The joule (symbol J), named for James Prescott Joule, 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:

[edit] 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.

[edit] Usage

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.

[edit] Confusion with newton metre

While it is dimensionally correct to express joules as newton metres or N·m, such use is discouraged^[1] by the SI authority to avoid confusion with torque. Torque and energy are fundamentally different physical quantities. For example, adding 1 N·m of torque to 1 N·m of energy gives a dimensionally consistent result of 2 N·m, but this quantity is physically meaningless.

[edit] 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.
• the kinetic energy of an adult human moving at a speed of about a handspan every second.
• the kinetic energy of a tennis ball moving at 23 km/h (14 mph).^[2]

[edit] Multiples

For additional examples, see: Orders of magnitude (energy)

[edit] Nanojoule

The nanojoule (nJ) is equal to one billionth of one joule. One nanojoule is about 1/160th of the kinetic energy of a flying mosquito.^[3]

[edit] Microjoule

The microjoule (μJ) is equal to one millionth of one joule. The Large Hadron Collider (LHC) is expected to produce collisions on the order of 1 microjoule (7 Tev) per particle.

[edit] Millijoule

The millijoule (mJ) is equal to one thousandth of one joule.

[edit] Kilojoule

The kilojoule (kJ) is equal to one thousand joules. Food labels in some countries express food energy in kilojoules. One kilojoule is about the amount of solar radiation received by one square metre of the Earth in one second.^[4]

[edit] Megajoule

The megajoule (MJ) is equal to one million joules, or approximately the kinetic energy of a one tonne vehicle moving at 160 km/h (100 mph).

[edit] Gigajoule

The gigajoule (GJ) is equal to one billion joules. 6 gigajoules is about the amount of chemical energy in a standard barrel of oil.^[5]

[edit] Terajoule

The terajoule (TJ) is equal to one trillion joules. About 60 terajoules were released by the bomb that exploded over Hiroshima.^[6]

[edit] Conversions

1 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 liter-atmosphere
• 1×10⁻⁴⁴ Foe (exactly)

Units defined exactly in terms of the joule include:

• 1 thermochemical calorie = 4.184 J^[7]
• 1 International Table calorie = 4.1868 J^[7]
• 1 watt hour = 3600 J
• 1 kilowatt hour = 3.6×10⁶ J (or 3.6 MJ)
• 1 ton TNT = 4.184 GJ

[edit] See also

• Conversion of units
• Orders of magnitude (energy)
• Fluence

[edit] References

[edit] External links

• Unit conversion from joule