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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 the force of one newton acting to move an object through a distance of one metre. In terms of dimensions:

:\rm 1\ J = 1\ N \cdot m = \left ( \frac{kg \cdot m}{s^2} \right ) \cdot m = \frac{kg \cdot m^2}{s^2}=Pa \cdot m^3= 1\ W \cdot s


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:

Usage

Confusion with newton metre

While it is dimensionally correct to express joules as newton metres or N·m, such use is discouraged 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.

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 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).


Multiples

For additional examples, see: Orders of magnitude

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.

Microjoule

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

Millijoule

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

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.

Megajoule

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

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.

Terajoule

The terajoule (TJ) is equal to one trillion joules. About 60 terajoules were released by the bomb that exploded over Hiroshimamarker.

Conversions

1 joule is equal to:

Units defined exactly 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 = (or 3.6 MJ)
  • 1 ton TNT = 4.184 GJ


See also



References

External links




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