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− | {{Wikia import|engineering|Energy}} |
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− | {{Directed link|Image:258px-Lightning_in_Arlington.jpg|thumb|right|258px|{{Directed link|Lightning}} is a prominent and highly visible form of energy transfer.}} |
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− | Hi this sight is dum'''Energy''' (from Latin ''Energia'' and Greek ''Ενεργεια'') is a measure of the ability to do {{Directed link|mechanical work}}.<ref>This definition is one of the most common; e.g. [http://observe.arc.nasa.gov/nasa/space/stellardeath/stellardeath_6.html Glossary at the NASA homepage]</ref> It is a fundamental {{Directed link|concept}} pertaining to the ability for {{Directed link|Action (physics)|action}}. In {{WPlink|physics}}, it is a {{Directed link|quantity}} that every {{WPlink|physical system}} possesses. This quantity is not absolute but relative to a state of the system known as its {{Directed link|Frame of reference|reference}} state or reference level. The energy of a physical system is defined as the amount of {{Directed link|mechanical work}} that the system can produce if it changes its state to its reference state; for example if a {{Directed link|liter}} of {{wikialink|chemistry|water}} cools down to 0{{Directed link|°C}}car hits a tree and decelerates from 120 {{Directed link|km/h}} to 0 {{Directed link|km/h}}. |
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− | == Types of energy == |
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− | Energy can be in several forms: mechanical {{Directed link|potential energy|potential}}—due to possible physical interactions with other objects (for example, {{Directed link|gravitational potential energy}}); {{Directed link|kinetic energy|kinetic}}—contained in macroscopic {{Directed link|motion}}; {{Directed link|chemical energy|chemical}}—potential stored in {{Directed link|chemical bonds}} between {{Directed link|atoms}}; {{Directed link|electrical energy|electrical}}—potential due to possible {{Directed link|electrical charge|charge}} interactions; {{Directed link|thermal energy|thermal}}—contained in the kinetic energy of individual {{WPlink|Molecule|molecules}}; {{WPlink|Nuclear energy|nuclear energy}}—potential stored between constituents of {{WPlink|Atomic nucleus|atomic nucleus}}. Light can be viewed as energy in the form of {{Directed link|photons}} or {{Directed link|wave}}s, depending on context. The theory of {{WPlink|General relativity|general relativity}} provides a framework to envision {{Directed link|mass}} itself as an expression of energy. |
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− | == Conservation of energy == |
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− | One form of energy can be readily transformed into another; for instance, a battery converts {{Directed link|chemical energy}} into {{Directed link|electrical energy}}, which can be converted into {{Directed link|thermal energy}}. Similarly, {{Directed link|potential energy}} is converted into {{Directed link|kinetic energy}} of moving {{wikialink|chemistry|water}} and {{Directed link|turbine}} in a {{Directed link|dam}}, which in turn transforms into {{Directed link|electric energy}} by {{Directed link|Electrical_generator|generator}}. The law of {{Directed link|conservation of energy}} states that in a {{Directed link|closed system}} the total amount of energy, corresponding to the sum of a system's constituent energy components, remains constant. This law follows from {{Directed link|translational symmetry}} of {{Directed link|time}}, which states the independence of any physical process on the moment it started. Some works, thus some forms of energy, are not easily measured by the unaided observer. |
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− | == Alternative uses of the term == |
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− | The term "energy" is also used in a {{Directed link|Spirituality|spiritual}} or non-scientific way that cannot be quantified, to make certain propositions appear more plausible, by imitating the scientific terminology. Usually this has something to do with {{Directed link|mystical}} and/or {{Directed link|healing}} type references such as {{Directed link|acupuncture}} and {{Directed link|reiki}}. Psychical researchers will often speak of so-called "{{Directed link|psychokinetic}} energy" when attempting to explain phenomena such as {{Directed link|poltergeist}} activity; this is likewise non science [http://www.whyprophets.com/prophets/non_science.htm]. |
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− | ==Forms of Energy== |
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− | *{{Directed link|Kinetic energy}}: the energy of moving objects |
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− | **{{Directed link|Thermal energy}}: the energy associated with heat |
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− | **{{Directed link|Sound|Sound energy}}: the energy of compression waves |
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− | **{{Directed link|Electrical energy}}: the energy of moving charged particles |
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− | *{{Directed link|Potential Energy}}: the energy that an object has due to position; also known as stored energy |
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− | **{{Directed link|Chemical energy}}: the stored energy of chemical substances |
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− | **{{Directed link|Nuclear energy}}: the stored energy of the atomic nucleus |
