calorie
Energyexact by definition
The calorie used here is the thermochemical calorie, defined as exactly 4.184 joules. That definition is exact, adopted so that the calorie would have a fixed value rather than depend on a water measurement. The original idea was the heat needed to raise one gram of water by one degree Celsius, which is only approximately true because water's specific heat varies with temperature.
Watch out: Several calories exist and they differ in the third or fourth digit. The thermochemical calorie is exactly 4.184 J, the International Table calorie is exactly 4.1868 J and underlies the BTU, the 15 °C calorie is about 4.1855 J, and the mean calorie about 4.190 J. Far more important, a nutritional Calorie with a capital C is 1000 of these, a factor of a thousand rather than a fraction of a percent.
| 1 cal | 4.184 J |
The unit dates to early nineteenth century French calorimetry, from the Latin calor for heat, and predates the recognition that heat and mechanical work are the same quantity.
About the calorie
The calorie is a unit in retirement that refuses to leave. Its original definition, the heat that raises one gram of water by one kelvin, sounds clean but is not well posed, because water's specific heat depends on the starting temperature. Raising water from 14.5 to 15.5 °C takes slightly more energy than raising it from 19.5 to 20.5 °C. Metrologists responded by defining several calories, each pinned to a different reference, and eventually by defining the thermochemical calorie as exactly 4.184 J and cutting the tie to water altogether.
The variant that matters outside the laboratory is capitalisation. The nutritional Calorie, sometimes written kcal or "large calorie", is 1000 thermochemical calories, or 4184 J. A food label reading "250 Calories" means 250 kcal, or just over a million joules. The convention arose in nineteenth century nutrition work where gram-scale calories were inconveniently small, and it has caused confusion ever since because the two units differ by a factor of 1000 and are distinguished only by a capital letter that most typography discards.