(commonly pronounced "jew'l") is the SI unit
in all forms, and it is one of the derived
SI units. It is also the unit for quantity of heat
, where it replaces the calorie
whenever possible. The symbol for joule is J
and the unit is defined as
the work done when the point of application of 1 MKS (meter, kilogram, second) unit of force (newton) moves a distance of one metre in the direction of the force
It is worth pointing out that 1 J = 1 Nm - which follows directly by the above definition - and the educated reader will immediately notice that newton meter (Nm
) is the SI unit for torque
. Torque and energy are however two different physical phenomena, which is why joule should be used only for energy, and newton meter should only be used for torque.
Furthermore, since the unit newton also is a derived SI unit, we can write
1 J = 1 Nm = 1 m2 kg/s2
This makes possible another definition of the unit expressed only in the base SI units. Keeping in mind that kinetic energy is defined as
Ek = m v2/2
we now can define the energy unit joule directly as
the kinetic energy of two kilograms moving at a velocity of one meter per second
which is pretty much like a small cat walking at a steady pace.
The unit is named in honor of James Prescott Joule (pronounced "jowl"), a British physicist who in addition to formulating Joule's law also helped Robert Mayer and Hermann von Helmholtz in developing the principle of energy as being indestructible.