# Energy

The energy is a capacity to transform a state. In the common direction energy indicates all that makes it possible to carry out a work, to manufacture Heat, Light, to produce a movement.

In Physical, it is a size Scalaire, expressed in -2}} (Joule S). Energy is the unified measurement of the various forms of movement. One generally distinguishes the kinetic energy, which correspond to the measurement of the movement of the matter particles, and the potential energy which correspond to the measurement of the movement of the virtual particles ensuring the interactions, i.e. in the beginning from the forces. They are the bosons mediators: the graviton for the gravitational force, the photon for the electromagnetic force, the W+ bosons, W and Z0 for the weak interaction, and let us gluons them for the strong interaction.

An important contribution of physics is the conservation of energy in the closed systems. This empirical principle was validated, well after its invention, by the Théorème of Noether. The law of the conservation of energy rises from the homogeneity of time. It states that the movement cannot be created and cannot be cancelled: it can only pass from a form to another. In order to give a quantitative characteristic of the qualitatively different forms of movement considered in physics, one introduces the forms of energy which correspond to them.

Not to confuse with the free energy, which is used isochoric transformations primarily in the case of.

## History

Word energy comes from low Latin energia which comes itself from the Greek ἐνέργεια ( energeia ), which means “force in action”, in opposition to δύναμις ( dynamis ) meaning “force in power”.

After having exploited its clean force, then that of the Slave S, animal and Natural (the Wind S and falls of Water), the man learned how to exploit the energies contained in nature and able to provide him an increasing quantity of mechanical work by the use of Machine S: machines Tool S, boilers and Moteur S. energy is then provided by a Carburant or fossil energy.

Energy is an old concept. The human experiment is that any work requires force and product of the Chaleur; that the more one “spends” of force per quantity of time, more quickly one can do a work, and more one warms up.

As energy necessary to is very undertaken human, the energy supply became one of major concerns of the human society.

A Greek of antiquity had on average five slaves. A modern household with a Electric meter of 6  kw has the energy equivalent of 36  slaves.

## Energetics

In the industrial society, the human activity passes by the supply of electrical energy produced by raw materials, mainly coal, Natural gas, Pétrole and Uranium; one speaks then about fossil energy; these raw materials are called by extension “energies”. One also speaks about renewable energies when the solar energy is used, the wind energy; the hydraulic power of the stoppings is most important of renewable energies. (See also: Energy policy.) Energy is an essential concept in Physique, which is specified since the 19th century.

One finds the concept of energy in all the branches of the Physique:

• in Mechanical;
• in Thermodynamic;
• in electromagnetism;
• in Mechanical quantum;
• but also in other disciplines, in particular in Chemistry.

## Approaches popularized

### A “universal” unit

Energy is a concept created by the human ones to quantify the interactions between very different phenomena; it is a little a common currency of exchange between the physical phenomena. These exchanges are controlled by the laws and principles of the Thermodynamique. The official unit of energy is the Joule.

When a phenomenon involves another phenomenon, the intensity of the second depends on the intensity of the first. For example, the chemical reactions in the muscles of a cyclist enable him to cause the displacement of the bicycle. The intensity of this displacement (i.e. speed) depends on the intensity of the chemical reactions of the muscles of the cyclist, which can be quantified (quantity of sugar “burned” by the Respiration, the Métabolisme of the muscle).

Let us take a more complex example. An Engine spark-ignition functions thanks to a chemical reaction: the Combustion (or “explosion”) which takes place inside a cylinder. Reaction of the Combustible (gasoline) with the Comburant (oxygen in air) produced gas with emission of heat and light, which results in an increase in the temperature and pressure in the cylinder; the difference in pressure between this gas and the atmosphere on other side of the piston move this last, which goes, through a mechanical drive, to make turn the wheels as well as a Alternateur which will produce electricity. In the passing, there will be Frottement S mechanics which will produce a heating and a wear.

