structural:expansion
Faraday's Law

where E {\displaystyle {\mathcal {E}}} is the electromotive force (or EMF, the voltage generated around a closed loop) and Φ is the magnetic flux —the product of the area times the magnetic field normal to that area. (This definition of magnetic flux is why B 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.