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The volt-ampere (SI symbol: VA, [1] sometimes V⋅A or V A) is the unit of measurement for apparent power in an electrical circuit. It is the product of the root mean square voltage (in volts) and the root mean square current (in amperes). [2] Volt-amperes are usually used for analyzing alternating current (AC) circuits.
The ampere is named for French physicist and mathematician André-Marie Ampère (1775–1836), who studied electromagnetism and laid the foundation of electrodynamics.In recognition of Ampère's contributions to the creation of modern electrical science, an international convention, signed at the 1881 International Exposition of Electricity, established the ampere as a standard unit of ...
"Sq" current of one daytime vortex within the ionospheric dynamo region: 180 kA Typical current used in electric arc furnace for ferroalloys [11] 10 6: 1 MA High range of Birkeland current: 5 MA Flux tube between Jupiter and Io (moon) [12] 26 MA Sandia National Laboratories, Z machine approximate firing current [13] since 2007 256 MA
In the International System of Units (SI), electric current is expressed in units of ampere (sometimes called an "amp", symbol A), which is equivalent to one coulomb per second. The ampere is an SI base unit and electric current is a base quantity in the International System of Quantities (ISQ).
In AC circuits, a current transformer can be used to convert the large current in the main circuit into a smaller current more suited to a meter. Some designs of transformer are able to directly convert the magnetic field around a conductor into a small AC current, typically either 1 A or 5 A at full rated current, that can be easily read by a ...
That is, the ratio of V/I is constant, and when current is plotted as a function of voltage the curve is linear (a straight line). If voltage is forced to some value V, then that voltage V divided by measured current I will equal R. Or if the current is forced to some value I, then the measured voltage V divided by that current I is also R.
Historically the "conventional" volt, V 90, defined in 1987 by the 18th General Conference on Weights and Measures [3] and in use from 1990 to 2019, was implemented using the Josephson effect for exact frequency-to-voltage conversion, combined with the caesium frequency standard. Though the Josephson effect is still used to realize a volt, the ...
The base value should only be a magnitude, while the per-unit value is a phasor. The phase angles of complex power, voltage, current, impedance, etc., are not affected by the conversion to per unit values. The purpose of using a per-unit system is to simplify conversion between different transformers.