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Electric potential is the
potential energy per unit of electric charge associated with a static (time-invariant)
electric field, also called the
electrostatic potential, typically measured in
volts. It is a scalar quantity. The difference of electrical potential between two points is known as
voltage.
There is also a generalized electric
scalar potential that is used in electrodynamics when time-varying electromagnetic fields are present. This generalized electric potential cannot be simply interpreted as a potential energy, however.
Explanation
Electric potential may be conceived of as "electric
pressure". Where this "pressure" is uniform, no current flows and nothing happens. This is similar to why people do not feel normal atmospheric air pressure: there is no difference between the pressure inside the body and outside, so nothing is felt. However, where this electrical pressure varies, it produces an electric field, which will create a
force (physics) on charged particles.
Mathematically, it is the potential φ (a scalar field) associated with the
irrotational vector field electric field \mathbf{E} (\mathbf{E}=-\mathbf{\nabla}\varphi) that occurs when the
magnetic field is time invariant (so that \mathbf{\nabla} \times \mathbf{E}=0 from
Faraday's law of induction).
Like any potential function, only the
potential difference (
voltage) between two points is physically meaningful (neglecting quantum
Aharonov-Bohm effects), since any constant can be added to φ without affecting \mathbf{E} (gauge invariance).
The electric potential φ is therefore measured in units of energy per unit of electric charge. In
SI units, this is:
joules/
coulombs =
volts.
The electric potential can also be generalized to handle situations with time-varying potential fields, in which case the electric field is not conservative and a potential function cannot be defined everywhere in space. There, an effective potential drop is included, associated with the inductance of the circuit. This generalized potential difference is also called the electromotive force (emf).
Introduction
Objects may possess a property known as electric charge. An electric field exerts a force on charged objects, accelerating them in the direction of the force, in either the same or the opposite direction of the electric field. If the charged object has a positive charge, the force and acceleration will be in the direction of the field. This force has the same direction as the
electric field vector, and its magnitude is given by the size of the charge multiplied with the magnitude of the electric field.
Classical mechanics explores the concepts such as
Force (physics), energy, potential etc. in more detail.
Force and potential energy are directly related. As an object moves in the direction that the force accelerates it, its potential energy decreases. For example, the gravitational potential energy of a cannonball at the top of a hill is greater than at the base of the hill. As the object falls, that potential energy decreases and is translated to motion, or inertial (kinetic) energy.
For certain forces, it is possible to define the "potential" of a field such that the potential energy of an object due to a field is dependent only on the position of the object with respect to the field. Those forces must affect objects depending only on the intrinsic properties of the object and the position of the object, and obey certain other mathematical rules.
Two such forces are the gravitational force (gravity) and the electric force in the absence of time-varying magnetic fields. The potential of an electric field is called the electric potential.
The electric potential and the
magnetic vector potential together form a four vector, so that the two kinds of potential are mixed under
Lorentz transformations.
Mathematical introduction
The concept of electric potential (denoted by: \phi, \phi_\mathrm{E} or
V) is closely linked with
potential energy, thus:
U_ \mathrm{E} = q\phi
where U_\mathrm{E} is the electric potential energy of a test charge
q due to the electric field. Note that the potential energy and hence also the electric potential is only defined up to an additive constant: one must
arbitrarily choose a position where the potential energy and the electric potential is zero.
The proper definition of the electric potential uses the electric field \mathbf{E}:
\phi_ \mathrm{E} = - \int_C \mathbf{E} \cdot \mathrm{d} \mathbf{S}
where E is equal to the electric field, ds is an unknown, and 'C' is an arbitrary path connecting the point with zero potential to the point under consideration. When \mathbf{\nabla} \times \mathbf{E} = 0, the line integral above does not depend on the specific path
C chosen but only on its endpoints. Equivalently, the electric potential determines the electric field via its
gradient:
\mathbf{E} = - \mathbf{\nabla} \phi_\mathrm{E}
and therefore, by Gauss's law, the potential satisfies
Poisson's equation:
\mathbf{\nabla} \cdot \mathbf{E} = \mathbf{\nabla} \cdot \left (- \mathbf{\nabla} \phi_\mathrm{E} \right ) = -\nabla^2 \phi_\mathrm{E} = \rho / \varepsilon_0
where ρ is the total charge density (including bound charge).
Note: these equations cannot be used if \mathbf{\nabla}\times\mathbf{E} \ne 0, i.e., in the case of a
nonconservative vector field electric field (caused by a changing magnetic field; see
Maxwell's equations). The generalization of electric potential to this case is described below.
