Electromagnetic Induction

Electromagnetic induction is the production of an emf, and so a current in a closed circuit, when the magnetic flux through a coil changes. The change can come from moving a magnet, moving the coil, or changing the current in a nearby coil. Michael Faraday discovered it in 1831.

Faraday's law says the induced emf in a coil equals the rate of change of magnetic flux through it, multiplied by the number of turns: ε = −N dΦ/dt. The faster the flux changes and the more turns there are, the bigger the emf.

Lenz's law says that the direction of an induced current is always such that it opposes the change in magnetic flux that produced it. It follows from conservation of energy and gives the minus sign in Faraday's law.

Induction depends on change. With the magnet at rest, the flux through the coil is large but constant, so its rate of change is zero, and so is the induced emf. The needle moves only while the magnet, or the coil, is moving.

Move the magnet faster, use a stronger magnet, use a coil with more turns, or wind the coil on a soft iron core. Each of these makes the flux through the coil change faster, so the emf is larger.

Power station generators, bicycle dynamos, transformers on electricity poles, induction cooktops, wireless phone chargers, metal detectors and the magnetic pickups in electric guitars all work by electromagnetic induction.