# Joule

(Redirected from Megajoule)
Joule
Unit system SI derived unit
Unit of Energy
Symbol J
Named after James Prescott Joule
Unit conversions
1 J in ... ... is equal to ...
SI base units    kgm2s−2
CGS units    1×107 erg
kilowatt hours    2.78×10−7 kW⋅h
kilocalories (thermochemical)    2.390×10−4 kcalth
BTUs    9.48×10−4 BTU
electronvolts    6.24×1018 eV

The joule (/l/); (symbol: J), is a derived unit of energy in the International System of Units.[1] It is equal to the energy transferred to (or work done on) an object when a force of one newton acts on that object in the direction of its motion through a distance of one metre (1 newton metre or N⋅m). It is also the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second. It is named after the English physicist James Prescott Joule (1818–1889).[2][3][4]

In terms firstly of base SI units and then in terms of other SI units:

${\displaystyle {\text{J}}={\frac {{\text{kg}}\cdot {\text{m}}^{2}}{{\text{s}}^{2}}}={\text{N}}\cdot {\text{m}}={\text{Pa}}\cdot {\text{m}}^{3}={\text{W}}\cdot {\text{s}}={\text{C}}\cdot {\text{V}},}$

where kg is the kilogram, m is the metre, s is the second, N is the newton, Pa is the pascal, W is the watt, C is the coulomb, and V is the volt.

One joule can also be defined as:

• 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 produce one watt of power for one second, or one "watt-second" (W⋅s) (compare kilowatt-hour – 3.6 megajoules). This relationship can be used to define the watt.

## Usage

This SI unit is named after James Prescott Joule. As with every International System of Units (SI) unit named for a person, the first letter of its symbol is upper case (J). However, when an SI unit is spelled out in English, it should always begin with a lower case letter (joule)—except in a situation where any word in that position would be capitalized, such as at the beginning of a sentence or in material using title case. Note that "degree Celsius" conforms to this rule because the "d" is lowercase.— Based on The International System of Units, section 5.2.

## Confusion with newton metre

In mechanics, the concept of force (in some direction) has a close analog in the concept of torque (about some angle):

Linear Angular
force torque
mass moment of inertia
distance angle

A result of this similarity is that the SI unit for torque is the newton metre, which works out algebraically to have the same dimensions as the joule. But they are not interchangeable. The CGPM has given the unit of energy the name joule, but has not given the unit of torque any special name, hence it is simply the newton metre (N⋅m) – a compound name derived from its constituent parts.[5] The use of newton metres for torque and joules for energy is helpful to avoid misunderstandings and miscommunications.[5]

The distinction may be seen also in the fact that energy is a scalar – the dot product of a vector force and a vector displacement. By contrast, torque is a vector – the cross product of a distance vector and a force vector. Torque and energy are related to one another by the equation

${\displaystyle E=\tau \theta \ ,}$

where E is energy, τ is (the vector magnitude of) torque, and θ is the angle swept (in radians). Since radians are dimensionless, it follows that torque and energy have the same dimensions.

## Practical examples

One joule in everyday life represents approximately:

• The energy required to lift a medium-size tomato (100 g) 1 m vertically from the surface of the Earth.[6]
• The energy released when that same tomato falls back down to the ground.
• The energy required to accelerate a 1 kg mass at 1 m⋅s−2 through a distance of 1 m.
• The heat required to raise the temperature of 1 g of water by 0.24 °C.[7]
• The typical energy released as heat by a person at rest every 1/60 s (approximately 17 ms).[8]
• The kinetic energy of a 50 kg human moving very slowly (0.2 m/s or 0.72 km/h).
• The kinetic energy of a 56 g tennis ball moving at 6 m/s (22 km/h).[9]
• The kinetic energy of an object with mass 1 kg moving at √2 ≈ 1.4 m/s.
• The amount of electricity required to light a 1 W LED for 1 s.

Since the joule is also a watt-second and the common unit for electricity sales to homes is the kW⋅h (kilowatt-hour), a kW⋅h is thus 1000 W × 3600 s = 3.6 MJ (megajoules).

## Multiples

For additional examples, see: Orders of magnitude (energy)
Submultiples Multiples Value SI symbol Name Value 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

### Zeptojoule

The zeptojoule (zJ) is equal to one sextillionth (10−21) of one joule. 160 zeptojoules is about one electronvolt.

### Picojoule

The Picojoule (pJ) is equal to one trillionth (10−12) of one joule.

### Nanojoule

The nanojoule (nJ) is equal to one billionth (10−9) of one joule. 160 nanojoules is about the kinetic energy of a flying mosquito.[10]

### Microjoule

The microjoule (μJ) is equal to one millionth (10−6) of one joule. The Large Hadron Collider (LHC) produces collisions of the microjoule order (7 TeV) per particle.

### Millijoule

The millijoule (mJ) is equal to one thousandth (10−3) of a joule.

### Kilojoule

The kilojoule (kJ) is equal to one thousand (103) joules. Nutritional food labels in most countries express energy in kilojoules (kJ).[11]

One square metre of the Earth receives about 1.4 kilojoules of solar radiation every second in full daylight.[12]

### Megajoule

The megajoule (MJ) is equal to one million (106) joules, or approximately the kinetic energy of a one megagram (tonne) vehicle moving at 161 km/h.

The energy required to heat 10 liters of liquid water at constant pressure from 0 °C (32 °F) to 100 °C (212 °F) is approximately 4.2 MJ.

One kilowatt hour of electricity is 3.6 megajoules.

