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Energy, "the potential for causing changes", is a concept used to understand and describe processes.


Introduction and terms

The etymology of the term "energy" is from Greek ενέ?γεια, εν- means "in" and έ?γον means "work"; the -ια suffix forms an abstract noun. The compound εν-ε?γεια in Epic Greek meant "divine action" or "magical operation"; it is later used by Aristotle in a meaning of "activity, operation" or "vigour", and by Diodorus Siculus for "force of an engine."

The word energy is used in several different contexts. The scientific use has a defined meaning, whilst the manyother technical uses often vary. Radiant energy is the energy of electromagnetic waves. Free energy may be categorised as renewable energy, although most renewable energy sources would not normally be called free energy sources or sources of perpetual motion. Free energy is energy which may be directly utilized (and returned) by a device from the surroundings (electromagnetic free energy is sometimes referred to as radiant energy). Free energy can also mean a primary energy source that is free (i.e. does not cost anything) for consumption. Examples include wind power, water power, telluric power, and solar power.

In physics, energy is the ability to do work and has many different forms (potential, kinetic, electromagnetic, etc.) No matter what its form, physical energy has the same units as work; a force applied through a distance. The SI unit of energy, the joule, equals one newton applied through one meter, for example. The conservation of energy is a fundamental law in science. It states that the total amount of energy (including potential energy) in a closed system remains constant (in contrast to a system whose border is permeable to energy and/or mass). In other words, energy can be converted from one form to another, but it cannot be created or destroyed. In modern physics, all forms of energy exhibit mass and all mass is a form of energy.

Energy is work

Energy is defined in terms of work and there are a variety of energy forms which are defined via type of work. Because of this, a definition of work is critical to the understanding of energy. Work is a defined by a generalization of force over distance evaluated to exactly one output along a curve. This is mathematically stated as:

 W = \int {F} \cdot {d}s

The equation defines that the work (W\,) is equal to the integral of the dot product of the force (F\,) on a body and the infinitesimal of the body's translation ({s}\,). Depending on the kind of force F\, involved, work of this force results in corresponding kind of energy (gravitational, electrostatic, kinetic, etc).

Science and energy

The energy of a closed system in a certain state is defined as the work needed to bring the system to that state from some reference state. Because work is defined via force involved, forms of energy are usually classified according to that force (elastic, gravitational, nuclear, electric, etc). Energy is a conserved quantity: it is neither created nor destroyed, but only transferred from place to place or from one form to another. The concept of energy change from one form to another, as a "driver" for natural processes, is useful in explaining many phenomena. In particular, since energy cannot be created or destroyed, the driver of energetic processes is not creation of energy per se, but rather the transformation of energy in such a way that the energy can diffuse in space toward areas of less energy concentration (that is, toward areas of less energy per volume). Such changes are associated with increases in entropy.


In physics, energy is the ability to do work (work is, simplistically, a force applied through a distance), and has several different forms. However, no matter what the form, physical energy uses the same units as work: a force applied through a distance. For example, kinetic energy is the amount of work to accelerate a body to a given velocity, gravitational potential energy is the amount of work to elevate or move a mass against a gravitational pull, etc. Because work is frame dependent it can only be defined relative to certain initial state or reference state of the system), energy also becomes frame dependent. For example, a speeding bullet has kinetic energy in the reference frame of non-moving observer, but it has zero kinetic energy in its proper (co-moving) reference frame -- because it takes zero work to accelerate a bullet from zero speed to zero speed. Of course, the selection of a reference state (or reference frame) is completely arbitrary - and usually is dictated to maximally simplify the problem to be dealt with. However, when a certain amount of total energy cannot be removed from a system by simple choice of frame, that energy is associated with an invariant mass in the system.


