Eddy Currents
Move a magnet over a conducting plate and watch circular eddy currents form in its wake. These induced currents arise from Faraday's law: a changing magnetic field creates an electric field that drives current loops through the conductor.
Move your mouse or finger across the canvas to move the magnet
Simulation Status
Key Relationships
How it works
When a magnet moves near a conducting plate, the magnetic flux through the conductor changes over time. According to Faraday's law of electromagnetic induction, ∇ × E = −∂B/∂t, this changing magnetic field induces a circulating electric field, which in turn drives circular currents through the conductor. These induced loops of current are called eddy currents.
Lenz's law tells us that the direction of the induced current is always such that it opposes the change that produced it. When the magnet approaches, the eddy currents flow in a direction that creates a magnetic field opposing the magnet's field (repulsion). When the magnet moves away, the currents reverse to try to maintain the field (attraction). This is why a magnet dropped through a copper tube falls slowly — the eddy currents create a braking force.
The eddy currents form closed loops because the electric field that drives them is curl-based (solenoidal). The intensity of the currents depends on the rate of change of the magnetic field (faster motion = stronger currents) and on the conductivity of the material. In a perfect conductor, eddy currents would persist forever; in real materials, they decay exponentially with a time constant τ = L/R determined by the material's inductance and resistance.
Eddy currents have many practical applications: electromagnetic braking in trains and roller coasters, induction heating in cooktops, metal detectors, eddy current testing for cracks in metal structures, and energy-loss reduction in transformer cores through lamination. They are also the reason why large metallic objects can interfere with magnetic instruments.