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 tension force Magnetic tension force The magnetic tension force is a restoring force (SI unit: Pa·m) that acts to straighten bent magnetic field lines. It equals: It is analogous to rubber bands and their restoring force. The force is directed antiradially. Although magnetic tension is referred to as a force, it is actually a pressure gradient (Pa m) which is also a force density (N m). The magnetic pressure is the energy density of the magnetic field which can be visualized as increasing as magnetic field lines converge in a given volume of space. In contrast, magnetic tension force is determined by