Faraday's law of induction
/ far-uh-DAY /
Move a magnet toward a coil of wire and a current flows in the coil, even though there is no battery anywhere. Faraday's law is the rule behind this small miracle: a changing magnetic flux through a loop creates a voltage, called an induced EMF (electromotive force). It answers the question, where does induced electricity come from?
Precisely, the induced EMF equals the negative rate of change of the magnetic flux: EMF = - dPhi/dt. For a coil with N turns the effect multiplies, EMF = - N dPhi/dt, so more turns give more voltage. The faster the flux changes, the bigger the EMF. The minus sign encodes the direction of the induced current, which is the content of Lenz's law. You can make the flux change in any of three ways — change the field B, change the area A, or change the angle theta.
This one law runs generators, transformers, induction cooktops, electric-guitar pickups, metal detectors, and contactless card readers — a huge slice of modern technology. The honest key point is that it is the CHANGE that matters, not the flux itself: a strong but steady field through a coil induces nothing, while a weak but rapidly changing field can induce a large voltage.
A 200-turn coil has its flux drop from 0.01 Wb to 0 in 0.1 s. The induced EMF has magnitude N dPhi/dt = 200 * (0.01 / 0.1) = 20 V.
200 turns with the flux falling 0.01 Wb in 0.1 s gives a 20 V pulse.
A steady magnet next to a still coil induces nothing; only a changing flux does. Winding many turns is exactly why coils, not single loops, are used to get a usable voltage.