In the
physical sciences,
potential difference is the difference in
potential between two points in a
conservative vector field. In engineering, it is sometimes described as the
across variable, where
flux is the through variable. Within the context of
electricity, the conservative vector field is the electrostatic field. Between any two points in the field, there is defined an electric potential difference where the unit is a
volt. This electric potential difference is referred to as the voltage between the two points.
Production
The product of the flux and the potential difference is the
power, which is the rate of change of the conserved quantity, e.g.,
energy. In fluid systems the potential difference is the difference in
pressure. In thermal systems the potential difference is the difference in
temperature. In mechanics, the potential difference is the difference in gravitational potential between two points. In electrical engineering the potential difference is the voltage, i.e. the difference between the initial and final points of an electrostatic potential.
Electrical definitions
A potential difference between two points gives rise to a "force" called an
electromotive force or
emf that tends to push
electrons or other charge-carriers from one point to the other. A potential difference is generated between the ends of an electrical
conductor that moves perpendicular to a
magnetic field. Between two points in an electrical circuit the potential difference is equal to the difference in their electrical potentials.
The potential difference is defined as the amount of
work per charge needed to move
electric charge from the second point to the first, or equivalently, the amount of work that unit charge flowing from the first point to the second can perform. In the
SI system of units, potential difference, electrical potential and electromotive force are measured in
volts, leading to the commonly used term
voltage and the symbol
V. Named after
Alessandro Volta, one
volt is defined to be one
joule of
energy per
coulomb of charge.
The potential difference between two points a and b is the line integral of the electric field
E:
:
If one thinks of an electrical circuit in
analogy to water circulating in a network of pipes, driven by pumps in the absence of
gravity, then the potential difference corresponds to the
pressure difference between two points. If there is a pressure difference between two points, then water flowing from the first point to the second will be able to do work, such as driving a turbine.
Voltage is additive in the following sense: the voltage between
A and
C is the sum of the voltage between
A and
B and the voltage between
B and
C. Two points in an electric circuit which are connected by an (ideal) conductor without resistance have a potential difference of zero. But other pairs of points may also have a potential difference of zero. If two such points are connected with a conductor, no current will flow through the connection. The various voltages in a circuit can be computed using
Kirchhoff's circuit laws.
Instruments
Common sources of emf are the
battery, the
electrical generator and the
capacitor. Instruments for measuring potential differences include the
voltmeter, the
potentiometer (measurement device), and the
oscilloscope. The voltmeter works by measuring the current through a fixed resistor, which, according to
Ohm's Law, is proportional to the potential difference across it. The potentiometer works by balancing the unknown voltage against a known voltage in a
bridge circuit. The cathode-ray oscilloscope works by amplifying the potential difference and using it to deflect an
electron beam from a straight path, so that the deflection of the beam is proportional to the potential difference.
History
In 1800, as the result of a professional disagreement over the galvanic response advocated by
Luigi Galvani,
Alessandro Volta developed the so-called
voltaic pile, a forerunner of the
battery, which produced a steady electric
current. Volta had determined that the most effective pair of dissimilar metals to produce electricity was
zinc and
silver. In the
1880s, the International Electrical Congress, now the
International Electrotechnical Commission (IEC), approved the volt for electromotive force. The volt was defined as the potential difference across a conductor when a current of one
ampere dissipates one
watt of power.
SI electricity units
Analogies
Voltage is
analogous to the
hydrostatic pressure of a fluid in a pipe. It also explains the "dimensions" of voltage. Consider the
potential energy of compressed air being pumped into tank. The energy increases with each new increment of air.
Pressure is that energy divided by the volume, which we can understand intuitively. Now consider the energy of electric charge (measured in
coulombs) being forced into a
capacitor. Voltage is that energy per charge, so voltage is analogous to a pressure-like sort of forcefulness. Also, dimensional analysis tells us that voltage ("energy per charge") is charge per distance, the distance being between the plates of the capacitor. (''See also
Hydraulic analogy)
Reference: page 16 of "Industrial Electronics," by D. J. Shanefield, Noyes Publications, Boston, 2001.
Another analogy can be made with energy by itself (or
work). You need 1
joule of
energy to produce 1 joule of work. (A joule of work is produced by constantly applying 1 newton of
force to move something 1 meter away). If we removed "C" in "1 V = 1 J/C" leading only to "1 J", we would think of voltage as simply energy ready to produce some work (moving a lot of electric charges). But when we talk about voltage, we don't need to know how many coulombs will actually move at the end, we just need to push all of them equally (as many as they are) while the device is on; we don't need the absolute value of energy, but the energy needed to move each electron (each coulomb). Thus, we normalize it by dividing over "C". If we removed some electrons without removing any of that energy, the voltage would be greater.
See also
External links
Category:Electricity
Category:Physical quantity
bg:Електрическо напрежение
ca:Diferència de potencial
cs:Elektrické napětí
da:Elektrisk spænding
de:Elektrische Spannung
es:Diferencia de potencial
it:Differenza di potenziale
nl:Elektrische spanning
ja:電圧
pl:Napięcie elektryczne
pt:Diferença de potencial elétrico
ru:Напряжение (электрическое)
sl:Električna napetost
fi:Jännite
sv:Elektrisk spänning
zh:電壓