Ohm
From Biocrawler, the free encyclopedia.
The ohm is the SI derived unit of electrical resistance (derived from the ampere and the watt). Its symbol is the Greek capital letter omega (Ω). The ohm is named for Georg Ohm, a German physicist who discovered the relation between voltage and current, expressed in Ohm's Law.
By definition from Ohm's Law, a device has a resistance of one ohm if a voltage of one volt will cause 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).
One thousand ohms is called a kilohm or kiloohm, and abbreviated kΩ. One million ohms is called a megohm or megaohm, and abbreviated MΩ. Other orders of magnitude follow standard SI conventions, and the last letter of the prefix is rarely omitted.
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.
The complex quantity impedance is a generalisation of resistance. Its real part is resistance and its imaginary part is reactance. For consistency, impedance, resistance and reactance all have units of ohms.
The ohm is since 1990 maintained internationally using the quantum Hall effect, where a conventional value is used for the 'von-Klitzing constant', fixed by the 18th CGPM as R{K-90} = 25812.807 Ω.
SI electricity units
| SI electromagnetism units edit (http://www.biocrawler.com/w/index.php?title=Template:SI_electromagnetism_units&action=edit) | |||
|---|---|---|---|
| Name | Symbol | Dimensions | Quantity |
| ampere (SI base unit) | A | A | Current |
| coulomb | C | A新 | Electric charge, Quantity of electricity |
| volt | V | J/C = kg搶2新−3嫂−1 | Potential difference |
| ohm | Ω | V/A = kg搶2新−3嫂−2 | Resistance, Impedance, Reactance |
| ohm metre | Ω搶 | kg搶3新−3嫂−2 | Resistivity |
| watt | W | V嫂 = kg搶2新−3 | Electrical power |
| farad | F | C/V = kg−1搶−2嫂2新4 | Capacitance |
| farad per metre | F/m | kg−1搶−3嫂2新4 | Permittivity |
| reciprocal farad | F−1 | kg1搶2嫂−2新−4 | Elastance |
| siemens | S | Ω−1 = kg−1搶−2新3嫂2 | Conductance, Admittance, Susceptance |
| siemens per metre | S/m | kg−1搶−3新3嫂2 | Conductivity |
| weber | Wb | V新 = kg搶2新−2嫂−1 | Magnetic flux |
| tesla | T | Wb/m2 = kg新−2嫂−1 | Magnetic flux density |
| ampere per metre | A/m | m−1嫂 | magnetic induction |
| ampere-turns per weber | A/Wb | kg−1搶−2新2嫂2 | Reluctance |
| henry | H | Wb/A = V新/A = kg搶2新−2嫂−2 | Inductance |
| henry per metre | H/m | kg搶新−2嫂−2 | Permeability |
| (dimensionless) | χ | - | Magnetic susceptibility |
See also
- Ohm's Law
- Scanned books of Georg Simon Ohm at the library of the University of Applied Sciences Nuremberg (http://www.fh-nuernberg.de/institutionen/bibliothek/bibsuche/texte_online_aufrufen/historische_buecher_der_gso_fh/werke_von_georg_simon_ohm/)
For the Hindu mantra syllable, see aum.ar:أوم ca:Ohm cs:Ohm da:Ohmes:Ohmio fr:Ohm it:Ohm nl:Ohm (eenheid) ja:オーム no:Ohm pl:Om (jednostka) pt:Ohm ru:Ом fi:Ohmi sl:ohm sv:Ohm sr:Ом zh:歐姆
