Ohm

From Freepedia

This article is about the SI derived unit. For the scientist, see Georg Ohm.
For the Hindu mantra syllable, see aum.

The ohm (symbol: Ω) is the SI unit of electric resistance.

Contents

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·s–3·A–2

SI multiples

Multiple Name Symbol Multiple Name Symbol
100 ohm Ω      
101 decaohm daΩ 10–1 deciohm dΩ
102 hectoohm hΩ 10–2 centiohm cΩ
103 kiloohm kΩ 10–3 milliohm mΩ
106 megaohm MΩ 10–6 microohm µΩ
109 gigaohm GΩ 10–9 nanoohm nΩ
1012 teraohm TΩ 10–12 picoohm pΩ
1015 petaohm PΩ 10–15 femtoohm fΩ
1018 exaohm EΩ 10–18 attoohm aΩ
1021 zettaohm ZΩ 10–21 zeptoohm zΩ
1024 yottaohm YΩ 10–24 yoctoohm yΩ

Origin

The ohm is named after Georg Ohm, a German physicist who discovered the relation between voltage and current, expressed in Ohm's Law.

Explanation

Image:Ohmslawvoltagesource.png 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/I2).

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. Its real part is resistance and its imaginary part is reactance. Impedance, resistance and reactance all have units of ohms.

The symbol for the ohm is the Greek capital letter omega (Ω). If the Greek letter cannot be used, the word ohm is used instead. The various guides for the use of the International System of Units (such as the official SI brochure and NIST Special Publication 811) 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. As a result, one is just about as likely to see "kilohm", "kiloohm" and even "kilo-ohm", and the same holds true for hecto-, micro-, nano-, pico-, femto-, atto-, zepto-, and yocto-. The only other SI unit to suffer from this kind of orthographic uncertainty is the ampere. 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. In the other direction, milliohms (or millohms) are seen where the resistance of cables, etc., are measured.

Conversions

A measurement in ohms is the reciprocal of a measurement in siemens, the SI unit of electrical conductance. Note that 'siemens' is both singular and plural. The reciprocal of the ohm is also called the mho, from ohm written backwards.

SI electricity units

SI electromagnetic units

edit

Quantity Name Symbol Dimensions
Current ampere (SI base unit) A A
Electric charge, Quantity of electricity coulomb C A·s
Potential difference volt V J/C = kg·m2·s−3·A−1
Resistance, Impedance, Reactance ohm Ω V/A = kg·m2·s−3·A−2
Resistivity ohm metre Ω·m kg·m3·s−3·A−2
Electrical power watt W V·A = kg·m2·s−3
Capacitance farad F C/V = kg−1·m−2·A2·s4
Elastance reciprocal farad F−1 kg·m2·A−2·s−4
Permittivity farad per metre F/m kg−1·m−3·A2·s4
Conductance, Admittance, Susceptance siemens S Ω−1 = kg−1·m−2·s3·A2
Conductivity siemens per metre S/m kg−1·m−3·s3·A2
Magnetic flux weber Wb V·s = kg·m2·s−2·A−1
Magnetic flux density tesla T Wb/m2 = kg·s−2·A−1
Magnetic induction ampere per metre A/m A·m−1
Reluctance ampere-turns per weber A/Wb kg−1·m−2·s2·A2
Inductance henry H Wb/A = V·s/A = kg·m2·s−2·A−2
Permeability henry per metre H/m kg·m·s−2·A−2
Magnetic susceptibility (dimensionless) χ -

See also

External links

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