In chemistry, the spontaneous exchange and transformation of energy with the environment is the cause and effect of all chemical transformations that a substance can undergo. These transformations can be a decomposition, synthesis or a reaction of molecules or atoms. A chemical transformation is possible only if so-called free energy considerations are fulfilled. The concept of free energy is a synthesis of energy and entropy, and in practice is entirely driven by entropy increases as energy is transferred to (or from) a reaction to its environment. Free energy is important in the context of chemistry, because energy considerations alone are not sufficient to decide whether a (net) chemical reaction will occur. Instead, this is determined by the total entropy of reactants and surroundings before and after the reaction, with the heat evolved or absorbed by the reaction taken into account only as it creates or destroys entropy (respectively). According to the second law of thermodynamics, the entropy of the universe must increase in all spontaneus processes (including chemical processes), and energy may be transmuted from any form to any other form (including from heat to any other form) so long as the second law is not violated. For example, a gas may expand and thus allow some of its heat to do work, but this is only possible because the net entropy of the universe increases due to the gas expansion, more than it decreases due to the disappearance of heat. The speed of a permitted spontaneous chemical reaction is also determined by another concept, activation energy. It refers to the minimum energy reactant molecules must have in order to be able to produce product molecules.

Basic forms of energy

There are various forms of energy and there exists relations between the different forms. In science, energy has different forms, forexample thermal, chemical, electrical, radiant, nuclear etc. They can all be, in fact, reduced to kinetic energy or potential energy. Thus energy can be divided into two broad categories.

Kinetic energy

Kinetic energy (Movement) is energy that a body possesses as a result of its motion. It is formally defined as the work needed to accelerate a body from rest to its current velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes. Negative work of the same magnitude would be required to return the body to a state of rest from that velocity. Kinetic energy is the energy of motion (an object which has speed can perform work on another object by colliding with it). The formula for kinetic energy is E_k= {1\over{2}} mv^2, v<<c where m\, is mass and v\, is velocity magnitude.

Kinetic thermal

Kinetic thermal energy is part of heat energy (which exists partly as kinetic energy in objects and partly in other forms of energy). Thermal energy (Heat) is the internal energy of a thermodynamic system at equilibrium. The flow of thermal energy from one system to another is called heat. If the systems are at different temperatures then part of the thermal energy flow can be converted into work. Heat is present in all objects in the universe. The average thermal energy per particle within a sample of matter is proportional to the sample's temperature. To raise the temperature of a sample of matter, work is required to accelerate the particles to higher kinetic energies, and also work is required to move particles against the electromagnetic forces which store their potential energy.

Thermal energy is a particularly diffused and randomly directed form of energy, which cannot be transformed to other types of energy in a closed system in thermal equilibrium. Thus, although some heat consists of kinetic energy, this kinetic energy is directed in random directions and cannot be used to perform work unless allowed to diffuse into a larger volume. Some heat may be turned into other types of energy if directed by allowing it to flow toward a region of lower temperature, but this is equivalent to allowing energy diffusion into a larger volume or space. On average, the kinetic part of total thermal energy is approximated by \overline{E_{kT}}= {3\over{2}} kT where k\, is the Boltzmann constant and T\, is absolute temperature. Other parts of thermal energy add to this (for example, in many solids at room temperature, potential thermal energy is about equal to kinetic thermal energy, so total thermal energy per particle is \overline{E_{T}}= 3kT).

Visible radiation

Visible energy (Light) is electromagnetic radiation with a wavelength that is visible to the eye (visible light) or, in a technical or scientific context, electromagnetic radiation of 625 nm to 380 nm wavelength (~ 405 to 790 THz). Light energy (sometimes refered to more broadly as "radiant energy") is the energy of photons and is responsible for the various sorts of electromagnetic radiation (work is required to create photons). Photons are the force-carrying particles of the electromagnetic force. Photons move at the speed of light in a perfect vacuum and carry energy and information with them. Other kinds of energy are stored in electric and magnetic fields which either do not change in time and space, or which change in ways which are not characteristic of simple electromagnetic radiation. These, too, however, count as energy. For example, a great deal of energy may be transferred between the windings of an electrical transformer, but it is not, strictly speaking, transferred by photons or electromagnetic radiation. Rather, it is transferred by other types of fluctuations in the electromagnetic field (see virtual particles). Light energy in photons is equal to {\!E_{kR}}= hf where f\, is the frequency of the photon and h\, is the Planck's constant.

Potential energy

Potential energy (Force; Work vs.) is "stored energy" which depends on mutual positions of bodies. Potential energy is unreleased energy (a positive quantity, like monetary savings), or else required energy (like monetary debt). The Potential energy is work against a specific force such as gravity, an elastic force of a spring in a clockwork motor, electric force, etc (and is usually named after that specific force).


