Watt

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watt
SI
Unit ofpower
SymbolW
Named afterJames Watt
Conversions
1 W in ...... is equal to ...
   
CGS units
   107 ergs−1
   English Engineering Units   0.7375621 ft⋅lbf/s = 0.001341022 hp

The watt (symbol: W) is the unit of

Newcomen engine with his own steam engine in 1776. Watt's invention was fundamental for the Industrial Revolution
.

Overview

When an object's

meter per second against a constant opposing force of one newton, the rate at which work
is done is one watt.

In terms of

real power
of an electrical circuit).

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

where ohm () is the
electrical resistance
.

Examples

  • 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 laborer 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

The watt is named after the Scottish inventor

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 within the 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 and Measures in 1948, the international watt was redefined from practical units to absolute units (i.e., using only length, mass, and time). Concretely, this meant that 1 watt was defined as the quantity of energy transferred in a unit of time, namely 1 J/s. In this new definition, 1 absolute watt = 1.00019 international watts. Texts written before 1948 are likely to be using the international watt, which implies caution when comparing numerical values from this period with the post-1948 watt.[5] In 1960, the 11th General Conference on Weights and Measures adopted the absolute watt into the International System of Units (SI) as the unit of power.[11]

Multiples

SI multiples of watt (W)
Submultiples Multiples
Value SI symbol Name Value SI symbol Name
10−1 W dW deciwatt 101 W daW decawatt
10−2 W cW centiwatt 102 W hW hectowatt
10−3 W mW milliwatt 103 W kW kilowatt
10−6 W μW microwatt 106 W MW megawatt
10−9 W nW nanowatt 109 W GW gigawatt
10−12 W pW picowatt 1012 W TW terawatt
10−15 W fW femtowatt 1015 W PW petawatt
10−18 W aW attowatt 1018 W EW exawatt
10−21 W zW zeptowatt 1021 W ZW zettawatt
10−24 W yW yoctowatt 1024 W YW yottawatt
10−27 W rW rontowatt 1027 W RW ronnawatt
10−30 W qW quectowatt 1030 W QW quettawatt
Common multiples are in bold face
Attowatt
The sound intensity in water corresponding to the international standard reference
μPa is approximately 0.65 aW/m2.[12]
Femtowatt
Powers measured in femtowatts are typically found in references to
FM tuner performance figures for sensitivity, quieting and signal-to-noise require that the RF energy applied to the antenna input be specified. These input levels are often stated in dBf (decibels referenced to 1 femtowatt). This is 0.2739 microvolts across a 75-ohm load or 0.5477 microvolt across a 300-ohm load; the specification takes into account the RF input impedance
of the tuner.
Picowatt
Powers measured in picowatts are typically used in reference to radio and radar receivers, acoustics and in the science of radio astronomy. One picowatt is the international standard reference value of sound power when this quantity is expressed in decibels.[13]
Nanowatt
Powers measured in nanowatts are also typically used in reference to radio and radar receivers.
Microwatt
Powers measured in microwatts are typically stated in
solar cells for devices such as calculators and watches are typically measured in microwatts.[14]
Milliwatt
A typical laser pointer outputs about five milliwatts of light power, whereas a typical hearing aid uses less than one milliwatt.[15] Audio signals and other electronic signal levels are often measured in dBm, referenced to one milliwatt.
Kilowatt
The kilowatt is typically used to express the output power of engines and the power of electric motors, tools, machines, and heaters. It is also a common unit used to express the electromagnetic power output of broadcast radio and television transmitters.
One kilowatt is approximately equal to 1.34 horsepower. A small electric heater with one heating element can use 1 kilowatt. The average electric power consumption of a household in the United States is about 1 kilowatt.[ii]
A surface area of 1 square meter on Earth receives typically about one kilowatt of sunlight from the Sun (the solar irradiance) (on a clear day at midday, close to the equator).[17]
Megawatt
Many events or machines produce or sustain the conversion of energy on this scale, including large electric motors; large warships such as aircraft carriers, cruisers, and submarines; large
diesel-electric locomotives typically produce and use 3 and 5 MW. U.S. nuclear power plants have net summer capacities between about 500 and 1300 MW.[18]: 84–101 
The earliest citing of the megawatt in the Oxford English Dictionary (OED) is a reference in the 1900 Webster's International Dictionary of the English Language. The OED also states that megawatt appeared in a 28 November 1947 article in the journal Science
(506:2).
A United States Department of Energy video explaining gigawatts
Gigawatt
A gigawatt is typical average power for an industrial city of one million habitants and also the output of a large power station. The GW unit is thus used for large power plants and power grids. For example, by the end of 2010, power shortages in China's Shanxi province were expected to increase to 5–6 GW[19] and the installation capacity of wind power in Germany was 25.8 GW.[20] The largest unit (out of four) of the Belgian Doel Nuclear Power Station has a peak output of 1.04 GW.[21] HVDC converters have been built with power ratings of up to 2 GW.[22]
Terawatt
The primary energy used by humans worldwide was about 160,000 terawatt-hours in 2019, corresponding to an average continuous power consumption of 18 TW that year.[23] The most powerful lasers from the mid-1960s to the mid-1990s produced power in terawatts, but only for nanosecond intervals. The average lightning strike peaks at 1 TW, but these strikes only last for 30 microseconds.
Petawatt
A petawatt can be produced by the current generation of lasers for time scales on the order of picoseconds. One such laser is the Lawrence Livermore's Nova laser, which achieved a power output of 1.25 PW by a process called chirped pulse amplification. The duration of the pulse was roughly 0.5 ps, giving a total energy of 600 J.[24] Another example is the Laser for Fast Ignition Experiments (LFEX) at the Institute of Laser Engineering (ILE), Osaka University, which achieved a power output of 2 PW for a duration of approximately 1 ps.[25][26]
Based on the average total solar irradiance of 1.361 kW/m2,[27] the total power of sunlight striking Earth's atmosphere is estimated at 174 PW. The planet's average rate of global warming, measured as Earth's energy imbalance, reached about 0.5 PW (0.3% of incident solar power) by 2019.[28]
Yottawatt
The power output of the Sun is 382.8 YW, about 2 billion times the power estimated to reach Earth's atmosphere.[29]

