| Millijoules (mJ) | Microjoules (µJ) |
|---|---|
| 1 Millijoule | 1000 µJ |
| 2 Millijoules | 2000 µJ |
| 3 Millijoules | 3000 µJ |
| 4 Millijoules | 4000 µJ |
| 5 Millijoules | 5000 µJ |
| 10 Millijoules | 10000 µJ |
| 20 Millijoules | 20000 µJ |
| 25 Millijoules | 25000 µJ |
| 50 Millijoules | 50000 µJ |
| 100 Millijoules | 100000 µJ |
| Reference | Millijoules (mJ) | Microjoules (µJ) |
|---|---|---|
| One food calorie (kcal) | 4184000 mJ | 4.184 × 109 µJ |
| An AA alkaline battery | 10000000 mJ | 1 × 1010 µJ |
| Daily adult food intake | 8.368 × 109 mJ | 8.368 × 1012 µJ |
| One unit on an electricity bill | 3.6 × 109 mJ | 3.6 × 1012 µJ |
| A lightning strike | 1 × 1012 mJ | 1 × 1015 µJ |
The millijoule is a unit of energy equal to one thousandth of a joule, or 0.001 joules. Its symbol is mJ. It is the first unit on the way down from the joule at which the human body stops being able to feel what is happening, and for that reason it appears wherever a small amount of energy has to be released deliberately and precisely.
Everyday mechanics reaches it easily. A paperclip weighing a gram, dropped from a metre, arrives with about ten millijoules. Pressing a keyboard key takes a couple, turning a page rather less, and the spark that jumps from a doorknob on a dry day carries between one and ten. That last figure is roughly where the threshold of sensation lies, which makes the millijoule the smallest energy a person routinely notices without being told.
Explosion protection is written here. The minimum ignition energy of methane in air is about 0.28 millijoules and of propane about 0.25, while clouds of flour, sugar or metal dust need tens of millijoules. Standards for hazardous areas set the permitted stored energy of a circuit against those numbers, so the whole framework of intrinsic safety turns on quantities in this range rather than on anything a voltmeter shows.
Medical and industrial lasers are rated in millijoules per pulse. A Q-switched laser used for tattoo removal or lithotripsy delivers a few hundred millijoules in a few nanoseconds, and the extreme brevity is what converts a modest energy into a peak power of megawatts. Ophthalmic capsulotomy uses a few millijoules, and dermatological devices are titrated in the same unit because the dose per pulse, not the session, determines the effect on tissue.
Small electrical and acoustic quantities land here too. A one-milliwatt laser pointer emits one millijoule every second. A person speaking loudly radiates roughly a millijoule of sound each second, and a smoke alarm sounder several. A capacitor of one microfarad charged to fifty volts stores 1.25 millijoules, which is the sort of figure printed on flash circuits, ignition modules and camera shutters.
Above it the joule takes over and effects become obviously mechanical; below it the microjoule takes over and effects become invisible. That makes the millijoule the practical boundary of perception, and explains why safety limits, sensitivity thresholds and dose specifications so often stop at three decimal places rather than going further.
One millijoule equals 0.001 joules, about 0.000239 calories, or about 0.000000278 watt-hours.
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.