joule

joule
Name	joule
Namedafter	James Prescott Joule
Units1	SI base units
Inunits1	kg⋅m2⋅s−2
Units2	CGS units
Inunits2	1 × 107 erg
Units3	British Gravitational System
Inunits3	≈ 0.737562 ft⋅lbf

joule. The joule is the derived unit of energy, work, and quantity of heat in the International System of Units. It is defined as the work done, or energy transferred, when a force of one newton acts through a displacement of one metre in the direction of the force. This fundamental unit is named in honor of the English physicist James Prescott Joule for his pioneering work in establishing the mechanical equivalent of heat.

Definition and equivalence

The joule is defined in terms of SI base units, specifically the kilogram, metre, and second. One joule equals one kilogram metre squared per second squared (1 J = 1 kg⋅m²⋅s⁻²). It is dimensionally equivalent to the energy required to exert a one-newton force over a one-metre distance. In terms of electrical units within the SI, one joule also equals one watt-second and is the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second. This equivalence bridges classical mechanics and electromagnetism, as formalized by the work of scientists like James Clerk Maxwell. The joule also has precise relationships in other unit systems, such as the CGS unit the erg, where one joule equals ten million ergs.

History

The unit was proposed in 1882 by the British Association for the Advancement of Science and officially adopted by the Second International Congress of Electricians in 1889. It was named for James Prescott Joule, whose famous experiments in the 1840s, such as using a paddle wheel to heat water, quantitatively demonstrated the relationship between mechanical work and heat. This work was crucial in formulating the first law of thermodynamics and overthrowing the caloric theory. The joule was later incorporated into the International System of Units upon its establishment in 1960. The General Conference on Weights and Measures has since refined its definition in line with fundamental constants, most recently as part of the 2019 SI base unit redefinition.

Practical examples

In everyday and scientific contexts, the joule represents modest amounts of energy. The kinetic energy of a two-kilogram mass moving at one metre per second is approximately one joule. The energy required to lift a small apple one metre against Earth's gravity is also about one joule. In dietary terms, a single food calorie (kilocalorie) is equivalent to about 4184 joules. A typical lightning bolt can release energy on the order of a billion joules. In particle physics, the energy of subatomic particles is often measured in electronvolts, with one electronvolt equaling approximately 1.602 × 10⁻¹⁹ joules. The Hiroshima atomic bomb released an estimated 63 terajoules of energy.

Multiples and submultiples

Common SI prefixes are used to denote multiples and submultiples of the joule for measurements across vast scales. The kilojoule (kJ, 10³ J) is frequently used in food energy and chemistry. The megajoule (MJ, 10⁶ J) is common in engineering and electricity billing, while the gigajoule (GJ, 10⁹ J) is used for larger industrial energy transactions. The terajoule (TJ, 10¹² J) and petajoule (PJ, 10¹⁵ J) describe national energy consumption, as tracked by organizations like the International Energy Agency. At the subatomic scale, the attojoule (aJ, 10⁻¹⁸ J) is a relevant unit. The zeptojoule (zJ, 10⁻²¹ J) approaches the magnitude of the Boltzmann constant times room temperature.

Confusion with newton-metre

Although dimensionally equivalent (both being kg⋅m²⋅s⁻²), the joule and the newton-metre are not interchangeable in all contexts to avoid confusion. The newton-metre is the unit for torque, a vector quantity representing a rotational force, while the joule is a scalar unit for energy or work. Using the joule for torque can obscure the fundamental physical difference between rotational mechanics and translational energy transfer. This distinction is maintained by international standards set by bodies like the International Bureau of Weights and Measures and is critical in fields such as mechanical engineering and structural analysis.

See also

* International System of Units * James Prescott Joule * Watt * Newton (unit) * Calorie * Electronvolt * British Thermal Unit

Category:Units of energy Category:SI derived units