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,
Radiant energy density Radiant energy density In radiometry, radiant energy density is the radiant energy per unit volume. The SI unit of radiant energy density is the joule per cubic metre (J/m). Radiant energy density, denoted "w" ("e" for "energetic", to avoid confusion with photometric quantities), is defined as where Because radiation always "transmits" the energy, it is useful to wonder what the speed of the transmission is. If all the radiation at given location propagates in the same direction, then the radiant flux through a unit area perpendicular to the propagation direction is given by the irradiance: where "c" is the radiation