Millijoule (mJ)

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.

Millijoule conversion table