Pronunciation: UK: /əʊm/ US: /oʊm/ Symbol: Ω
The ohm (symbol: Ω) is the International System of Units (SI) derived unit of electrical resistance, named after German physicist Georg Simon Ohm. Although several empirically derived standard units for expressing electrical resistance were developed in connection with early telegraphy practice, the British Association for the Advancement of Science proposed a unit derived from existing units of mass, length and time and of a convenient size for practical work as early as 1861. The definition of the ohm was revised several times. Today the definition of the ohm is expressed from the quantum Hall effect.
An ohm is equal to the resistance of a circuit in which a potential difference of one volt produces a current of one ampere (1Ω = 1 V/A); or, the resistance in which one watt of power is dissipated when one ampere flows through it.
The ohm is defined as an electrical resistance between two points of a conductor when a constant potential difference of one volt, applied to these points, produces in the conductor a current of one ampere, the conductor not being the seat of any electromotive force. Reduced to base SI units, one ohm is the equivalent of one kilogram meter squared per second cubed per ampere squared (1 kg times m 2 · s -3 · A -2 . The ohm is also the equivalent of a volt per ampere (V/A).
- A volt is he unit of electromotive force or electric pressure. It is the electromotive force(pd) which, if steadily applied to a circuit having a resistance of one ohm, will produce a current of one ampere
- An amp is the standard unit of electric current. The current produced by a pressure of one volt in a circuit having a resistance of one ohm
In a direct-current (DC) circuit (like you would find in an e-cigarette), a component has a resistance of one ohm when a potential difference of one volt produces a current of one ampere through the component.
In many cases the resistance of a conductor in ohms is approximately constant within a certain range of voltages, temperatures, and other parameters. These are called linear resistors. In other cases resistance varies (e.g., thermistors).