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 quantum Magnetic flux quantum The magnetic flux, represented by the symbol , threading some contour or loop is defined as the magnetic field multiplied by the loop area , i.e. . Both and can be arbitrary and so is . However, if one deals with the superconducting loop or a hole in a bulk superconductor, it turns out that the magnetic flux threading such a hole/loop is quantized. The (superconducting) magnetic flux quantum is a combination of fundamental physical constants: the Planck constant and the electron charge . Its value is, therefore, the same for any superconductor. The phenomenon of flux