Changing magnetic flux and induced electromotive force
Faraday's law quantifies how a changing magnetic field generates an electric field and drives current through a conductor. When magnetic flux through a loop changes, an electromotive force (EMF) appears around the loop, pushing charge through any connected circuit. The magnitude of this induced EMF is proportional to the rate of flux change: a rapidly changing field induces a strong EMF, while a slowly changing field induces a weak one. The direction of the induced current follows Lenz's law: the current flows in a direction that opposes the change in flux. This opposition explains why an electromagnet and a magnet repel when brought toward each other, and why conducting loops experience a force when placed in changing magnetic fields.
The foundation of electrical power generation and conversion
Every electrical generator exploits Faraday's law by rotating a magnet around a coil or spinning a coil in a magnetic field. As the loop rotates, the flux through it continuously changes, inducing an alternating EMF that drives current through external circuits. This principle scales from tiny generators powering flashlights to massive turbines in power plants. Transformers use mutual induction: current flowing in a primary coil creates a changing magnetic field that induces current in a secondary coil without direct electrical connection. The voltage ratio between coils depends on the ratio of turns, allowing transformers to step voltage up or down efficiently. This versatility makes Faraday's law indispensable to modern electrical infrastructure.