| Microjoules (µJ) | Terajoules (TJ) |
|---|---|
| 1 Microjoule | 1 × 10-18 TJ |
| 2 Microjoules | 2 × 10-18 TJ |
| 3 Microjoules | 3 × 10-18 TJ |
| 4 Microjoules | 4 × 10-18 TJ |
| 5 Microjoules | 5 × 10-18 TJ |
| 10 Microjoules | 1 × 10-17 TJ |
| 20 Microjoules | 2 × 10-17 TJ |
| 25 Microjoules | 2.5 × 10-17 TJ |
| 50 Microjoules | 5 × 10-17 TJ |
| 100 Microjoules | 1 × 10-16 TJ |
| Reference | Microjoules (µJ) | Terajoules (TJ) |
|---|---|---|
| One food calorie (kcal) | 4.184 × 109 µJ | 0.000000004184 TJ |
| An AA alkaline battery | 1 × 1010 µJ | 0.00000001 TJ |
| Daily adult food intake | 8.368 × 1012 µJ | 0.000008368 TJ |
| One unit on an electricity bill | 3.6 × 1012 µJ | 0.0000036 TJ |
| A lightning strike | 1 × 1015 µJ | 0.001 TJ |
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 terajoule is a unit of energy equal to a thousand gigajoules, or 1,000,000,000,000 joules. Its symbol is TJ. It is the unit in which countries account for their energy, and it appears far more often in official statistics than in any engineering catalogue, because it is sized for the annual consumption of a town rather than of a machine.
International energy accounting runs on it. The International Energy Agency publishes national energy balances in terajoules, and greenhouse-gas inventories submitted under the United Nations climate framework report fuel use in the same unit. Emission factors are expressed to match: burning a terajoule of natural gas releases about 56 tonnes of carbon dioxide, diesel about 74, and hard coal about 95, which turns any energy figure directly into an emissions figure.
Generation is described here too. A three-megawatt wind turbine running at a realistic capacity factor produces roughly 33 terajoules in a year. A single cargo of liquefied natural gas in a large carrier holds close to 3800 terajoules, which is why one shipment can cover a small country's gas demand for weeks. Grid-scale batteries, by contrast, are still measured in fractions of a terajoule.
Destructive energy reaches the scale as well. A kilotonne of TNT equals 4.184 terajoules exactly, so the bomb dropped on Hiroshima released roughly 63 terajoules. An earthquake of magnitude five radiates about two terajoules of seismic energy, and each whole step up the magnitude scale multiplies that by about thirty-two, which is what makes a magnitude seven event a thousand times the size of a magnitude five one.
The unit also marks a boundary in how energy is discussed. Below it, quantities belong to buildings, vehicles and appliances, and people argue about efficiency. At and above it, quantities belong to policy: fuel imports, subsidy schemes, emission targets and grid planning. The same physics is being measured, but the terajoule is where the conversation stops being technical and becomes political.
Above the terajoule the ladder continues with the petajoule, used for whole national sectors, and the exajoule, used for world totals: global primary energy consumption runs to something over 600 exajoules a year. Reading any of these back down is a matter of moving the decimal point three places at a time.
One terajoule equals 1,000,000,000,000 joules, about 277.8 megawatt-hours, or about 239 gigacalories.