Ohms

Ohms is the SI unit of electrical resistance, symbolized by the Greek letter omega (Ω). The unit is named after the German physicist Georg Simon Ohm, who established Ohm’s law. In practical use, the plural is “ohms,” and the unit describes how strongly a component resists the flow of electric current.

The resistance R between two points is defined by Ohm’s law as R = V / I, where V is the potential difference and I is the current. A one-ohm resistor passes one ampere of current when a one-volt potential difference is applied (assuming constant temperature and no electromotive forces). The ohm is also expressed as one volt per ampere. Its dimensional formula is voltage divided by current, or equivalently electrical conductance is the reciprocal in siemens (S).

Uniform conductors have resistance that depends on material properties, length, and cross-sectional area: R = ρL/A, where ρ is resistivity, L is length, and A is cross-sectional area. This relation explains why longer or thinner wires exhibit higher resistance and why different materials have different resistivities.

Standards and definitions have evolved with the SI system. Since 2019, the ohm is realized through fixed fundamental constants, notably the relationship between voltage and frequency via the Josephson effect and the quantum Hall effect that defines the von Klitzing constant R_K = h/e^2 as an exact value. In practice, the ohm is realized with quantum electrical standards to maintain precise, reproducible resistance measurements.

Ohms are used to characterize electrical components such as resistors, sensors, and circuitry. Common examples range from fractions of an ohm in low-value shunts to kiloohms or megohms in signaling and insulation applications. See also resistivity, conductance, and Ohm’s law.