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Joule

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Wikipedia

The joule (written in lower case, 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 metre 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:

, 1, mathrm{J}=1, mathrm{N} cdot mathrm{m}

, 1, mathrm{J}=1, mathrm{kg} cdot frac{mathrm{m}^{2}}{mathrm{s}^{2}}

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
The kinetic energy of a 2 kg mass moving at a velocity of 1 m/sec. The energy is linear in the mass but quadratic in the velocity, being given by E = ½mv².
The potential energy of a 1 kg mass at an elevation of 1 m above a reference point in a gravitational field of 1 m/s². Earth's gravity being 9.81 m/s² at sea level, 1 kg at 1 m above the Earth's surface has a potential energy of 9.8 joules relative to that surface. When dropped the potential energy gradually becomes kinetic energy, with the conversion being complete at the instant the mass passes the reference point. Whereas kinetic energy is relative to an inertial frame, in this case that of the earth, potential energy is relative to a position, in this case the Earth's surface.

Conversions

1 joule is exactly 10⁷ ergs.

1 joule is approximately equal to:

6.2415 eV (electronvolts)
0.2390 cal (calorie) (small calories, lower case c)
2.3901 kilocalorie, Calories (food energy, upper case C)
9.4782 BTU (British thermal unit)
0.7376 ft·lbf (foot-pound force)
23.7 ft·pdl (foot poundals)
2.7778 kilowatt hour
2.7778 watt hour
9.8692 litre-atmosphere

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 J (or 3.6 MJ)
1 ton TNT = 4.184 GJ

Useful to remember:

1 joule = 1 newton metre = 1 watt second

Practical examples

One joule in everyday life is approximately:

the energy required to lift a small apple one metre 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 receive by drinking a single drop of malt whiskey.
the kinetic energy of an adult human moving a distance of about a handspan every second.

SI multiples

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.