There is thus a rearrangement of the molecules (rupture and re-creation of chemical bonds) which causes an increase in the momentum of the molecules (what results in an increase in the temperature of gas and thus an increase in its pressure). This last causes the movement of a solid (the piston), which will involve a system of transmission, and to thus be able on the one hand to make turn an axis, which can be for example connected to the wheels of a car or to an alternator. The drive of the moving part of this alternator will make turn a magnet which, by induction within a reel, will cause a displacement of electrons (electric current).

The concept of energy will make it possible to calculate the intensity of the various phenomena (for example the speed of the car and quantity of electricity produced by the alternator) according to the intensity of the initial phenomenon (quantity of gas and the heat produced by the chemical reaction of combustion).

; Remarks

• In the general public applications, and in particular in the field of the Nutrition, one frequently expresses energy in Calorie S; the calorie is in any rigor the energy which should be provided to make heat one gram of water of a Degree Celsius, but the nutritionnists name by simplification “calorie” what the physicists name “kilogram calorie”.

• In electricity, one uses the Watt-heure (Wh), power consumption during one hour by an apparatus having a power of a Watt, or its multiple the kilowatt-hour (kWh) which is worth: 1000  Wh. This one is not very far away from the work which a horse in one hour (736  can carry out; Wh by convention) except in terms of cost, because it returns to France in 2005 to 7  centimes of euro.
• For reasons Thermodynamique S (second principle), any real transformation energy is irreversible, which wants to say that by reversing the operation (example: retransformer moving via an electrical motor the energy produced by the dynamo of a bicycle) one does not find the quantity the power consumption at the beginning. That is related to the losses.

### Energy and the industrial revolution

The concept of energy is fundamental for the study of the phenomena of transformation (like the Chimie and the Métallurgie) and of mechanical drive, which is the base of the Industrial revolution. The physical concept of energy logically was thus born at the 19th century.

In 1686, Leibniz watch that the quantity m·v, called “lifeblood”, is preserved. In 1788, Lagrange watch the invariance of the sum of two quantities, which one will call later “kinetic energy” and “potential energy”.

At the 19th century, one manages by a series of experiments to highlight reports or laws :

• One notes that the fall of a given weight the same height always produces the same heating (calorimetry);
• And that if final speed is not null, the rise of temperature is less, as so only part of the fall was converted of speed and the remainder into heat;
• In the same way a heating will be able to produce a dilation, an increase in pressure, which itself will make it possible “to produce a work” for example by moving a Masse;
• the total is always preserved: thus is born the scientific Concept of energy, still unspecified “thing” but which one postulates a Propriété:
energy is preserved in all the phenomena, becoming in turn, Chaleur, Pression, Speed, Height, etc

Thus, thanks to energy, one can connect observations as different as a Mouvement, a Rotation, a Température, the Couleur of a body or a Lumière, a Consommation of Sucre or coal, a Usure, etc

It also appears that if energy is preserved and transformed, certain transformations are easy or reversible and others not.

For example, it is easy to transform drop height into heating, one can do it completely, on the other hand the reverse is difficult (are needed complex apparatuses) and part of “energy” will have to be diffused and thus lost. This observation will be at the base of the idea of Entropie.

Starting from the concept of conservation of energy (in quantity), one will be able to look of an eye different from the complex systems (in particular Biologique S and Chimique S) which violate apparently this law and, one will arrive, realizing new scientific progresses, with always validating the postulate or principle of conservation of energy.

### Energy and esotericism

Energy is thus “something” which is preserved. However, this concept of “something” rather fuzzy and is rather well illustrated by the joke:

principle −1 of thermodynamics: energy exists, the proof, it is that it is paid

(reference to the Principles of thermodynamics).

This fuzzy concept left the image in many spirits of a kind of fluid which would pass from one object to the other during the transformations, reminiscence of the concept of Phlogistique (a “immaterial fluid” supposed to convey heat). This vision, known as “substantialist” was subjacent a long time for example in the theories of heat (concept of “Heat-substance” or Calorique), until the middle of the 19th century. Besides one finds it in the modern terminology of “Heat-storage capacity”, “Latent heat”, etc

For lack of a vocabulary more adapted, the term “energy” frequently returns in the speeches pseudo-scientists (with the Onde S). One thus hears about “pure” energy (whereas energy does nothing but describe the state of something of other), or of a “still unknown energy”…

The difference between “energies” of the speech pseudo-scientist is at the level of the definition: in physics, energy is a precisely definite, quantifiable and measurable size. This implies that one can be able to measure precisely energy (kinetic, potential…) or its variations, at least from the theoretical point of view. This is not the case of pseudo-energies such as “energy psychokinetic” or “cosmic” which are not verifiable nor refutable, their existence not being able to be proven and thus not scientists.

