Eddy Currents and Their Applications
On this page
Direct answer
Drop a strong magnet down a thick copper pipe and it drifts in slow motion: the changing flux through the copper walls induces circulating currents — eddy currents — whose own magnetic field, by Lenz's law, opposes the motion creating them. Whenever a bulk conductor experiences changing flux, loops of induced current swirl in planes perpendicular to that flux, dissipating energy as heat (I^2R) and producing retarding forces. The effect is engineered both ways: exploited in magnetic braking, deadbeat galvanometers, induction furnaces, induction cooktops and metal detectors, and suppressed in transformer and motor cores by laminating the steel into thin insulated sheets. The direction of every application follows one rule — the induced effect fights the change.
What you must remember
- Origin: any bulk conductor in changing magnetic flux develops closed loops of induced current in its body; unlike a circuit's wires, the path is the metal itself.
- Lenz's law direction: eddy currents flow so as to oppose the flux change causing them — an approaching magnet's north face induces currents presenting a north face back.
- Magnetic braking: eddy currents in a metal wheel or track create a drag that rises with speed yet involves no contact or wear — used in trains, roller coasters and exercise bikes; the braking weakens as the speed falls.
- Electromagnetic damping: the coil of a deadbeat galvanometer is wound on a metal frame so its eddy currents settle the pointer in a moment instead of oscillating.
- Induction furnace: very high-frequency alternating flux melts metals through eddy heating; the same principle cooks dinner on an induction cooktop, heating only the ferromagnetic utensil.
- Laminated cores: transformers and motors use thin sheets of silicon steel insulated by varnish — the loops get tiny cross-sections and higher resistance, cutting eddy loss drastically.
- Energy trail: eddy currents always convert mechanical or field energy into heat — useful in a furnace, a nuisance in a core.
Why lamination starves the eddies
Consider a solid transformer core carrying alternating flux. Each circular path through the iron encloses changing flux and so acts as a short-circuited secondary winding: the induced emf around a large loop is large, and the resistance of that fat iron path is small — a recipe for enormous currents and severe heating. Now slice the same core into thin sheets stacked with insulating varnish between them. Any would-be current loop must thread its way across the insulated interfaces, and the effective loops within each sheet have small cross-section and much higher resistance, so the induced currents shrink by a large factor; since the heating goes as I^2R, and the current is what collapsed, the power loss plummets even though R per path rose. The same argument explains the direction of design everywhere in this chapter: slots cut into the rim of a braking wheel lengthen the current paths; a coil wound on a copper frame damps quickly because its loops are allowed to exist.
Where the exam probes the concept
The conceptual separator is energy bookkeeping: the magnet falling in the copper pipe loses gravitational potential energy to eddy heat, so it reaches a small terminal speed — candidates who think "no battery, no current" miss that the changing flux itself is the source. The second probe is asymmetry of motion: an eddy-braked wheel stops smoothly and the braking vanishes at rest, so the technique cannot hold a load stationary — magnetic braking supplements, not replaces, mechanical brakes, a standard assertion-reason point. Third, material matters: the copper pipe works far better than a plastic one, and an induction cooktop needs a ferromagnetic or suitable utensil — questions test such everyday juxtapositions. Finally, the unwanted-versus-wanted classification is itself examinable: eddy heating in a transformer core is a loss to be minimised, in a furnace the entire purpose — the same physics on either side of the intention.
Frequently asked questions
Why does a magnet fall slowly through a copper pipe?
The changing flux induces eddy currents in the pipe walls, and by Lenz's law their field opposes the magnet's motion, producing a retarding force that balances gravity at a low terminal speed.
What are eddy currents and how are they minimised in machines?
They are loops of induced current swirling in the body of a bulk conductor facing changing flux; laminating cores into thin insulated sheets shrinks the loops and starves the currents.
How do eddy currents provide braking without contact?
A moving conductor near a magnet develops eddy currents whose field opposes the motion, dissipating kinetic energy as heat with no friction surfaces to wear.
Why is the coil of a deadbeat galvanometer wound on a metal frame?
The frame's eddy currents damp the oscillations electromagnetically so the pointer settles at once on the reading.
How does an induction cooktop heat food?
High-frequency alternating fields induce eddy currents directly in the utensil's metal, heating the vessel itself while the cooktop surface stays comparatively cool.