Original: A corollary of Faraday 's Law , together with AmpÃ re 's law and Ohm 's law is Lenz 's law : The EMF induced in an electric circuit always acts in such a direction that the current it drives around the circuit opposes the change in magnetic flux which produces the EMF .
Simplified: The negative sign in both equation above is a result of Lenz 's law , named after Heinrich Lenz . His law states that the electromotive force ( EMF ) produces a current that opposes the motion of the changing magnetic flux .