Thus when one speaks “about kinetic energy” of a body, this one can be precisely defined, for a body considered as specific, and in traditional Mécanique by the formula: $E_ \left\{K\right\} = \ frac \left\{1\right\} \left\{2\right\} mv^ \left\{2\right\} \,$, $v \,$ being the speed of the body in the reference frame of motion study. The quantity thus is clearly defined, with a field of precise validity (here v   ≪  C and except quantum field, specific body). No formula (nor through any fact) will never give the expression, even approximate, of “psychokinetic energy” in the beliefs of the esotericism…

Only the mathematisation of the concept of energy makes it possible to avoid confusions and contradictions inherent in the old substantialist and holistic vision. Thus energy in general cannot be defined: it is not other than a size physical, numerical, associated with a concrete situation (for example, the movement of a body for the kinetic energy, an interaction for a form of potential energy, etc). It is by the number that the concept of energy reaches an adequate degree of objectivity in modern physics.

Confusion is partly maintained by simplifications language, where by convenience one states sometimes that:

• a wave is a transport of energy without transport of matter;
- or -
• the mass is a form of energy: $E = m \ mathrm \left\{C\right\} ^2$;

whereas more precise formulations (but sometimes longer) would be:

• a wave propagates a disturbance, whose intensity can be expressed like an energy, without transporting matter;
- and -
• the mass can be transformed into Photon S (disintegration), in nuclear Liaison (the mass of the Atomic nucleus is lower than the sum of the masses of the Nucléon S individually taken), of the photons can be transformed into mass (transformation of a photon gamma into pair electron - Positron); the intensity of the mass and can thus like all these phenomenon be expressed in the form of an energy.

One cannot thus separate the concept of energy from the form in which it is stored.

### Energy and martial arts

In much of martial arts it is question of energy (or of IQ ). However, it is necessary to see rather there a metaphor of the will or the training of the practitioners, which does not have any relationship with the scientific term energy which is a quantifiable and measurable physical size. The IQ is a spiritual concept, it is a Chinese word which have as a translation “spirit”, “vapor”, “exhalation”, “fluid”, “impulse”, “energy”. The Indian concept which approaches some is the Prana. The IQ can also be called spiritual energy.

## Energy in physical sciences

It is a size in ml 2 T −2 (Joules).

In physics, energy is a manner of expressing the intensity of the phenomena; it is in fact a measurable quantity, and which is expressed in a different way according to the transformations that undergoes a system (Chemical reaction, shock, movement, nuclear Reaction etc). Energy being defined in a different way according to the phenomena, one can made define various “forms of energy” (see further).

In addition, according to the law of causality, a phenomenon has a cause; it is the variation of intensity of phenomenon-causes which causes the variation of the intensity of the phenomenon-effect. If the intensities of the phenomena causes and effect are expressed in the form of an energy, it is seen whereas energy is preserved (see hereafter).

The unit of the international System to measure energy is the joule ( J ).

Certain activities use other units, in particular the electronvolt (1  eV  =  1,602·10 −19   J), the Kilowatt-hour (1  kWh  =  3,6   MJ), the calorie (4,18  J), the Calorie (food: : 4180  J; note C capital), and the kilogram in relativistic physics.

Thermodynamics is the discipline which studies the energy conversions which utilize thermal energy. The first principle affirms that energy is preserved, the second principle imposes limitations on the output of the thermal conversion energy into mechanical, electric or different energy.

### Energy, power and force

The word “energy” comes from the Greek word meaning “work”. But the word “work” is also used in physics to indicate the energy provided by the action of a force.

