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| Because [[energy]] is defined via [[Mechanical work|work]], the [[SI]] unit for '''energy''' is the same as the unit of work – the [[joule]] (J), named in honor of [[James Prescott Joule]] and his experiments on the [[mechanical equivalent of heat]]. In slightly more fundamental terms, 1 joule is equal to 1 [[newton (unit)|newton]]-[[metre]] and, in terms of [[SI base unit]]s
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| :<math>1\ \mathrm{J} = 1\ \mathrm{kg} \left( \frac{\mathrm{m}}{\mathrm{s}} \right ) ^ 2 = 1\ \frac{\mathrm{kg} \cdot \mathrm{m}^2}{\mathrm{s}^2}</math>
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| An energy unit that is used in [[atomic physics]], [[particle physics]] and [[high energy physics]] is the [[electronvolt]] (eV). One eV is equivalent to [[1 E-19 J|1.60217653×10<sup>−19</sup> J]]. In [[spectroscopy]] the unit cm<sup>−1</sup> = 0.000123986 eV is used to represent energy since energy is inversely proportional to wavelength from the equation <math> E = h \nu = h c/\lambda </math>.
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| In discussions of energy production and consumption, the units [[barrel of oil equivalent]] and [[ton of oil equivalent]] are often used.
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| When discussing amounts of energy released in [[explosion]]s or [[bolide]] [[impact event]]s, the [[TNT equivalent]] unit is often used. 1 ton of TNT equivalent is equal to 4.2 × 10<sup>9</sup> joules. Therefore, 1 kt TNT is 4.2 × 10<sup>12</sup> joules, and 1 Mt TNT is 4.2 × 10<sup>15</sup> joules.
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| Note that [[torque]], the "rotational force" or "angular force" which causes a change in rotational motion is typically expressed in newton-metres. This is not a simple coincidence: a torque of 1 newton-metre applied on 1 [[radian]] requires exactly 1 newton-metre = 1 joule of energy.
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| ==Other units of energy==
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| In [[cgs]] units, one [[erg]] is 1 [[gram|g]] [[centimetre|cm]]<sup>2</sup> [[second|s]]<sup>−2</sup>, equal to 1.0×10<sup>−7</sup> J.
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| ===US units===
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| The [[imperial units|imperial]]/[[United States customary units|U.S. units]] for both energy and work include the [[foot-pound force]] (1.3558 J), the [[British thermal unit]] (Btu) which has various values in the region of 1055 J, and the [[horsepower]]-hour (2.6845 MJ).
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| ===Electricity===
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| The energy unit used for everyday [[electricity]], particularly for utility bills, is the [[kilowatt-hour]] (kWh), and one kWh is equivalent to 3.6×10<sup>6</sup> J (3600 kJ or 3.6 MJ). Electricity usage is often given in units of kilowatt-hours per year (kWh/yr). This is actually a measurement of average [[Electric power|power]] consumption, i.e., the average rate at which energy is transferred.
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| ===Natural gas===
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| Natural gas in the US is sold in [[Therm]]s or 100 cubic feet. One Therm is equal to about 105.5 [[Joule|megajoules]]. In the rest of the world, natural gas is sold in [[Joule|gigajoules]].
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| ===Food industry===
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| The [[calorie]] equals the amount of [[thermal energy]] necessary to raise the [[temperature]] of one [[gram]] of [[water]] by 1 [[Celsius]] degree, at a [[pressure]] of 1 [[atmospheric pressure|atm]]. For thermochemistry a calorie of 4.184 J is used, but other calories have also been defined, such as the International Steam Table calorie of 4.1868 J. [[Food energy]] is measured in large calories or kilocalories, often simply written capitalized as "Calories" (= 10<sup>3</sup> calories).
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| ===Atom physics and chemistry===
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| In physics and chemistry, it is still common to measure energy on the atomic scale in the non-SI, but convenient, units [[electronvolt]]s (eV). The [[Hartree]] (the [[atomic unit]] of energy) is commonly used in calculations. Historically [[Rydberg]] units have been used.
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| ===Spectroscopy===
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| In [[spectroscopy]] and related fields it is common to measure energy levels in units of [[Wavenumber|reciprocal centimetres]]. These units (cm<sup>−1</sup>) are strictly speaking not energy units but units proportional to energies, with <math>\ hc\sim 2\cdot 10^{-23}\ J\ cm</math> being the proportionality constant.<ref name=nist_recip_cm>{{cite web|url=http://cccbdb.nist.gov/wavenumber.asp |title=CCCBDB What's a cm-1? |publisher=Cccbdb.nist.gov |date= |accessdate=2013-11-13}}</ref>
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| ===Explosions===
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| A gram of [[trinitrotoluene|TNT]] releases 980–1100 [[calorie]]s upon explosion. To define the '''tonne of TNT''', this was arbitrarily standardized by letting 1000 thermochemical calories = 1 gram TNT = 4184 [[Joule|J]] (exactly).<ref>[http://physics.nist.gov/Pubs/SP811/appenB8.html NIST Guide for the Use of the International System of Units (SI): Appendix B8—Factors for Units Listed Alphabetically]</ref>
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| == See also ==
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| {{Portal|Energy}}
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| * [[Energy consumption]]
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| * [[Conversion of units of temperature]]
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| * [[Conversion of units#Energy, work, or amount of heat]]
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| * [[Orders of magnitude (energy)]]
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| == References==
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| {{reflist}}
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| {{Footer energy}}
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| [[Category:Units of energy| ]]
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| [[Category:Conversion of units of measurement]]
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