Lenz's law
The equation for electromagnetic induction can be written as
A factor N has been inserted here to include the case of multiple turns of wire.
The negative sign represents Lenz's Law, which tells which way the induced current flows, based on the horse-sense observation that the magnetic field generated by an induced current must oppose the change in the flux that caused it. This has to be so, because otherwise the induced current would make the flux change even more and soon the current would become infinitely large.