Energy Conservation in Lenz's Law

Lenz's law is not an independent postulate -- it is forced on us by the law of conservation of energy. There are two distinct physical situations that induce a current: (i) a conductor physically moving through a magnetic field, and (ii) a stationary circuit sitting in a magnetic field that is itself changing with time.

In case (i), the induced current in the moving conductor always experiences a magnetic force (from the very field it sits in) that is directed opposite to the conductor's velocity -- i.e. the induced current's own magnetic interaction always resists the motion that produces it, so external work must be done to keep the conductor moving.

In case (ii), the induced current sets up its OWN magnetic field within the area bounded by the circuit; this self-field is opposite to the (increasing) original field, or in the same direction as the (decreasing) original field -- either way, always acting to oppose the CHANGE in flux, never the flux itself. …