volt
Voltageexact by definition
The volt is the SI unit of electric potential difference, defined as one joule of energy transferred per coulomb of charge moved, equivalently one watt per ampere. A potential difference of one volt across a component means every coulomb that passes through it gives up exactly one joule. Since the volt is a coherent derived SI unit built from the second, metre, kilogram and ampere, its relation to the base units is exact by definition rather than measured.
| 1 V | 1 V |
Named for Alessandro Volta, who in 1800 announced the voltaic pile, the first source of steady electric current and the ancestor of every battery since.
About the volt
Voltage is the electrical equivalent of pressure, and the analogy survives more scrutiny than most analogies do. A volt is one joule per coulomb, \(V = W/Q\), so it measures how much energy each unit of charge carries rather than how much charge is moving. That is why a 9 V battery and a 9 V wall adapter offer the same push while delivering wildly different currents, and why the phrase "it's the amps that kill you, not the volts" is only half right: without volts to drive it, no current flows through you at all.
The volt is also where the practical electrical units meet mechanics. One volt times one ampere is one watt, so \(P = VI\); one volt across one ohm drives one ampere, so \(V = IR\). Since 2019 the volt has been realised through the Josephson effect, where a superconducting junction irradiated at a known frequency produces voltage steps of exactly \(nf h / 2e\). With both \(h\) and \(e\) now fixed by definition, that makes the volt reproducible in any national laboratory to parts in \(10^{10}\) without shipping a physical standard cell anywhere.