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− | *{{Directed link|Radiant energy}}: the energy of {{Directed link|Electromagnetism|electromagnetic waves}}, including light |
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− | ==Units== |
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− | ===SI=== |
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− | The {{Directed link|SI}} unit for both '''energy''' and work is the {{Directed link|joule}} (J), named in honour of {{Directed link|James Prescott Joule}} and his experiments on the {{Directed link|mechanical equivalent of heat}}. In slightly more fundamental terms, 1 joule is equal to 1 {{Directed link|newton}}-{{Directed link|meter}} and, in terms of {{Directed link|SI base unit}}s: |
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− | <math>1\ \mathrm{J} = 1\ \mathrm{kg} \left( \frac{\mathrm{m}}{\mathrm{s}} \right ) ^ 2 = 1\ \frac{\mathrm{kg} \cdot \mathrm{m}^2}{\mathrm{s}^2}</math> |
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− | An energy unit that is used in {{WPlink|particle physics}} is the {{Directed link|electronvolt}} (eV). One eV is equivalent to {{Directed link|1 E-19 J|1.60217653×10<small><sup>−19</sup></small> J}}. |
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− | In {{Directed link|spectroscopy}} the unit cm<sup>-1</sup> = 0.0001239 eV is used to represent energy since energy is inversely proportional to wavelength from the equation <math> E = h \nu = h c/\lambda </math>. |
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− | (Note that {{Directed link|torque}}, which is typically expressed in newton-meters, has the same dimension and this is not a simple coincidence: a torque of 1 newton-meter applied on 1 radian requires exactly 1 newton-meter=joule of energy.) |
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− | ===Other units of energy=== |
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− | In [[Absolute centimeter-gram-second system|cgs]] units, one [[erg]] is 1 [[gram|g]] [[centimeter|cm]]<small><sup>2</sup></small> [[second|s]]<small><sup>−2</sup></small>, equal to 1.0×10<small><sup>−7</sup></small> [[joule|J]]. |
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− | The {{Directed link|imperial units|imperial}}/{{Directed link|US customary units|US units}} for both energy and work include the {{Directed link|foot-pound force}} (1.3558 J), the {{Directed link|British thermal unit}} (Btu) which has various values in the region of 1055 J, and the {{Directed link|horsepower}}-hour (2.6845 MJ). |
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− | The energy unit used for everyday {{Directed link|electricity}}, particularly for utility bills, is the {{Directed link|kilowatt-hour}} (kW h), and one kW h is equivalent to {{Directed link|1 E6 J|3.6×10<small><sup>6</sup></small> J }} (3600 kJ or 3.6 MJ; the metric units usually are self-consistent, and this particular one may seem arbitrary; it's not, the metric measurement for time is the second, and there are 3,600 seconds in an hour -- in other words, 1 kW second = 1 kJ, but the kW h is a more convenient unit for everyday use). |
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− | The {{Directed link|calorie}} is mainly used in nutrition and equals the amount of {{Directed link|heat}} necessary to raise the {{Directed link|temperature}} of one {{Directed link|kilogram}} of {{wikialink|chemistry|water}} by 1 {{Directed link|Celsius}} degree, at a {{Directed link|pressure}} of 1 {{Directed link|atmospheric pressure|atm}}. This amount of heat depends somewhat on the initial temperature of the water, which results in various different units sharing the name of "calorie" but having slightly different energy values. It is equal to {{Directed link|1 E0 J|4.1868 kJ}}. |
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− | The calories used for {{Directed link|food energy}} in nutrition are the large calories based on the kilogram rather than the gram, often identified as ''food calories''. These are sometimes called kilocalories with that calorie being the small calorie based on the gram, and as a result the prefixes are generally avoided for the large calories (i.e., 1 kcal is 4.184 kJ, never 4.184 MJ, even if "calories" are also used for the other, larger unit in the same document or the same nutrition label). Food calories are sometimes noted as ''C''alories (1000 calories) or simply abbreviated Cal with the capital C, but that convention is more often found in chemistry or physics textbooks—which do not use these large calories—than it is in real-world applications by those who do use these calories. (This convention is also, of course, useless when the word calorie appears in a location where it would ordinarily be capitalized, as at the beginning of a sentence or in the first column of a nutrition label as a substitute for the quantity being measured, which is energy, when all the other quantities such as "Iron" and "Sugars" are also capitalized.) |
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− | ==Transfer of energy== |
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− | ===Work=== |
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− | main|Mechanical work |
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− | ''Work'' is a defined as a [path integral] of [force] F over distance s: |
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− | <math> W = \int \mathbf{F} \cdot \mathrm{d}\mathbf{s}</math> |
