Watt

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, and is used to quantify the rate of energy transfer. In dimensional analysis, power is described by M L 2 T − 3 {displaystyle {mathsf {M}}{mathsf {L}}^{2}{mathsf {T}}^{-3}} . 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, and is used to quantify the rate of energy transfer. In dimensional analysis, power is described by M L 2 T − 3 {displaystyle {mathsf {M}}{mathsf {L}}^{2}{mathsf {T}}^{-3}} . 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. 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). Two additional unit conversions for watt can be found using the above equation and Ohm's Law. Where ohm ( Ω {displaystyle Omega } ) is the SI derived unit of electrical resistance. The watt is named after the Scottish inventor James Watt. This unit 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. 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. 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.' In October 1908, at the International Conference on Electric Units and Standards in London, so-called 'international' definitions were established for practical electrical units. Siemens' definition was adopted as the 'international' watt. (Also used: 1 ampere2 x 1 ohm.) The watt was defined as equal to 107 units of power in the 'practical system' of units. 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 now 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. 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. 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. 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 references 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.