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
Collider (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 Hiroshima.
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