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This is the definition declared in the modern International System of Units in 1960. [13] The definition of the joule as J = kg⋅m 2 ⋅s −2 has remained unchanged since 1946, but the joule as a derived unit has inherited changes in the definitions of the second (in 1960 and 1967), the metre (in 1983) and the kilogram . [14]
Energy is defined via work, so the SI unit of energy is the same as the unit of work – the joule (J), named in honour of James Prescott Joule [1] and his experiments on the mechanical equivalent of heat. In slightly more fundamental terms, 1 joule is equal to 1 newton metre and, in terms of SI base units
Thus, one joule is one watt-second, and 3600 joules equal one watt-hour. The CGS energy unit is the erg and the imperial and US customary unit is the foot pound . Other energy units such as the electronvolt , food calorie or thermodynamic kcal (based on the temperature change of water in a heating process), and BTU are used in specific areas of ...
The SI unit for specific energy is the joule per kilogram (J/kg). Other units still in use worldwide in some contexts are the kilocalorie per gram (Cal/g or kcal/g), mostly in food-related topics, and watt-hours per kilogram (W⋅h/kg) in the field of batteries.
However, good approximations can be made for gases in many states using simpler methods outlined below. For many solids composed of relatively heavy atoms (atomic number > iron), at non-cryogenic temperatures, the heat capacity at room temperature approaches 3R = 24.94 joules per kelvin per mole of atoms (Dulong–Petit law, R is the gas constant).
joule per square metre per hertz J⋅m −2 ⋅Hz −1: M⋅T −1: Radiant exposure of a surface per unit frequency or wavelength. The latter is commonly measured in J⋅m −2 ⋅nm −1. This is sometimes also called "spectral fluence". H e,λ [nb 4] joule per square metre, per metre J/m 3: M⋅L −1 ⋅T −2: See also:
The erg, a unit of energy equal to 10 −7 joules (100 nJ), is not an SI unit. (A joule is an SI unit and one joule equals 10 million ergs.) The erg originated in the Centimetre–gram–second system of units (CGS) and is symbolized as erg. Its name is derived from ergon (ἔργον), a Greek word meaning 'work' or 'task'. [1]
In 1845, Joule published a paper entitled The Mechanical Equivalent of Heat, in which he specified a numerical value for the amount of mechanical work required to "produce a unit of heat", based on heat production by friction in the passage of electricity through a resistor and in the rotation of a paddle in a vat of water. [35]