# Voltage

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.: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.:177f,197f Voltage or electric potential difference is denoted symbolically by V, simplified V, or U, for instance in the context of Ohm's or Kirchhoff's circuit laws.

Voltage 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. 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).