In physics, force and energy are two different manners to model the phenomena. For example, one will be able to treat the fall of an object is:

• with the forces: by applying the Laws of the movement of Newton, by writing that acceleration is proportional to the force and inversely proportional to the mass;
• or with energies: while formulating that the reduction in the potential energy gravity is equal to the increase in the kinetic energy.

Work thus indicates the energy of a phenomenon which can also be modelled by a force, i.e. a phenomenon which causes an action directed in a direction.

However, certain phenomena have a disordered, chaotic action; for example, the agitation of the molecules of a gas at rest (without wind), or the agitation of the atoms of a solid. This disordered agitation causes the feeling of “heat”, and it is measured by a parameter called Température. The bound energy on this disordered agitation is called thermal energy.

### Output

The energy “released” by a phenomenon disperses between several other phenomena.

Thus, in a flame (chemical reaction), part of released energy becomes heat, another light, another fraction is stored in complex molecules, etc

One names Rendement the quotient between the energy having the form which interests us and energy spent to obtain it.

In the case of an engine, for example, which interests us is the produced mechanical movement. Remainder of energy is at best regarded as lost (case what leaves in heat in exhaust fumes), at worst the vermin (case what leaves in work of physical strain or chemical the engine).

An ideal engine, which would convert all the energy of combustion of the gasoline into mechanical movement of the vehicle, would have an output of 1 (or 100  %). Actually this one is in the neighborhoods of 25  % only for one engine 4 times, and a little more for the turbines, in particular industrial.

The real output is of course always lower than 1.

In certain case, it can appear to a “output” apparent superior with 1:

• a Heat pump (or a reversed air-conditioner) usually gives 3 times more heat than electrical energy was injected to him. It is simply because instead of dissipating this energy in heat by Joule effect, it went to seek calories outside (was this in a water with 2°C, which makes all the same still 275,15  Kelvin S). The energetic efficiency is in fact equal to 1 (by definition, since energy is preserved), and one prefers to name coefficient of performance the report/ratio of the calories placed at the disposal by the heat pump with that which the only Joule effect would have ensured.
• Another case of apparent output higher than 1 comes from an undervaluation of the energy injected for historical reasons. Thus, the boilers traditionally have as a reference energy “NCV” (Lower Calorific value) of the fuel, which supposes a combustion producing only gases. The boilers with condensation, able to recover the thermal energy of the transformation of the steam into liquid, thus could post apparent outputs higher than 1.

### Law of conservation

Energy is a quantity which preserves.

The concept of conservation is relatively simple to include/understand.

If one puts in a volume something and that one closes the box well, one expects to find there, when it later on is opened, which one put at it. This in physics is called a principle of conservation; the box is the whole of the phenomena considered. If all is not found, it is that a part could leave in a form or another or even as what misses (or is moreover) changed form and that one did not realize from there. One has in fact “forgotten to put an element in the box”, one neglected to include a phenomenon in the system.

This principle is so strong in physics that with each time it appeared not to be checked that led to important discoveries. Each time it seemed that energy was not preserved, it was acted in fact of its transformation into a new form. For example, the radioactivity has a time interpreted as the réémission of something which was received outside and the explanation came from equivalence masses energy.

Energy in a volume of office is thus preserved, by principle, and if it decreases in volume, it is that a part left there… or that it was transformed into something that it is necessary for us to identify: heat, mass, radiation, etc the loss of energy, even tiny, is frequently due to its transformation into thermal energy.

One is tempted to write:

“energy changes of a form into another, but never disappears. ”
The exact formulation would be:
“When the intensity of a phenomenon varies, that can be done only by the variation of another phenomenon; the sum of energies representing the intensity of these phenomena is a constant. ”

In the radioactive processes S, the movement of the ejected particle, or the impulse of the photon created, comes from the disappearance of the mass; one often writes by a short cut that “the energy of mass transforms into kinetic energy”.

The energy of a Réaction Chimique corresponds to a variation of too negligible mass to be measurable, which made accept a time the Conservation of the mass in the chemical reactions. In fact, one always currently considers that the mass is preserved during a chemical reaction, but it is known that it is an approximation.

