Electromagnetic induction

Electromagnetic induction is the production of an electromotive force (emf)—a voltage—in a conductor or circuit when the magnetic flux linked with it changes. Magnetic flux measures the magnetic field passing through a chosen surface. For a simple loop in a uniform field, flux changes if the field strength, the loop’s area, or its orientation relative to the field changes. Moving a magnet relative to a coil is one familiar way to change the flux; changing the current in a nearby coil can do so as well. A changing flux can produce an emf in an open circuit, but a sustained induced current requires a closed conducting path.

Faraday’s law states that the induced emf is proportional to how quickly the magnetic flux through a circuit changes. For a coil with N turns, it is commonly written ε = −N dΦB/dt, where ΦB is magnetic flux and the minus sign indicates the direction of the induced emf. Lenz’s law gives that direction: the induced current’s magnetic effect opposes the change in flux that produced it. It does not necessarily oppose the original magnetic field; for example, if the flux is decreasing, the induced effect tends to resist that decrease. This directional rule is consistent with conservation of energy. A faster flux change or more coil turns can produce a larger emf, all else being equal.

Michael Faraday’s experiments in 1831 showed that changing magnetic conditions can produce an emf. In one arrangement, moving a magnet into or out of a coil induced an emf; Faraday also observed induction between two nearby circuits when the current in one was switched on or off. The relative motion of magnet and coil matters in the first arrangement, while the second illustrates induction through a changing magnetic field without moving the circuits. American scientist Joseph Henry independently discovered electromagnetic induction soon afterward. James Clerk Maxwell later expressed the relationship mathematically as Faraday’s law within the broader theory of electromagnetism.

Electric generators use induction to convert mechanical motion into electrical energy. In a transformer, a changing current in one coil creates changing magnetic flux that induces a voltage in another coil, allowing electrical energy to be transferred between circuits. Induction is also used in electric motors and in wireless charging systems, where changing magnetic fields transfer energy without a direct electrical connection between the charging unit and device. In each case, the surrounding apparatus supplies or transfers energy; induction does not create energy from nothing.