Lenz's law states that an induced emf acts in a direction that opposes the change in magnetic flux that produced it. This opposition is required by energy conservation: if the induced effect reinforced the change, energy could be produced without any external work.
The Mechanism
In IB Physics D.4 Induction (HL only), Faraday's law is written as
where is the induced emf, is the number of turns, and is the magnetic flux through one turn. The minus sign represents Lenz's law.
Suppose the north pole of a magnet moves toward a conducting coil. The magnetic flux through the coil increases, so an emf is induced. If the circuit is closed, the induced current creates its own magnetic field that opposes this increase: the near face of the coil becomes a north pole and repels the approaching magnet.
The person moving the magnet must therefore apply a force and do mechanical work against this repulsion. That energy is transferred into electrical energy and, because the coil has resistance, eventually into thermal energy. Moving the magnet faster increases , producing a larger induced emf and a stronger opposing effect, so more work must be done.
If the induced field instead assisted the increasing flux, the magnet would accelerate toward the coil while producing an increasing current. The system would gain kinetic and electrical energy without an equivalent energy input, violating conservation of energy.
A common misconception is that the induced magnetic field always opposes the original magnetic field. It actually opposes the change in magnetic flux: if flux is decreasing, the induced field acts to maintain it.
Exam Technique
For an Explain question, state the flux change, determine the induced field that opposes that change, infer the current direction using a hand rule, and link the required external work to the electrical or thermal energy produced. Do not write only “the current opposes the field.”