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− | The equation above says that the work (<math>W</math>) is equal to the integral of the {{Directed link|dot product}} of the {{Directed link|force}} (<math>\mathbf{F}</math>) on a body and the {{Directed link|infinitesimal}} of the body's {{Directed link|position}} (<math>\mathbf{s}</math>). |
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− | ===Heat=== |
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− | main|Heat |
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− | ''Heat'' is the common name for {{Directed link|thermal energy}} of an object that is due to the motion of the {{Directed link|atoms}} and {{Directed link|molecules}} that constitute the object. This motion can be {{Directed link|translational}} (motion of molecules or atoms as a whole); {{Directed link|vibrational}} (relative motion of atoms within molecules) or {{Directed link|rotational}} (motion of the atoms of a molecule about a common centre). It is the form of energy which is usually linked with a change in {{Directed link|temperature}} or in a change in {{Directed link|phase}} of {{Directed link|matter}}. In {{Directed link|chemistry}}, heat is the amount of energy which is absorbed or released when atoms are rearranged between various molecules by a {{Directed link|chemical reaction}}. |
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− | The relationship between heat and energy is similar to that between work and energy. Heat flows from areas of high {{Directed link|temperature}} to areas of low temperature. All objects (matter) have a certain amount of internal energy that is related to the random motion of their atoms or molecules. This internal energy is directly proportional to the temperature of the object. When two bodies of different {{Directed link|temperature}} come in to thermal contact, they will exchange internal energy until the {{Directed link|temperature}} is equalised. The amount of energy transferred is the amount of heat exchanged. It is a common misconception to confuse heat with internal energy, but there is a difference: the change of the internal energy is the heat that flows from the surroundings into the system plus the work performed by the surroundings on the system. Heat Energy is transferred in three different ways: {{Directed link|Heat conduction|conduction}}, {{Directed link|convection}} and/or {{Directed link|Thermal radiation|radiation}}. |
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− | ===Conservation of energy=== |
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− | The first law of {{Directed link|thermodynamics}} says that the total inflow of energy into a system must equal the total outflow of energy from the system, plus the change in the energy contained within the system. This law is used in all branches of physics, but frequently violated by quantum mechanics (see {{Directed link|off shell}}). {{Directed link|Noether's theorem}} relates the {{Directed link|conservation of energy}} to the {{Directed link|time invariance}} of physical laws. |
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− | An example of the conversion and {{Directed link|conservation of energy}} is a {{Directed link|pendulum}}. At its highest points the {{Directed link|kinetic energy}} is zero and the {{Directed link|potential gravitational energy}} is at its maximum. At its lowest point the {{Directed link|kinetic energy}} is at its maximum and is equal to the decrease of {{Directed link|potential energy}}. If one unrealistically assumes that there is no {{Directed link|friction}}, the energy will be conserved and the {{Directed link|pendulum}} will continue swinging forever. (In practice, available energy is '''never''' perfectly conserved when a system changes state; otherwise, the creation of {{Directed link|perpetual motion}} machines would be possible.) |
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− | Another example is a {{Directed link|Chemical_explosive|chemical explosion}} in which {{Directed link|potential chemical energy}} is converted to {{Directed link|kinetic energy}} and {{Directed link|heat}} in a very short time. |
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− | == Relations between different forms of energy == |
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− | All forms of energy: {{Directed link|thermal energy|thermal}}, {{Directed link|chemical energy|chemical}}, {{Directed link|electrical energy|electrical}}, {{Directed link|radiant energy|radiant}}, {{Directed link|nuclear energy|nuclear}} etc. can be in fact reduced to {{Directed link|kinetic energy}} or {{Directed link|potential energy}}. For example {{Directed link|thermal energy}} is essentially {{Directed link|kinetic energy}} of {{Directed link|atoms}} and {{Directed link|molecules}}; {{Directed link|chemical energy}} can be visualized to be the {{Directed link|potential energy}} of {{Directed link|atoms}} within {{Directed link|molecules}}; {{Directed link|electrical energy}} can be visualized to be the {{Directed link|potential energy|potential}} and {{Directed link|kinetic energy}} of {{Directed link|electrons}}; similarly {{Directed link|nuclear energy}} is the {{Directed link|potential energy}} of {{Directed link|nucleons}} in {{Directed link|atomic nucleii}}. |
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− | === Kinetic energy === |
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− | {{main|Kinetic energy}} |
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− | {{Directed link|Kinetic energy}} is the portion of energy related to motion. |
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− | :<math>E_k = \int \mathbf{v} \cdot \mathrm{d}\mathbf{p}</math> |
