iPedia Free Information Center

Information about Ohm (unit)

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


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


Enlarge picture
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

An ohm is the electrical resistance offered by a current-carrying element that produces a voltage drop of one volt when a current of one ampere is flowing through it.

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. 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.

Units of ohms, kilohms (103 Ω) and megohms (106 Ω) are used in electronic design documentation. On schematic diagrams and parts lists kilohms are abbreviated "K" and megohms are abbreviated "M". Thus, 33 kilohms would be rendered as 33K, and 5.1 megohms would be 5.1M. Another commonly used convention is that the multiplier is used to replace the decimal point, so that 5.1 megohms can also be represented as 5M1. This convention is used because a decimal point can be difficult to see in small or cluttered print. Values less than 1K are rendered either (a) without any symbol, or (b) with an "R", following the number; so 680 ohms can be shown as 680 or 680R, and 4.7 ohms as 4R7. Resistors are usually identified by a reference designator, R, and a cardinal number, e.g., R12.

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). But when an SI unit is spelled out, it should always be written in lowercase (ohm), unless it begins a sentence or is the name "degree Celsius".
— Based on The International System of Units, section 5.2.

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 (simply put, 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:
:
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.

See also

External links

Si, si, or SI may refer to (all SI unless otherwise stated):

In language:
  • One of two Italian words:
  • sì (accented) for "yes"
  • si

..... Click the link for more information.
Ohm may refer to:
  • Georg Ohm (1789–1854), German physicist
  • Ohm (unit) (Ω), the SI unit of electrical resistance named after Georg Ohm
  • Martin Ohm (1792–1872), German mathematician
  • Ohm River, a river in Germany

..... Click the link for more information.
Si, si, or SI may refer to (all SI unless otherwise stated):

In language:
  • One of two Italian words:
  • sì (accented) for "yes"
  • si

..... Click the link for more information.
Electrical impedance, or simply impedance, describes a measure of opposition to a sinusoidal alternating current (AC). Electrical impedance extends the concept of resistance to AC circuits, describing not only the relative magnitudes of the voltage and current, but also the
..... Click the link for more information.
Direct current (DC or "continuous current") is the constant flow of electric charge. This is typically in a conductor such as a wire, but can also be through semiconductors, insulators, or even through a vacuum as in electron or ion beams.
..... Click the link for more information.
Electrical resistance is a measure of the degree to which an object opposes an electric current through it. The SI unit of electrical resistance is the ohm. Its reciprocal quantity is electrical conductance measured in siemens.
..... Click the link for more information.
Georg Simon Ohm

Born March 16 1789(1789--)
Erlangen, Germany
Died July 6 1854 (aged 65)
Munich, Germany
..... Click the link for more information.
volt (symbol: V) is the SI derived unit of electric potential difference or electromotive force.[1][2] It is named in honor of the Italian physicist Alessandro Volta (1745–1827), who invented the voltaic pile, the first modern chemical battery.
..... Click the link for more information.
ampere, in practice often shortened to amp, (symbol: A) is a unit of electric current, or amount of electric charge per second. The ampere is an SI base unit, and is named after André-Marie Ampère, one of the main discoverers of electromagnetism.
..... Click the link for more information.
Ohm's law states that, in an electrical circuit, the current passing through a conductor between two points is proportional to the potential difference (i.e. voltage drop or voltage) across the two points, and inversely proportional to the resistance between them.
..... Click the link for more information.
Device may refer to:

Computing and electronics

  • Computer hardware
  • Peripheral device, any device attached to a computer that expands its functionality
  • Device file, an interface for a device driver

..... Click the link for more information.
WATT

City of license Cadillac, Michigan
Broadcast area [1]
Branding NewsTalk 1240
First air date 1945
Frequency 1240 kHz
Format News-Talk-Sports
Power 1,000 watts
Class C
Owner MacDonald Garber Broadcasting
..... Click the link for more information.
20th century - 21st century
1960s  1970s  1980s  - 1990s -  2000s  2010s  2020s
1987 1988 1989 - 1990 - 1991 1992 1993

