Lenz's law

Read the statement slowly. The induced field opposes the change in flux, not the flux. Getting that wrong is the single most common error on induction questions, and it is invisible until you meet the case below.

The case that catches people. A loop sits in a field pointing into the page, and that field is switched down toward zero. The flux into the page is decreasing, so the change is a loss, and the induced current opposes the loss: it flows clockwise to make more into-the-page field inside the loop. A student holding "opposes the flux" answers counterclockwise and drops the question. Growing flux into the page gives counterclockwise; shrinking flux into the page gives clockwise.

Four steps, in order. Name the field direction through the loop. Say whether the flux is growing, shrinking, or steady, and stop there if it is steady. Decide which way the induced field must point inside the loop: against the existing flux if it is growing, along with it if it is shrinking. Then point your right thumb that way and let your fingers give the current.

This is not a second law bolted onto Faraday's. It is energy conservation. An induced current that reinforced its own cause would grow without limit and hand you free energy, so the sign has to be the one that resists. That is also why dragging a conducting rod through a field takes work.

The CED warns that naming Lenz's law is not a complete free-response justification. Write the flux direction, the direction of its change, and the right-hand rule step. Topic 12.4 works examples.