Magnetic flux area S is where "B" is the magnitude of the magnetic field (the magnetic flux density) having the unit of Wb/m (tesla), "S" is the area of the surface, and "θ" is the angle between the magnetic field lines and the normal (perpendicular) to S. For a varying magnetic field, we first consider the magnetic flux through an infinitesimal area element dS, where we may consider the field to be constant: A generic surface, S, can then be broken into infinitesimal elements and the total magnetic flux through the surface is then the surface integral From the definition of the
Magnetic tension force as the divergence of the Maxwell tensor: which gives the electromagnetic force density in terms of Maxwell stress tensor, formula_24, and the Poynting vector, formula_25. Now, the magnetic tension is implicitly included inside formula_24. The implication of the above relation is the conservation of momentum. Here, formula_27 is the momentum flux density and plays a role similar to formula_28 in Poynting's theorem. 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