In electromagnetism, electric flux is the measure of the distribution of the electric field through a given surface, although an electric field in itself cannot flow. It is a way of describing the electric field strength at any distance from the charge causing the field.
The electric field E can exert a force at any point in space. The electric field is proportional to the gradient of the voltage. The electric field is capable of moving point charged particles. The ratio of the force over the flow is an impedance.
An electric "charge," such as a single electron in space, has an electric field surrounding it. In pictorial form, this electric field is shown as a dot, the charge, radiating "lines of flux". These are called Gauss lines. Electric Flux Density is the amount of electric flux, the number of "lines," passing through a given area. Units are Gauss/square meter. Electric flux is proportional to the number of electric field lines going through a normally perpendicular surface. If the electric field is uniform, the electric flux passing through a surface of vector area S is
where E is the electric field (having units of V/m), E is its magnitude, S is the area of the surface, and θ is the angle between the electric field lines and the normal (perpendicular) to S.
For a non-uniform electric field, the electric flux dΦE through a small surface area dS is given by
(the electric field, E, multiplied by the component of area perpendicular to the field). The electric flux over a surface S is therefore given by the surface integral:
where E is the electric field and dS is a differential area on the closed surface S with an outward facing surface normal defining its direction.
For a closed Gaussian surface, electric flux is given by:
- E is the electric field,
- S is any closed surface,
- Q is the total electric charge inside the surface S,
- ε0 is the electric constant (a universal constant, also called the "permittivity of free space") (ε0 ≈ 8.854 187 817... x 10−12 farads per meter (F·m−1)).
While the electric flux is not affected by charges that are not within the closed surface, the net electric field, E, in the Gauss' Law equation, can be affected by charges that lie outside the closed surface. While Gauss' Law holds for all situations, it is only useful for "by hand" calculations when high degrees of symmetry exist in the electric field. Examples include spherical and cylindrical symmetry.
Its dimensional formula is [L3MT−3I−1].
- Purcell, p22-26
- Purcell, p5-6.
- Browne, p223.