In physics, force is a name given to a net influence that causes a free body with mass to accelerate. A net (or resultant) force which causes such acceleration may be the non-zero additive sum of many different forces acting on a body. Force is a vector quantity defined as the rate of change of momentum induced in a free body by the net force acting on it, and therefore force has a direction associated with it. The SI unit of force is the newton. There is a form of potential energy for each of the four basic forces in nature: gravity, electromagnetic, and strong and weak nuclear forces.

A potential energy may be positive or negative because it can represent work done on a system (against a restoring force) or work done by a system as a force result. Negative energy is a "mathematical construct", which may be representive of a real entity, in reference to another system. For instance, using the power of a compressed spring to launch a dart uses the elastic potential energy stored within the spring. When the spring is released, this energy is converted into kinetic energy, and work is performed.

Potential elastic energy is the energy stored in the elastic nature of objects. In the ideal case, of Hooke's Law, the energy is equal to \!E_{pE} = {\frac{1}{2} k x^2} where k\, is the spring constant, dependant on the individual spring, and x\, is the deformation of the object.


Gravitational potential energy is seen when are masses are moved apart (such as when a crate is lifted ), or when masses move together (as when a meteorite falls to Earth). If the masses of the objects are considered point masses, gravitational potential energy is equal to E_{pG} = - {GmM \over r} where  m\, and  M\, are the two masses in question,  r\, is the distance between them, and G\, is the Gravitational constant.

Electric and magnetic

Electromagnetic potential energy results from moving charges against a field, and also includes the common chemical potential energies (energy required to break chemical bonds or obtained from forming them. The energy released in lightning or from burning a litre of fuel oil, are some common kinds of electromagnetic potential energy . Electromagnetic potential energy is equal to E_{pE} = {q Q \over 4\pi\epsilon_0 r} where q\, and Q\, are the electric charges on the objects in question, r\, is the distance between them, and \epsilon_0\, is the electric constant of a perfect vacuum (eg., permittivity of vacuum, a physical constant).

Electrical energy (Current; Charge; Ampere) is a form of energy present in any electric field or in any volume containing electromagnetic radiation. The SI unit of electrical energy is the joule, while the unit used by electrical utility companies is the watt-hour (W·h) or the kilowatt-hour (kW·h). Electrical potential energy (static time-invariant electric field) is the potential energy per unit of charge associated with a static (time-invariant) electric field, also called the electrostatic potential, typically measured in volts.

Electrostatic potential (EMF; Voltage; Volt) is the "electric pressure" between two points (neglecting quantum Aharonov-Bohm effects). In physics, the magnetic potential is a method of representing the magnetic field by using a potential value instead of the actual vector field. There are two methods of relating the magnetic field to a potential field and they give rise to two possible types of magnetic potential.


Potential thermal energy results from the electromagnetic potential energy when kinetic energy interacts with various electromagnetic fields between atoms, which contain it (this results in energy storage: in a solid, heat energy is about evenly divided between kinetic and potential energy; for gasses the division increasingly favors kinetic energy).


Potential chemical energy is the energy stored in the bonds of chemical structures. It is released in chemical reactions. Chemical energy is a form of potential energy related to the breaking and forming of chemical bonds.


Nuclear energy (Atomic) is the controlled use of nuclear reactions to release energy for work including propulsion, heat, and the generation of electricity. Nuclear forces act to bind nuclear particles more strongly and closely. After a reaction has completed (nuclear particles like protons and neutrons are not destroyed in fission and fusion processes, but collections "have" less mass than individually free atoms). Weak nuclear forces provide the potential energy for certain kinds of radioactive decay, such as beta decay. Ultimately, the energy released in nuclear processes is according to \!E = {\Delta m c^2} where {\Delta m}\, is the amount of rest mass released into the surroundings as active energy (heat, light, kinetic energy), and c\, is the speed of light in a vacuum.

Other forms of energy

Free electrochemical energy is energy which can be converted into work in accordance with the second law of thermodynamics:

  • Helmholtz "free" energy is a thermodynamic potential which measures the "useful" work obtainable from constant temperature, constant volume thermodynamic systems. It is sometimes known as the "work content". Many sources do not use the superceded term "Helmholtz free energy", instead using Helmholtz energy.
  • Gibbs "free" energy is the energy portion of a closed thermodynamic system available to do work. The Gibbs free energy is a thermodynamic potential and is therefore a state function of a thermodynamic system.

