Georg Simon Ohm

Ohm, Georg Simon, 1787-1854, German physicist. He was professor at Munich from 1852. His study of electric current led to his formulation of the law now known as Ohm's law. The unit of electrical resistance (see ohm) was named for him. He also made studies in acoustics and in crystal interference. His writings include The Galvanic Current Investigated Mathematically (1827, tr. 1891).

(born March 16, 1789, Erlangen, Bavaria—died July 6, 1854, Munich) German physicist. While teaching mathematics at the Jesuits' College in Cologne (1817–27), he discovered that the flow of electric current through a conductor is directly proportional to the potential difference, or voltage, and inversely proportional to the resistance. He resigned when his theory (Ohm's law) was coldly received. His theory soon came to be widely recognized, and he subsequently taught in Nürnberg (1833–49) and Munich (1849–54). The physical unit measuring electrical resistance was named for him.

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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 defined as 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.

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

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 resistors. In other cases resistance varies (e.g., thermistors).

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

Conversions

• The SI unit of electrical conductance is the siemens, formerly known as the mho (ohm written 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 and Joule's law:

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

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 not applicable to devices whose resistance varies with current.

References and notes

See also

• Ohm's law
• Resistor
• Resistivity
• abohm
• Siemens (unit)

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