The
ohm is the
SIThe International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
unit of
electrical resistanceThe electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
, named after German
physicistA physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole...
Georg Simon OhmGeorg Simon Ohm was a German physicist. As a high school teacher, Ohm began his research with the recently-invented electrochemical cell, invented by Italian Count Alessandro Volta. Using equipment of his own creation, Ohm determined that there is a direct proportionality between the potential...
.
Definition
The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1
voltThe volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
, applied to these points, produces in the conductor a current of 1
ampereThe ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
, the conductor not being the seat of any
electromotive forceIn physics, electromotive force, emf , or electromotance refers to voltage generated by a battery or by the magnetic force according to Faraday's Law, which states that a time varying magnetic field will induce an electric current.It is important to note that the electromotive "force" is not a...
.
In many cases the resistance of a conductor in ohms is approximately constant within a certain range of voltages, temperatures, and other parameters; one speaks of
linearIn mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...
resistorA linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
s. In other cases resistance varies (e.g.,
thermistorA thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor...
s).
Commonly used multiples and submultiples in electrical and electronic usage are the milliohm, ohm, kilohm, megohm, and gigaohm.
In alternating current circuits,
electrical impedanceElectrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
is also measured in ohms.
Conversions
The
SIThe International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
unit of electrical conductance is the
siemensThe siemens is the SI derived unit of electric conductance and electric admittance. Conductance and admittance are the reciprocals of resistance and impedance respectively, hence one siemens is equal to the reciprocal of one ohm, and is sometimes referred to as the mho. In English, the term...
, also known as the mho (ohm spelled backwards, symbol is ℧); it is the
reciprocalIn mathematics, a multiplicative inverse or reciprocal for a number x, denoted by 1/x or x−1, is a number which when multiplied by x yields the multiplicative identity, 1. The multiplicative inverse of a fraction a/b is b/a. For the multiplicative inverse of a real number, divide 1 by the...
of resistance in ohms.
Power as a function of resistance
The power dissipated by a linear resistor may be calculated from its resistance, and voltage or current. The formula is a combination of
Ohm's lawOhm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points...
and Joule's laws:
where
P is the power in
wattThe watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
s,
R the resistance in ohms,
V the
voltageVoltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
across the resistor, and
I the current through it.
This formula is applicable to devices whose resistance varies with current.
Use of the Ω symbol in electronic documents
Care should be taken when preparing documents (including HTML documents) that make use of the symbol Ω. Some document editing software will attempt to use the
symbolSymbol is one of the four standard fonts available on all PostScript-based printers, starting with Apple's original LaserWriter . It contains a complete unaccented Greek alphabet and a selection of commonly used mathematical symbols...
typefaceIn typography, a typeface is the artistic representation or interpretation of characters; it is the way the type looks. Each type is designed and there are thousands of different typefaces in existence, with new ones being developed constantly....
to render the character. Where the font is not supported, a
WThe watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
is displayed instead (a "10 W" resistor instead of a "10 Ω" resistor, for instance). As this represents the SI unit of
powerIn physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
, not resistance, this can lead to confusion.
UnicodeUnicode is a computing industry standard for the consistent encoding, representation and handling of text expressed in most of the world's writing systems...
encodes an ohm symbol distinct from Greek omega among
letterlike symbolsLetterlike Symbols are graphemes which are constructed mainly from the glyphs of one or more letters.In Unicode, Letterlike Symbols are placed in the block U+2100–214F, as in the following table.-See also:*Mapping of Unicode characters...
, but it is only included for backwards compatibility and the Greek uppercase omega character is preferred.. Many text editors allow the use of ALT 234 to produce the Ω symbol.
External links