Generalization to electrodynamics
When time-varying magnetic fields are present (which is true whenever there are time-varying electric fields and vice versa), one cannot describe the electric field simply in terms of a scalar potential φ because the electric field is no longer conservative: \int \mathbf{E}\cdot \mathrm{d}\mathbf{S} is path-dependent because \mathbf{\nabla} \times \mathbf{E}\neq 0.
Instead, one can still define a scalar potential by also including the magnetic vector potential \mathbf{A}. In particular, \mathbf{A} is defined by:
\mathbf{B} = \mathbf{\nabla} \times \mathbf{A}
where \mathbf{B} is the
magnetic flux density. One can always find such an \mathbf{A} because \mathbf{\nabla} \cdot \mathbf{B} = 0 (the absence of magnetic monopoles). Given this, the quantity \mathbf{F} = \mathbf{E} + \partial\mathbf{A}/\partial t
is a conservative field by Faraday's law of induction and one can therefore write:
\mathbf{E} = -\mathbf{\nabla}\phi - \frac{\partial\mathbf{A-->{\partial t}
where φ is the scalar potential defined by the conservative field \mathbf{F}.
The electrostatic potential is simply the special case of this definition where \mathbf{A} is time-invariant. On the other hand, for time-varying fields, note that \int_a^b \mathbf{E} \cdot \mathrm{d}\mathbf{S} \neq \phi(a) - \phi(b), unlike electrostatics.
Note that this definition of φ depends on the gauge fixing for the vector potential \mathbf{A} (the gradient of any scalar field can be added to \mathbf{A} without changing \mathbf{B}). One choice is the
Coulomb gauge, in which we choose \mathbf{\nabla} \cdot \mathbf{A} = 0. In this case, we obtain -\nabla^2 \phi = \rho/\varepsilon_0, where ρ is the charge density, just as for electrostatics. Another common choice is the
Lorenz gauge, in which we choose \mathbf{A} to satisfy \mathbf{\nabla} \cdot \mathbf{A} = - \frac{1}{c^2} \frac{\partial\phi}{\partial t}.
Special cases and computational devices
The electric potential at a point \mathbf{l} due to a constant electric field \mathbf{E} can be shown to be:
\phi_\mathrm{E} = - \int \mathbf{E} \cdot \mathrm{d}\mathbf{l}
The electric potential created by a point charge
q, at a distance
r from the charge, can be shown to be, in
SI units:
\phi_\mathbf{E} = \frac{q} {4 \pi \epsilon_o r}
The electric potential due to a system of point charges is equal to the sum of the point charges' individual potentials. This fact simplifies calculations significantly, since addition of potential (scalar) fields is much easier than addition of the electric (vector) fields.
The electric potential created by a tridimensional spherically symmetric
Gaussian distribution charge density \rho(r) given by:
\rho(r) = \frac{q}{\sigma^3\sqrt{2\pi}^3}\,e^{-\frac{r^2}{2\sigma^2-->,
where
q is the total charge, is obtained by solving the
Poisson's equation (in
cgs units):
\nabla^2 \phi_\mathbf{E} = - 4 \pi \rho.
The solution is given by:
\phi_\mathbf{E}(r) = \frac{q}{r}\,\mbox{erf}\left(\frac{r}{\sqrt{2}\sigma}\right)
where erf(
x) is the error function.This solution can be checked explicitly by a careful manual evaluation of \nabla^2 \phi_\mathbf{E}.Note that, for
r much greater than σ, erf(
x) approaches unity and the potential \phi_\mathbf{E} approaches the point charge potential\frac{q}{r} seen above, as expected.
Applications in electronics
This electric potential, typically measured in
volts, provides a simple way to analyze
electrical networks without requiring detailed knowledge of the circuit shape or the fields within it.
The electric potential provides a simple way to analyze
electrical networks with the help of
Kirchhoff's circuit laws#Kirchhoff's voltage law, without solving the detailed Maxwell's equations for the fields of the circuit.
Units
The
SI unit of electric potential is the
volt (in honour of
Alessandro Volta), which is so widely used that the terms
voltage and
electric potential are almost synonymous. Older units are rarely used nowadays. Variants of the centimeter gram second system of units included a number of different units for electric potential, including the
Conversion of units#Electromotive_force and the statvolt.