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− | The equation above says that the kinetic energy (<math>E_k</math>) is equal to the integral of the {{Directed link|dot product}} of the {{Directed link|velocity}} (<math>\mathbf{v}</math>) of a body and the {{Directed link|infinitesimal}} of the body's {{Directed link|momentum}} (<math>\mathbf{p}</math>). |
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− | For non-{{Directed link|special relativity|relativistic}} velocities, that is velocities much smaller than the {{Directed link|speed of light}}, we can use the {{Directed link|Newtonian approximation}} |
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− | :<math>E_k = \begin{matrix} \frac{1}{2} \end{matrix} mv^2</math> |
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− | where |
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− | ''E''<sub>k</sub> is {{Directed link|kinetic energy}} |
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− | ''m'' is {{Directed link|mass}} of the body |
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− | ''v'' is {{Directed link|velocity}} of the body |
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− | At near-light velocities, we use the correct {{Directed link|relativistic}} formula: |
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− | :<math>E_k = m c^2 (\gamma - 1) = \gamma m c^2 - m c^2 \;\!</math> |
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− | :<math>\gamma = \frac{1}{\sqrt{1 - (v/c)^2}} </math> |
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− | where |
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− | ''v'' is the {{Directed link|velocity}} of the body |
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− | ''m'' is its {{Directed link|rest mass}} |
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− | ''c'' is the {{Directed link|speed of light}} in a {{Directed link|vacuum}}, which is approximately 300,000 kilometers per second |
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− | <math>\gamma m c^2 \,</math> is the ''total energy'' of the body |
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− | <math>m c^2 \,</math> is again the {{Directed link|rest mass}} energy. |
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− | See also, {{Directed link|E=mc²}}. |
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− | In the form of a {{Directed link|Taylor series}}, the {{Directed link|relativistic}} formula can be written as: |
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− | :<math>E_k = \frac{1}{2} mv^2 - \frac{3}{8} \frac{mv^4} {c^2} + \cdots </math> |
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− | Hence, the second and higher terms in the series correspond with the "inaccuracy" of the Newtonian approximation for kinetic energy in relation to the {{Directed link|relativistic}} formula. |
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− | However, the phrase "conservation of energy" is often confusing to a non scientist. This is so, because of the common usage of the terms "save energy" or conserve energy" used in campaigns for conservation of energy resources like electricity or fossil fuels. |
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− | === Potential energy === |
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− | {{main|Potential energy}} |
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− | In contrast to {{Directed link|kinetic energy}}, which is the energy of a {{Directed link|system}} due to its {{Directed link|motion}}, or the internal motion of its particles, the {{Directed link|potential energy}} of a system is the energy associated with the spatial configuration of its components and their interaction with each other. Any number of particles which exert forces on each other automatically constitute a system with potential energy. Such forces, for example, may arise from {{Directed link|electrostatic}} interaction (see {{Directed link|Coulomb's law}}), or {{Directed link|gravity}}. |
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− | In an isolated system consisting of two stationary objects that exert a force <math>f(x)</math> on each other and lie on the x-axis, their {{Directed link|potential energy}} is most generally defined as |
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− | :<math>E_p = -\int f(x) \, dx</math> |
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− | where the force between the objects varies only with {{Directed link|distance}} <math>x</math> and is {{Directed link|integrated}} along the line connecting the two objects. |
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− | To further illustrate the relationship between {{Directed link|force}} and {{Directed link|potential energy}}, consider the same system of two objects situated along the x-axis. If the {{Directed link|potential energy}} due to one of the objects at any point <math>x</math> is <math>U(x)</math>, then the force on that object at <math>x</math> is |
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− | :<math>f(x) = -\frac{dU(x)}{dx}</math> |
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− | This mathematical relationship demonstrates the direct connection between {{Directed link|force}} and {{Directed link|potential energy}}: the {{Directed link|force}} between two objects is in the direction of decreasing {{Directed link|potential energy}}, and the magnitude of the {{Directed link|force}} is proportional to the extent to which {{Directed link|potential energy}} decreases. A large {{Directed link|force}} is associated with a large decrease in {{Directed link|potential energy}}, while a small {{Directed link|force}} is associated with a small decrease in {{Directed link|potential energy}}. Notice how, in this case, the {{Directed link|force}} on an object depends entirely on its {{Directed link|potential energy}}. |
