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{{Use dmy dates|date=July 2012}} |
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{{for|other types of power|Power (disambiguation)}} |
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{{Infobox physical quantity |
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| unit = [[watt]] |
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| symbols = ''P'' |
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}} |
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{{Classical mechanics}} |
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In [[physics]], '''power''' (symbol: ''P'') is defined as the amount of energy consumed per unit time. In the [[MKS system]], the unit of power is the [[joule]] per second (J/s), known as the [[watt]] (in honour of [[James Watt]], the eighteenth-century developer of the [[Watt steam engine|steam engine]]). For example, the rate at which a light bulb converts electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit time.<ref>{{Cite book|chapter= 6. Power|authors= Halliday and Resnick|title=Fundamentals of Physics|year= 1974}}</ref><ref>Chapter 13, § 3, pp 13-2,3 ''[[The Feynman Lectures on Physics]]'' Volume I, 1963</ref> |
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Energy transfer can be used to do [[Work (physics)|work]], so power is also the rate at which this work is performed. The same amount of work is done when carrying a load up a flight of stairs whether the person carrying it walks or runs, but more power is expended during the running because the work is done in a shorter amount of time. The output power of an electric motor is the product of the [[torque]] the motor generates and the angular velocity of its output shaft. The power expended to move a vehicle is the product of the traction force of the wheels and the velocity of the vehicle. |
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The integral of power over time defines the [[Work (physics)|work]] done. Because this integral depends on the trajectory of the point of application of the force and torque, this calculation of work is said to be [[Nonholonomic system|path dependent]]. |
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==Units== |
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[[File:Ansel Adams - National Archives 79-AAB-02.jpg|right|thumb|alt=Ansel Adams photograph of electrical wires of the Boulder Dam Power Units|Ansel Adams photograph of electrical wires of the Boulder Dam Power Units, 1941–1942]] |
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The dimension of power is energy divided by time. The [[International System of Units|SI]] unit of power is the [[watt]] (W), which is equal to one [[joule]] per second. Other units of power include [[erg]]s per second (erg/s), [[horsepower]] (hp), metric horsepower ([[horsepower#PS|Pferdestärke]] (PS) or [[horsepower#CV and cv|cheval vapeur]], CV), and [[foot-pound force|foot-pounds]] per minute. One horsepower is equivalent to 33,000 foot-pounds per minute, or the power required to lift 550 [[Pound (mass)|pounds]] by one foot in one second, and is equivalent to about 746 watts. Other units include [[dBm]], a relative logarithmic measure with 1 milliwatt as reference; (food) [[calorie]]s per hour (often referred to as [[kilocalorie]]s per hour); [[Btu]] per hour (Btu/h); and [[refrigeration ton|tons of refrigeration]] (12,000 Btu/h). |
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==Average power== |
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As a simple example, burning a kilogram of [[coal]] releases much more energy than does detonating a kilogram of [[Trinitrotoluene|TNT]],<ref>Burning coal produces around 15-30 [[megajoule]]s per kilogram, while detonating TNT produces about 4.7 megajoules per kilogram. For the coal value, see {{cite web | last = Fisher | first = Juliya | title = Energy Density of Coal | work = The Physics Factbook | url = http://hypertextbook.com/facts/2003/JuliyaFisher.shtml|year=2003|accessdate =30 May 2011}} For the TNT value, see the article [[TNT equivalent]]. The coal value does not include the weight of oxygen used during combustion, while the TNT number if TNT only.</ref> but because the TNT reaction releases energy much more quickly, it delivers far more power than the coal. |
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If Δ''W'' is the amount of [[mechanical work|work]] performed during a period of [[time]] of duration Δ''t'', the '''average power''' ''P''<sub>avg</sub> over that period is given by the formula |
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:<math> |
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P_\mathrm{avg} = \frac{\Delta W}{\Delta t}\,. |
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</math> |
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It is the average amount of work done or energy converted per unit of time. The average power is often simply called "power" when the context makes it clear. |
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The '''instantaneous power''' is then the limiting value of the average power as the time interval Δ''t'' approaches zero. |
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:<math> |
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P = \lim _{\Delta t\rightarrow 0} P_\mathrm{avg} = \lim _{\Delta t\rightarrow 0} \frac{\Delta W}{\Delta t} = \frac{dW}{dt}\,. |
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</math> |
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In the case of constant power ''P'', the amount of work performed during a period of duration ''T'' is given by: |
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:<math> |
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W = PT\,. |
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</math> |
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In the context of energy conversion, it is more customary to use the symbol ''E'' rather than ''W''. |
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==Mechanical power== |
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Power in mechanical systems is the combination of forces and movement. In particular, power is the product of a force on an object and the object's velocity, or the product of a torque on a shaft and the shaft's angular velocity. |
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Mechanical power is also described as the time derivative of work. In [[mechanics]], the [[mechanical work|work]] done by a force '''F''' on an object that travels along a curve ''C'' is given by the [[line integral]]: |
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: <math>W_C = \int_{C} \bold{F} \cdot \mathrm{d}\bold{x}</math> |
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Which, when the path is a straight line, can also be written as: |
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:<math>\int\bold{F}\cdot \bold{v}dt,</math> |
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where '''x''' defines the path ''C'' and '''v''' is the velocity along this path. Applying the gradient theorem to the first equation (and remembering that force is the negative of the gradient of the potential energy) yields: |
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:<math>W_C = U(B)-U(A)</math> |
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Where A and B are the beginning and end of the path along which the work was done. |
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Thus the power developed along a path is the time derivative of this: |
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:<math>P(t) = \frac{\mathrm{d}U(B)}{\mathrm{d}t}-\frac{\mathrm{d}U(A)}{\mathrm{d}t}</math> |
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In one dimension and with a constant velocity, this can be simplified to: |
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:<math>P(t) = F\cdot v.</math> |
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In rotational systems, power is the product of the [[torque]] <var>τ</var> and [[angular velocity]] <var>ω</var>, |
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:<math>P(t) = \boldsymbol{\tau} \cdot \boldsymbol{\omega}, \,</math> |
