Search

# Electric potential

Electric potential is the potential energy per unit charge associated with a static (time-invariant) electric field, also called the electrostatic potential, typically measured in volts. Metaphorically, electric potential may be conceived of as "electric pressure" that can push electric charges to different locations. Technically, it is the potential φ (a scalar field) associated with the conservative electric field E (E = −φ) that occurs when the magnetic field is time invariant (so that ∇ × 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 E.

The electric potential 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 sitations with time-varying magnetic 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).

 Contents

## Introduction

Objects may possess a property known as electric charge. In the presence of an electric field, a force is exerted on such objects, accelerating them in the direction of the force. 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, energy, potential etc. in more detail.

There is a direct relationship between force and potential energy. 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 tower is greater than at the base of the tower. As the object falls, that potential energy decreases and is translated to motion, or inertial 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 vector of dimension 4, so that the two kinds of potential are mixed under Lorentz transformations.

## Mathematical introduction

The concept of electric potential (denoted by: φ or V) is closely linked with potential energy. Specifically, one definition for electric potential is:

$\phi = \frac{U} {q}$

where U is the potential energy of a small charge q due to the electric field. Here, q must be a test charge infinitesimally small, so as to not significantly affect the distribution of charges elsewhere. φE will only depend on the position of q but not on its size. Note that the potential energy and hence also the electric potential is only defined up to an additive constant: one may arbitrarily choose one position where the potential energy and the electric potential is zero.

The electric potential can also be calculated using the electric field E, thus:

$\phi_E = - \int_s \mathbf{E} \cdot d\mathbf{s}$

where s is an arbitrary path connecting the point with zero potential to the point under consideration. Note: this equation cannot be used and the electric potential is not defined if ×E ≠ 0, i.e., in the case of a changing magnetic field (see Maxwell's equations). When ×E = 0, the above integral does not depend on the specific path s chosen but only on its endpoints because then:

$\oint_s \mathbf{E} \cdot d\mathbf{s} = 0$

for any closed path s (this equation, in a simplified form, is extremely useful in electrical engineering as one of Kirchhoff's circuit laws).

If E is constant, then φE looks like this:

$\phi_\mathbf{E} = - \mathbf{E} \cdot \mathbf{s}$

where s is the displacement vector from the point of zero potential to the point under consideration.

The electric potential created by a point charge q can be shown to have the following form, in SI units:

$\phi_\mathbf{E} = \frac{q} {4 \pi \epsilon_o r}$

where r is the distance of the point under consideration from the point charge.

The electric potentials due to a system of point charges may be computed as the sum of the respective potentials, which simplifies calculations significantly since adding scalar fields is very much easier than adding the electric fields, which are vector fields.

## Applications in electronics

This electric potential, typically measured in Volts, provides a simple way to analyze electric circuits 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 voltage law, without solving the detailed Maxwell's equations for the fields of the circuit.