Ohm_(unit)

This article is about the SI derived unit. For other meanings, see Ohm (disambiguation).

A multimeter can be used to measure resistance in ohms. It can also be used to measure capacitance, voltage, current, etc.

Several resistors. Their resistance, in ohms, is marked using a color code.

The ohm (symbol: Ω) is the SI unit of electrical impedance or, in the direct current case, electrical resistance, named after Georg Ohm.

Definition

The ohm is the electric resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere, the conductor not being the seat of any electromotive force.BIPM SI Brochure: Appendix 1, p. 144

$\backslash Omega\; =\; \backslash dfrac\{\backslash mbox\{V\}\}\{\backslash mbox\{A\}\}\; =\; \backslash dfrac\{\backslash mbox\{m\}^2\; \backslash cdot\; \backslash mbox\{kg\}\}\{\backslash mbox\{s\}^\{3\}\; \backslash cdot\; \backslash mbox\{A\}^2\}$

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.Dennis Lee Electrical measurement standards based upon quantum phenomena, p. 6

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 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 non-SI unit, the mho (ohm written backwards), is equivalent to siemens but is mostly obsolete and rarely used.
• Ohms to watts: The power dissipated by a resistor may be calculated using resistance and voltage. The formula is a combination of Ohm\'s law and Joule\'s law:

$P=\{V^2\; \backslash over\; R\}$

where P is the power in watts, R is the resistance in ohms and V is the voltage across the resistor.

This method is not reliable for determining the power of an incandescent light bulb, resistance heater or electric short since all these devices operate at high temperatures and the resistance measured using a meter will not represent the operating resistance. For these conditions instead multiply V by current in amperes to get power in watts.

References and notes

See also

• Ohm\'s law
• Resistor
• abohm

External links

• Scanned books of Georg Simon Ohm at the library of the University of Applied Sciences Nuernberg
• Official SI brochure
• NIST Special Publication 811
• History of the ohm at sizes.com

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