If a coil of wire is placed in a changing magnetic field, a current will be induced in the wire. ... A changing magnetic field through a coil of wire therefore must induce an emf in the coil which in turn causes current to flow.

When a magnet is moved into a coil of wire, changing the magnetic field and magnetic flux through the coil, a voltage will be generated in the coil according to Faraday's Law. ... The polarity of the induced emf is such that it produces a current whose magnetic field opposes the change that produces it.

As the magnet approaches the coil, it causes the left-pointing magnetic field in the coil to increase. Lenz's law tells us that the emf induced in the coil will drive a current in the direction that creates a magnetic field pointing to the right.

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