Joule

Joule, James Prescott, 1818-89, English physicist. His scientific researches began in his youth when he invented an electromagnetic engine. Joule made valuable contributions to the fields of heat, electricity, and thermodynamics. His work established the mechanical theory of heat, and he was the first to determine the relationship between heat energy and mechanical energy (the mechanical equivalent of heat). Joule discovered the first law of thermodynamics, which is a form of the law of conservation of energy (see conservation laws). He was one of the great experimental scientists of the 19th cent. The mechanical unit of work is named for him.

joule, abbr. J, unit of work or energy in the mks system of units, which is based on the metric system; it is the work done or energy expended by a force of 1 newton acting through a distance of 1 meter. The joule is named for James P. Joule.

(born Dec. 24, 1818, Salford, Lancashire, Eng.—died Oct. 11, 1889, Sale, Cheshire) English physicist. After studying under John Dalton, in 1840 he described “Joule's law,” which stated that the heat produced in a wire by an electric current is proportional to the product of the resistance of the wire and the square of the current. In 1843 he published his value for the amount of work required to produce a unit of heat, called the mechanical equivalent of heat, and established that heat is a form of energy. He established that the various forms of energy are basically the same and can be changed from one into another, a discovery that formed the basis of the law of conservation of energy, the first law of thermodynamics. In his honour, the value of the mechanical equivalent of heat is usually represented by the letter math.J, and a standard unit of work is called the joule.

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(born Dec. 24, 1818, Salford, Lancashire, Eng.—died Oct. 11, 1889, Sale, Cheshire) English physicist. After studying under John Dalton, in 1840 he described “Joule's law,” which stated that the heat produced in a wire by an electric current is proportional to the product of the resistance of the wire and the square of the current. In 1843 he published his value for the amount of work required to produce a unit of heat, called the mechanical equivalent of heat, and established that heat is a form of energy. He established that the various forms of energy are basically the same and can be changed from one into another, a discovery that formed the basis of the law of conservation of energy, the first law of thermodynamics. In his honour, the value of the mechanical equivalent of heat is usually represented by the letter math.J, and a standard unit of work is called the joule.

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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 SDJEJ IS THE COOLEST GUT ALIVE _l_* 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 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 receive by drinking a drop of beer.
• the kinetic energy of an adult human moving a distance of about a handspan every 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