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,
Permeability (electromagnetism) example in a Newton–Raphson iterative solution scheme that computes the changing saturation of a magnetic circuit. Permeability is the inductance per unit length. In SI units, permeability is measured in henries per metre (H·m = J/(A·m) = N·A). The auxiliary magnetic field H has dimensions current per unit length and is measured in units of amperes per metre (A·m). The product "µ" H thus has dimensions inductance times current per unit area (H·A/m). But inductance is magnetic flux per unit current, so the product has dimensions magnetic flux per unit area, that is, magnetic flux density. This is the magnetic