Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential between two points, which (in a static electric field) is defined as the work needed per unit of charge to move a test charge between the two points. In the International System of Units, the derived unit for voltage (potential difference) is named volt.[1]:166 In SI units, work per unit charge is expressed as joules per coulomb, where 1 volt = 1 joule (of work) per 1 coulomb (of charge). The old SI definition for volt used power and current; starting in 1990, the quantum Hall and Josephson effect were used, and recently (2019) fundamental physical constants have been introduced for the definition of all SI units and derived units.[1]:177f,197f Voltage or electric potential difference is denoted symbolically by V, simplified V,[2] or U,[3] for instance in the context of Ohm's or Kirchhoff's circuit laws.

Batteries are sources of voltage in many electric circuits.
Common symbols
V , V , U , U
SI unitvolt
In SI base unitskg⋅m2⋅s−3⋅A−1
Derivations from
other quantities
Voltage = Energy / charge
DimensionM L2 T−3 I−1

Electric potential differences between points can be caused physically by electric charge build up or imbalance (e.g. well known "static" and electronic capacitor) also by electric current through a magnetic field, and by time-varying magnetic fields (e.g. dynamo or generator), or some combination of these three.[4][5] Additionally on a macroscopic scale potential difference can be caused by electrochemical processes (cells and batteries) and pressure induced piezoelectric effect and heat induced emf across metal-metal junctions. These latter processes at microscopic level have the physical origins previously mentioned. A voltmeter can be used to measure the voltage (or potential difference) between two points in a system; often a common reference potential such as the ground of the system is used as one of the points. A voltage may represent either a source of energy (electromotive force) or lost, used, or stored energy (potential drop).