Watt: Difference between revisions – Wikipedia

 

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{{About|the unit of power}}

{{About|the unit of power}}

{{redirect|MWT|the former Australian theatre company|Melbourne Workers Theatre}}

{{redirect|MWT|the former Australian theatre company|Melbourne Workers Theatre}}

{{hatnote|”Joules per second” redirects here, it should not be confused with Joule-second}}

{{hatnote|”Joules per second” redirects here, it should not be confused with Joule-second}}

{{Infobox Unit

{{Infobox Unit

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SI derived unit of power

The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m²⋅s⁻³.^[1][2][3] It is used to quantify the rate of energy transfer. The watt is named in honor of James Watt (1736–1819), an 18th-century Scottish inventor, mechanical engineer, and chemist who improved the Newcomen engine with his own steam engine in 1776. Watt’s invention was fundamental for the Industrial Revolution.

Overview[edit]

When an object’s velocity is held constant at one meter per second against a constant opposing force of one newton, the rate at which work is done is one watt.

${displaystyle mathrm {1~W=1~J{/}s=1~N{cdot }m{/}s=1~kg{cdot }m^{2}{cdot }s^{-3}} }$

In terms of electromagnetism, one watt is the rate at which electrical work is performed when a current of one ampere (A) flows across an electrical potential difference of one volt (V), meaning the watt is equivalent to the volt-ampere (the latter unit, however, is used for a different quantity from the real power of an electrical circuit).

${displaystyle mathrm {1~W=1~Vtimes 1~A} }$

Two additional unit conversions for watt can be found using the above equation and Ohm’s law.

${displaystyle mathrm {1~W=1~V^{2}/Omega =1~A^{2}{cdot }Omega } }$

where ohm ($Omega$) is the SI derived unit of electrical resistance.

Examples[edit]

• A person having a mass of 100 kg who climbs a 3-meter-high ladder in 5 seconds is doing work at a rate of about 600 watts. Mass times acceleration due to gravity times height divided by the time it takes to lift the object to the given height gives the rate of doing work or power.^[i]
• A labourer over the course of an eight-hour day can sustain an average output of about 75 watts; higher power levels can be achieved for short intervals and by athletes.^[4]

Origin and adoption as an SI unit[edit]

The watt is named after the Scottish inventor James Watt.^[5] The unit name was proposed initially by C. William Siemens in August 1882 in his President’s Address to the Fifty-Second Congress of the British Association for the Advancement of Science.^[6] Noting that units in the practical system of units were named after leading physicists, Siemens proposed that watt might be an appropriate name for a unit of power.^[7] Siemens defined the unit consistently within the then-existing system of practical units as “the power conveyed by a current of an Ampère through the difference of potential of a Volt”.^[8]

In October 1908, at the International Conference on Electric Units and Standards in London,^[9] so-called “international” definitions were established for practical electrical units.^[10] Siemens’ definition was adopted as the “international” watt. (Also used: 1 A² × 1 Ω.)^[5] The watt was defined as equal to 10⁷ units of power in the “practical system” of units.^[10] The “international units” were dominant from 1909 until 1948. After the 9th General Conference on Weights…