SI unit of electrical resistance; symbol ?; a resistance of 1 ohm exists between two points of a conductor if a potential difference of 1 volt causes a current of 1 amp to flow between them; named after Georg Ohm; commonly used as kilohms (k?, 103 ohms) and megohms (M?, 106 ohms). In 1990 the von Klitzing constant, RK-90, became the international standard of resistance measurement.
This article is about the SI derived unit. For other meanings, see Ohm (disambiguation).The ohm (symbol: Ω) is the SI unit of electrical impedance or, in the direct current case, electrical resistance, named after Georg Ohm.
Definition
An ohm is a resistance that produces a potential difference of one volt when a current of one ampere is flowing through it.
1 Ω = 1 V/A = 1 m²·kg·sOrigin
| This SI unit is named after Georg Ohm. As for all SI units whose names are derived from the proper name of a person, the first letter of its symbol is uppercase (Ω). |
Ohm, a German physicist, discovered the relation between voltage and current in a metallic conductor, expressed in Ohm's Law.
The unit was first suggested as the ohma by Charles Tilston Bright and Latimer Clark in 1861, although the spelling is recorded in 1864 as ohmad. Ohm was adopted into the SI by CIPM Resolution 2, 1946.
Explanation
By definition from Ohm's Law, a device has a resistance of one ohm if a voltage of one volt causes a current of one ampere to flow (R = V/I). Alternatively and equivalently, a device that dissipates one watt of power with one ampere of current flowing through it has a resistance of one ohm (R = P / I 2).
Since 1990, the ohm has been maintained internationally using the quantum Hall effect, where a conventional value is used for the 'von-Klitzing constant', fixed by the 18th General Conference on Weights and Measures as R{K-90} = 25812.807 Ω.
The complex quantity impedance is a generalisation of resistance. Impedance, resistance and reactance all have units of ohms.
The symbol for the ohm is the Greek capital letter omega (Ω). The various guides for the use of the International System of Units do not explicitly forbid the elision of the final "o" of some SI prefixes, although there is nothing in them to suggest that it is allowable, either. In the particular case of the ohm, one even sees the "a" prefixes lose that vowel: hence megohm and gigohm. Higher prefixes are rarely used with ohm.
Units of ohms, kilohms (10 Ω) are used in electronic design documentation. so 680 ohms can be shown as 680 or 680R.
Conversions
A measurement in ohms is the reciprocal of a measurement in siemens, the SI unit of electrical conductance.
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