Calorie

# Calorie

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{{About|the unit of energy|its use in nutrition and food labeling regulations|Food energy|the study of reduced calorie intake|CALERIE}} {{Redirect|Calories|the 1951 short story|Calories (story)}} The calorie is a pre-[[SI]] [[metric system|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 In most fields its use is archaic, having been replaced by the SI unit of energy, the [[joule]].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 [[Celsius|°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 [[kilojoule]]s. 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. Despite metrication, the calorie is still often encountered in contexts such as bond and conformational energies in molecular modeling. The gram calorie, however, is a very small unit for use in nutritional contexts. Instead, the kilocalorie (symbol: kcal) or large calorie is used. In such context calorie and kilocalorie are equivalent.

## Variations

The energy needed to increase the temperature of a given mass of water by 1 °C 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 of units#Energy, work, or amount of heat|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. NEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINENEWLINE
Name Symbol Conversions Notes
[[Thermochemistry|Thermochemical]] calorie calth ≡ {{val|4.184|ul=J}} ≈ {{val|0.003964|ul=BTU}} ≈ {{val|1.163|e=-6|ul=kWh}} ≈ {{val|2.611|e=19|ul=eV}}
4 °C calorie cal4 ≈ 4.204 J ≈ {{val|0.003985|u=BTU}} ≈ {{convert|4.204|J|kWh|disp=output only}} ≈ {{convert|4.204|J|eV|disp=output only}} 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 cal15 ≈ 4.1855 J ≈ {{val|0.0039671|u=BTU}} ≈ {{convert|4.1855|J|kWh|disp=output only}} ≈ {{convert|4.1855|J|eV|disp=output only}} the amount of energy required to warm one gram of air-free water from 14.5 °C to 15.5 °C at [[Atmospheric pressure#Standard atmospheric pressure|standard atmospheric pressure]] (101.325 [[kilopascal|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 cal20 ≈ 4.182 J ≈ {{val|0.003964|u=BTU}} ≈ {{convert|4.182|J|kWh|disp=output only}} ≈ {{convert|4.182|J|eV|disp=output only}} 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.
Mean calorie calmean ≈ 4.190 J ≈ {{val|0.003971|u=BTU}} ≈ {{convert|4.190|J|kWh|disp=output only}} ≈ {{convert|4.190|J|eV|disp=output only}} {{frac|100}} 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#Steam tables|Steam Table]] calorie (1929) ≈ 4.1868 J ≈ {{val|0.0039683|u=BTU}} ≈ {{convert|4.1868|J|kWh|disp=output only}} ≈ {{convert|4.1868|J|eV|disp=output only}} {{frac|860}} international watt hours = {{frac|180|43}} international joules exactly.
International Steam Table calorie (1956) calIT ≡ 4.1868 J ≈ {{val|0.0039683|u=BTU}} ≈ {{convert|4.1868|J|kWh|disp=output only}} ≈ {{convert|4.1868|J|eV|disp=output only}} 1.163 mW·h = 4.1868 J exactly. This definition was adopted by the Fifth International Conference on Properties of Steam (London, July 1956).
[[International Union of Nutritional Sciences|IUNS]] calorie ≡ 4.182 J ≈ {{val|0.003964|u=BTU}} ≈ {{convert|4.182|J|kWh|disp=output only}} ≈ {{convert|4.182|J|eV|disp=output only}} This is a ratio adopted by the Committee on Nomenclature of the International Union of Nutritional Sciences.
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