Calorie
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of heat, entering French and English dictionaries between 1841 and 1867.[1] In most fields its use is archaic, having been replaced by the SI unit of energy, the joule.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)[2] 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 or food calorie (symbol: Cal)[2] approximates the energy needed to increase the temperature of 1 kilogram of water by 1 °C. This is exactly 1000 small calories or about 4.2 kilojoules.
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 is sometimes impossible). 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 gram calorie. In nutritional contexts, however, a larger unit is more useful. In such contexts the term calorie can be taken to refer to the kilogram calorie. Food energy may alternatively be given in kilocalories (symbol: kcal). The term kilocalorie can generally be taken to refer to 1000 gram calories since metric prefixes tend to be used only with the gram calorie.[citation needed]
Contents |
[edit] Variations
The energy needed to increase the temperature of a gram 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 factors used to convert measurements in calories to their equivalents in joules are numerically equivalent to expressions of the specific heat capacity of water in joules per gram or kilojoules per kilogram.
| Name | Symbol | Equivalent in Joules | Notes |
|---|---|---|---|
| Thermochemical calorie | calth | ≡ 4.184 J | [3] |
| 4 °C calorie | cal4 | ≈ 4.204 J | 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 | 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.[3] |
| 20 °C calorie | cal20 | ≈ 4.182 J | 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 | 1⁄100 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 Table calorie (1929) | ≈ 4.1868 J | 1⁄860 international watt hours = 180⁄43 international joules exactly.[4] | |
| International Steam Table calorie (1956) | calIT | ≡ 4.1868 J | 1.163 mW·h = 4.1868 J exactly. This definition was adopted by the Fifth International Conference on Properties of Steam (London, July 1956).[3] |
| IUNS calorie | ≡ 4.182 J | This is a ratio adopted by the Committee on Nomenclature of the International Union of Nutritional Sciences.[5] |
[edit] Conversions
The conversion factor between calories and joules is numerically equivalent to the specific heat capacity of liquid water (in SI units).
One gram calorie is approximately:
- 4.184 J (joules)
- 0.003964 BTU (British thermal units)
- 1.163×10−6 kW·h (kilowatt hours)
- 2.611×1019 eV (electron volts)
One kilogram calorie (food calorie) is approximately:
- 4.184 kJ
- 3.964 BTU
- 0.001163 kW·h
- 2.611×1022 eV
[edit] 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
- ^ 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."
