Problem
Imagine a world in which the induced current had the opposite sense to that prescribed by Lenz: the loop would favour the motion of the magnet instead of opposing it. Explain in five lines why such a world would violate energy conservation, and what would happen if you placed a simple closed loop next to a stationary magnet.
Solution
If the induced current favoured the motion of the magnet, every displacement of it would be accelerated: its kinetic energy would grow and, at the same time, the loop would dissipate a growing amount of electrical energy through Joule heating. Both energies would appear out of nowhere, with no source: this is a perpetual motion machine of the first kind, which violates the first law of thermodynamics. With a closed loop placed next to a stationary magnet, the smallest thermal fluctuation would trigger a current that pushes the magnet, whose motion amplifies the current, which pushes the magnet harder, and so on: the system would self-accelerate indefinitely. This is precisely the absurdity that Lenz’s minus sign exists to prevent.
Links
Topics: Induzione elettromagnetica Concepts: Legge di Lenz · Legge di Faraday-Neumann-Lenz Skills: Analisi di casi limite e fantafisica