Lenz's Law

• Lenz's law: the induced e.m.f. is in a direction that opposes the change that caused it. The direction claim is M1, the oppose-the-change completion is A1 (9702/42/O/N/24 Q1(b)(i)).

• The energy argument: if the coil's near face became a south pole instead, the magnet would be pulled in, accelerating while also generating current — energy from nowhere. Opposition is what makes the pusher do the work that becomes the electrical energy.

• Every “which way / which end is positive” question walks the same three marks (9702/41/O/N/25 Q7(b)(iv)) (9702/42/M/J/23 Q6(c)):

1. The force on the induced current must oppose the motion — state that force's direction.
2. Fleming's left-hand rule (with that force and the field) fixes the current's direction in the conductor.
3. For the polarity: inside a source, current runs from − to + — so the end the current flows towards (inside) is the positive terminal.

• In the 2024 wheel this gave current from axle to rim and the rim end at the higher potential; in the 2025 aircraft, wingtip P came out positive. Same template, new scenery.
• Eddy currents are Lenz's law loose in solid metal: a conductor moving through a non-uniform field has circulating currents induced inside it, and their forces always oppose the motion — used deliberately in electromagnetic braking, where the oscillation or rotation dies away as the energy becomes heat.

Lenz = energy conservation with a direction: induced effects oppose their cause. Template: opposing force → left-hand rule → −-to-+ inside the source.