The joule (pronounced /ˈdʒuːl/ or /ˈdʒaʊl/) (symbol: J) is the SI unit of energy measuring heat, electricity and mechanical work. It was named after English physicist James Prescott Joule.

Description

One joule is the work done, or energy expended, by a force of one newton moving one meter along the direction of the force. This quantity is also denoted as a newton metre with the symbol N·m. Note that torque also has the same units as work, but the quantities are not the same. In elementary units:

One joule is also:

• The work required to move an electric charge of one coulomb through an electrical potential difference of one volt; or one coulomb volt, with the symbol C·V.
• The work done to produce power of one watt continuously for one second; or one watt second (compare kilowatt hour), with the symbol W·s. Thus a kilowatt hour is 3,600,000 joules or 3.6 megajoules

History

A joule is the mechanical equivalent of heat meaning the number of units of work in which the unit of heat can perform. Its value was found by James Prescott Joule in experiments that showed the mechanical energy Joule's equivalent, and represented by the symbol J. The term was first introduced by Dr. Mayer of Heilbronn.

Conversions

Main article: Conversion of units#Energy, work, or heat

1 joule is exactly 10⁷ ergs.

1 joule is approximately equal to:

• 6.24150636309 ×10¹⁸ eV (electronvolts)
• 0.238845896628 cal (calorie) (small calories, lower case c)
• 2.390 ×10⁻⁴ kilocalorie, Calories (food energy, upper case C)
• 9.47817120313 ×10⁻⁴ BTU (British thermal unit)
• 0.737562149277 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 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 amount of energy, as heat, that a quiet person generates 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 get by drinking a single drop of beer.
• the kinetic energy of an adult human moving a distance of about a handspan every second.

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 metre = 1 watt second

SI multiples

See also

• Conversion of units
• SI prefixes
• Orders of magnitude
• Orders of magnitude (energy)
• Electronvolt
• Kilowatt hour
• Fluence
• Foe

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

• Unit conversion from joule
• Online Joule Converter
• Joule in E=mc²