Electric field

In physics, an electric field or E-field is an effect produced by an electric charge that exerts a force on charged objects in its vicinity. The units of the electric field are newtons per coulomb or volts per meter (both are equivalent). Electric fields are composed of photons and contain electrical energy with energy density proportional to the square of the field intensity. In the static case, an electric field is composed of virtual photons being exchanged by the charged particle(s) creating the field. In the dynamic case the electric field is accompanied by a magnetic field, by a flow of energy, and by real photons.

Definition and derivation

The mathematical definition of the electric field is developed as follows. Coulomb's law gives the force between two point charges (infinitesimally small charged objects) as

Related Topics:
Field - Coulomb's law

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:

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mathbf{F} = rac{1}{4 pi epsilon_0}rac{q_1 q_2}{r^2}mathbf{hat r} (1)

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where

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• $epsilon\_0$ (pronounced epsilon-nought) is a physical constant, the permittivity of free space;
• $q\_1$ and $q\_2$ are the electric charges of the objects;
• $r$ is the magnitude of the separation vector between the objects;
• $hat\; r$ is the unit vector representing the direction from one charge to the other.

In the SI system of units, force is given in newtons, charge in coulombs, and distance in metres. Thus, epsilon_0 has units of C²/(N·m²).

Related Topics:
SI - Newton - Coulomb - Metre

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This was known empirically. Suppose one of the charges is taken to be fixed, and the other one to be a moveable "test charge". Note that according to this equation, the force on the test object is proportional to its charge. The electric field is defined as the proportionality constant between charge and force:

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:

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mathbf{F} = qmathbf{E}

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:

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mathbf{E} = rac{1}{4 pi epsilon_0}rac{Q}{r^2}mathbf{hat r}

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However, note that this equation is only true in the case of electrostatics, that is to say, when there is nothing moving. The more general case of moving charges causes this equation to become the Lorentz equation. When we speak of a "moveable test charge", this means that the charge can be moved to, and held at, any position.

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