- Units of energy
energyis defined via work, the SIunit for energy is the same as the unit of work – the joule(J), named in honour of James Prescott Jouleand his experiments on the mechanical equivalent of heat. In slightly more fundamental terms, 1 joule is equal to 1 newton- metreand, in terms of SI base units:
An energy unit that is used in
atomic physics, particle physicsand high energy physicsis the electronvolt(eV). One eV is equivalent to 1.60217653×10−19 J. In spectroscopythe unit cm-1 = 0.0001239 eV is used to represent energy since energy is inversely proportional to wavelength from the equation .
In discussions of energy production and consumption, the units
barrel of oil equivalentand ton of oil equivalentare often used.
When discussing amounts of energy released in
explosions or bolide impact events, the TNT equivalentunit is often used. 1 ton of TNT equivalent is equal to 4.2 × 109 joules. Therefore, 1 kT TNT is 4.2 × 1012 joules, and 1 MT TNT is 4.2 × 1015 joules.
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.
Other units of energy
The imperial/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).
The energy unit used for everyday
electricity, particularly for utility bills, is the kilowatt-hour(kWh), and one kWh is equivalent to 3.6×106 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 power consumption, i.e., the average rate at which energy is transferred.
calorieequals the amount of thermal energynecessary to raise the temperatureof one gramof waterby 1 Celsiusdegree, at a pressureof 1 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 energyis measured in large calories or kilocalories, often simply written capitalized as "Calories" (= 103 calories).
In physics and chemistry, it is still common to measure energy on the atomic scale in the non-SI, but convenient, units
electronvolts (eV). The Hartree(the atomic unitof energy) is commonly used in calculations. Historically Rydbergunits have been used.
spectroscopyand related fields it is common to measure energy levels in units of reciprocal centimetres. These units (cm-1) are strictly speaking not energy units but units proportional to energies, with hc being the proportionality constant.
Conversion of units
For conversion of units of energy, see
Conversion of units#Energy, work, or heat.
* [http://www.pneumofore.com/support/tools Unit Converter Tool also for Energy]
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