Eddy currents are circulating loops of electric charge that appear inside conductors whenever they experience a changing magnetic environment. They arise from one of the most fundamental principles in electromagnetism—Faraday’s law of induction—which states that a changing magnetic flux induces an electromotive force in a conductor. Instead of forming a single clean path like current in a wire, the induced charges swirl around in closed, vortex-like patterns, much like water eddies that form behind a rock in a flowing river. These loops of current exist even when the conductor has no external electrical connections, making them an intrinsic consequence of electromagnetic change.
To understand why eddy currents form, consider a metal plate moving through a magnetic field. As different parts of the plate enter or exit the magnetic region, the magnetic flux through those regions changes. According to Faraday’s law, this change induces local electric fields, which drive electrons into circular trajectories governed by Lenz’s law. Lenz’s law tells us that the induced current will always produce its own magnetic field opposing the original change that created it. Therefore, eddy currents act like a built-in brake: they resist the motion responsible for their creation. This is why pushing a strong magnet toward a thick aluminum or copper plate feels strangely sluggish even though those metals are not magnetically attracted. The resistance is purely due to the induced currents opposing your motion.
While eddy currents can be useful, they also pose challenges. In electrical machines such as transformers, electric motors, and generators, alternating magnetic fields are unavoidable. If these machines were made from solid metal cores, the changing fields would induce strong eddy currents, wasting energy as heat and lowering efficiency. This is why transformer cores and motor laminations are constructed from thin, insulated sheets of metal rather than a single solid block. The layers restrict the size of each possible current loop, dramatically reducing eddy-current losses.
Despite their drawbacks in certain applications, eddy currents are extremely valuable in others. One important use is in electromagnetic braking systems found in high-speed trains and roller coasters. When a conductor moves through a magnetic field, the eddy currents generated in it create a strong drag force that slows the motion smoothly and without mechanical contact. Because there is no friction between solid parts, this form of braking is quiet, reliable, and resistant to wear. Eddy currents also play a key role in metal detectors, where the presence of a conductive object distorts the detector’s magnetic field, altering the induced currents in a way that can be measured. In non-destructive testing, eddy-current probes identify cracks or flaws inside metal components; defects change how the induced currents circulate, providing inspectors with a map of structural irregularities.
The physics of eddy currents also ensures that magnetic fields are partially “excluded” from the interiors of good conductors when those fields change rapidly. This phenomenon, called the skin effect, forces alternating currents to concentrate near the surface of a conductor. At high frequencies, nearly all the current flows in a thin outer layer, because deeper penetration would require stronger and stronger eddy currents to oppose the changing field. This behavior shapes the design of high-frequency electronics, radio-frequency coils, and even microwave ovens, where conductive surfaces confine electric fields and prevent them from passing through.
Although eddy currents cannot be avoided whenever magnetic fields and conductors interact dynamically, understanding the principles behind them allows engineers and physicists to control and exploit them. By tailoring geometry, material conductivity, magnetic-field strength, and frequency, one can either suppress eddy currents to minimize energy loss or harness them to create forces, heat, or diagnostic signals. Their presence is a vivid example of the intertwined nature of electricity and magnetism, demonstrating how motion, fields, and materials respond to one another in ways that are both practically powerful and conceptually elegant.