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,
Spectral flux density 1 MW m), W·m·Hz, Jansky or solar flux units. The terms irradiance, radiant exitance, radiant emittance, and radiosity are closely related to spectral flux density. The terms used to describe spectral flux density vary between fields, sometimes including adjectives such as "electromagnetic" or "radiative", and sometimes dropping the word "density". Applications include: For the flux density received from a remote unresolvable "point source", the measuring instrument, usually telescopic, though not able to resolve any detail of the source itself, must be able to optically resolve enough details of the sky around the point source, so as to record radiation coming