Ohm

Overview

**ohm**is the SI

International System of Units

The 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 resistance

Electrical resistance

The 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 physicist

Physicist

A 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 Ohm

Georg Ohm

Georg 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...

.

The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt

Volt

The 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 ampere

Ampere

The 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 force

Electromotive force

In 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 linear

Linear

In mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...

resistor

Resistor

A 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.

Unanswered Questions

Encyclopedia

The

unit of electrical resistance

, named after German physicist

Georg Simon Ohm

.

, applied to these points, produces in the conductor a current of 1 ampere

, the conductor not being the seat of any electromotive force

.

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 linear

resistor

s. In other cases resistance varies (e.g., thermistor

s).

Commonly used multiples and submultiples in electrical and electronic usage are the milliohm, ohm, kilohm, megohm, and gigaohm.

In alternating current circuits, electrical impedance

is also measured in ohms.

unit of electrical conductance is the siemens

, also known as the mho (ohm spelled backwards, symbol is ℧); it is the reciprocal

of resistance in ohms.

and Joule's laws:

where

s,

across the resistor, and

This formula is applicable to devices whose resistance varies with current.

typeface

to render the character. Where the font is not supported, a W

is displayed instead (a "10 W" resistor instead of a "10 Ω" resistor, for instance). As this represents the SI unit of power

, not resistance, this can lead to confusion.

Unicode

encodes an ohm symbol distinct from Greek omega among letterlike symbols

, 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.

**ohm**is the SIInternational System of Units

The 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 resistance

Electrical resistance

The 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 physicist

Physicist

A 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 Ohm

Georg Ohm

Georg 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 voltVolt

The 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 ampere

Ampere

The 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 force

Electromotive force

In 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 linear

Linear

In mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...

resistor

Resistor

A 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., thermistor

Thermistor

A 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 impedance

Electrical impedance

Electrical 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 SIInternational System of Units

The 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 siemens

Siemens (unit)

The 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 reciprocal

Multiplicative inverse

In 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

Ohm'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 wattWatt

The 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

Voltage, 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 (typeface)

Symbol 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...

typeface

Typeface

In 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 W

Watt

The 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 power

Power (physics)

In 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.

Unicode

Unicode

Unicode 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 symbols

Letterlike Symbols

Letterlike 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.