| Reference | Joules (J) | Microjoules (µJ) |
|---|---|---|
| One food calorie (kcal) | 4184 J | 4.184 × 109 µJ |
| An AA alkaline battery | 10000 J | 1 × 1010 µJ |
| Daily adult food intake | 8368000 J | 8.368 × 1012 µJ |
| One unit on an electricity bill | 3600000 J | 3.6 × 1012 µJ |
| A lightning strike | 1 × 109 J | 1 × 1015 µJ |
The joule is the SI unit of energy, work and heat. Its symbol is J. One joule is the work done when a force of one newton moves its point of application through one metre, and equally the energy dissipated when one watt of power flows for one second.
That equivalence is the reason the unit exists. Before the nineteenth century, heat and mechanical work were treated as different things measured in different units, heat in calories and work in foot-pounds. James Prescott Joule spent years demonstrating that a fixed amount of mechanical work always produces the same amount of heat, most famously by stirring water with a paddle wheel driven by falling weights and measuring the temperature rise. Establishing that exchange rate, the mechanical equivalent of heat, unified mechanics and thermodynamics into one subject.
Because it is defined through the base units, the joule connects everything. It is a newton metre, a watt second, a coulomb volt and a kilogram metre squared per second squared, and every one of those forms appears in ordinary use depending on which branch of physics is speaking. Electrical, mechanical, thermal and chemical energy are all counted in the same unit, which is what makes energy accounting across a system possible at all.
The scale is human-sized in a useful way. Lifting an apple one metre takes about one joule. A heartbeat uses roughly one joule. Dropping a textbook from a desk releases about ten. Below that, joules become clumsy and the prefixed units take over; above it, kilojoules and megajoules do the same.
Food energy is properly measured in joules, and labels in Australia, New Zealand and the European Union give kilojoules alongside or instead of calories. The calorie survives in general use mainly through habit, and the conversion, one calorie equal to 4.184 joules, is fixed by definition rather than measured.
Nutrition, ballistics, battery capacity, laser output, explosive yield and the energy released by an earthquake are all quoted in joules or its multiples, which allows quantities from completely different domains to be compared directly.
One joule equals one newton metre, one watt second, about 0.239 calories, or about 0.000278 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.