State Lenz's law. Use it to find the direction of the induced current when a magnet approaches a coil, and show that it follows from the law of conservation of energy.

1. 1 · State the law. The induced emf has a polarity that tends to produce a current opposing the change in flux that produced it.
2. 2 · Apply it. N pole approaching: flux rises, the near face becomes a north pole, and the current is anticlockwise seen from the magnet.
3. 3 · Draw the diagram. [Coil with near face marked N; bar magnet with N facing it; arrow for motion; anticlockwise current arrows on the coil]
4. 4 · Link to energy. The coil repels the magnet, so work must be done to push it; this work becomes electrical energy and heat.
5. 5 · Argue the opposite case. If the current aided the motion, energy would grow from nothing (perpetual motion), which is impossible.
6. 6 · Real life. Regenerative braking in metro trains uses this opposing force.
7. 7 · Close in one line. Therefore, Lenz's law is the conservation of energy applied to induction.

Scoring tip: Use the word 'change' in the statement: the current opposes the change in flux, not the flux.