In physics, energy (from the Greek – energeia, "activity, operation", – energos, "active, working"[1]) is a scalar physical quantity that describes the amount of work that can be performed by a force. Energy is an attribute of objects and systems that is subject to a conservation law. Several different forms of energy exist to explain all known natural phenomena. These forms include (but are not limited to) kinetic, potential, thermal, gravitational, sound, light, elastic, and electromagnetic energy. The forms of energy are often named after a related force.

Any form of energy can be transformed into another form, but the total energy always remains the same. This principle, the conservation of energy, was first postulated in the early 19th century, and applies to any isolated system. According to Noether's theorem, the conservation of energy is a consequence of the fact that the laws of physics do not change over time.[2]

Although the total energy of a system does not change with time, its value may depend on the frame of reference. For example, a seated passenger in a moving airplane has zero kinetic energy relative to the airplane, but non-zero kinetic energy relative to the Earth.

The following outline is provided as an overview of and topical guide to energy:

## Essence of energy

Main article: Energy
• Mass–energy equivalence (E=mc2)

## Forms of energy

• Potential energy
• Gravitational potential energy
• Elastic potential energy
• Kinetic energy
• Thermal energy
• Electric energy
• Magnetic energy
• Electromagnetic fields
• Chemical energy
• Nuclear energy
• Surface energy

## Energy units (terms)

See Units of energy

• Barrel of oil equivalent
• British thermal unit
• Calorie
• Current solar income – the amount of solar energy that falls as sunlight
• Electronvolt – (symbol: eV) is the amount of energy gained by a single unbound electron when it falls through an electrostatic potential difference of one volt
• Planck energy, 1.22 × 1019 GeV (billion electron volts)
• Enthalpy
• Erg – (symbol "erg") unit of energy and mechanical work in the centimetre-gram-second (CGS) system of units
• EU energy label
• Fill factor – defined as the ratio of the maximum power (Vmp x Jmp) divided by the short-circuit current (Isc) and open-circuit voltage (Voc) in light current density – voltage (J-V) characteristics of solar cells.
• Foot-pound – (symbol ft·lbf or ft·lbf) is an Imperial and U.S. customary unit of mechanical work, or energy, although in scientific fields one commonly uses the equivalent metric unit of the joule (J). There are approximately 1.356 J/(ft·lbf).
• Gigaton – Metric Unit of mass, equal to 1,000,000,000 (1 billion) metric tons, 1,000,000,000,000 (1 trillion) kilograms
• Any of various units of energy, such as gigatons of TNT equivalent, gigatons of coal equivalent, gigatons petroleum equivalent.
• Gray (unit) – (symbol: Gy), is the SI unit of energy for the absorbed dose of radiation. One gray is the absorption of one joule of radiation energy by one kilogram of matter. One gray equals 100 rad, an older unit.
• Heat
• Joule – (symbol J, also called newton meter, watt second, or coulomb volt)
• Kilowatt-hour – (symbol: kW·h) corresponds to one kilowatt (kW) of power being used over a period of one hour.
• Mass-energy equivalence – where mass has an energy equivalence, and energy has a mass equivalence
• Megawatt
• Net energy gain
• Power factor – of an AC electric power system is defined as the ratio of the real power to the apparent power.
• Therm – (symbol thm) a non-SI unit of heat energy. It is approximately the heat equivalent of burning 100 cubic feet of natural gas. In the US gas industry it is defined as exactly 100,000 BTU59°F or 105.4804 megajoules.
• Ton of oil equivalent
• TPE – Ton Petroleum Equivalent, 45.217 GJ, see ton of oil equivalent

## History of energy

Main article: History of energy

## Energy infrastructure

See especially Category:Electric power and Category:Fuels for a large number of conventional energy related topics.

## Physics of energy

• Energy
• Activation energy explains the differences in the speeds of various chemical reactions
• Alternative Energy Index
• Bioenergetics
• Chemical energetics
• Energy in physical cosmology
• Energy in Earth science that is responsible for the macroscopic transformations on the planet Earth
• Electricity
• Exergy
• Green energy
• Orders of magnitude (energy) – list describes various energy levels between 10−31 joules and 1070 joules
• Thermodynamics
• Perpetual motion
• Heat
• History of energy
• Energy forms, the forms in which energy can be defined
• Energy transformation, relating to energy's changes from one form to another.
• Energy (signal processing), the inner product of a signal in the time domain
• Energy density spectrum, relating to the distribution of signal energy over frequencies.
• Potential energy, the form of energy that is due to position of an object
• Kinetic energy, the form of energy as a consequence of the motion of an object or its constituents
• Mechanical energy, the potential energy and kinetic energy present in the components of a mechanical system.
• Binding energy, a concept explaining how the constituents of atoms or molecules are bound together
• Bond energy, a measure of the strength of a chemical bond
• Blue energy, energy retrieved from the difference in the salt concentration between seawater and river water
• Nuclear energy, energy that is the consequence of decomposition or combination of atomic nuclei
• Gibbs free energy, a related concept in chemical thermodynamics that incorporates entropy considerations too
• Helmholtz free energy, a thermodynamic potential that measures the "useful" work obtainable from a closed thermodynamic system at a constant temperature, useful for studying explosive chemical reactions
• Elastic energy, which causes or is released by the elastic distortion of a solid or a fluid
• Ionization energy – the (IE) of an atom is the energy required to strip it of an electron.
• Interaction energy, the contribution to the total energy that is a result of interaction between the objects being considered
• Internal energy – (abbreviated E or U) the total kinetic energy due to the motion of molecules (translational, rotational, vibrational) and the total potential energy associated with the vibrational and electric energy of atoms within molecules.
• Negative energy
• Energy conversion – process of converting energy from one form to another
• Dark energy, used to explain some cosmological phenomena
• Energy quality, empirical experience of the characteristics of different energy forms as they flow and transform
• Energy density, amount of energy stored in a given system or region of space per unit volume, or per unit mass
• Energy flow, flow of energy in an ecosystem through food chains
• Energetics, the scientific study of energy flows under transformation
• Stress-energy tensor, the density and flux of energy and momentum in space-time; the source of the gravitational field in general relativity
• Food energy, energy in food that is available
• Primary energy – Energy contained in raw fuels and any other forms of energy received by a system as input to the system.
• Radiant energy – energy that is transported by waves
• Rotational energy – An object's rotational energy or angular kinetic energy is part of its total kinetic energy
• Solar radiation – radiant energy emitted by the sun, particularly electromagnetic energy