Conventions in the electric power industry

In the

combined heat and power station such as Avedøre Power Station.[34]

When describing alternating current (AC) electricity, another distinction is made between the watt and the volt-ampere. While these units are equivalent for simple resistive circuits, they differ when loads exhibit electrical reactance.

Radio transmission

half-wave dipole antenna would need to radiate to match the intensity of the transmitter's main lobe
.

Distinction between watts and watt-hours

The terms power and energy are closely related but distinct physical quantities. Power is the rate at which energy is generated or consumed and hence is measured in units (e.g. watts) that represent energy per unit time.

For example, when a

watt hours (W·h), 0.1 kilowatt hour, or 360 kJ
. This same amount of energy would light a 40-watt bulb for 2.5 hours, or a 50-watt bulb for 2 hours.

terawatt hours
for a given period; often a calendar year or financial year. One terawatt hour of energy is equal to a sustained power delivery of one terawatt for one hour, or approximately 114 megawatts for a period of one year:

Power output = energy / time
1 terawatt hour per year = 1×1012 W·h / (365 days × 24 hours per day) ≈ 114 million watts,

equivalent to approximately 114 megawatts of constant power output.

The

watt-second is a unit of energy, equal to the joule
. One kilowatt hour is 3,600,000 watt seconds.

While a watt per hour is a unit of rate of change of power with time,[iii] it is not correct to refer to a watt (or watt-hour) as a watt per hour.[35]

See also

Explanatory notes

  1. ^ The energy in climbing the stairs is given by mgh. Setting m = 100 kg, g = 9.8 m/s2 and h = 3 m gives 2940 J. Dividing this by the time taken (5 s) gives a power of 588 W.
  2. ^ Average household electric power consumption is 1.19 kW in the US, 0.53 kW in the UK. In India it is 0.13 kW (urban) and 0.03 kW (rural) – computed from GJ figures quoted by Nakagami, Murakoshi and Iwafune.[16]
  3. power plants on an electric grid to compensate for loss of output from other sources, such as when solar power generation drops to zero as the sun sets. See duck curve
    .

