Energy
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| In [[physics]], '''energy''' is an indirectly observed quantity that is often understood as the ability of a [[physical system]] to do [[Work (physics)|work]] on other physical systems. However, this must be understood as an overly simplified definition, as the laws of [[thermodynamics]] demonstrate that not all energy can perform work. Depending on the boundaries of the physical system in question, energy as understood in the above definition may sometimes be better described by concepts such as [[exergy]], [[emergy]] and [[thermodynamic free energy]]. Therefore, in the words of [[Richard Feynman]], ''"It is important to realize that in physics today, we have no knowledge what energy is. We do not have a picture that energy comes in little blobs of a definite amount."'' However, it is clear that energy is always an indispensable prerequisite for performing work, and the concept has great importance in [[natural science]]. | In [[physics]], '''energy''' is an indirectly observed quantity that is often understood as the ability of a [[physical system]] to do [[Work (physics)|work]] on other physical systems. However, this must be understood as an overly simplified definition, as the laws of [[thermodynamics]] demonstrate that not all energy can perform work. Depending on the boundaries of the physical system in question, energy as understood in the above definition may sometimes be better described by concepts such as [[exergy]], [[emergy]] and [[thermodynamic free energy]]. Therefore, in the words of [[Richard Feynman]], ''"It is important to realize that in physics today, we have no knowledge what energy is. We do not have a picture that energy comes in little blobs of a definite amount."'' However, it is clear that energy is always an indispensable prerequisite for performing work, and the concept has great importance in [[natural science]]. | ||
| - | Since work is defined as a [[force]] acting through a distance (a length of space), energy is always equivalent to the ability to exert pulls or pushes against the basic forces of nature, along a path of a certain length. The total energy contained in an object is identified with its [[mass]], and energy cannot be created or destroyed ([[thermodynamic free energy]], however, can be destroyed). When [[matter]] (ordinary material particles) is changed into energy (such as energy of motion, or into radiation), the '''mass''' of the system does not change through the transformation process. However, there may be mechanistic limits as to how much of the matter in an object may be changed into other types of energy and thus into [[Work (thermodynamics)|work]], on other systems. Energy, like mass, is a [[scalar (physics)|scalar]] physical quantity. In the [[International System of Units]] (SI), energy is measured in [[joule]]s, but in many fields other units, such as [[kilowatt-hour]]s and [[kilocalorie]]s, are customary. All of these units translate to units of work, which is always defined in terms of forces and the distances that the forces act through. | + | Energy is necessary for things to change. All living things require [[available energy]] to stay alive; humans get such energy from [[food]], along with the oxygen needed to metabolize the food. Human civilization requires a continual supply of energy to function, [[energy resource]]s such as [[fossil fuel]]s are a vital topic in economics and politics. Earth's [[climate]] and [[ecosystem]] are driven by the radiant energy Earth receives from the sun, and are delicately sensitive to changes in the amount received. |
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| - | A system can transfer energy to another system by simply transferring matter to it (since matter is equivalent to energy, in accordance with its mass). However, when energy is transferred by means other than matter-transfer, the transfer produces changes in the second system, as a result of work done on it. This work manifests itself as the effect of force(s) applied through distances within the target system. For example, a system can emit energy to another by transferring (radiating) [[electromagnetic energy]], but this creates forces upon the particles that absorb the radiation. Similarly, a system may transfer energy to another by physically impacting it, but in that case the energy of motion in an object, called [[kinetic energy]], results in forces acting over distances (new energy) to appear in another object that is struck. Transfer of [[thermal energy]] by [[heat]] occurs by both of these mechanisms: heat can be transferred by electromagnetic radiation, or by physical contact in which direct particle-particle impacts transfer kinetic energy. | + | |
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| - | Energy may be stored in systems without being present as matter, or as kinetic or electromagnetic energy. Stored energy is created whenever a particle has been moved through a field it interacts with (requiring a force to do so), but the energy to accomplish this is stored as a new position of the particles in the field—a configuration that must be "held" or fixed by a different type of force (otherwise, the new configuration would resolve itself by the field pushing or pulling the particle back toward its previous position). This type of energy "stored" by force-fields and particles that have been forced into a new physical configuration in the field by doing work on them by another system, is referred to as [[potential energy]]. A simple example of potential energy is the work needed to lift an object in a gravity field, up to a support. Each of the basic forces of nature is associated with a different type of potential energy, and all types of potential energy (like all other types of energy) appears as system [[mass]], whenever present. For example, a compressed spring will be slightly more massive than before it was compressed. Likewise, whenever energy is transferred between systems by any mechanism, an associated mass is transferred with it. | + | |
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| - | Any form of energy may be [[energy transformation|transformed]] into another form. For example, all types of potential energy are converted into kinetic energy when the objects are given freedom to move to different position (as for example, when an object falls off a support). When energy is in a form other than thermal energy, it is theoretically possible to transform it with very high efficiency to any other type of energy, including [[electricity]] or production of new particles of matter. (''Exactly'' 100% efficiency is impossible only because of friction and similar losses.) By contrast, there are strict limits to how efficiently thermal energy can be converted into other forms of energy, as described by [[Carnot's theorem (thermodynamics)|Carnot's theorem]] and the [[second law of thermodynamics]]. | + | |
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| - | In all such [[energy transformation]] processes, the total energy remains the same, and a transfer of energy from one system to another, results in a loss to compensate for any gain. 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. | + | |
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| - | 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 (and higher total energy) relative to the [[Earth]]. | + | |
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| ==See also== | ==See also== | ||
Revision as of 22:38, 14 September 2013
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In physics, energy is an indirectly observed quantity that is often understood as the ability of a physical system to do work on other physical systems. However, this must be understood as an overly simplified definition, as the laws of thermodynamics demonstrate that not all energy can perform work. Depending on the boundaries of the physical system in question, energy as understood in the above definition may sometimes be better described by concepts such as exergy, emergy and thermodynamic free energy. Therefore, in the words of Richard Feynman, "It is important to realize that in physics today, we have no knowledge what energy is. We do not have a picture that energy comes in little blobs of a definite amount." However, it is clear that energy is always an indispensable prerequisite for performing work, and the concept has great importance in natural science.
Energy is necessary for things to change. All living things require available energy to stay alive; humans get such energy from food, along with the oxygen needed to metabolize the food. Human civilization requires a continual supply of energy to function, energy resources such as fossil fuels are a vital topic in economics and politics. Earth's climate and ecosystem are driven by the radiant energy Earth receives from the sun, and are delicately sensitive to changes in the amount received.
See also
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