### Allegorical, esoteric, and pseudoscientific

• Energy (esotericism), invoked by spiritualists for alternative modes of healing the human body as well as a spirit that permeates all of reality.
• Orgone, Wilhelm Reich discovered this energy and tried to use it to cure various physical ailments and control the weather.
• Qi a concept from Oriental medicine that is sometimes translated as "energy" in the West.
• Vitalism, often referred to as "energy"
• Cold fusion, nuclear fusion at conditions close to room temperature.
• Bubble fusion, also known as Sonofusion, energy from acoustic collapse of bubbles.
• Water-fuelled car, powering a car using water as fuel.

## Energy industry

• World energy resources and consumption
• List of energy resources, substances like fuels, petroleum products and electricity
• Energy crisis, the need to conserve energy resources
• Energy development, development of energy resources
• Embodied energy, the sum total of energy expended to deliver a good or service as it travels through the economy
• Energy conservation, tips for conserving energy resources
• Energy economics, as the foundation of other relationships
• Energy policy, government policies and plans for energy supply
• Energy storage, methods commonly used to store energy resources for later use
• Biosphere
• Ecological energetics
• Ecology
• Energy balance
• Earth Day
• U4energy, a pan European school challenge on energy education launched in September 2010. U4energy is an initiative funded under the IEE programme to improve energy consumption in schools and their local communities.
• Energy development – Ongoing effort to provide abundant and accessible energy, through knowledge, skills and constructions.
• Energy speculation
• Free energy suppression
• Future energy development – Provides a general overview of future energy development.
• History of perpetual motion machines
• Hubbert peak theory, also known as peak oil – the theory that world oil production will peak (or has peaked), and will then rapidly decline, with a corresponding rapid increase in prices.
• Primary production
• Power harvesting
• Renewable energy development

## Energy politics

### Energy Issues

• 2000 Watt society
• Environmental concerns with electricity generation
• Fuel poverty
• Greasestock, American showcase of vehicles and technologies powered by alternative energy
• Low-carbon economy
• Peak Oil
• Soft energy path – an energy use and development strategy delineated and promoted by some energy experts and activists
• Strategic Petroleum Reserve

### Energy Policies and Use – National and International

#### International

• Energy policy – Introductory article
• Energy and Environmental Security Initiative (EESI)

#### Regional and national

• Energy law – overview of many energy laws from various countries and states
• New York energy law
• Energy Tax Act – United States energy-related legislation. See also : Category:United States federal energy legislation
• United Kingdom:
• Energy policy of the United Kingdom
• Energy use and conservation in the United Kingdom

## Energy economics

### Energy companies

• Exxon Mobil
• Enercon GmbH – Company based in Germany that operates in the wind turbine industry. One of the biggest producers in the world.
• Saudi Aramco
• Sasol
• United States Enrichment Corporation – contracts with the United States Department of Energy to produce enriched uranium.

### Non-profit organizations

• Musicians United for Safe Energy

### Industry associations

• OPEC – Organization of Petroleum-exporting Countries
• IEA – International Energy Agency
• CAPP – Canadian Association of Petroleum Producers
• World LP Gas Association – WLPGA

## Energy technology inventors

• Alessandro Volta
• Charles Kettering
• Farrington Daniels – solar energy
• Georges Leclanché – battery
• John Frederic Daniell – Daniell cell
• Rudolf Diesel – compression ignition internal combustion engine
• Georges Imbert – wood gas
• Leonardo da Vinci
• Moritz von Jacobi
• Nikolaus Otto – internal combustion engine
• Robert Stirling – Stirling engine (external combustion)
• Nikola Tesla
• James Watt – steam engine with separate condensor

## References

1. Harper, Douglas. "Energy". Online Etymology Dictionary. Retrieved May 1, 2007.
2. Lofts, G; O'Keeffe D; et al (2004). "11 — Mechanical Interactions". Jacaranda Physics 1 (2 ed.). Milton, Queensland, Australia: John Willey & Sons Australia Ltd.. pp. 286. ISBN 0 7016 3777 3.
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