightning strike | 1 × 1015 µJ | 1000 MJ |
The microjoule is a unit of energy equal to one millionth of a joule, or 0.000001 joules. Its symbol is µJ. It is the scale of small living things, of short laser pulses and of the electricity a sensor spends waking up, and it is the smallest energy unit that still describes events an unaided person can notice happening.
Insects live here. A flea of half a milligram leaping twenty centimetres spends about one microjoule, and a mosquito in level flight carries roughly a quarter of one. Lifting a gram by a tenth of a millimetre costs the same. These are the quantities that make the unit intuitive: not invisible in principle, merely too small to matter unless something repeats them millions of times a second.
Ignition safety is written in this range, and the numbers decide how equipment may be built. The minimum energy that will ignite a hydrogen and air mixture is about 17 microjoules; for most hydrocarbon vapours it is a few hundred. Intrinsically safe apparatus for refineries, grain stores and mines is certified by proving that no fault can release more than the mixture requires, which is why circuits destined for hazardous areas are audited in microjoules rather than in volts.
Pulsed light instruments are specified in microjoules almost universally. A vehicle lidar fires pulses of a few microjoules many thousand times a second, laser marking systems use tens to hundreds, and femtosecond amplifiers destined for micromachining sit at the top of the range. The energy per pulse sets what the beam can cut, image or safely illuminate, and the average power alone does not answer that question.
Low-power electronics budgets in the same terms. A passive radio-frequency identification tag harvests a few microjoules from the reader's field and must complete its whole exchange inside that. A short-range radio advertisement costs tens of microjoules, a quartz watch movement about one per second, and a temperature sensor that wakes, measures and sleeps again may spend under ten. Battery life on such devices is arithmetic performed in microjoules.
Above it the millijoule takes over and the effects become mechanical: a spark you feel, a click you hear, a small object dropped. Below it the nanojoule takes over and the effects become optical and electronic. The microjoule occupies the narrow band between, which is why so many disciplines that otherwise share nothing converge on the same prefix.
One microjoule equals 0.000001 joules, about 6.24 × 10¹² electronvolts, or about 2.78 × 10⁻¹⁰ watt-hours.
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.