Electric field
An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field for a system of charged particles. Electric fields originate from electric charges, or from time-varying magnetic fields. Electric fields and magnetic fields are both manifestations of the electromagnetic force, one of the four fundamental forces (or interactions) of nature. Electric fields are important in many areas of physics, and are exploited practically in electrical technology. In atomic physics and  chemistry, for instance, the electric field is the attractive force holding the atomic nucleus and electrons together in atoms. It is also the force responsible for chemical bonding between atoms that result in molecules. The electric field is defined mathematically as a vector field that associates to each point in space the (electrostatic or Coulomb) force per unit of charge exerted on an infinitesimal positive test charge at rest at that point. The derived SI units for the electric field are volts per meter (V/m), exactly equivalent to newtons per coulomb (N/C). Description. The electric field is defined  at each point in space as the force (per unit charge) that would be experienced by a vanishingly small positive test charge if held at that point. As the electric field is defined in terms of force, and force is a vector (i.e. having both magnitude and direction), it follows that an electric field is a vector field. Vector fields of this form are sometimes referred to as force fields. The electric field acts between two charges similarly to the way the gravitational field acts between two masses, as they both obey an inverse-square law with distance. This is  the basis for Coulomb's law, which states that, for stationary charges, the electric field varies with the source charge and varies inversely with the square of the distance from the source. This means that if the source charge were doubled, the electric field would double, and if you move twice as far away from the source, the field at that point would be only one-quarter its original strength. The electric field can be visualized with a set of lines whose direction at each point is the same as the field's, a concept introduced by Michael Faraday, whose  term 'lines of force' is still sometimes used. This illustration has the useful property that the field's strength is proportional to the density of the lines. The field lines are the paths that a point positive charge would follow as it is forced to move within the field, similar to trajectories that masses follow within a gravitational field. Field lines due to stationary charges have several important properties, including always originating from positive charges and terminating at negative charges, they enter all good conductors at right angles, and they never cross or close in on themselves. The field lines  are a representative concept; the field actually permeates all the intervening space between the lines. More or fewer lines may be drawn depending on the precision to which it is desired to represent the field. The study of electric fields created by stationary charges is called electrostatics. Faraday's law describes the relationship between a time-varying magnetic field and the electric field. One way of stating Faraday's law is that the curl of the electric field is equal to the negative time derivative of the magnetic field. In the absence of time-varying magnetic field, the electric field  is therefore called conservative (i.e. curl-free). This implies there are two kinds of electric fields: electrostatic fields and fields arising from time-varying magnetic fields. While the curl-free nature of the static electric field allows for a simpler treatment using electrostatics, time-varying magnetic fields are generally treated as a component of a unified electromagnetic field. The study of time varying magnetic and electric fields is called electrodynamics. Mathematical formulation. Electric fields are caused by electric charges, described by Gauss's law, and time varying magnetic fields, described by Faraday's law of induction. Together, these laws are  enough to define the behavior of the electric field. However, since the magnetic field is described as a function of electric field, the equations of both fields are coupled and together form Maxwell's equations that describe both fields as a function of charges and currents. Electrostatics. In the special case of a steady state (stationary charges and currents), the Maxwell-Faraday inductive effect disappears. The resulting two equations (Gauss's law formula_1 and Faraday's law with no induction term formula_2), taken together, are equivalent to Coulomb's law, which states that a particle with electric charge formula_3 at  position formula_4 exerts a force on a particle with charge formula_5 at position formula_6 of: formula_7 where formula_8 is the unit vector in the direction from point formula_4 to point formula_6, and is the electric constant (also known as "the absolute permittivity of free space") with units C⋅m⋅N. Note that formula_11, the vacuum electric permittivity, must be substituted with formula_12, permittivity, when charges are in non-empty media. When the charges formula_5 and formula_3 have the same sign this force is positive, directed away from the other charge, indicating the particles repel each other. When the charges have unlike signs  the force is negative, indicating the particles attract. To make it easy to calculate the Coulomb force on any charge at position formula_6 this expression can be divided by formula_5 leaving an expression that only depends on the other charge (the "source" charge) formula_17 This is the "electric field" at point formula_6 due to the point charge formula_3; it is a vector-valued function equal to the Coulomb force per unit charge that a positive point charge would experience at the position formula_6. Since this formula gives the electric field magnitude and direction at any point formula_6 in space (except  at the location of the charge itself, formula_4, where it becomes infinite) it defines a vector field. From the above formula it can be seen that the electric field due to a point charge is everywhere directed away from the charge if it is positive, and toward the charge if it is negative, and its magnitude decreases with the inverse square of the distance from the charge. The Coulomb force on a charge of magnitude formula_23 at any point in space is equal to the product of the charge and the electric field at that point formula_24 The units of  the electric field in the SI system are newtons per coulomb (N/C), or volts per meter (V/m); in terms of the SI base units they are kg⋅m⋅s⋅A. Superposition principle. Due to the linearity of Maxwell's equations, electric fields satisfy the superposition principle, which states that the total electric field, at a point, due to a collection of charges is equal to the vector sum of the electric fields at that point due to the individual charges. This principle is useful in calculating the field created by multiple point charges. If charges formula_25 are stationary in space at  points formula_26, in the absence of currents, the superposition principle says that the resulting field is the sum of fields generated by each particle as described by Coulomb's law: formula_27 where formula_28 is the unit vector in the direction from point formula_29 to point formula_30. Continuous charge distributions. The