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The watt (symbol: W) is the SI derived unit of power, equal to one joule per second. A human being climbing a flight of stairs is doing work at the rate of about 200 watts; a trained athlete can work at up to 900 watts for short periods. An automobile engine produces work at a rate of around 100 000 watts. A typical household, incandescent lightbulb uses 40 to 100 watts.
Definition One watt is one joule (the SI unit of energy) per second. 1 W = 1 J/s = 1 newton meter per second = 1 kg·m2·s−3 Origin 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 Conférence Générale des Poids et Mesures in 1960. Use in electrical circuits In an electrical circuit, the watt is a very convenient unit since the power, P, expressed in watts, is equal to the voltage or electromotive force, E, expressed in volts, times the current, I, expressed in amperes: P = E times I, or, P=E I SI multiples For example, the power usage of a device would be computed as follows: (Note: Since the domestic voltage is usually alternating current (AC) as opposed to direct current (DC) you would imagine the need to apply a reduction by approximately one-third. This is because the average power in an AC sine wave is an Root mean square (RMS) value of its equivalent DC voltage. However most domestic voltages are already quoted in RMS values so there is in fact no need). If the device is wired for North American current (120V) P= 120 volts × 2.4 amps P= 288 watts Cost (Approx.) = (Power consumption × Cost = (288W × 24h)/1000 × .13¤/kWh × 30 (Assuming the device is run continuously. This will also give the maximum cost possible) Cost = 6912Wh/1000 × .13¤/kWh × 30 Cost = 6.912kWh × .13¤/kWh × 30 Cost = 0.89856¤ × 30 Cost = 26.9568¤ Note: is less. Megawatt The megawatt (symbol: MW) is equal to one million (106) watts. Few things can sustain the transfer or consumption of energy on this scale; some of these events or entities include: lightning strikes, naval craft (such as aircraft carriers and submarines), engineering hardware, and some scientific research equipment (such as the supercollider and large lasers). The productive capacity of electrical generators operated by utility companies is often measured in MW. Modern high-powered diesel-electric railroad locomotives typically have a peak power output of (3 to 5) MW, whereas a typical modern nuclear power plant produces a peak output on the order of 500 to 2000 MW. 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 (506:2). Watts electrical and thermal Watt electrical (abbreviation: We) is a term that refers to power produced as electricity. SI prefixes can be used, for example megawatt electrical (MWe) and gigawatt electrical (GWe). Watt thermal (abbreviation: Wt or Wth) refers to thermal power produced; again SI prefixes can be used, for example megawatt thermal (MWt) and gigawatt thermal (GWt). These terms are used to disambiguate the electric output of a thermal power station versus the (larger) thermal output. For example, the Embalse nuclear power plant in Argentina uses a fission reactor to generate 2109 MWth of heat, which creates steam to drive a turbine, which generates 648 MWe of electricity. Kilowatt-hour A kilowatt-hour (kWh) is a unit of energy, equal to 3 600 000 joules (3.6 MJ) or the energy that would be transferred at a constant rate of one kilowatt per hour. This unit is often used in the context of power plants and home energy bills. This is, however, not an SI unit because the hour is not an SI unit. Trivia In the popular film series Back to the Future, Doc Emmett Brown is known for his pronunciation of the word "gigawatt" in the famous line, "1.21 gigawatts!" (He pronounces it as if it started with a "j"). Kevin Self reports that in the 1920s giga- was pronounced with a hard g, as in golf. For some period in the 20th century, Doc's soft g was widespread, but by 1995 the pronunciation had returned to the hard g. See also Notes | ||||||||
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