Joules
Physical unit | |
---|---|
Unit name | Joules |
Unit symbol | |
Physical quantity (s) | Energy , work , internal energy , warmth |
Formula symbol | |
dimension | |
system | International system of units |
In SI units | |
In CGS units | |
Named after | James Prescott Joule |
Derived from | Kilogram , meter , second |
Joule [ dʒuːl ] is the derived SI unit of energy . Named after James Prescott Joule , this unit is used today for all forms of energy , including work and heat .
Like any derived unit, the joule can be expressed using base units . With the units kg , m and s the following applies:
illustration
One joule is equal to the energy required to:
- a body with a weight of 1 Newton - this corresponds to a mass of approx. 0.102 kilograms , about a bar of chocolate - to lift one meter ( 1 Newton meter ) or
- to accelerate a body with a mass of 2 kilograms from rest to a speed of 1 m / s or
- the power of one watt - that is roughly the power of the human heart - to produce for one second ( 1 watt- second ) or
- at an electrical voltage of one volt to allow an electrical current of one ampere to flow for one second ( 1 volt ampere second ) or
- to heat one gram of water by approx. 0.239 Kelvin .
In nutrition labels , kilojoules (kJ) - based on 100 g , 100 ml or 1 serving - are used to indicate the physiological calorific value .
pronunciation
The pronunciation of the word “Joule” is inconsistent: In the German-speaking world, the pronunciation [ dʒuːl ] is common, and this is probably how James Prescott Joule pronounced his name.
history
At the first International Electricity Congress on September 20 and 21, 1881 in Paris, not only was the CGS system declared binding, but in addition to this “absolute system” also a “practical system” consisting of ohms , amperes , volts , Coulomb and Farad , which provides more manageable orders of magnitude for practical use in electrical engineering than units derived directly from the CGS system. Both systems differ only in factors from powers of ten.
In his inaugural address as President of the British Association for the Advancement of Science on August 23, 1882 , Carl Wilhelm Siemens proposed the joule as a unit for heat in addition to the Weber , Watt and Gauss units to supplement the practical system . Derived from the other electromagnetic units, it should be defined as the amount of heat that a current of 1 ampere produces at a resistance of 1 ohm within 1 second , and thus equivalent to 10 ^{7 }erg , the unit of energy in the CGS system. ^{}
“The unit of heat has hitherto been taken variously as the heat required to raise a pound of water at the freezing point through 1 ° Fahrenheit or Centigrade, or, again, the heat necessary to raise a kilogram of water 1 ° Centigrade. The inconvenience of a unit so entirely arbitrary is sufficiently apparent to justify the introduction of one based on the electro-magnetic system, viz. the heat generated in one second by the current of an Ampère flowing through the resistance of an Ohm. In absolute measure its value is 10 ^{7} CGS units, and, assuming Joule's equivalent as 42,000,000, it is the heat necessary to raise 0.238 grammes of water 1 ° Centigrade, or, approximately, the ^{1} ⁄ _{1000} th part of the arbitrary unit of a pound of water raised 1 ° Fahrenheit and the ^{1} ⁄ _{4000} th of the kilograms of water raised 1 ° Centigrade. Such a heat unit, if found acceptable, might with great propriety, I think, be called the Joule, after the man who has done so much to develop the dynamical theory of heat. "
“The unit of heat has been defined in various ways, for example as the heat required to heat a pound of water from freezing point by 1 ° Fahrenheit or Celsius, or as the heat required to raise a kilogram of water by 1 ° Celsius. The inadequacy of such a completely random unit is evident enough to justify the introduction of a unit based on the electromagnetic system, defined as the heat generated during one second when 1 ampere flows through a 1 ohm resistor. In the CGS system, this value has the measure of 10 ^{7} and, assuming the Joules mechanical heat equivalent of 42,000,000, is equal to the heat required to heat 0.238 grams of water by 1 ° Celsius, or roughly a thousandth of that by chance defined unit of one pound of water that is heated by 1 ° Fahrenheit and about a four-thousandth of a kilogram of water that is heated by 1 ° Celsius. If such a unit of heat is generally accepted, it should be called "Joule" in my opinion, since he has done so much for the development of dynamic heat theory. "
The second international electricity congress in Paris decided on the joule on August 31, 1889 as a unit in the proposed form, next to the watt and the Henry (at that time still called the quadrant ). The joule was thus defined both via the CGS base units and via ohms and amperes, which in turn can be derived from the CGS base units, but were also specified with direct measurement rules.
The fourth International Electricity Congress in Chicago in August 1893 therefore also introduced the international joule , which is derived from the specifications for the international ohm and the international ampere , which had also been slightly changed from the old ohms and amps. In contrast, the absolute joule or theoretical joule has given the true value based on the CGS system.
At the 5th General Conference on Weights and Measures in 1913, together with the “Newton” and the “Joule”, the “watt” in its current definition as 1 W = 1 J / s was proposed.