superposition principle allows for the calculation of the electric field due to a continuous distribution of charge formula_31 (where formula_32 is the charge density in coulombs per cubic meter). By considering the charge formula_33 in each small volume of space formula_34 at point formula_35 as a point charge, the resulting  electric field, formula_36, at point formula_30 can be calculated as formula_38 where formula_39 is the unit vector pointing from formula_35 to formula_30. The total field is then found by "adding up" the contributions from all the increments of volume by integrating over the volume of the charge distribution formula_42: formula_43 Similar equations follow for a surface charge with continuous charge distribution formula_44 where formula_45 is the charge density in coulombs per square meter formula_46 and for line charges with continuous charge distribution formula_47 where formula_48 is the charge density in coulombs per meter. formula_49 Electric potential. If a system is  static, such that magnetic fields are not time-varying, then by Faraday's law, the electric field is curl-free. In this case, one can define an electric potential, that is, a function formula_50 such that This is analogous to the gravitational potential. The difference between the electric potential at two points in space is called the potential difference (or voltage) between the two points. In general, however, the electric field cannot be described independently of the magnetic field. Given the magnetic vector potential, , defined so that one can still define an electric potential formula_51 such that: formula_52 where  formula_53 is the gradient of the electric potential and formula_54 is the partial derivative of A with respect to time. Faraday's law of induction can be recovered by taking the curl of that equation formula_55 which justifies, a posteriori, the previous form for . Continuous vs. discrete charge representation. The equations of electromagnetism are best described in a continuous description. However, charges are sometimes best described as discrete points; for example, some models may describe electrons as point sources where charge density is infinite on an infinitesimal section of space. A charge formula_23 located at formula_57 can be described mathematically  as a charge density formula_58, where the Dirac delta function (in three dimensions) is used. Conversely, a charge distribution can be approximated by many small point charges. Electrostatic fields. Electrostatic fields are electric fields that do not change with time. Such fields are present when systems of charged matter are stationary, or when electric currents are unchanging. In that case, Coulomb's law fully describes the field. Parallels between electrostatic and gravitational fields. Coulomb's law, which describes the interaction of electric charges: formula_59). This suggests similarities between the electric field E and the gravitational field g, or their associated  potentials. Mass is sometimes called "gravitational charge". Electrostatic and gravitational forces both are central, conservative and obey an inverse-square law. Uniform fields. A uniform field is one in which the electric field is constant at every point. It can be approximated by placing two conducting plates parallel to each other and maintaining a voltage (potential difference) between them; it is only an approximation because of boundary effects (near the edge of the planes, electric field is distorted because the plane does not continue). Assuming infinite planes, the magnitude of the electric field "E" is: formula_60 where Δ"V" is the  potential difference between the plates and "d" is the distance separating the plates. The negative sign arises as positive charges repel, so a positive charge will experience a force away from the positively charged plate, in the opposite direction to that in which the voltage increases. In micro- and nano-applications, for instance in relation to semiconductors, a typical magnitude of an electric field is in the order of , achieved by applying a voltage of the order of 1 volt between conductors spaced 1 µm apart. Electrodynamic fields. Electrodynamic fields are electric fields which do change with time, for instance  when charges are in motion. In this case, a magnetic field is produced in accordance with Ampère's circuital law (with Maxwell's addition), which, along with Maxwell's other equations, defines the magnetic field, formula_61, in terms of its curl: formula_62 where formula_63 is the current density, formula_64 is the vacuum permeability, and formula_11 is the vacuum permittivity. That is, both electric currents (i.e. charges in uniform motion) and the (partial) time derivative of the electric field directly contributes to the magnetic field. In addition, the Maxwell–Faraday equation states formula_66 These represent two of Maxwell's four equations  and they intricately link the electric and magnetic fields together, resulting in the electromagnetic field. The equations represent a set of four coupled multi-dimensional partial differential equations which, when solved for a system, describe the combined behavior of the electromagnetic fields. In general, the force experienced by a test charge in an electromagnetic field is given by the Lorentz force law: formula_67 Energy in the electric field. The total energy per unit volume stored by the electromagnetic field is formula_68 where is the permittivity of the medium in which the field exists, formula_69 its magnetic permeability, and and are  the electric and magnetic field vectors. As and fields are coupled, it would be misleading to split this expression into "electric" and "magnetic" contributions. In particular, an electrostatic field in any given frame of reference in general transforms into a field with a magnetic component in a relatively moving frame. Accordingly, decomposing the electromagnetic field into an electric and magnetic component is frame-specific, and similarly for the associated energy. The total energy "U" stored in the electromagnetic field in a given volume "V" is formula_70 The electric displacement field. Definitive equation of vector fields. In the presence of matter , it is helpful to extend the notion of the electric field into three vector fields: formula_71 where P is the electric polarization – the volume density of electric dipole moments, and is the electric displacement field. Since E and P are defined separately, this equation can be used to define . The physical interpretation of D is not as clear as E (effectively the field applied to the material) or (induced field due to the dipoles in the material), but still serves as a convenient mathematical simplification, since Maxwell's equations can be simplified in terms of free charges and currents. Constitutive  relation. The E and D fields are related by the permittivity of the material, "ε". For linear, homogeneous, isotropic materials E and D are proportional and constant throughout the region, there is no position dependence: formula_72 For inhomogeneous materials, there is a position dependence throughout the material: formula_73 For anisotropic materials the and fields are not parallel, and so and are related by the permittivity tensor (a 2nd order tensor field), in component form: formula_74 For non-linear media, and are not proportional. Materials can have varying extents of linearity, homogeneity and isotropy.