References

  1. . §2.3.4, Table 4.
  2. .
  3. (PDF) from the original on 2021-06-04, retrieved 2021-12-16
  4. .
  5. ^ .
  6. ^ "Address by C. William Siemens". Report of the Fifty-Second meeting of the British Association for the Advancement of Science. Vol. 52. London: John Murray. 1883. pp. 1–33.
  7. ^ Siemens supported his proposal by asserting that Watt was the first who "had a clear physical conception of power, and gave a rational method for measuring it." "Siemens, 1883, p. 6"
  8. ^ "Siemens", 1883, p. 5"
  9. .
  10. ^ a b Fleming, John Ambrose (1911). "Units, Physical" . In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 27 (11th ed.). Cambridge University Press. pp. 738–745, see page 742.
  11. ^ "Resolution 12 of the 11th CGPM (1960)". Bureau International des Poids et Mesures (BIPM). Archived from the original on April 20, 2020. Retrieved 9 April 2018.
  12. ^ Ainslie, M. A. (2015). A century of sonar: Planetary oceanography, underwater noise monitoring, and the terminology of underwater sound. Acoustics Today.
  13. ^ Morfey, C.L. (2001). Dictionary of Acoustics.
  14. ^ "Bye-Bye Batteries: Radio Waves as a Low-Power Source", The New York Times, Jul 18, 2010, archived from the original on 2017-03-21.
  15. ^ Stetzler, Trudy; Magotra, Neeraj; Gelabert, Pedro; Kasthuri, Preethi; Bangalore, Sridevi. "Low-Power Real-Time Programmable DSP Development Platform for Digital Hearing Aids". Datasheet Archive. Archived from the original on 3 March 2011. Retrieved 8 February 2010.
  16. ^ Nakagami, Hidetoshi; Murakoshi, Chiharu; Iwafune, Yumiko (2008). International Comparison of Household Energy Consumption and Its Indicator (PDF). ACEEE Summer Study on Energy Efficiency in Buildings. Pacific Grove, California: American Council for an Energy-Efficient Economy. Figure 3. Energy Consumption per Household by Fuel Type. 8:214–8:224. Archived (PDF) from the original on 9 January 2015. Retrieved 14 February 2013.
  17. , p.153
  18. ^ "Appendix A | U.S. Commercial Nuclear Power Reactors" (PDF). 2007–2008 Information Digest (Report). Vol. 19. United States Nuclear Regulatory Commission. 1 August 2007. pp. 84–101. Archived from the original (PDF) on 16 February 2008. Retrieved 27 December 2021.
  19. ^ Bai, Jim; Chen, Aizhu (11 November 2010). Lewis, Chris (ed.). "China's Shanxi to face 5–6 GW power shortage by yr-end – paper". Peking: Reuters.
  20. ^ "Not on my beach, please". The Economist. 19 August 2010. Archived from the original on 24 August 2010.
  21. ^ "Chiffres clés" [Key numbers]. Electrabel. Who are we: Nuclear (in French). 2011. Archived from the original on 2011-07-10.
  22. ^ Davidson, CC; Preedy, RM; Cao, J; Zhou, C; Fu, J (October 2010), "Ultra-High-Power Thyristor Valves for HVDC in Developing Countries", 9th International Conference on AC/DC Power Transmission, London: IET.
  23. ^ Hannah Ritchie; Max Roser (2020). "Global Direct Primary Energy Consumption". Our World in Data. Published online at OurWorldInData.org. Retrieved 2020-02-09.
  24. ^ "Crossing the Petawatt threshold". Livermore, CA: Lawrence Livermore National Laboratory. Archived from the original on 15 September 2012. Retrieved 19 June 2012.
  25. ^ World's most powerful laser: 2 000 trillion watts. What's it?, IFL Science, 12 August 2015, archived from the original on 2015-08-22.
  26. ^ Eureka alert (publicity release), Aug 2015, archived from the original on 2015-08-08.
  27. ^ "Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present". CH: PMODWRC. Archived from the original on 2011-08-30. Retrieved 2005-10-05.
  28. .
  29. ^ Williams, David R. "Sun Fact Sheet". nasa.gov. NASA. Retrieved 26 February 2022.
  30. ^ Rowlett, Russ. "How Many? A Dictionary of Units of Measurement. M". University of North Carolina at Chapel Hill. Archived from the original on 2011-09-04. Retrieved 2017-03-04.
  31. ^ Cleveland, CJ (2007). "Watt". Encyclopedia of Earth.
  32. ^ "Solar Energy Grew at a Record Pace in 2008 (excerpt from EERE Network News".
    US: Department of Energy). 25 March 2009. Archived
    from the original on 18 October 2011.
  33. (PDF) from the original on 2021-06-04, retrieved 2021-12-16
  34. DONG Energy. Archived from the original
    on 2014-03-17. Retrieved 2014-03-17.
  35. ^ "Inverter Selection". Northern Arizona Wind and Sun. Archived from the original on 1 May 2009. Retrieved 27 March 2009.

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