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,
Flux quantum state ψ(r, t) have a probability density defined as So the probability of finding a particle in a differential volume element dr is Then the number of particles passing perpendicularly through unit area of a cross-section per unit time is the probability flux; This is sometimes referred to as the probability current or current density, or probability flux density. As a mathematical concept, flux is represented by the surface integral of a vector field, where F is a vector field, and d"A" is the vector area of the surface "A", directed as the surface normal.For the second,n is the