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− | These two relationships – the definition of {{Directed link|potential energy}} based on {{Directed link|force}}, and the dependence of {{Directed link|force}} on {{Directed link|potential energy}} – show how the concepts of {{Directed link|force}} and {{Directed link|potential energy}} are intimately linked: if two objects do not exert forces on each other, there is no {{Directed link|potential energy}} between them. If two objects do exert forces on each other, then {{Directed link|potential energy}} naturally arises in the system as part of the system's total energy. Since {{Directed link|potential energy}} arises from forces, any change in the system's spatial configuration will either increase or decrease the system's {{Directed link|potential energy}} as the objects are repositioned. |
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− | When a system moves to a lower {{Directed link|potential energy}} state, energy is either released in some form or converted into another form of energy, such as {{Directed link|kinetic energy}}. The {{Directed link|potential energy}} can be "stored" as {{Directed link|gravitational energy}}, {{Directed link|elastic energy}}, {{Directed link|chemical energy}}, {{Directed link|rest mass energy}} or {{Directed link|electrical energy}}, but arises in all cases from the spatial positioning and interaction of objects within a system. Unlike {{Directed link|kinetic energy}}, which exists in any moving body, {{Directed link|potential energy}} exists in any body which is interacting with another object. |
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− | For example a {{Directed link|mass}} released above the {{Directed link|Earth}} initially has {{Directed link|potential energy}} resulting from the {{Directed link|gravity|gravitational attraction}} of the Earth, which is transferred to {{Directed link|kinetic energy}} as the {{Directed link|gravitational force}} acts on the object and its {{Directed link|potential energy}} is decreased as it falls. |
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− | Equation: |
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− | :<math>E_p = mgh \;</math> |
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− | where ''m'' is the mass, ''h'' is the {{Directed link|height}} and ''g'' is the value of {{Directed link|acceleration}} due to {{Directed link|gravity}} at the Earth's surface (see {{Directed link|gee}}). |
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− | ===Internal energy=== |
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− | {{main|Internal energy}} |
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− | ''Internal energy'' is the {{Directed link|kinetic energy}} associated with the motion of {{Directed link|molecule}}s, and the {{Directed link|potential energy}} associated with the {{Directed link|rotation|rotational}}, {{Directed link|vibration|vibrational}} and {{Directed link|electric}} energy of {{Directed link|atom}}s within molecules. {{Directed link|Internal energy}}, like energy, is a quantifiable {{Directed link|state function}} of a system. |
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− | ==History== |
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− | In the past, energy was discussed in terms of easily observable effects it has on the {{Directed link|property|properties}} of objects or changes in state of various systems. Basically, if something changed, some sort of energy was involved in that change. 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; examples are the {{Directed link|electrical energy}} stored in a battery, the {{Directed link|chemical energy}} stored in a piece of food, the {{Directed link|thermal energy}} of a water heater, or the {{Directed link|kinetic energy}} of a moving train. To simply say energy is "change or the potential for change", however, misses many important examples of energy as it exists in the physical world. |
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− | The concept of energy and work are relatively new additions to the physicist’s toolbox. Neither {{WPlink|Galileo}} nor {{Directed link|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. |
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− | The development of {{Directed link|steam engines}} required engineers to develop concepts and formulas that would allow them to describe the {{Directed link|mechanical}} and {{Directed link|thermal}} efficiencies of their systems. Engineers such as {{WPlink|Sadi Carnot}} and {{WPlink|James Prescott Joule}}, mathematicians such as {{WPlink|Benoît Paul Émile Clapeyron}} and {{WPlink|Hermann von Helmholtz}}, and amateurs such as {{WPlink|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 {{Directed link|caloric theory|caloric}}) or merely a physical quantity, such as {{Directed link|momentum}}. |
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− | William Thomson ({{Directed link|Lord Kelvin}}) amalgamated all of these laws into his laws of {{Directed link|thermodynamics}}, which aided in the rapid development of energetic descriptions of chemical processes by {{Directed link|Rudolf Clausius}}, {{Directed link|Josiah Willard Gibbs}}, {{Directed link|Walther Nernst}}. In addition, this allowed {{Directed link|Ludwig Boltzmann}} to describe entropy in mathematical terms, and to discuss, along with {{Directed link|Jožef Stefan}}, the laws of {{Directed link|radiant energy}}. |
