The joule is a derived unit of energy in the International System of Units. It is equal to the energy transferred to (or work done on) an object when a force of one newton acts on that object in the direction of the force's motion through a distance of one metre (1 newton metre or N⋅m). It is also the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second. It is named after the English physicist James Prescott Joule (1818–1889).
One joule can also be defined as the following:
- The work required to move an electric charge of one coulomb through an electrical potential difference of one volt, or one coulomb-volt (C⋅V). This relationship can be used to define the volt.
- The work required to produce one watt of power for one second, or one watt-second (W⋅s) (compare kilowatt-hour – 3.6 megajoules). This relationship can be used to define the watt.
The joule is named after James Prescott Joule. As with every SI unit named for a person, its symbol starts with an upper case letter (J), but when written in full it follows the rules for capitalisation of a common noun; i.e., "joule" becomes capitalised at the beginning of a sentence and in titles, but is otherwise in lower case.
One joule represents (approximately)
- The kinetic energy of a 2 kg mass traveling at 1 m/s
- The energy required to lift a medium-sized tomato up 1 metre (3 ft 3 in) (assume the tomato has a mass of approximately 100 grams (3.5 oz)).
- The energy required to accelerate a 1 kg mass at 1 m⋅s−2 through a distance of 1 m.
- The heat required to raise the temperature of 1 g of water by 0.24 °C.
- The typical energy released as heat by a person at rest every 1/60 s (approximately 17 ms).
- The kinetic energy of a 50 kg human moving very slowly (0.2 m/s or 0.72 km/h).
- The kinetic energy of a 56 g tennis ball moving at 6 m/s (22 km/h).
- The amount of electricity required to light a 1 W LED for 1 s.
A kilowatt-hour is 3.6 megajoules.
Multiples of joule
|Value||SI symbol||Name||Value||SI symbol||Name|
|10−1 J||dJ||decijoule||101 J||daJ||decajoule|
|10−2 J||cJ||centijoule||102 J||hJ||hectojoule|
|10−3 J||mJ||millijoule||103 J||kJ||kilojoule|
|10−6 J||µJ||microjoule||106 J||MJ||megajoule|
|10−9 J||nJ||nanojoule||109 J||GJ||gigajoule|
|10−12 J||pJ||picojoule||1012 J||TJ||terajoule|
|10−15 J||fJ||femtojoule||1015 J||PJ||petajoule|
|10−18 J||aJ||attojoule||1018 J||EJ||exajoule|
|10−21 J||zJ||zeptojoule||1021 J||ZJ||zettajoule|
|10−24 J||yJ||yoctojoule||1024 J||YJ||yottajoule|
|Common multiples are in bold face|
1×107 erg (exactly)
0.2390 cal (gram calories)
2.390×10-4 kcal (food calories)
0.7376 ft⋅lb (foot-pound)
23.7 ft⋅pdl (foot-poundal)
2.7778×10-7 kW⋅h (kilowatt-hour)
2.7778×10−4 W⋅h (watt-hour)
9.8692×10−3 l⋅atm (litre-atmosphere)
11.1265×10−15 g (by way of mass-energy equivalence)
1×10−44 foe (exactly)
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