{"id":2087,"date":"2026-09-28T11:48:02","date_gmt":"2026-09-28T11:48:02","guid":{"rendered":"https:\/\/physicsfundamentalsinfo.com\/blog\/?p=2087"},"modified":"2026-09-28T11:48:04","modified_gmt":"2026-09-28T11:48:04","slug":"eddy-currents","status":"publish","type":"post","link":"https:\/\/physicsfundamentalsinfo.com\/blog\/electromagnetism\/eddy-currents\/","title":{"rendered":"Eddy Currents: What They Are, What They Waste and Where They Help"},"content":{"rendered":"\n
Definition<\/div>

\n\nEddy currents are loops of current that a changing magnetic field drives inside a solid conductor, circulating in the bulk metal itself with no wire to guide them. In a thin sheet they waste power in proportion to the square of the field, the square of the frequency and the square of the thickness, divided by the resistivity.\n\n<\/p><\/div>\n\n

Slide a strong magnet down the inside of a thick copper pipe and it refuses to fall properly. It drifts, as though the pipe were full of honey. Nothing is touching it, and copper is not magnetic in any ordinary sense.<\/p>\n\n

What holds it back is current \u2014 current running in circles inside the pipe wall, induced by the magnet’s own moving field and pushing back against the thing that made it. The same circulating current is why a transformer runs warm, why an induction hob heats the pan and not the glass, and why a metal core is almost never one solid lump.<\/p>\n\n

What Is an Eddy Current?<\/h2>\n\n

An eddy current is a closed loop of current induced inside a solid conductor by a changing magnetic flux, flowing through the bulk metal itself rather than round a wire.<\/p>\n\n

The name is borrowed from water. Drop a stone in a stream and the flow curls into little whirlpools that close on themselves; do the equivalent to a block of metal with a changing field and the current does the same thing. Nobody chose the path. It is simply the route the metal offers.<\/p>\n\n

That is the part students find strange, so it is worth saying flatly. Faraday’s law of induction<\/a> does not require a wire. Every closed path you can imagine drawing inside a lump of metal encloses some flux, and if that flux changes, an EMF appears round that path.<\/p>\n\n

The metal’s own conductivity does the rest. An EMF round a path that is made of conductor drives a current round it, and since the block is solid, there are uncountably many such paths, nested inside one another.<\/p>\n\n

Two familiar laws already cover half the story. Lenz’s law<\/a>, whose own post hands this topic over rather than putting a number on it, settles which way the loops turn: always the way that opposes whatever change produced them, which is why the falling magnet is slowed rather than sped up. Faraday’s law in its EMF form<\/a> settles how big the driving voltage is.<\/p>\n\n

Neither can tell you the one thing an engineer actually needs. How much power<\/em> do the loops burn? That question needs two quantities neither law contains: how thick the metal is, and how readily it conducts.<\/p>\n\n

One more condition, easy to lose sight of. The flux through the metal has to be changing<\/strong>. A steady direct current<\/a> makes a steady field and induces nothing at all; it is alternating current, or relative motion between the metal and the magnet, that does the work.<\/p>\n\n

The Eddy Current Loss Formula<\/h2>\n\n

For a thin sheet of non-magnetic metal sitting in a uniform alternating field that lies along the sheet, the power dissipated per unit volume is given by one compact expression.<\/p>\n\n

P\/V = \u03c0\u00b2B\u00b2f\u00b2d\u00b2 \/ (6\u03c1)<\/div>\n\n