The watt (symbol: W) is a unit of power. In the International System of Units (SI) it is defined as a derived unit of 1 joule per second,[1] and is used to quantify the rate of energy transfer. In dimensional analysis it is described by M L 2 T − 3 displaystyle mathsf ML ^ 2 mathsf T ^ -3 . Contents 1 Examples 2 Origin and adoption as an SI unit 3 Multiples 3.1 Attowatt 3.2 Femtowatt 3.3 Picowatt 3.4 Nanowatt 3.5 Microwatt 3.6 Milliwatt 3.7 Kilowatt 3.8 Megawatt 3.9 Gigawatt 3.10 Terawatt 3.11 Petawatt 4 Conventions in the electric power industry
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Examples[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 1 watt. 1 W = 1 J s = 1 N ⋅ m s = 1 k g ⋅ m 2 s 3 displaystyle mathrm 1~W=1~ frac J s =1~ frac N cdot m s =1~ frac kg cdot m^ 2 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). 1 W = 1 V ⋅ 1 A displaystyle mathrm 1~W=1~Vcdot 1~A Two additional unit conversions for watt can be found using the above equation and Ohm's Law. 1 W = 1 V 2 Ω = 1 A 2 ⋅ Ω displaystyle mathrm 1~W=1~ frac V^ 2 Omega =1~A^ 2 cdot Omega Where ohm ( Ω displaystyle Omega ) is the
A person having a mass of 100 kilograms 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 8-hour day can sustain an average output of about 75 watts; higher power levels can be achieved for short intervals and by athletes.[2] Origin and adoption as an SI unit[edit]
The watt is named after the Scottish inventor
For additional examples of magnitude for multiples and submultiples of the watt, see Orders of magnitude (power) SI multiples for 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 Common multiples are in bold face Attowatt[edit]
The attowatt (aW) is equal to 10−18 watt. The sound intensity in
water corresponding to the international standard reference sound
pressure of 1 μPa is approximately 0.65 aW/m2.[3]
Femtowatt[edit]
The femtowatt (fW) is equal to one quadrillionth (10−15) of a watt.
Technologically important powers that are measured in femtowatts are
typically found in reference(s) to radio and radar receivers. For
example, meaningful 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 microvolt 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[edit]
The picowatt (pW), not to be confused with the much larger petawatt
(PW), is equal to one trillionth (10−12) of a watt. Technologically
important powers that are 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 as a
level in decibels.[4]
Nanowatt[edit]
The nanowatt (nW) is equal to one billionth (10−9) of a watt.
Important powers that are measured in nanowatts are also typically
used in reference to radio and radar receivers.
Microwatt[edit]
The microwatt (µW) is equal to one millionth (10−6) of a watt.
Important powers that are measured in microwatts are typically stated
in medical instrumentation systems such as the
Power factor
Volt-ampere
Notes[edit] ^ 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. ^ 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.[7] ^ Watts per hour would properly refer to a rate of change of power being used (or generated). Watts per hour might be useful to characterize the ramp-up behavior of power plants, or slow-reacting plant where their power could only change slowly. For example, a power plant that changes its power output from 1 MW to 2 MW in 15 minutes would have a ramp-up rate of 4 MW/h. References[edit] ^
External links[edit] Look up watt in Wiktionary, the free dictionary. Borvon, Gérard, History of the electrical units, FR: Free . Nelson, Robert A (February 2000), The International System of Units: Its History and Use in Science and Industry, Via Satellite, ATI courses . v t e SI units Authority:
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