In June 1935, the International Electrotechnical Commission , the successor to the International Electricity Congress, decided to adopt the Giorgi system, an MBS system in which the existing "practical units" without the powers of ten as conversion factors find a natural place in which the magnetic field constant matches is defined. The joule also had its place here.
In 1946, as part of the Meter Convention , the International Committee for Weights and Measures redefined the joule as the work that is done by the point at which 1 MFS unit of force (not officially named as Newton at the time ) begins 1 meter is moved in the direction of which. The joule was also expressly intended as a unit of energy and for use in an electromagnetic context.
The General Conference on Weights and Measures ratified this definition in 1948 at its 9th Conference and in particular also required that the joule be used in calorimetry whenever possible instead of the calorie . The joule has been part of the International System of Units (SI) since 1960 .
In the EU , the EC directive 71/354 / EEC of October 18, 1971 declared the joule as "finally approved" like the other SI units. The EU member states had to introduce compulsory use within five years ; the use of the competing calorie was to be prohibited by the end of 1977 at the latest. The fate of the Erg was initially left open; its use was to be prohibited until the end of 1979.
Directive 80/181 / EEC of December 20, 1979 (valid since October 1981) confirmed the SI with the joule, but granted the member states the possibility of re-admitting other units for a limited period as long as they are used in addition and the SI unit is highlighted. The deadline was initially set at the end of 1989 and was later extended to the end of 1999 and then to the end of 2009. Since January 1, 2010, the additional indication of calories in addition to joules has been permitted in the EU for an unlimited period.
In the case of foodstuffs, the Food Information Regulation (LMIV) has stipulated the specification of the physiological calorific value in the unit kJ per 100 g in EU goods traffic since December 13, 2014 . In addition to being given in kJ, the calorific value can also be given in the outdated unit kcal per 100 g.
Related units
A conversion table for common energy units can be found in the article Energy .
Important units for energy are:
- 1 MJ = kWh
- 1 kWh = 3.6 MJ
Units of apparent electrical energy are:
- 1 volt ampere second (VAs) = 1 J
- 1 volt ampere hour (VAh) = 3.6 kJ
- 1 kilovoltamp hour (kVAh) = 3.6 MJ
Units of electrical reactive energy are:
- 1 var second (vars) = 1 year
- 1 varhour (varh) = 3.6 kJ
- 1 kilovar hour (kvarh) = 3.6 MJ
See also
- Order of magnitude (energy) - compilation of everyday and non-everyday energies.
Web links
Individual evidence
- ^ The legal units in Germany. (PDF; 1.6 MB) Physikalisch-Technische Bundesanstalt , June 2012, p. 12 , accessed on September 27, 2019 .
- ↑ ^{a } ^{b } Directive 2009/3 / EC of the European Parliament and of the Council of March 11, 2009 amending Directive 80/181 / EEC of the Council on the harmonization of the laws of the member states on units in metrology .
- ↑ Quotation ( Oxford English Dictionary ): “ Although some people of this name call themselves dʒaʊl , and others dʒəʊl , it is almost certain that JP Joule (and at least some of his relatives) used dʒuːl . ”
- ^ Report of the Fifty-Second Meeting of the British Association for the Advancement of Science. P. 6 f.
- ↑ Pat Naughtin: . A chronological history of the modern metric system on: metricationmatters.com. (PDF file; 1.4 MB), 2009.
- ↑ Minutes of the 5th General Conference on Weights and Measures , 1913, page 59, presentation of a project by the French government to define the base and derived units, accessed on Feb. 24, 2020
- ↑ Arthur E. Kennelly: Adoption of the Meter-Kilogram-Mass-Second (MKS) Absolute System of Practical Units by the International Electrotechnical Commission (IEC), Bruxelles, June, 1935. (PDF file; 440 kB), 9. August 1935.
- ^ CIPM, 1946, Resolution 2, Definitions of electric units. On: bipm.org.
- ↑ 9th CGPM, Resolution 3: Triple point of water; thermodynamic scale with a single fixed point; unit of quantity of heat (joule). On: bipm.org.
- ↑ Directive 71/354 / EEC of the Council of October 18, 1971 on the approximation of the laws of the member states on units in metrology .
- ↑ Directive 76/770 / EEC of the Council of July 27, 1976 amending Directive 71/354 / EEC on the harmonization of the laws of the member states on units in metrology .
- ↑ Directive 80/181 / EEC of the Council of December 20, 1979 on the approximation of the laws of the member states on units in metrology and the repeal of Directive 71/354 / EEC in the consolidated version of May 27, 2009 .
- ↑ Council Directive 89/617 / EEC of November 27, 1989 amending Directive 80/181 / EEC on the harmonization of the laws of the member states on units in metrology .
- ↑ Directive 99/103 / EC of the European Parliament and of the Council of January 24, 2000 amending Directive 80/181 / EEC on the harmonization of the laws of the member states on units in metrology .