Eddy currents are loops of electrical current that form inside a conductor whenever it experiences a changing magnetic field. The core cause is electromagnetic induction: when the magnetic environment around a piece of metal shifts, whether because a magnet is moving, a nearby current is fluctuating, or the metal itself is in motion, the changing magnetic flux forces electrons in the conductor to circulate in closed loops. These swirling currents got their name because they resemble small whirlpools, or eddies, in water.
How Changing Magnetic Fields Create Eddy Currents
The process starts with a principle Michael Faraday identified in 1840: electric effects only exist when something is changing. A static magnet held next to a sheet of aluminum does nothing special. But move that magnet toward the sheet, pull it away, or vary the strength of a magnetic field passing through the metal, and circulating currents appear inside it.
Here’s what happens step by step. A magnetic field passes through a conductive material, and the total amount of that field threading through the metal (called magnetic flux) changes over time. That change creates a circulating electric field inside the conductor. Because the material can conduct electricity, that electric field pushes electrons into closed loops of current. Unlike current in a wire that follows a single path, eddy currents spread continuously through the body of the metal, flowing in flat, circular patterns perpendicular to the magnetic field.
The direction of these loops isn’t random. A principle called Lenz’s law dictates that the induced current always flows in the direction that opposes the change that created it. If a magnet’s north pole approaches a sheet of aluminum, the eddy currents circulate in a direction that creates their own magnetic field pointing back toward the magnet, effectively pushing against it. This opposition is why eddy currents produce drag forces and why they always resist the change in flux rather than amplify it.
What Determines How Strong They Are
Several factors control the size and intensity of eddy currents. The strength of the magnetic field matters directly: a stronger field produces larger currents. The rate of change is equally important. A slowly approaching magnet induces weak currents, while a rapidly oscillating field drives much stronger ones. Power lost to eddy currents scales with the square of both the field strength and the frequency, so doubling the frequency quadruples the energy dissipated.
The material itself plays a major role. Metals with high electrical conductivity, like copper and aluminum, allow eddy currents to flow more freely, producing stronger current loops. But there’s a tradeoff: higher conductivity also means the currents concentrate closer to the surface of the material rather than penetrating deeply. In magnetic metals like iron or steel, magnetic permeability amplifies the effect so strongly that it can dominate over conductivity, making the interaction between the field and the material much more complex.
Temperature also shifts the balance. As a metal heats up, its conductivity generally drops, which weakens eddy currents. This is one reason eddy current behavior can change during prolonged exposure to alternating magnetic fields.
How Geometry Shapes the Current Loops
The physical shape and size of a conductor directly affect how eddy currents form. The current in any given loop is proportional to the area of that loop, so a large, solid piece of metal allows wide-sweeping current paths that carry more energy. A thin sheet or narrow wire restricts the loops, reducing their strength.
This relationship is why engineers use lamination to control unwanted eddy currents. In a transformer core, for example, instead of using a single solid block of iron, the core is built from many thin sheets stacked together with thin insulating layers between them. Electrons can’t jump across the insulating gaps, so they’re forced into much smaller loops within each individual lamination. Modern high-performance transformers use laminations as thin as 0.18 to 0.23 mm. The thinner the sheets, the greater the suppression of eddy currents, because the power lost scales with the square of the lamination thickness.
The Skin Effect
At higher frequencies, eddy currents don’t penetrate evenly through a conductor. They crowd toward the surface, a phenomenon called the skin effect. The depth at which current effectively flows depends on the frequency of the changing field, the material’s conductivity, and its magnetic permeability.
The numbers can be dramatic. For a standard 60 Hz power line, the skin depth in copper is about 8.4 mm, meaning current flows through most of a typical wire’s cross-section. At 700 MHz (a common cellphone frequency), the skin depth in copper shrinks to just 2.5 microns, a tiny fraction of a hair’s width. This is why high-frequency applications require careful attention to conductor design: the current is essentially flowing in a thin shell on the metal’s surface.
Eddy Currents as Heat Sources
When eddy currents flow through a material with electrical resistance, the collisions between moving charges and the atoms of the metal convert electrical energy into heat. This is the same process that makes a wire warm up when current passes through it, just happening in swirling loops instead of a straight path.
Induction cooktops exploit this directly. A coil beneath the glass surface carries a rapidly alternating current, creating a changing magnetic field. When you place a steel or iron pan on the cooktop, that field induces eddy currents in the pan’s base. The metal’s resistance converts the induced current into heat, warming the pan from within rather than from an external flame or heating element. The cooktop surface itself stays relatively cool because the heat is generated inside the cookware, not transferred to it from outside.
Braking Without Contact
The drag force that Lenz’s law predicts has a practical payoff: braking without friction. In an eddy current brake, a conductive surface (like a metal rail or disk) moves past a stationary magnet. The motion induces eddy currents in the conductor, and those currents generate their own magnetic field that opposes the motion. The result is a braking force proportional to the speed of the moving object.
This type of brake has no mechanical contact between parts, which means no wear, no noise, and no overheating from friction. Eddy current brakes slow high-speed trains and bring roller coaster cars to a smooth stop at the end of a ride. They’re also used in semi-trailer trucks as a supplement to conventional brakes, reducing wear on brake pads during long descents, and in power tools to stop spinning blades quickly when the motor is shut off.
Detecting Hidden Flaws
Because eddy currents are sensitive to anything that disrupts their flow path, they serve as a powerful inspection tool. In eddy current testing, a probe coil generates a changing magnetic field near the surface of a conductive part. Eddy currents form in the material, and if the metal is uniform, those currents flow smoothly in predictable patterns.
When the currents encounter a crack, void, or inclusion, they’re forced to detour around the flaw. That longer path reduces the current flow, which in turn changes the electrical properties the probe measures. By reading those changes, a trained technician can identify surface and near-surface defects without cutting into or damaging the part. NASA has relied on this technique for over a decade to inspect bolts, nuts, and fastener holes on spacecraft and launch hardware, checking for cracks and gouges that would be invisible to the eye but dangerous under stress.