Three widespread technologies exploit the dissipation of eddy currents.
Eddy current brake (Foucault)
High-speed trains, heavy vehicles and gym equipment (professional exercise bikes) use contactless electromagnetic brakes. A rotating metal disc (fixed to the wheel or flywheel) passes between the pole pieces of an electromagnet: the more we energise the electromagnet, the more eddy currents arise in the disc, the more kinetic energy is converted into heat. There is no wear or mechanical friction, but — by Lenz’s law — at zero speed the braking force vanishes: these brakes must always be paired with a mechanical brake to bring the vehicle to a complete stop.
Magnetic levitation (Maglev)
Electrodynamic-type magnetic levitation trains (the Japanese JR-Maglev) exploit the same mechanism as the magnet in the copper tube, upside down: it is the track that is the conductor (aluminium), while the train carries superconducting electromagnets on board. As the train moves, eddy currents arise in the track that, by Lenz’s law, repel it upward. At very low speed the levitation is not enough (the Lenz effect is proportional to speed): this is why the train starts out resting on wheels and “lifts off” once it exceeds about km/h.
Induction cooking
An induction hob is essentially an open transformer: beneath the glass-ceramic surface lies a coil carrying a high-frequency alternating current ( kHz). The base of the pan, if made of ferromagnetic material, acts as the “secondary”: very intense eddy currents form within it, heating it by Joule heating. A glass or pure-aluminium pan does not heat up (glass has too little magnetic permeability; aluminium’s conductivity is too high and its thickness too small, so at those frequencies it becomes “transparent” owing to the skin effect). The hob itself remains cool to the touch, because it is the pan that acts as the dissipative conductor (Bloomfield 2016). See also Riferimenti bibliografici.
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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