A general law for electromagnetic induction
The definition of the induced $emf$ as the integral over a closed loop of the Lorentz force acting on a unit positive charge leads immediately to a general law for electromagnetic induction phenomena. The general law is applied to three significant cases: moving bar, Faraday's and Corbino's disc. This last application illustrates the contribution of the drift velocity of the charges to the induced $emf$: the magneto-resistance effect is obtained without using microscopic models of electrical conduction. Maxwell wrote down `general equations of electromotive intensity' that, integrated over a closed loop, yield the general law for electromagnetic induction, if the velocity appearing in them is correctly interpreted. The flux of the magnetic field through an arbitrary surface that have the circuit as contour {\em is not the cause} of the induced $emf$. The flux rule must be considered as a calculation shortcut for predicting the value of the induced $emf$ when the circuit is filiform. Finally, the general law of electromagnetic induction yields the induced $emf$ in both reference frames of a system composed by a magnet and a circuit in relative uniform motion, as required by special relativity.