20.3 Lenz's Law and Direction Workflows

Lenz's law says induced current produces magnetic effects that oppose the change in magnetic flux, not necessarily the original field itself. That wording matters. If inward flux is increasing, induced current must create outward field to resist the increase. If inward flux is decreasing, induced current must reinforce inward field to resist the decrease.

Direction solving is fastest in three moves: identify whether external flux through your chosen loop direction is increasing or decreasing; decide the direction of induced field needed to oppose that trend; then convert induced-field direction to current direction with right-hand grip rule.

When a loop enters a field region, overlap area grows and flux magnitude typically increases. When it leaves, overlap shrinks and flux magnitude decreases. The current direction therefore flips between entry and exit for the same external field orientation. This is one of the most common exam patterns.

The same logic applies when a magnet approaches or recedes from a coil, and when current in a nearby wire changes. Always phrase the situation as flux trend first. That single sentence usually determines direction before any equation is written.

Induction is never free energy. If your setup creates current, the source driving flux change must provide power. In motional emf systems you push against magnetic reaction forces. In transformers and generators you provide mechanical or electrical input that sustains changing flux.