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Ohm

 
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Ohm
 
 
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The ohm (symbol: O
O

O is the fifteenth letter of the modern Latin alphabet. Its name in English language is spelled o , plural oes ....
) is the SI
Si

Si, si, or SI may refer to :...
 unit of electrical impedance
Electrical impedance

Electrical impedance, or simply impedance, describes a measure of opposition to a sinusoidal alternating current . Electrical impedance extends the concept of Electrical resistance to AC circuits, describing not only the relative amplitudes of the voltage and Electric current, but also the relative Phase ....
 or, in the direct current
Direct current

Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as battery , thermocouples, solar cells, and commutator-type electric machines of the dynamo type....
 case, electrical resistance
Electrical resistance

The electrical resistance of an object is a measure of its opposition to the passage of a steady electrical current. An object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross-sectional area, and proportional to the resistivity of the material....
, named after 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....
.

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

The volt is the SI SI derived unit of electric potential difference or electromotive force, commonly known as voltage. It is named in honor of the Lombard physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery ....
, applied to these points, produces in the conductor a current of 1 ampere
Ampere

The ampere is the International System of Units unit of electric current. The ampere, in practice often shortened to amp, is an SI base unit, and is named after Andr?-Marie Amp?re, one of the main discoverers of electromagnetism....
, the conductor not being the seat of any electromotive force
Electromotive force

Electromotive force is a term used to characterize electrical devices, such as voltaic cells, Thermoelectric effects, electrical generators and transformers, and even resistors....
.

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

The word linear comes from the Latin word linearis, which means created by lines.In mathematics, a linear map or function f is a function which satisfies the following two properties......
 resistor
Resistor

|- align = "center"||width = "25"|| |- align = "center"||| Potentiometer|- align = "center"| || |- align = "top"| Resistor|| Variable resistor...
s.






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Electronic Multi Meter
Aeldre Kulmassemodstande
The ohm (symbol: O
O

O is the fifteenth letter of the modern Latin alphabet. Its name in English language is spelled o , plural oes ....
) is the SI
Si

Si, si, or SI may refer to :...
 unit of electrical impedance
Electrical impedance

Electrical impedance, or simply impedance, describes a measure of opposition to a sinusoidal alternating current . Electrical impedance extends the concept of Electrical resistance to AC circuits, describing not only the relative amplitudes of the voltage and Electric current, but also the relative Phase ....
 or, in the direct current
Direct current

Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as battery , thermocouples, solar cells, and commutator-type electric machines of the dynamo type....
 case, electrical resistance
Electrical resistance

The electrical resistance of an object is a measure of its opposition to the passage of a steady electrical current. An object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross-sectional area, and proportional to the resistivity of the material....
, named after 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....
.

Definition

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

The volt is the SI SI derived unit of electric potential difference or electromotive force, commonly known as voltage. It is named in honor of the Lombard physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery ....
, applied to these points, produces in the conductor a current of 1 ampere
Ampere

The ampere is the International System of Units unit of electric current. The ampere, in practice often shortened to amp, is an SI base unit, and is named after Andr?-Marie Amp?re, one of the main discoverers of electromagnetism....
, the conductor not being the seat of any electromotive force
Electromotive force

Electromotive force is a term used to characterize electrical devices, such as voltaic cells, Thermoelectric effects, electrical generators and transformers, and even resistors....
.

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

The word linear comes from the Latin word linearis, which means created by lines.In mathematics, a linear map or function f is a function which satisfies the following two properties......
 resistor
Resistor

|- align = "center"||width = "25"|| |- align = "center"||| Potentiometer|- align = "center"| || |- align = "top"| Resistor|| Variable resistor...
s. In other cases resistance varies (e.g., thermistor
Thermistor

A thermistor is a type of resistor with electrical resistance proportional to its temperature. The word is a portmanteau of Thermal and resistor....
s).

The most commonly used multiples and submultiples in electrical and electronic usage are the milliohm, ohm, kilohm, and megohm.

Conversions

  • The SI
    Si

    Si, si, or SI may refer to :...
     unit of electrical conductance
    Electrical conductance

    Electrical conductance is a measure of how easily electricity flows along a certain path through an electrical element. The SI derived unit of conductance is the Siemens ....
     is the siemens
    Siemens (unit)

    The siemens is the SI SI derived unit of electric conductance. It is equal to inverse ohm. It is named after the Germany inventor and industrialist Ernst Werner von Siemens, and was previously called the #Mho....
    , formerly known as the mho (ohm spelled backwards); it is the reciprocal 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 law
Ohm's law

Ohm's law applies to electrical circuits; it states that the electric current through a conductor between two points is directly Proportionality to the potential difference or voltage across the two points, and inversely proportional to the Electrical resistance between them....
 and Joule's law
Joule's law

Joule's laws are a pair of laws concerning the heat produced by a current and the energy dependence of an ideal gas to that of pressure, volume, and temperature, respectively....
: where P is the power in watts, R the resistance in ohms, V the voltage across the resistor, and I the current through it.

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

Use of the O symbol in electronic documents

Care should be taken when preparing documents (including HTML documents) which make use of the O symbol. Some document editing software will attempt to use the Symbol
Symbol (typeface)

Symbol is one of the four standard fonts available on most PostScript-based printers. It contains a complete unaccented Greek alphabet and a selection of commonly used mathematical symbols as well....
 typeface
Typeface

In typography, a typeface is a set of one or more fonts, in one or more sizes, designed with stylistic unity, each comprising a coordinated set of glyphs....
 to render the character. Where the font is not supported, a W
WATT

WATT is a radio station broadcasting a News radio-Talk radio-Sports radio format. Licensed to Cadillac, Michigan, it first began broadcasting in 1945....
 is displayed instead. As this represents the SI unit of power
Power (physics)

In physics, power is the rate at which mechanical work is performed or energy is transmitted, or the amount of energy required or expended for a given unit of time....
, not resistance, this can lead to confusion.

See also

  • Ohm's law
    Ohm's law

    Ohm's law applies to electrical circuits; it states that the electric current through a conductor between two points is directly Proportionality to the potential difference or voltage across the two points, and inversely proportional to the Electrical resistance between them....
  • Resistor
    Resistor

    |- align = "center"||width = "25"|| |- align = "center"||| Potentiometer|- align = "center"| || |- align = "top"| Resistor|| Variable resistor...
  • Resistivity
    Resistivity

    Electrical resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electrical charge....
  • abohm
    Abohm

    The abohm is the basic unit of electrical resistance in the centimeter gram second system of units. One abohm is equal to 10-9 ohms in the International System of Units; one abohm is a nanoohm....
  • Siemens (unit)
    Siemens (unit)

    The siemens is the SI SI derived unit of electric conductance. It is equal to inverse ohm. It is named after the Germany inventor and industrialist Ernst Werner von Siemens, and was previously called the #Mho....


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