| Reference | Joules (J) | Megajoules (MJ) |
|---|---|---|
| One food calorie (kcal) | 4184 J | 0.004184 MJ |
| An AA alkaline battery | 10000 J | 0.01 MJ |
| Daily adult food intake | 8368000 J | 8.368 MJ |
| One unit on an electricity bill | 3600000 J | 3.6 MJ |
| A lightning strike | 1 × 109 J | 1000 MJ |
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 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.