Year 1990 (MCMXC) was a common year starting on Monday (link displays the 1990 Gregorian calendar).
..... Click the link for more information.
The quantum Hall effect is a quantum-mechanical version of the Hall effect, observed in two-dimensional electron systems subjected to low temperatures and strong magnetic fields, in which the Hall conductance takes on the quantized values


..... Click the link for more information.
Klaus von Klitzing

Born May 28 1943 (1943--) (age 64)

Nationality German
Field Physics
..... Click the link for more information.
The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures (CGPM, never GCWM). It is one of the three organizations established to maintain the International System of Units (SI) under the terms of the Convention
..... Click the link for more information.
In mathematics, a complex number is a number of the form


where a and b are real numbers, and i is the imaginary unit, with the property i ² = −1.
..... Click the link for more information.
Electrical impedance, or simply impedance, describes a measure of opposition to a sinusoidal alternating current (AC). Electrical impedance extends the concept of resistance to AC circuits, describing not only the relative magnitudes of the voltage and current, but also the
..... Click the link for more information.
Reactance is the imaginary part of electrical impedance, a measure of opposition to a sinusoidal alternating current. Reactance arises from the presence of inductance and capacitance within a circuit, and is denoted by the symbol , the SI unit is the ohm.
..... Click the link for more information.
Greek alphabet
Child systems Gothic
Glagolitic
Cyrillic
Coptic
Old Italic alphabet
Latin alphabet

ISO 15924 Grek

Note: This page may contain IPA phonetic symbols in Unicode.
..... Click the link for more information.
OMEGA is the premier counter-terrorism unit of Latvia.

Founded in 1992, OMEGA cooperates with many other counter-terrorism units over the world. Its equipment and weaponry includes the Heckler & Koch MP5, the Steyr AUG and the Makarov PM.
..... Click the link for more information.
ampere, in practice often shortened to amp, (symbol: A) is a unit of electric current, or amount of electric charge per second. The ampere is an SI base unit, and is named after André-Marie Ampère, one of the main discoverers of electromagnetism.
..... Click the link for more information.
Si, si, or SI may refer to (all SI unless otherwise stated):

In language:
  • One of two Italian words:
  • sì (accented) for "yes"
  • si

..... Click the link for more information.
Georg Simon Ohm

Born March 16 1789(1789--)
Erlangen, Germany
Died July 6 1854 (aged 65)
Munich, Germany
..... Click the link for more information.
Symbols are objects, characters, or other concrete representations of ideas, concepts, or other abstractions. For example, in the United States, Canada and Great Britain, a red octagon is a symbol for the traffic sign meaning "STOP".
..... Click the link for more information.
Capital letters or majuscules (in the Roman alphabet: A, B, C, D, ...) are one type of case in a writing system. Capital letters (also simply called capitals or caps) are also known as upper case
..... Click the link for more information.
For the minimalist musical sub-genre, see .
Lower case or lowercase or minuscule letters are the smaller form of letters, as opposed to capital letters: for example, the letter "a" is lower case while the letter "A"
..... Click the link for more information.
Celsius is, or relates to, the Celsius temperature scale (previously known as the centigrade scale). The degree Celsius (symbol: °C) can refer to a specific temperature on the Celsius scale
..... Click the link for more information.
The siemens (symbol: S) is the SI derived unit of electric conductance. It is named after the German inventor and industrialist Ernst Werner von Siemens, and is equivalent to the previously used designation of this unit, the mho.
..... Click the link for more information.
Si, si, or SI may refer to (all SI unless otherwise stated):

In language:
  • One of two Italian words:
  • sì (accented) for "yes"
  • si

..... Click the link for more information.


This article is copied from an article on Wikipedia.org - the free encyclopedia created and edited by online user community. The text was not checked or edited by anyone on our staff. Although the vast majority of the wikipedia encyclopedia articles provide accurate and timely information please do not assume the accuracy of any particular article. This article is distributed under the terms of GNU Free Documentation License.
Herod_Archelaus


page counter