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where '''ω''' measured in radians per second. |
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In fluid power systems such as hydraulic actuators, power is given by |
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:<math> P(t) = pQ, \!</math> |
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where ''p'' is [[pressure]] in [[pascal (unit)|pascals]], or N/m<sup>2</sup> and ''Q'' is [[volumetric flow rate]] in m<sup>3</sup>/s in SI units. |
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===Mechanical advantage=== |
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If a mechanical system has no losses then the input power must equal the output power. This provides a simple formula for the [[mechanical advantage]] of the system. |
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Let the input power to a device be a force ''F<sub>A</sub>'' acting on a point that moves with velocity ''v<sub>A</sub>'' and the output power be a force ''F<sub>B</sub>'' acts on a point that moves with velocity ''v<sub>B</sub>''. If there are no losses in the system, then |
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:<math>P = F_A v_A = F_B v_B, \!</math> |
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and the [[mechanical advantage]] of the system is given by |
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: <math> \mathrm{MA} = \frac{F_B}{F_A} = \frac{v_A}{v_B}. </math> |
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A similar relationship is obtained for rotating systems, where ''T<sub>A</sub>'' and ''ω<sub>A</sub>'' are the torque and angular velocity of the input and ''T<sub>B</sub>'' and ''ω<sub>B</sub>'' are the torque and angular velocity of the output. If there are no losses in the system, then |
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:<math>P = T_A \omega_A = T_B \omega_B, \!</math> |
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which yields the [[mechanical advantage]] |
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:<math> \mathrm{MA} = \frac{T_B}{T_A} = \frac{\omega_A}{\omega_B}.</math> |
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These relations are important because they define the maximum performance of a device in terms of [[velocity ratio]]s determined by its physical dimensions. See for example [[gear ratio]]s. |
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==Power in optics== |
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In [[optics]], or [[radiometry]], the term ''power'' sometimes refers to [[radiant flux]], the average rate of energy transport by electromagnetic radiation, measured in [[watt]]s. In other contexts, it refers to [[optical power]], the ability of a [[lens (optics)|lens]] or other optical device to [[focus (optics)|focus]] light. It is measured in [[dioptre]]s (inverse [[metre]]s), and equals the inverse of the [[focal length]] of the optical device. |
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==Electrical power== |
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{{main|Electric power}} |
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The instantaneous electrical power ''P'' delivered to a component is given by |
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:<math> |
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P(t) = I(t) \cdot V(t) \, |
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</math> |
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where |
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:''P''(''t'') is the instantaneous power, measured in [[watt]]s ([[joule]]s per [[second]]) |
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:''V''(''t'') is the [[potential difference]] (or voltage drop) across the component, measured in [[volt]]s |
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:''I''(''t'') is the [[Electric current|current]] through it, measured in [[ampere]]s |
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If the component is a [[resistor]] with time-invariant [[voltage]] to [[electric current|current]] ratio, then: |
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:<math> |
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P=I \cdot V = I^2 \cdot R = \frac{V^2}{R} \, |
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</math> |
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where |
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:<math> |
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R = \frac{V}{I} \, |
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</math> |
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is the [[electrical resistance|resistance]], measured in [[ohm]]s. |
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==Peak power and duty cycle== |
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[[File:peak-power-average-power-tau-T.png|right|thumb|right|In a train of identical pulses, the instantaneous power is a periodic function of time. The ratio of the pulse duration to the period is equal to the ratio of the average power to the peak power. It is also called the duty cycle (see text for definitions).]] |
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In the case of a periodic signal <math>s(t)</math> of period <math>T</math>, like a train of identical pulses, the instantaneous power <math>p(t) = |s(t)|^2</math> is also a periodic function of period <math>T</math>. The ''peak power'' is simply defined by: |
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:<math> |
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P_0 = \max [p(t)] |
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</math>. |
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The peak power is not always readily measurable, however, and the measurement of the average power <math>P_\mathrm{avg}</math> is more commonly performed by an instrument. If one defines the energy per pulse as: |
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:<math> |
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\epsilon_\mathrm{pulse} = \int_{0}^{T}p(t) \mathrm{d}t \, |
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</math> |
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then the average power is: |
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:<math> |
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P_\mathrm{avg} = \frac{1}{T} \int_{0}^{T}p(t) \mathrm{d}t = \frac{\epsilon_\mathrm{pulse}}{T} \, |
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</math>. |
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One may define the pulse length <math>\tau</math> such that <math>P_0\tau = \epsilon_\mathrm{pulse}</math> so that the ratios |
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:<math> |
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\frac{P_\mathrm{avg}}{P_0} = \frac{\tau}{T} \, |
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</math> |
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are equal. These ratios are called the ''duty cycle'' of the pulse train. |
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==See also== |
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* [[Simple machines]] |
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* [[Mechanical advantage]] |
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* [[Motive power]] |
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* [[Orders of magnitude (power)]] |
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* [[Pulsed power]] |
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* [[Intensity (physics)|Intensity]] — in the radiative sense, power per area |
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* [[Power gain]] — for linear, two-port networks. |
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==References== |
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<references/> |
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[[Category:Concepts in physics]] |
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[[Category:Power (physics)| ]] |
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Revision as of 18:21, 23 September 2013
Hey Guys, GET A LIFE!