Coordinates | 41°52′55″N87°37′40″N |
---|---|
Name | Joule |
Standard | SI derived unit |
Quantity | Energy |
Symbol | J |
Namedafter | James Prescott Joule |
Units1 | SI base units |
Inunits1 | 1 kg·m2/s2 |
Units2 | CGS units |
Inunits2 | erg |
Units3 | kilowatt hours |
Inunits3 | |
Units3 | kilocalories |
Inunits3 |
In terms firstly of base SI units and then in terms of other SI units: :
where N is the newton, m is the metre, kg is the kilogram, s is the second, Pa is the pascal, and W is the watt.
One joule can also be defined as:
::
:where E is the energy, τ is magnitude of the torque, and θ is the angle moved (in radians). Since radians are dimensionless, it follows that torque and energy have the same dimensions.
The use of newton-metres for torque and joules for energy is useful in helping avoid misunderstandings and miscommunications.
Torque and energy have the same dimension because both torque and work (a type of energy) can be calculated by multiplying a force by a distance. However, the details are quite different in the two cases. For torque, the multiplication of force and distance is a vector cross product, while for work it is a dot product. Also, for torque, the distance involved is the length of the lever arm, while for energy it is the distance traveled by the object undergoing the force.
One kilojoule per second (1000 watts) is approximately the amount of solar radiation received by one square metre of the Earth in full daylight.
Because 1 watt times one second equals one joule, 1 kilowatt-hour is 1000 watts times 3600 seconds, or 3.6 megajoules.
Units defined exactly in terms of the joule include: 1 thermochemical calorie = 4.184 J 1 International Table calorie = 4.1868 J
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