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− | For further information, see the {{WPlink|Timeline_of_thermodynamics}}, statistical mechanics, and random processes. |
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− | ==Energy and Economy== |
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− | Main articles: {{WPlink|Energy development}} and {{Directed link|Energy policy}} |
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− | The way in which humans use energy is one of the defining characteristics of an economy. The progression from animal power to {{Directed link|steam power}}, then the {{Directed link|internal combustion engine}} and {{Directed link|electricity}}, are key elements in the development of modern civilization. {{Directed link|Future energy development}}, for example of {{Directed link|renewable energy}}, may be key to avoiding the {{Directed link|effects of global warming}}.<!--- etc etc much more to be said --> |
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− | ==See also== |
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− | *{{Directed link|Principles of energetics}} |
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− | *{{Directed link|List of energy topics}} |
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− | === Energy in natural sciences === |
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− | * {{Directed link|Energy conversion}} |
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− | * {{Directed link|Enthalpy}} |
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− | * {{Directed link|Energy quality}} |
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− | * {{Directed link|Exergy}} |
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− | * {{Directed link|Power (physics)}} |
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− | * {{Directed link|Specific orbital energy}} |
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− | * {{Directed link|Solar radiation}} |
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− | * {{Directed link|Thermodynamics}} |
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− | * {{Directed link|Thermodynamic entropy}} |
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− | ===Energy resources=== |
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− | *{{Directed link|List of energy resources|List}} |
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− | *{{Directed link|Embodied energy}} |
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− | *{{Directed link|Emergy}} |
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− | *{{Directed link|Energy crisis|Crisis}} |
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− | *{{Directed link|Energy development|Development}} |
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− | *{{Directed link|Energy policy|Policy}} |
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− | *{{Directed link|Renewable energy|Renewable}} |
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− | *{{Directed link|Energy balance}} |
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− | *{{Directed link|Energy demand management|Management}} |
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− | *{{Directed link|Energy storage|Storage}} |
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− | *{{Directed link|Energy transmission|Transmission}} |
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− | *{{Directed link|EU Energy Label}} |
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− | *{{Directed link|EU Intelligent Energy}}, |
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− | *{{Directed link|energy efficiency|Efficiency}} |
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− | == Further reading == |
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− | *{{Directed link|Richard Feynman|Feynman, Richard}}. ''Six Easy Pieces: Essentials of Physics Explained by Its Most Brilliant Teacher''. Helix Book. See the chapter "conservation of energy" for Feynman's explanation of what energy is and how to think about it. |
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− | *{{Directed link|Albert Einstein|Einstein, Albert}} (1952). ''Relativity: The Special and the General Theory (Fifteenth Edition)''. ISBN 0-517-88441-0 |
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− | *{{Directed link|Alfred J. Lotka}} (1956). ''Elements of Mathematical Biology'', forerly published as 'Elements of Physical Biology', Dover, New York. |
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− | == Notes == |
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− | {{reflist}} |
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− | == External links == |
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− | *[http://www.energy-business-review.com Energy Business Review] |
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− | *[http://www.physicsweb.org/article/world/15/7/2 What does energy really mean? From Physics World] |
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− | *[http://www.energy.ca.gov/glossary/ Glossary of Energy Terms] |
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− | * [http://www.iea.org International Energy Agency IEA - {{Directed link|OECD}}] |
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− | * [http://arxiv.org/pdf/physics/0004055.pdf 'Actual' (First-Law) Energy in Relation to Free Energy and Entropy] |
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[[Category:Measurable quantities]] |
[[Category:Measurable quantities]] |
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