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 field the area times the magnetic field normal to that area. (This definition of magnetic flux is why is often referred to as "magnetic flux density".) The negative sign represents the fact that any current generated by a changing magnetic field in a coil produces a magnetic field that "opposes" the "change" in the magnetic field that induced it. This phenomenon is known as Lenz's law. This integral formulation of Faraday's law can be converted into a differential form, which applies under slightly different conditions. This form is covered as one of Maxwell's equations below. Similar to the way that a