Watt
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225878468
2008-07-15T20:46:00Z
Gerry Ashton
1479481
[[WP:UNDO|Undid]] revision 225878138 by [[Special:Contributions/162.34.231.16|162.34.231.16]] ([[User talk:162.34.231.16|talk]])
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The '''watt''' (symbol: '''W''') is the [[SI]] [[SI derived unit|derived unit]] of [[power (physics)|power]], equal to one [[joule]] of energy per [[second]]. It measures a rate of energy use or production.
A [[human]] climbing a flight of stairs is doing work at a rate of about 200 watts. A typical [[automobile]] engine produces mechanical energy at a rate of 25,000 watts (approximately 33.5 [[horsepower]]) while cruising. A typical household [[incandescent light bulb]] uses electrical energy at a rate of 25 to 100 watts, while [[compact fluorescent light]]s typically consume 5 to 30 watts.
== Definition ==
One watt is the rate at which work is done when an object is moving at one meter per second against a force of one newton.
:<math>
1~\rm{W} = 1~\dfrac{\rm{J}}{\rm{s}} = 1~\dfrac{\rm{kg} \cdot \rm{m^2}}{\rm{s^3}} = 1~\dfrac{\rm{N\cdot m}}{\rm{s}} \,
</math>.
By the definition of the units of [[ampere]] and [[volt]], work is done at a rate of one watt when one ampere flows through a potential difference of one volt. <ref>{{cite web | title=Amps, Volts, Watts, Ohms | accessdate=2007-04-17 | url=http://ebtx.com/mech/ampvolt.htm}}</ref>
== Origin and adoption as an SI unit ==
The '''watt''' is named after [[James Watt]] for his contributions to the development of the [[steam engine]], and was adopted by the Second Congress of the British Association for the Advancement of Science in 1889 and by the 11th [[General Conference on Weights and Measures]] in 1960 as the unit of power incorporated in the [[International System of Units]] (or "SI").
{{SI unit lowercase|James Watt|watt|W}}
== Derived and qualified units for power distribution ==
===Microwatt===
The '''microwatt''' (symbol:'''μW''') is equal to one millionth (10<sup>-6</sup>) of a watt.
===Milliwatt===
The '''milliwatt''' (symbol:'''mW''') is equal to one thousandth (10<sup>-3</sup>) of a watt.
=== Kilowatt ===
The '''kilowatt''' (symbol: '''kW'''), equal to one thousand watts, is typically used to state the power output of [[engine]]s and the power consumption of tools and machines. A kilowatt is approximately equivalent to 1.34 [[horsepower]]. An electric heater with one heating-element might use 1 kilowatt.
=== Megawatt ===
The '''megawatt''' (symbol: MW) is equal to one [[million]] (10<sup>6</sup>) watts.
Many things can sustain the transfer or consumption of energy on this scale; some of these events or entities include: [[lightning]] strikes, large electric motors, naval craft (such as [[aircraft carrier]]s and [[submarine]]s), engineering hardware, and some scientific research equipment (such as the [[supercollider]] and large [[laser]]s). A large residential or retail building may consume several megawatts in electric power and heating energy.
The productive capacity of [[electrical generator]]s operated by [[utility company|utility companies]] is often measured in MW. Modern high-powered [[diesel-electric]] railroad [[locomotive]]s typically have a peak power output of 3 to 5 MW, whereas U.S. [[nuclear power plant]]s have net summer capacities between about 500 and 1300 MW.<ref>[[Nuclear Regulatory Commission]]. (2007). [http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1350/v19/sr1350v19.pdf ''2007–2008 Information Digest'']. Retrieved on [[2008-01-27]]. Appendix A.</ref>
According to the ''[[Oxford English Dictionary]]'', the earliest citing for "megawatt" is a reference in the 1900 ''[[Webster's]] International Dictionary of English Language''. The OED also says "megawatt" appeared in a [[28 November]], [[1847]], article in ''[[Science (journal)|Science]]'' (506:2).
===Gigawatt===
The '''gigawatt''' (symbol: GW) is equal to one [[1,000,000,000 (number)|billion]] (10<sup>9</sup>) watts. This unit is sometimes used with large power plants or power grids.
===Terawatt===
The '''terawatt''' (symbol: TW) is equal to one [[trillion]] (10<sup>12</sup>) watts. The average energy usage by humans (about 15 TW) is commonly measured in these units. The most powerful [[laser]]s from the mid 1960s to the mid 1990s produced power in terawatts, but only for nanoseconds.
=== Electrical and thermal ===
In the electric power industry, '''Megawatt electrical''' (abbreviation: MW<sub>e</sub>{{Fact|date=January 2008}} or MWe<ref>[http://www.unc.edu/~rowlett/units/dictM.html How Many? A Dictionary of Units of Measurement]</ref>) is a term that refers to [[electric power]], while '''megawatt thermal''' (abbreviations: MW<sub>t</sub>, MW<sub>th</sub>, MWt, or MWth) refers to thermal power produced. Other [[SI prefix]]es are sometimes used, for example ''gigawatt electrical'' (GW<sub>e</sub>).
<ref> 'Megawatt electrical' and 'megawatt thermal' are not SI units,[http://physics.nist.gov/Pubs/pdf.html Taylor 1995, Guide for the Use of the International System of Units (SI), NIST Special Publication SP811] The [[International Bureau of Weights and Measures]] states that unit symbols should not use subscripts to provide additional information about the quantity being measured, and regards these symbols as incorrect. International Bureau of Weights and Measures. (2006). [http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf The International System of Units (SI)]. 132.</ref>
For example, the [[Embalse nuclear power plant]] in Argentina uses a [[fission reactor]] to generate 2109 MW<sub>t</sub> of heat, which creates steam to drive a turbine, which generates 648 MW<sub>e</sub> of electricity. The difference is heat lost to the surroundings.
== Confusion of watts and watt-hours ==
[[Power (physics)|Power]] and [[energy]] are frequently confused in the general media. Power is the rate at which energy is used. A watt is one joule of energy per second. For example, if a 100 watt light bulb is turned on for one hour, the energy used is 100 [[watt-hour]]s or 0.1 kilowatt-hour, or 360,000 joules. This same quantity of energy would light a 40 watt bulb for 2.5 hours. A power station would be rated in watts, but its annual energy sales would be in watt-hours (or kilowatt-hours or megawatt-hours). A kilowatt-hour is the amount of energy equivalent to a steady power of 1 kilowatt running for 1 hour:
:(1 kW·h)(1000 W/kW)(3600 s/h) = 3,600,000 W·s = 3,600,000 J = 3.6 [[megajoule|MJ]].
== See also ==
{{portal|Energy}}
<div style="-moz-column-count:2; column-count:2;">
*[[Volt-ampere]]
*[[Conversion of units]]
*[[James Watt]]
*[[Declared net capacity]] (power plants)
*[[Orders of magnitude (power)]]
*[[Power factor]]
*[[Root mean square]] (RMS)
*[[Watt balance]]
*[[Watt-hour]]
*[[Wattmeter]]
</div>
== References ==
{{reflist}}
== External links ==
*Nelson, Robert A., "''[http://www.aticourses.com/international_system_units.htm The International System of Units] Its History and Use in Science and Industry''". Via Satellite, February 2000.
[[Category:SI derived units]]
[[Category:Units of power]]
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