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1824 as a unit of heat, entering French and English dictionaries between 1841 and 1867.[1] In most fields its use has been replaced by the SI unit of energy, the joule. However, it remains a commonly used unit for energies in the field of chemistry,[2][citation needed] and in many countries it remains in common use as a unit of food energy.
Definitions of the calorie fall into two classes:
- The small calorie or gram calorie (symbol: cal)[3] approximates the energy needed to increase the temperature of 1 gram of water by 1 kelvin at standard atmospheric pressure (101.325 kPa). This is approximately 4.2 joules.
- The large calorie, kilogram calorie, dietary calorie, nutritionist's calorie or food calorie (symbol: Cal)[3] approximates the energy needed to increase the temperature of 1 kilogram of water by 1 kelvin. This is exactly 1,000 small calories or approximately 4.2 kilojoules.
The calorie, the kilocalorie, and the kilojoule are in common use in nutritional contexts as units of food energy. As used in these contexts the calorie (unprefixed) is based on the kilogram whereas the kilocalorie is based on the gram. That is, the nutritional calorie is the kilogram calorie and the kilocalorie is one thousand gram calories. Thus, in nutrition, the terms calorie and kilocalorie refer to equivalent units.
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 experimental calorimetry, and commonly used to specify bond and conformational energies in molecular modeling.[4]
[edit] Definitions
The energy needed to increase the temperature of a given mass of water by 1 °C at standard 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.
| Name | Symbol | Conversions | Notes |
|---|---|---|---|
| Thermochemical calorie | calth | ≡ 4.184 J | [5] |
| 4 °C calorie | cal4 | ≈ 4.204 J
≈ 0.003985 BTU ≈ 1.168×10−6 kWh ≈ 2.624×1019 eV |
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
≈ 0.0039671 BTU ≈ 1.1626×10−6 kWh ≈ 2.6124×1019 eV |
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. 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.[5] |
| 20 °C calorie | cal20 | ≈ 4.182 J
≈ 0.003964 BTU ≈ 1.162×10−6 kWh ≈ 2.610×1019 eV |
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
≈ 0.003971 BTU ≈ 1.164×10−6 kWh ≈ 2.615×1019 eV |
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
≈ 0.0039683 BTU ≈ 1.163×10−6 kWh ≈ 2.6132×1019 eV |
1⁄860 international watt hours = 180⁄43 international joules exactly.[6] | |
| International Steam Table calorie (1956) | calIT | ≡ 4.1868 J
≈ 0.0039683 BTU ≈ 1.163×10−6 kWh ≈ 2.6132×1019 eV |
1.163 mW·h = 4.1868 J exactly. This definition was adopted by the Fifth International Conference on Properties of Steam (London, July 1956).[5] |
| IUNS calorie | ≡ 4.182 J
≈ 0.003964 BTU ≈ 1.162×10−6 kWh ≈ 2.610×1019 eV |
This is a ratio adopted by the Committee on Nomenclature of the International Union of Nutritional Sciences.[7] |
[edit] Notes and references
- ^ Etymology: French calorie, from Latin calor meaning "heat".
- ^ http://www.chemistrydaily.com/chemistry/Calorie
- ^ 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."
