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For other uses, see Watt (disambiguation).
"Kilowatt" redirects here. For the community in Kern County, California, see Kilowatt, California.
The watt (pronounced /ˈwɒt/ wot; symbol: W) is a derived unit of power in the International System of Units (SI), named after the Scottish engineer James Watt (1736–1819). The unit, defined as one joule per second, measures the rate of energy conversion.
Definition
* In terms of classical mechanics, one watt is the rate at which work is done when an object's velocity is held constant at one meter per second against constant opposing force of one newton.
\mathrm{W = \frac{J}{s} = \frac{N\cdot m}{s} = \frac{kg\cdot m^2}{s^3}}
* In terms of electromagnetism, one watt is the rate at which work is done when one ampere (A) of current flows through an electrical potential difference of one volt (V).
\mathrm{W = V \cdot A}
Two additional unit conversions for watt can be found using the above equation and Ohm's Law.
\mathrm{W = \frac{V^2}{\Omega} = A^2\cdot\Omega}
Where ohm (Ω) is the SI derived unit of electrical resistance.
Volt
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For other uses, see Volt (disambiguation).
Josephson junction array chip developed by NIST as a standard volt.
The volt (symbol: V) is the SI derived unit of electromotive force, commonly called "voltage".[1] It is also the unit for the related but slightly different[citation needed] quantity electric potential in a point (voltage as related to a reference ground) and electric potential difference (also called "electrostatic potential difference"[citation needed]). It is named in honor of the Italian physicist Alessandro Volta (1745–1827), who invented the voltaic pile, possibly the first chemical battery.
Definition
The volt is defined as the value of the voltage across a conductor when a current of one ampere dissipates one watt of power in the conductor.[2] It can be written in terms of SI base units as: m2 · kg · s−3 · A−1. It is also equal to one joule of energy per coulomb of charge, J/C.
\mbox{V} = \dfrac{\mbox{W}}{\mbox{A}} = \sqrt{\mbox{W} \cdot \Omega} = \dfrac{\mbox{J}}{\mbox{A} \cdot \mbox{s}} = \dfrac{\mbox{N} \cdot \mbox{m} }{\mbox{A} \cdot \mbox{s}} = \dfrac{\mbox{kg} \cdot \mbox{m}^2}{\mbox{A} \cdot \mbox{s}^{3}} = \dfrac{\mbox{kg} \cdot \mbox{m}^2}{\mbox{C} \cdot \mbox{s}^2} = \dfrac{\mbox{N} \cdot \mbox{m}} {\mbox{C}} = \dfrac{\mbox{J}}{\mbox{C}}
[edit] Josephson junction definition
Between 1990 and 1997 the volt was calibrated using the Josephson effect for exact voltage-to-frequency conversion, combined with cesium-133 time reference, as decided by the 18th General Conference on Weights and Measures. The following value for the Josephson constant is used:
K{J-90} = 2e/h = 0.4835979 GHz/µV.
This is typically used with an array of several thousand or tens of thousands of junctions, excited by microwave signals between 10 and 80 GHz (depending on the array design).[3] Empirically, several experiments have shown that the method is independent of device design, material, measurement setup, etc, , and no correction terms are required in a practical implementation. [4] However, as of July 2007, this is not the official BIPM definition of Volt.[5]]
Ampere
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For other uses, see Ampere (disambiguation).
Current can be measured by a galvanometer, via the deflection of a magnetic needle in the magnetic field created by the current.
The ampere (symbol: A) is the SI unit of electric current[1] (symbol: I) and is one of the seven[2] SI base units. It is named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics. In practice, its name is often shortened to amp.
In practical terms, the ampere is a measure of the amount of electric charge passing a point per unit time. Around 6.241 × 1018 electrons, or one coulomb, passing a given point each second constitutes one ampere.
Definition
Ampère's force law[4][5] states that there is an attractive force between two parallel wires carrying an electric current. This force is used in the formal definition of the ampere which states that it is "the constant current which will produce an attractive force of 2 × 10–7 newton per metre of length between two straight, parallel conductors of infinite length and negligible circular cross section placed one metre apart in a vacuum".[1][6]
In terms of Ampère's force law,
so
The SI unit of charge, the coulomb, "is the quantity of electricity carried in 1 second by a current of 1 ampere."[7] Conversely, a current of one ampere is one coulomb of charge going past a given point per second:
That is, in general, charge Q is determined by steady current I flowing for a time t as Q = It.
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