Electric potential is the potential energy per unit of electric charge associated with a static (time-invariant) electric field, also called the
electrostatic potential, typically measured in
volts. It is a scalar quantity. The difference of electrical potential between two points is known as
voltage.
There is also a generalized electric
scalar potential that is used in electrodynamics when time-varying electromagnetic fields are present. This generalized electric potential cannot be simply interpreted as a potential energy, however.
Explanation
Electric potential may be conceived of as "electric pressure". Where this "pressure" is uniform, no current flows and nothing happens. This is similar to why people do not feel normal atmospheric air pressure: there is no difference between the pressure inside the body and outside, so nothing is felt. However, where this electrical pressure varies, it produces an electric field, which will create a
force (physics) on charged particles.
Mathematically, it is the
potential φ (a
scalar field) associated with the irrotational vector field
electric field \mathbf{E} (\mathbf{E}=-\mathbf{\nabla}\varphi) that occurs when the magnetic field is time invariant (so that \mathbf{\nabla} \times \mathbf{E}=0 from Faraday's law of induction).
Like any potential function, only the potential difference (
voltage) between two points is physically meaningful (neglecting quantum Aharonov-Bohm effects), since any constant can be added to φ without affecting \mathbf{E} (gauge invariance).
The electric potential φ is therefore measured in units of energy per unit of electric charge. In SI units, this is:
joules/coulombs =
volts.
The electric potential can also be generalized to handle situations with time-varying potential fields, in which case the electric field is not conservative and a potential function cannot be defined everywhere in space. There, an effective potential drop is included, associated with the inductance of the circuit. This generalized potential difference is also called the
electromotive force (emf).
Introduction
Objects may possess a property known as electric charge. An electric field exerts a force on charged objects, accelerating them in the direction of the force, in either the same or the opposite direction of the electric field. If the charged object has a positive charge, the force and acceleration will be in the direction of the field. This force has the same direction as the electric field vector, and its magnitude is given by the size of the charge multiplied with the magnitude of the electric field.
Classical mechanics explores the concepts such as
Force (physics), energy, potential etc. in more detail.
Force and potential energy are directly related. As an object moves in the direction that the force accelerates it, its potential energy decreases. For example, the gravitational potential energy of a cannonball at the top of a hill is greater than at the base of the hill. As the object falls, that potential energy decreases and is translated to motion, or inertial (kinetic) energy.
For certain forces, it is possible to define the "potential" of a field such that the potential energy of an object due to a field is dependent only on the position of the object with respect to the field. Those forces must affect objects depending only on the intrinsic properties of the object and the position of the object, and obey certain other mathematical rules.
Two such forces are the gravitational force (
gravity) and the electric force in the absence of time-varying magnetic fields. The potential of an electric field is called the electric potential.
The electric potential and the
magnetic vector potential together form a
four vector, so that the two kinds of potential are mixed under Lorentz transformations.
Mathematical introduction
The concept of electric potential (denoted by: \phi, \phi_\mathrm{E} or
V) is closely linked with potential energy, thus:
U_ \mathrm{E} = q\phi
where U_\mathrm{E} is the
electric potential energy of a
test charge q due to the electric field. Note that the potential energy and hence also the electric potential is only defined up to an additive constant: one must
arbitrarily choose a position where the potential energy and the electric potential is zero.
The proper definition of the electric potential uses the
electric field \mathbf{E}:
\phi_ \mathrm{E} = - \int_C \mathbf{E} \cdot \mathrm{d} \mathbf{S}
where E is equal to the electric field, ds is an unknown, and 'C' is an arbitrary path connecting the point with zero potential to the point under consideration. When \mathbf{\nabla} \times \mathbf{E} = 0, the line integral above does not depend on the specific path
C chosen but only on its endpoints. Equivalently, the electric potential determines the electric field via its
gradient:
\mathbf{E} = - \mathbf{\nabla} \phi_\mathrm{E}
and therefore, by
Gauss's law, the potential satisfies
Poisson's equation:
\mathbf{\nabla} \cdot \mathbf{E} = \mathbf{\nabla} \cdot \left (- \mathbf{\nabla} \phi_\mathrm{E} \right ) = -\nabla^2 \phi_\mathrm{E} = \rho / \varepsilon_0
where ρ is the total
charge density (including bound charge).
Note: these equations cannot be used if \mathbf{\nabla}\times\mathbf{E} \ne 0, i.e., in the case of a
nonconservative vector field electric field (caused by a changing magnetic field; see
Maxwell's equations). The generalization of electric potential to this case is described below.