A major result of the theoretical physics basing on the Lagrangian formalism, the Théorème of Noether, watch that the fact that energy preserve is equivalent to the symmetry of translation in the time of the equations of physics.

This quantity is made up of various elements (thermal energy, energy Cinétique, energy of mass, etc), which are exchanged in a play which is always with null sums. The Theorem of Noether watch which this characteristic is equivalent to the symmetry of the physical equations compared to a translation in time or space.

The conservation of the mass can be seen like a form of conservation of energy. It is the direction of the $E there = mc^2$ of Einstein.

### Forms of energy

In practice, one distinguishes often various “forms” from energy. However, it is necessary to be conscious that energy is used to measure the intensity of a phenomenon, this division is only one manner of making correspond energy to the phenomenon which it measures. In addition, this distinction does not have anything absolute, but depends only on the position of the observer: the principle of relativity also applies to energy, so that the same phenomenon could be analyzed in term of kinetic”, “electromagnetic”, or “potential” energy “…

The forms of energy classically considered are:

• kinetic Energy: energy associated with the movement with a body or a particle; that also includes/understands the electromagnetic energy transported by the Photon S (Lumière, waves radio, X-rays and γ…) or by particles charged (electrical energy);
• thermal Energy: kinetic energy of a whole at rest;
• One can say that the other types of energy are potential energies: with the help of a small change, possible without work, an unstable system is transformed into a more stable system, with release of the difference in energy between the two systems (most stable having a less energy);
• mechanical potential Energy (potential energy of gravity or elastic energy potential) which what is called forms with the kinetic energy the mechanical energy;
• chemical potential energy;
• electromagnetic potential Energy (electrostatic potential energy or magnetostatic): unstable position of one or more particle (S) charged (S) in a electromagnetic Field, for example the energy stored in a condenser or an electric reel;
• Latent heat;
• Free energy.

In the Theory of relativity, Einstein only establishes the existence of two forms of energy:

• kinetic Energy, due to the Mass and the relative Speed of the body;
• Energy of mass: mass and energy at rest are equivalent (the famous E = mc ²). This form of energy includes all the forms of preceding energies in the traditional vision: a “traditional” contribution of energy - such as the tension of an arc - the mass of the generally negligible system of way increases, except within the framework of the nuclear reactions. For example, at the time of Nuclear fission, the total mass of Matière decreases slightly. The mass “missing”, immaterial, is in the form of kinetic energy of the particles or thermal energy. In the nuclear plants, this thermal energy is then recovered for the Electrical production.

Not-consumed or saved energy (one speaks then about Négawatt) can also be regarded as energy arisings. The valorization of such layers is often very profitable and more creative of employment than the search for new resources.

The fatal Energy: it is the energy ineluctably present or trapped in a Processus or a product, which sometimes and to some extent can be easily recovered and developed; Example: France produced in the years 2000 more 25  million t/an of household waste of which 40  %, following delays in the installation of recycling were still treated by incineration. The calorific value of this waste is a form of fatal energy. Without recovery (recovery of heat, methane, hydrogen and/or electricity, etc, possibly with Co-or tri-generation, this energy would be lost in the environment (in the discharges) or is rejected into the atmosphere. The combustion of waste can produce vapor which can feed from the greenhouses, factories or an urban network of heat. The methanisation of organic waste can produce substantial quantities of methane, and a compost which may undergo beneficiation in agriculture.

### Energy and power

Energy spent to create a phenomenon measures the extent of the final phenomenon. This energy is provided by another phenomenon, called “driving phenomenon”.

Certain driving phenomena will do the work quickly, others more slowly; for example, a warehouseman runt will take a long time before assembling breeze blocks one by one in top of the scaffolding, whereas a muscular warehouseman carries several at the same time and is faster (on the other hand, the end result will be exactly the same one).

This capacity to mobilize much energy in a given time is called power of the driving phenomenon:

the power is the energy provided by a phenomenon divided by the duration of the phenomenon, P   =  of / dt .
The power is measured in Watt S (1  W  =  1  J/s)

See the detailed article Power.

