Joule

The joule (symbol: J) is the SI unit of energy, or work. It is named in honour of the physicist James Prescott Joule (1818–1889).

Definition

The joule is a derived unit defined as the work done, or energy required, to exert a force of one newton for a distance of one metre, so the same quantity may be referred to as a newton metre or newton-metre (also with meter spelling), with the symbol N·m or N m. It can also be written as kg·m²·s⁻². However, the newton metre is usually used as a measure of torque, not energy.

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

Conversions

1 joule is exactly 10⁷ erg.

1 joule is approximately equal to:

• 6.241506363 × 10¹⁸ eV (electron-volts)
• 0.239 cal (calorie) (small calories)
• 2.390 × 10⁻⁴ Calorie or kilocalorie (food)
• 9.48 × 10⁻⁴ BTU (British thermal unit)
• 0.738 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
• the energy required to lift a small apple (102 g) one metre against Earth's gravity

Units defined in terms of the joule include:

• 1 thermochemical calorie = 4.184 J (exact)
• 1 International Table calorie = 4.1868 J (exact)
• 1 watt-hour = 3600 J (exact)

See also

• conversion of units
• SI prefixes
• orders of magnitude
• orders of magnitude (energy)
• electronvolt
• watt-hour