The joule (symbol J) is the derived unit of energy in the International System of Units. It is the energy exerted by a force of one newton acting to move an object through a distance of one metre. In terms of dimensions:
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Definition
One joule is defined as the amount of work done by a force of one newton moving an object through a distance of one metre. Other relationships are:
- The work required to move an electric charge of one coulomb through an electrical potential difference of one volt; or one coulomb volt (C·V). (This relationship can be used to define the volt);
- The work required to continuously produce one watt of power for one second; or one watt second (W·s) (compare kilowatt hour). (This relationship can be used to define the watt)
Conversions
Main article: Conversion of units#Energy1 joule is equal to:
- 1×107 ergs (exactly)
- 6.24150974×1018 eV (electronvolts)
- 0.2390 cal (thermochemical gram calories or small calories)
- 2.3901×10−4 kcal (thermochemical kilocalories, kilogram calories, large calories or food calories)
- 9.4782×10−4 BTU (British thermal unit)
- 0.7376 ft·lbf (foot-pound force)
- 23.7 ft·pdl (foot-poundals)
- 2.7778×10−7 kilowatt-hour
- 2.7778×10−4 watt-hour
- 9.8692×10−3 litre-atmosphere
- 1×10−44 Foe (exactly)
Units defined in terms of the joule include:
- 1 thermochemical calorie = 4.184 J
- 1 International Table calorie = 4.1868 J
- 1 watt hour = 3600 J
- 1 kilowatt hour = 3.6×106 J (or 3.6 MJ)
- 1 ton TNT = 4.184 GJ
Useful to remember:
- 1 joule = 1 newton × 1 metre = 1 watt × 1 second
Practical examples
One joule in everyday life is approximately:
- the energy required to lift a small apple one meter straight up.
- the energy released when that same apple falls one meter to the ground.
- the energy released as heat by a person at rest, every hundredth of a second.
- the energy required to heat one gram of dry, cool air by 1 degree Celsius.
- one hundredth of the energy a person can receive by drinking a drop of beer[citation needed].
- the kinetic energy of an adult human moving at a speed of about a handspan every second.
SI multiples
| Submultiples | Multiples | |||||
|---|---|---|---|---|---|---|
| Value | Symbol | Name | Value | Symbol | Name | |
| 10–1 J | dJ | decijoule | 101 J | daJ | decajoule | |
| 10–2 J | cJ | centijoule | 102 J | hJ | hectojoule | |
| 10–3 J | mJ | millijoule | 103 J | kJ | kilojoule | |
| 10–6 J | µJ | microjoule | 106 J | MJ | megajoule | |
| 10–9 J | nJ | nanojoule | 109 J | GJ | gigajoule | |
| 10–12 J | pJ | picojoule | 1012 J | TJ | terajoule | |
| 10–15 J | fJ | femtojoule | 1015 J | PJ | petajoule | |
| 10–18 J | aJ | attojoule | 1018 J | EJ | exajoule | |
| 10–21 J | zJ | zeptojoule | 1021 J | ZJ | zettajoule | |
| 10–24 J | yJ | yoctojoule | 1024 J | YJ | yottajoule | |
| Common multiples are in bold face | ||||||
This SI unit is named after James Prescott Joule. As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is uppercase (J). When an SI unit is spelled out in English, it should always begin with a lowercase letter (joule), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase.
—Based on The International System of Units, section 5.2.
See also
References
- The adoption of joules as units of energy, FAO/WHO Ad Hoc Committee of Experts on Energy and Protein, 1971. A report on the changeover from calories to joules in nutrition.
External links
Categories: SI derived units | Units of energy
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Q. Red light has a wavelength of 800nm. a)What is the frequency of this light? b)What is the ENERGY in joules of one photon of red light? c)What is the ENERGY in joules of one mole of photons of red light? I'm confused with b) and c) I solved a) multiplying the speed of light by the wavelength. I hope I'm not mistaken. Thank you for your help.
Asked by Pancha Pistolas - Mon Nov 19 15:33:31 2007 - - 1 Answers - 1 Comments
A. a. frequency is just 1/wavelength (this is a general relationship between the two that always holds), but your way works too. b. E = hf = h/lambda, since f = 1/lambda, as you saw in part A. h is Planck's constant. That's for one photon. c. A mole just means 6.022E23 of that something, so if you know the energy of one photon, just x it by 6.022E23 to find the energy of a mole of it.
Answered by Knows what he is talking about - Mon Nov 19 15:46:48 2007