## Energy in the field of the alive one

At the living organisms, energy takes the form of chemical energy either directly available to the enzymatic components of the cells (Adénosine tri-phosphate), or stored in the form of Sucre S simple or ramified (Amidon), of grease in the animals, of||oil]] S at the plants.

## Heat transfers

The Heat transfers belong to a field of irreversible thermodynamics called thermodynamic , i.e., to simplify, that the phenomenon cannot retrogress.

Transferred energy is presented primarily in the form of heat which goes spontaneously from a hot zone towards a cold zone (Second principle of thermodynamics). This transfer of heat can be accompanied by a mass transfer. This phenomenon arises in three different forms:

• conduction;
• Convection;
Each one of these three modes is dominating in its universe of predilection: conduction in the solids, convection in the fluids moving (liquid, gas), the radiation in the vacuum (where it is the only possible mode).

### Conduction

Thermal conduction is the phenomenon by which the temperature of a medium is homogenized. It corresponds to the transmission of thermal agitation between molecules and occurs in a solid, a liquid or a gas. Example: the temperature of a bar heated at an end tends to be standardized by thermal conduction.

### Convection

The convection is the transfer of heat caused by the movement of the particles of a fluid. It occurs in a fluid moving. Example: the hot air, less dense, goes up, transporting the heat of bottom upwards.

The radiation is the transfer of heat by wave propagation electromagnetic or radioactive decay. It can occur in all the mediums, empties including. Example: the Earth is heated by the radiation of the sun.

## Energy supply

The energy sources used by the man are of origin renewable or not:

• energies of fossil origin (gas, oil, coal) on the cars, the aircraft, the thermo plants… ;

• nuclear energy of origin obtained by Nuclear fission (the nuclear Fusion not being possible in a foreseeable future in the short run);
• the energy of biomassic origin (dry biomass, wet biomass and biocarburants);
• energy of the hydraulic of the rivers, stoppings and pressure pipes, renewable origin;
• the energy of wind origin;
• the solar energy of origin (luminous energy transformation into heat or electricity: solar photovoltaic; solar thermics; solar thermodynamics);
• the energy of kinetic and potential origin related to the displacement of sea surface under the action of the swell;
• geothermal energy of origin;
• the energy of tidal origin;
• the energy of marethermic origin;
• the muscular energy of origin (conversion of Sugars and/or Lipid S and/or Starch S into heat and).

## Some interesting figures

In 1960,50  % of the electricity produced in France came from renewable sources (hydroelectricity). The doubling of consumption was envisaged every ten years (law checked since the beginning of the century), and this positive ratio could not be maintained, all the favorable sites being equipped.

The changing was ensured by the nuclear power which provides 80% of electricity today. Regularity of the doubly in 10  years at the time of the Oil crisis of 1973 ended.

Germany recently made climb its wind electrical production of 38  % per annum during two consecutive years. It contributes for 10  % of its needs.

To fix the ideas, the joined together engines of the rocket Saturn V in years 1960 alone consumed during the few minutes of their combustion an energy equivalent to thousandths of what was burned out of oil on planet during same time. (source: the energy saving , Yves Manguy, Dunod)

The oil price is in September 2004 close to 50  dollars the barrel. It remains with approximately 50  dollars in March 2005. Experts announced the June 7th 2004 that this price could not be maintained in a viable way and that in the short run a rise with 180 dollars the barrel would be probable. October 29th, 2007, the oil price rose until 93.80  dollars the barrel.

Increase in the courses of 25  % between June and September 2004 again draws the attention to their communication of the time:

• Quadrupling of the course in the short run announced in June 2004 (BBC)

• Consumption of energy of some industrialized countries, according to the figures of the state of the world 2004 , Paris, the Discovery, 2003: it is observed that countries with cold climate (Scandinavia) and the immense countries (the United States, Canada, Australia) consume the most energy.

According to the site of ECA, a park of 4 engines with fusion of the type ITER for a supply uninterrupted of 4×: 1500  MW (: 600000  people) would occupy 1  km, provided technology is one day controlled.

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