Dark energy is a hypothetical form of energy which permeates all of space and has strong negative pressure. According to some geometrical theories of gravitation, the effect of such a negative pressure is qualitatively similar to a force acting in opposition to gravity at large scales. Invoking such an effect is currently the most popular method for explaining recent observations that the universe appears to be expanding at an accelerating rate, as well as accounting for a significant portion of the missing mass in the universe.

In physics, the zero point energy is the lowest possible energy that a quantum mechanical physical system may possess; it is the energy of the ground state of the system. All quantum mechanical systems have a zero point energy. The term arises commonly in reference to the ground state of the quantum harmonic oscillator. In quantum field theory, it is a synonym for the vacuum energy, an amount of energy associated with the vacuum of empty space. In cosmology, the vacuum energy is taken to be the origin of the cosmological constant. Experimentally, the zero-point energy of the vacuum leads directly to the Casimir effect, and is directly observable in nanoscale devices. Because zero point energy is the lowest possible energy a system can have, this energy cannot be removed from the system. Despite the definition, the concept of zero-point energy, and the hint of a possibility of extracting "free energy" from the vacuum, has attracted the attention of amateur inventors. Numerous devices, often called free energy devices, exploiting the idea, have been proposed. As a result of this activity, and its intriguing theoretical explanation, it has taken on a life of its own in popular culture, appearing in science fiction books, games and movies.

Closed energy systems

As a consequence of energy conservation law, one form of energy can often be readily transformed into another - for instance, a battery converts chemical energy into electrical energy in a closed system. An example is the gravitational potential energy which is converted into the kinetic energy which is then transformed into electric energy. In all cases, as long as no energy is allowed to escape from the closed system, the sum of all the different energies in the closed system remains constant, no matter how many changes take place.

In practice, available energy is rarely perfectly conserved when a closed system changes state; in large systems, some energy will be converted into 'useless' (non-available) energies, such as those associated with heat. This fraction, however, may be reduced arbitrarily toward zero. In large systems with little friction, motion may continue nearly indefinitely because useable energy is traded between usable kinetic and potential energies with so little conversion into heat. In small closed systems, where there may be no friction, the possibility of indefinite motion, with perpetual conversion of kinetic and potential energy, is the case.

While energy in forms other than heat may be freely converted to other forms (including into heat) with efficiency approaching or even equaling 100%, once energy has been converted into heat, there are severe limitations in re-converting this energy into other useful forms, and efficiency never reaches 100%. If this were not so, the easy creation of free energy devices (those which evolve heat, but use that heat to continue running) would be possible.

Heat, therefore, deserves to be placed in a special class of energy, which has been "degraded" by giving it access to all parts of a closed system. While most heat consists of kinetic and potential energies associated with atomic motion, or with certain kinds of radiant energy (i.e., electromagnetic energy with a blackbody spectrum), the energy associated with heat is in a "diffused" and non-direction form, in which the energy has spread out to occupy all of the possible states of a system which can store it. This happens at a certain equilibrium temperature, where "temperature" is a measure of energy concentration in a system. When all parts of a system reach the same temperature, the energy of heat cannot be directed into particular other kinds of energy (or used to do work), unless the closed system is "enlarged" in some fashion which allows the heat is allowed to diffuse into a particular direction, in which it is even less concentrated (such as when the heat is allowed to flow to a region of lower temperature). Thus we see that heat is energy which has already reached a sort of minimal concentration or diffusion in the closed system it is in, and is useless in thee closed system for doing any kind of work unless the closed system is opened in such a way as to let the heat have access to a surrounding system.

Energy is always conserved in closed systems, if heat is taken into account. But the amount of useful energy is usually not conserved, since once energy is converted to heat, it loses some of its ability to do work, and therefore its ability to be convertable to other kinds of energy.