Generalization to electrodynamics
When time-varying magnetic fields are present (which is true whenever there are time-varying electric fields and vice versa), one cannot describe the electric field simply in terms of a scalar potential φ because the electric field is no longer conservative: \int \mathbf{E}\cdot \mathrm{d}\mathbf{S} is path-dependent because \mathbf{\nabla} \times \mathbf{E}\neq 0.
Instead, one can still define a scalar potential by also including the magnetic vector potential \mathbf{A}. In particular, \mathbf{A} is defined by:
\mathbf{B} = \mathbf{\nabla} \times \mathbf{A}
where \mathbf{B} is the
magnetic flux density. One can always find such an \mathbf{A} because \mathbf{\nabla} \cdot \mathbf{B} = 0 (the absence of magnetic monopoles). Given this, the quantity \mathbf{F} = \mathbf{E} + \partial\mathbf{A}/\partial t
is a conservative field by
Faraday's law of induction and one can therefore write:
\mathbf{E} = -\mathbf{\nabla}\phi - \frac{\partial\mathbf{A-->{\partial t}
where φ is the scalar potential defined by the conservative field \mathbf{F}.
The electrostatic potential is simply the special case of this definition where \mathbf{A} is time-invariant. On the other hand, for time-varying fields, note that \int_a^b \mathbf{E} \cdot \mathrm{d}\mathbf{S} \neq \phi(a) - \phi(b), unlike electrostatics.
Note that this definition of φ depends on the
gauge fixing for the vector potential \mathbf{A} (the
gradient of any scalar field can be added to \mathbf{A} without changing \mathbf{B}). One choice is the Coulomb gauge, in which we choose \mathbf{\nabla} \cdot \mathbf{A} = 0. In this case, we obtain -\nabla^2 \phi = \rho/\varepsilon_0, where ρ is the charge density, just as for electrostatics. Another common choice is the Lorenz gauge, in which we choose \mathbf{A} to satisfy \mathbf{\nabla} \cdot \mathbf{A} = - \frac{1}{c^2} \frac{\partial\phi}{\partial t}.
Special cases and computational devices
The electric potential at a point \mathbf{l} due to a constant electric field \mathbf{E} can be shown to be:
\phi_\mathrm{E} = - \int \mathbf{E} \cdot \mathrm{d}\mathbf{l}
The electric potential created by a point charge
q, at a distance
r from the charge, can be shown to be, in SI units:
\phi_\mathbf{E} = \frac{q} {4 \pi \epsilon_o r}
The electric potential due to a system of point charges is equal to the sum of the point charges' individual potentials. This fact simplifies calculations significantly, since addition of potential (scalar) fields is much easier than addition of the electric (vector) fields.
The electric potential created by a tridimensional spherically symmetric
Gaussian distribution charge density \rho(r) given by:
\rho(r) = \frac{q}{\sigma^3\sqrt{2\pi}^3}\,e^{-\frac{r^2}{2\sigma^2-->,
where
q is the total charge, is obtained by solving the Poisson's equation (in
cgs units):
\nabla^2 \phi_\mathbf{E} = - 4 \pi \rho.
The solution is given by:
\phi_\mathbf{E}(r) = \frac{q}{r}\,\mbox{erf}\left(\frac{r}{\sqrt{2}\sigma}\right)
where erf(
x) is the
error function.This solution can be checked explicitly by a careful manual evaluation of \nabla^2 \phi_\mathbf{E}.Note that, for
r much greater than σ, erf(
x) approaches unity and the potential \phi_\mathbf{E} approaches the point charge potential\frac{q}{r} seen above, as expected.
Applications in electronics
This electric potential, typically measured in
volts, provides a simple way to analyze
electrical networks without requiring detailed knowledge of the circuit shape or the fields within it.
The electric potential provides a simple way to analyze
electrical networks with the help of
Kirchhoff's circuit laws#Kirchhoff's voltage law, without solving the detailed
Maxwell's equations for the fields of the circuit.
Units
The
SI unit of electric potential is the
volt (in honour of Alessandro Volta), which is so widely used that the terms
voltage and
electric potential are almost synonymous. Older units are rarely used nowadays. Variants of the
centimeter gram second system of units included a number of different units for electric potential, including the Conversion of units#Electromotive_force and the statvolt.
Electric potential - Wikipedia, the free encyclopedia
At a point in space, the electric potential is the potential energy per unit of charge that is associated with a static (time-invariant) electric field.
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