The calorie is a pre-SI metric unit of energy. It was first defined by Nicolas Cl ment 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 In most fields its use is archaic, having been replaced by the SI unit of energy, the joule.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, 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. It is also called the nutritionist's calorie.
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.
The gram calorie, however, is too small a unit for use in nutritional contexts. Instead, the kilocalorie (symbol: kcal) or large calorie is used. In this context calorie and kilocalorie are equivalent.
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.
|4 C calorie
||the amount of energy required to warm one gram of air-free water from 3.5 C to 4.5 C at standard atmospheric pressure.
|15 C calorie
||the amount of energy required to warm one gram of air-free water from 14.5 C to 15.5 C at standard atmospheric pressure (101.325 kPa). Experimental values of this calorie ranged from 4.1852 J to 4.1858 J. The CIPM in 1950 published a mean experimental value of 4.1855 J, noting an uncertainty of 0.0005 J.
|20 C calorie
||the amount of energy required to warm one gram of air-free water from 19.5 C to 20.5 C at standard atmospheric pressure.
||of the amount of energy required to warm one gram of air-free water from 0 C to 100 C at standard atmospheric pressure.
|International Steam Table calorie (1929)
international watt hours = international joules exactly.
|International Steam Table calorie (1956)
||1.163 mW h = 4.1868 J exactly. This definition was adopted by the Fifth International Conference on Properties of Steam (London, July 1956).
||This is a ratio adopted by the Committee on Nomenclature of the International Union of Nutritional Sciences.
Notes and references
- Empty calorie
- Basal metabolic rate
- Food energy
- Nutrition facts label
- Conversion of units of energy
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