Energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...

is defined via

work Work may refer to: * Work (human activity), intentional activity people perform to support themselves, others, or the community ** Manual labour, physical work done by humans ** House work, housework, or homemaking ** Working animal, an animal t ...

, so the SI unit of energy is the same as the unit of work – the

joule The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applie ...

(J), named in honour of

James Prescott Joule James Prescott Joule (; 24 December 1818 11 October 1889) was an English physicist, mathematician and brewer, born in Salford, Lancashire. Joule studied the nature of heat, and discovered its relationship to mechanical work (see energy). ...

and his experiments on the mechanical equivalent of heat. In slightly more fundamental terms, is equal to 1

newton metre The newton-metre (also newton metre or newton meter; symbol N⋅m or N m) is the unit of torque (also called ) in the International System of Units (SI). One newton-metre is equal to the torque resulting from a force of one newton applie ...

and, in terms of SI base units :

1\ \mathrm = 1\ \mathrm \left( \frac \right ) ^ 2 = 1\ \frac

An energy unit that is used in atomic physics,

particle physics Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...

and

high energy physics Particle physics or high energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standa ...

is the

electronvolt In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in vacuum ...

(eV). One eV is equivalent to . In spectroscopy the unit cm⁻¹ ≈ is used to represent energy since energy is inversely proportional to wavelength from the equation

E = h \nu = h c/\lambda

. In discussions of energy production and consumption, the units

barrel of oil equivalent The barrel of oil equivalent (BOE) is a unit of energy based on the approximate energy released by burning one barrel (, or ) of crude oil. The BOE is used by oil and gas companies in their financial statements as a way of combining oil and nat ...

and

ton of oil equivalent The tonne of oil equivalent (toe) is a unit of energy defined as the amount of energy released by burning one tonne of crude oil. It is approximately 42 gigajoules or 11.630 megawatt-hours, although as different crude oils have different calorifi ...

are often used.

British imperial / US customary units

The British imperial units and U.S. customary units for both energy and work include the

foot-pound force The foot-pound force (symbol: ft⋅lbf, ft⋅lbf, or ft⋅lb ) is a unit of work or energy in the engineering and gravitational systems in United States customary and imperial units of measure. It is the energy transferred upon applying a ...

(1.3558 J), the British thermal unit (BTU) which has various values in the region of 1055 J, the

horsepower Horsepower (hp) is a unit of measurement of power, or the rate at which work is done, usually in reference to the output of engines or motors. There are many different standards and types of horsepower. Two common definitions used today are t ...

-hour (2.6845 MJ), and the gasoline gallon equivalent (about 120 MJ).

Electricity

A unit of electrical energy, particularly for utility bills, is the kilowatt-hour (kWh); one kilowatt-hour is equivalent to . Electricity usage is often given in units of kilowatt-hours per year or other time period. This is actually a measurement of average

power Power most often refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power * Power (social and political), the ability to influence people or events ** Abusive power Power may a ...

consumption, meaning the average rate at which energy is transferred. One kilowatt-hour per year is about 0.11 watts.

Natural gas

Natural gas is often sold in units of energy content or by volume. Common units for selling by energy content are

s or therms. One therm is equal to about . Common units for selling by volume are cubic metre or cubic feet. Natural gas in the US is sold in therms or 100 cubic feet (100 ft³ = 1 Ccf). In Australia, natural gas is sold in cubic metres. One cubic metre contains about 38 megajoules. In the most of the world, natural gas is sold in gigajoules.

Food industry

The calorie is defined as the amount of

thermal energy The term "thermal energy" is used loosely in various contexts in physics and engineering. It can refer to several different well-defined physical concepts. These include the internal energy or enthalpy of a body of matter and radiation; heat, de ...

necessary to raise the

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...

of one

gram The gram (originally gramme; SI unit symbol g) is a unit of mass in the International System of Units (SI) equal to one one thousandth of a kilogram. Originally defined as of 1795 as "the absolute weight of a volume of pure water equal to th ...

of water by 1 Celsius degree, from a temperature of , at a

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

of . For thermochemistry a calorie of is used, but other calories have also been defined, such as the International Steam Table calorie of . In many regions,

food energy Food energy is chemical energy that animals (including humans) derive from their food to sustain their metabolism, including their muscular activity. Most animals derive most of their energy from aerobic respiration, namely combining the carbohy ...

is measured in large calories or kilocalories equalling , sometimes written capitalized as . In the European Union, food energy labeling in joules is mandatory, often with calories as supplementary information.

Atom physics and chemistry

In physics and chemistry, it is common to measure energy on the atomic scale in the non-SI, but convenient, units

s (eV). 1 eV is equivalent to the kinetic energy acquired by an electron in passing through a potential difference of 1 volt in a vacuum. It is common to use the SI magnitude prefixes (eg milli- , mega- etc) with electronvolts. Because of the relativistic equivalence between mass and energy, the eV is also sometimes used as a unit of mass. The Hartree (the

atomic unit The Hartree atomic units are a system of natural units of measurement which is especially convenient for atomic physics and computational chemistry calculations. They are named after the physicist Douglas Hartree. By definition, the following ...

of energy) 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 (cm⁻¹) are strictly speaking not energy units but units proportional to energies, with

\ hc\sim 2\cdot 10^\ \mathrm\ \mathrm

being the proportionality constant.

Explosions

A gram of

TNT Trinitrotoluene (), more commonly known as TNT, more specifically 2,4,6-trinitrotoluene, and by its preferred IUPAC name 2-methyl-1,3,5-trinitrobenzene, is a chemical compound with the formula C6H2(NO2)3CH3. TNT is occasionally used as a reagen ...

releases upon explosion. To define the tonne of TNT, this was standardized to giving a value of for the tonne of TNT.

References

{{Energy footer Conversion of units of measurement