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This article is about the unit of energy. For its use in nutrition and food labeling regulations, see
Food energy. For the study of reduced calorie intake, see
CALERIE.
The calorie is a pre-SI metric unit of energy. It was first defined by Nicolas Clément in 1824 as a unit of heat, entering French and English dictionaries between 1841 and 1867. In most fields its use is archaic, having been replaced by the SI unit of energy, the joule. However, in many countries it remains in common use as a unit of food energy.
Definitions of a calorie fall into two classes:
- The small calorie or gram calorie (symbol: cal) approximates the energy needed to increase the temperature of 1 gram of water by 1 °C. This is about 4.2 joules.
- The large calorie, kilogram calorie, dietary calorie, nutritionist's calorie or food calorie (symbol: Cal) approximates the energy needed to increase the temperature of 1 kilogram of water by 1 °C. This is exactly 1,000 small calories or about 4.2 kilojoules.
The gram calorie is not used in nutritional contexts. Instead, the large calorie is used. In this context calorie and kilocalorie are equivalent.
In an attempt to avoid confusion, the large calorie is sometimes written as Calorie (with a capital C). This convention, however, is not always followed, and not explained to the average person clearly (and is sometimes ambiguous, such as at the beginning of a sentence). Whether the large or small calorie is intended often must be inferred from context. When used in scientific contexts, the term calorie refers to the small calorie; it is often encountered in contexts such as bond and conformational energies in molecular modeling.
Variations
The energy needed to increase the temperature of a given mass of water by 1 °C at atmospheric pressure depends on the starting temperature and is difficult to measure precisely. Accordingly, there have been several definitions of the calorie. The two perhaps most popular definitions used in older literature are the 15 °C calorie and the thermochemical calorie.
The conversion factors used to convert calories to joules are numerically equivalent to expressions of the specific heat capacity of water in joules per gram or kilogram.
Notes and references
- ^ Etymology: French calorie, from Latin calor meaning "heat".
- ^ a b Merriam-Webster's Online Dictionary Def 1a http://www.merriam-webster.com/dictionary/calorie
- ^ http://www.ch.ic.ac.uk/local/organic/t5.html
- ^ a b c International Standard ISO 31-4: Quantities and units – Part 4: Heat. Annex B (informative): Other units given for information, especially regarding the conversion factor. International Organization for Standardization, 1992.
- ^ Figure depends on the conversion factor between international joules and absolute (modern) joules. Using the mean international ohm and volt (1.00049 Ω, 1.00034 V [1]), the international joule is about 1.00019 J, using the US international ohm and volt (1.000495 Ω, 1.000330 V) it is about 1.000165 J, giving 4.18684 J and 4.18674 J, respectively
- ^ FAO (1971). "The adoption of joules as units of energy". http://www.fao.org/docrep/meeting/009/ae906e/ae906e17.htm. "While the nutritional calorie has not been defined, basically it is the thermochemical calorie. The standards used in calorimetric work in nutrition is ultimately the heat of combustion of an internationally graded standard benzoic acid. This is primarily expressed as joules per gramme mole and secondarily as thermochemical calories per mole derived by dividing by 4.182, a factor which has been approved by the Committee on Nomenclature of the IUNS."
See also