When the conductor is not a wire but an extended mass (a disc, a plate, a cylinder), the induced currents are not confined to a single path: they close into vortices inside the material, forming eddy currents or Foucault currents (after the French physicist Léon Foucault, who studied them in 1855).
Principle — Eddy currents
In an extended conductor subjected to a varying magnetic field (or moving relative to a field), induced currents form as closed loops within the mass itself. The power they dissipate through Joule heating is proportional to:
- the square of the frequency of the flux variation,
- the square of the amplitude of ,
- the thickness (squared, for thin plates) of the conductor,
- the electrical conductivity of the material.
It is worth clarifying the names: “Foucault” and “Lenz” denote the same phenomenon seen from two sides. The currents are named after Foucault; the law that explains their direction — always opposing the change — is Lenz’s law.
These four dependencies explain all the practical applications: frequency, field, thickness and conductivity are increased when one wants to dissipate energy (brakes, cooking), and the current paths are broken up (laminations, slots) when the dissipation is unwanted.
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Topics: Electromagnetic induction Concepts: Eddy currents · Joule heating
Related exercises: Problem — The magnet in the copper tube · Worked exercise — magnet in a tube, terminal velocity · Worked exercise — Waltenhofen’s disc