Economics and politics

Ther are a variety of energy resources, substances like fuels, petroleum products, and electric power installations Energy consumption is a measure of the rate of energy use such as fuels or electricity. The conventional energy industry is a generic term for all of the industries involved the production and sale of energy for consumption, including fuel extraction, manufacturing fuel and refining, and fuel distribution. Modern society consumes large amounts fuel, and the energy industry is a crucial part of the infrastructure and maintenance of society in almost all countries. In particular, the energy industry comprises:

  • the petroleum industry, include oil companies, petroleum refiners, fuel transport and end-user sales at gas stations
  • the gas industry, including natural gas extraction, and coal gas manufacture, as well as distribution and sales
  • the electrical power industry, including electricity generation, electric power distribution and sales
  • the coal industry
  • the nuclear power industry
  • alternative energy and sustainable energy companies, including those involved in hydroelectric power, wind power, and solar power generation, and the manufacture, distribution and sale of alternative fuels.
  • traditional energy industry based on the collection and distribution of firewood, the use of which, for cookng and heating, is particulary common in poorer countries

Essentially energy consumption seeks to quantify dynamic processes which create entropy. Typically rates of energy consumption are used to compare development between nations. Energy crisis is any great shortfall (or price rise) in the supply of energy resources to an economy. It usually refers to the shortage of oil and additionally to electricity or other natural resources. A crisis often has effects on the rest of the economy, with many recessions being caused by an energy crisis in some form. In particular, the production costs of electricity rise, which raises manufacturing costs. For the consumer, the price of gasoline (petrol) and diesel for cars and other vehicles rises, leading to reduced consumer confidence and spending, higher transportation costs and general price rising.

Energy development is the ongoing effort to provide abundant and accessible energy resources through knowledge, skills, and constructions. When harnessing energy from primary energy sources and converting them into ever more convenient secondary energy forms, such as electrical energy and cleaner fuels, both quantity (harnessing more energy) and quality (more efficient use) are important. Energy development is the ongoing effort to provide abundant and accessible energy, through knowledge, skills and construction. When converting energy from primary energy sources into more convenient energy forms, such as electrical energy. Harnessing more primary energy and more efficient conversion are both essential.

Energy conservation is the practice of decreasing the quantity of energy used while achieving a similar outcome of end use. This practise may result in increase of national security, personal security, financial capital, human comfort and environmental value. Individuals and organizations that are direct consumers of energy may want to conserve energy in order to reduce energy costs and promote environmental values. Industrial and commercial users may want to increase efficiency and maximize profit. Energy storage is the storing of some form of energy that can be drawn upon at a later time to perform some useful operation.

Energy policy is the manner a given entity (often governmental) has decided to address issues of energy production, distribution and consumption. The attributes of energy policy may include legislation, international treaties, incentives to investment, guidelines for energy conservation, taxation and other public policy techniques.


Energy, in the distant past, was discussed in terms of easily observable effects it has on the properties of objects or changes in state of various systems. It was generally construed that behind all changes, some sort of energy was involved. As it was realized that energy could be stored in objects, the concept of energy came to embrace the idea of the potential for change as well as change itself. Such effects (both potential and realized) come in many different forms. While in spiritualism they were reflected in changes in a person, in physical sciences it is reflected in different forms of energy itself. For example, electrical energy stored in a battery, the chemical energy stored in a piece of food, the thermal energy of a water heater, or the kinetic energy of a moving train.

The concept of energy and work are relatively new additions to the physicist’s toolbox. Neither Galileo nor Newton made any contributions to the theoretical model of energy, and it was not until the middle of the 19th century that these concepts were introduced.

The development of steam engines required engineers to develop concepts and formulas that would allow them to describe the mechanical and thermal efficiencies of their systems. Engineers such as Sadi Carnot and James Prescott Joule, mathematicians such as Émile Claperyon and Hermann von Helmholtz , and amateurs such as Julius Robert von Mayer all contributed to the notions that the ability to perform certain tasks, called work, was somehow related to the amount of energy in the system. The nature of energy was elusive, however, and it was argued for some years whether energy was a substance (the caloric) or merely a physical quantity, such as momentum.

William Thomson (Lord Kelvin) amalgamated all of these laws into his laws of thermodynamics, which aided in the rapid development of energetic descriptions of chemical processes by Rudolf Clausius, Josiah Willard Gibbs, Walther Nernst. In addition, this allowed Ludwig Boltzmann to describe entropy in mathematical terms, and to discuss, along with Jožef Stefan, the laws of radiant energy.

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