Because energy is defined via the SI unit for energy is the same as the unit of work
– the joule, 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-metre
and, in terms of SI base units
An energy unit that is used in atomic physics, particle physics and high energy physics is
the electronvolt. One eV is equivalent to 1.60217653×10−19 J. In spectroscopy the
unit cm−1 = 0.000123986 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 equivalent and ton of oil equivalent are often used.
When discussing amounts of energy released in explosions or bolide impact events, the
TNT equivalent unit 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.
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.
Other units of energy In cgs units, one erg is 1 g cm2 s−2,
equal to 1.0×10−7 J. US units
The imperial/U.S. units for both energy and work include the foot-pound force, the British
thermal unit which has various values in the region of 1055 J, and the horsepower-hour.
Electricity The energy unit used for everyday electricity,
particularly for utility bills, is the kilowatt-hour; one kWh is equivalent to 3.6×106 J . Electricity
usage is often given in units of kilowatt-hours per year. This is actually a measurement of
average power consumption, i.e., the average rate at which energy is transferred.
Natural gas Natural gas in the US is sold in Therms or
100 cubic feet. One Therm is equal to about 105.5 megajoules. In the rest of the world,
natural gas is sold in gigajoules. Food industry
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 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". Atom physics and chemistry
In physics and chemistry, it is still common to measure energy on the atomic scale in the
non-SI, but convenient, units electronvolts. The Hartree is commonly used in calculations.
Historically Rydberg units have been used. Spectroscopy
In spectroscopy and related fields it is common to measure energy levels in units of reciprocal
centimetres. These units are strictly speaking not energy units but units proportional to
energies, with being the proportionality constant. Explosions
A gram of TNT releases 980–1100 calories upon explosion. To define the tonne of TNT,
this was arbitrarily standardized by letting 1000 thermochemical calories = 1 gram TNT
= 4184 J. See also
Energy consumption Conversion of units of temperature
Conversion of units#Energy, work, or amount of heat
Orders of magnitude References