### Gigajoule

The gigajoule (GJ) is equal to one billion (109) joules. 6 GJ is about the chemical energy of combusting 1 barrel (159 l) of crude oil.[13] 2 GJ is about the Planck energy unit.

### Terajoule

The terajoule (TJ) is equal to one trillion (1012) joules; or about 0.278 GWh (which is often used in energy tables). About 63 TJ of energy was released by the atomic bomb that exploded over Hiroshima.[14] The International Space Station, with a mass of approximately 450 megagrams and orbital velocity of 7.7 km/s,[15] has a kinetic energy of roughly 13 TJ. In 2017 Hurricane Irma was estimated to have a peak wind energy of 112 TJ.[16][17]

### Petajoule

The petajoule (PJ) is equal to one quadrillion (1015) joules. 210 PJ is about 50 megatons of TNT. This is the amount of energy released by the Tsar Bomba, the largest man-made explosion ever.

### Exajoule

The exajoule (EJ) is equal to 1018 (one quintillion) joules. The 2011 Tōhoku earthquake and tsunami in Japan had 1.41 EJ of energy according to its rating of 9.0 on the moment magnitude scale. Yearly U.S. energy consumption amounts to roughly 94 EJ.

### Zettajoule

The zettajoule (ZJ) is equal to one sextillion (1021) joules. The human annual global energy consumption is approximately 0.5 ZJ.

### Yottajoule

The yottajoule (YJ) is equal to one septillion (1024) joules. This is approximately the amount of energy required to heat all the water on Earth by 1 °C. The thermal output of the Sun is approximately 400 YJ per second.

## Conversions

1 joule is equal to:

• 1×107 erg (exactly)
• 6.24150974×1018 eV
• 0.2390 cal (gram calories)
• 2.390×10−4 kcal (food calories)
• 9.4782×10−4 BTU
• 0.7376 ft⋅lb (foot-pound)
• 23.7 ft⋅pdl (foot-poundal)
• 2.7778×10−7 kW⋅h (Kilowatt hour)
• 2.7778×10−4 W⋅h (Watt hour)
• 9.8692×10−3 l⋅atm (litre-atmosphere)
• 11.1265×10−15 g (by way of mass-energy equivalence)
• 1×10−44 foe (exactly)

Units defined exactly in terms of the joule include:

• 1 thermochemical calorie = 4.184 J[18]
• 1 International Table calorie = 4.1868 J[19]
• 1 W⋅h = 3600 J (or 3.6 kJ)
• 1 kW⋅h = 3.6×106 J (or 3.6 MJ)
• 1 W⋅s = 1 J
• 1 ton TNT = 4.184 GJ

## Notes and references

1. ^ International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), p. 120, ISBN 92-822-2213-6, archived (PDF) from the original on 2017-08-14
2. ^ American Heritage Dictionary of the English Language, Online Edition (2009). Houghton Mifflin Co., hosted by Yahoo! Education.
3. ^ The American Heritage Dictionary, Second College Edition (1985). Boston: Houghton Mifflin Co., p. 691.
4. ^ McGraw-Hill Dictionary of Physics, Fifth Edition (1997). McGraw-Hill, Inc., p. 224.
5. ^ a b "Units with special names and symbols; units that incorporate special names and symbols". International Bureau of Weights and Measures. Archived from the original on 28 June 2009. Retrieved 18 March 2015. A derived unit can often be expressed in different ways by combining base units with derived units having special names. Joule, for example, may formally be written newton metre, or kilogram metre squared per second squared. This, however, is an algebraic freedom to be governed by common sense physical considerations; in a given situation some forms may be more helpful than others. In practice, with certain quantities, preference is given to the use of certain special unit names, or combinations of unit names, to facilitate the distinction between different quantities having the same dimension.
6. ^ "How much is 100 grams?". HealthAliciousNess. Retrieved 2016-03-11.
7. ^ "Units of Heat – BTU, Calorie and Joule". Engineeringtoolbox.com. Retrieved 2013-09-16.
8. ^ This is called the basal metabolic rate. It corresponds to about 5,000 kJ (1,200 kcal) per day. The kilocalorie (symbol kcal) is also known as the dietary calorie. "At rest" means awake but inactive.
9. ^ Ristinen, Robert A.; Kraushaar, Jack J. (2006). Energy and the Environment (2nd ed.). Hoboken, NJ: John Wiley & Sons. ISBN 0-471-73989-8.
10. ^ "Physics - CERN". public.web.cern.ch.
11. ^ "You Say Calorie, We Say Kilojoule: Who's Right?". Retrieved 2 May 2017.
12. ^ "Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present". Retrieved 2005-10-05.
13. ^ "Energy Units - Energy Explained, Your Guide To Understanding Energy - Energy Information Administration". www.eia.gov.
14. ^ Malik, John (September 1985). "Report LA-8819: The yields of the Hiroshima and Nagasaki nuclear explosions" (PDF). Los Alamos National Laboratory. Archived from the original (PDF) on 11 October 2009. Retrieved 18 March 2015.
15. ^ "International Space Station Final Configuration" (PDF). European Space Agency. Archived from the original (PDF) on 21 July 2011. Retrieved 18 March 2015.
16. ^ Bonnie Berkowitz; Laris Karklis; Reuben Fischer-Baum; Chiqui Esteban (11 September 2017). "Analysis - How big is Hurricane Irma?". Washington Post. Retrieved 2 November 2017.
17. ^ "Irma unleashes its fury on south Florida", Financial Times, accessed 10-Sept-2017 (subscription required)
18. ^ 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.
19. ^ Feynman, Richard (1963). "Physical Units". Feynman's Lectures on Physics. Retrieved 2014-03-07.