Physics · Ch 12 — Electromagnetic Induction
Lenz's Law
Lenz's Law
Faraday's second law gives the magnitude of the induced emf but says nothing about its direction. That gap was filled by H.F.E. Lenz (1804-1864), who -- working independently and without knowledge of Faraday's and Henry's results -- arrived at the same conclusions, and is credited with the rule for direction that now bears his name.
Lenz's law states: the direction of the induced current in a circuit is always such that the magnetic field it produces opposes the very change in magnetic flux that induced it. Equivalently, since the direction of the induced emf and the induced current coincide, the induced emf always tends to drive a current whose effect opposes the change that causes it. This is sometimes stated even more broadly as: 'every effect of induction acts in opposition to the cause that produces it.'
Lenz's law requires a closed circuit for an actual induced CURRENT to flow; if the conductor is open, we mentally close it with an imagined external circuit, apply Lenz's law to that imagined loop to find the sense of the current that WOULD flow, and from that read off which end of the real open conductor is at the higher potential. …
Worked out. A coil of wire is wound around a vertical iron core, and a light metallic ring is placed loosely over the top of the core, resting on it. When a switch closes and current begins to flow through the coil, the metallic ring -- initially at rest on top of the core -- is suddenly flung several metres up into the air. Explanation: before the coil's current is switched on, zero magnetic flux passes through the ring; the instant current flows, upward-directed flux appears in the core and rises rapidly, inducing (by Lenz's law) a current in the ring flowing in the direction OPPOSITE to the coil's own current. Since anti-parallel (opposite-direction) currents repel one another, the ring is repelled strongly enough to leap into the air -- a direct, …