StellarElectromagnetic induction occurs when a changing magnetic flux through a loop produces an induced emf. This is described by Faraday’s law:
and for a coil of N turns,
The negative sign is the statement of Lenz’s law: the induced emf drives a current whose magnetic effects oppose the change in flux that caused it. The emf is therefore not trying to oppose the field itself, but the change in flux.
The calculus here is essential. Induction depends on the time rate of change of flux, not simply on the flux value. A large flux can produce no emf if it is constant, while a small flux changing rapidly can produce a large emf. Flux can change because the magnetic field changes, the area changes, or the angle changes. This is why moving loops, rotating coils, and changing fields can all generate emf.