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The voltage (U) between two points in an electric circuit or electric field is equal to the work done by the electric field in transferring a positive unit charge from one point to the other. In a conservative electric field, the work does not depend on the displacement path of the charge; in this case the voltage between the two points coincides with the potential difference between the points.
In a nonconservative field, voltage depends on the displacement path of the charge between the two points. Nonconservative forces are active within any source of direct current (generators, storage batteries, galvanic cells, and the like). The voltage across the terminals of a current source is always assumed to be the work done by the electric field in transferring a positive unit charge along a path that lies outside the source. In this case the voltage is equal to the potential difference between the terminals of the source and is defined by Ohm’s law U = IR – ℰ, where I is the current, R is the internal resistance of the source, and ℰ is the electromotive force (emf) of the source. Under open-circuit conditions (I = 0) the absolute value of the voltage is equal to the emf of the source. Therefore the emf of the source is often defined as the open-circuit voltage across the terminals of the source.
For alternating currents, voltage is usually characterized by its effective value, which is the root-mean-square voltage value of one period. The voltage across the terminals of an AC source or an inductance coil is measured by the work done by the electric field in transferring a positive unit charge along a path outside the source or the coil. A rotational (nonconservative) electric field is particularly nonexistent along the path, and the voltage is equal to the potential difference.
Voltage is usually measured with a voltmeter. The unit of voltage in the International System of Units is the volt.
REFERENCESTamm, I. E. Osnovy leorii elektrichestvo, 9th ed. Moscow, 1976. Chapters 3 and 6.
Kalashnikov, S. G. Elektrichestvo, 4th ed. (Obshchii kurs fiziki.) Moscow, 1977. Chapter 3,7, and 21.
G. IA. MIAKISHEV
Voltages are often given relative to "earth" or "ground" which is taken to be at zero Volts. A circuit's earth may or may not be electrically connected to the actual earth.
The voltage between two points is also given by the charge present between those points in Coulombs divided by the capacitance in Farads. The capacitance in turn depends on the dielectric constant of the insulators present.
Yet another law gives the voltage across a piece of circuit as its inductance in Henries multiplied by the rate of change of current flow through it in Amperes per second.
A simple analogy likens voltage to the pressure of water in a pipe. Current is likened to the amount of water (charge) flowing per unit time.