Flux surface surrounding a charge "q" is "q"/ε. In free space the electric displacement is given by the constitutive relation D = ε E, so for any bounding surface the D-field flux equals the charge "Q" within it. Here the expression "flux of" indicates a mathematical operation and, as can be seen, the result is not necessarily a "flow", since nothing actually flows along electric field lines. The magnetic flux density (magnetic field) having the unit Wb/m (Tesla) is denoted by B, and magnetic flux is defined analogously: with the same notation above. The quantity arises in Faraday's law of induction,
Magnetic flux boundary ∂Σ and, secondly, as the change of magnetic flux through the open surface Σ. This equation is the principle behind an electrical generator. By way of contrast, Gauss's law for electric fields, another of Maxwell's equations, is where Note that the flux of E through a closed surface is "not" always zero; this indicates the presence of "electric monopoles", that is, free positive or negative charges. Magnetic flux In physics, specifically electromagnetism, the magnetic flux (often denoted or ) through a surface is the surface integral of the normal component of the magnetic field B passing through that surface.