Physics · Ch 4 — Electromagnetic Induction and Alternating Current
Eddy Currents
Eddy Currents
According to Faraday's law, an emf is induced in ANY conductor whenever the magnetic flux passing through it changes -- and the conductor need not be a thin wire loop at all. Even a solid conducting sheet or plate has an emf induced in it when the linked flux changes, but because a solid sheet has no single definite wire path for a current to follow, the induced current instead circulates in closed, roughly circular loops within the body of the conductor. Because these swirling current patterns resemble the eddies seen in flowing water, they are named eddy currents (also called Foucault currents, after the French physicist Jean Bernard Léon Foucault who first studied them). A simple pendulum demonstration makes eddy currents visible: a conducting disc swinging between the poles of a powerful electromagnet is barely damped at all when the magnet is off (air friction alone slows it, over many oscillations), but is damped very rapidly -- stopping within just a few swings -- once the electromagnet is switched on, because strong eddy currents are now induced in the moving disc and their magnetic drag opposes its motion; cutting radial slots in the disc breaks up the paths available for these currents and restores many more oscillations before it stops, directly proving that the rapid damping was due to eddy currents and not some other effect.
Eddy currents cause a genuine, unavoidable energy LOSS (dissipated as Joule heat within the conductor), and this is deliberately minimised in devices like transformer cores and electric motor armatures by building them from many thin, mutually INSULATED laminations (or, for motor windings, bundles of thin insulated wires) rather than one solid block of material -- confining any eddy currents to small loops within each thin layer, rather than allowing them to circulate across the device's full cross-section. …
What this figure shows. A flat metal plate sits in an alternating magnetic field (labelled 'Alternating magnetic field'), and the figure draws several concentric closed circular current loops within the body of the plate itself, labelled 'Eddy currents'. Because the plate has no thin, definite wire path for current to follow the way a coil does, the changing flux instead drives induced currents that circulate in these closed, roughly circular paths throughout the conductor's bulk -- resembling the swirling eddies seen in flowing water, which is exactly where the name 'eddy currents' comes from (they are also called …
What this figure shows. A flat conducting disc, mounted as the bob of a pendulum, swings between the poles of a powerful electromagnet, shown across three panels. Panel (a) shows the pendulum's set-up with the disc positioned between the N and S poles of the magnet. Panel (b) shows what happens with the electromagnet switched ON: the oscillating disc, moving through the strong field, has large eddy currents induced in it, and these currents' own magnetic drag brings the pendulum to rest within just a few swings -- much faster than the slow decay air friction alone would cause. Panel (c) shows the same disc but with several radial slots cut into it: cutting slots breaks up the closed loops available for eddy currents to circulate in, so far smaller eddy currents are induced, and the slotted pendulum keeps swinging for many more oscillations before stopping, directly demon …
What this figure shows. Two related sub-figures show how eddy current losses are minimised in real machines. Figure 4.12(a) compares a solid, single block of core material (which would allow large eddy-current loops throughout its whole cross-section) against a 'Laminated Core' built from many thin sheets ('Single Lamination') stacked together but electrically insulated from one another -- breaking the core into many thin slices confines any eddy currents to small loops within each thin lamina, drastically cutting the total eddy-current loss, and this laminated-core construction is used inside transformers. Figure 4.12(b) shows the analogous idea for an electric motor's armature: instead of using one thick block of copper for the winding, the winding is made from a bundle of thin insulated wires bundled together, so that eddy currents i …
What this figure shows. A cooking pan sits on top of a 'Cooking Zone', beneath which is hidden a tightly wound coil of insulated wire; arrows labelled 'High frequency Alternating magnetic field' rise from the coil through the base of the pan. When the stove is switched on, an alternating current (raised in frequency from the domestic 50 Hz supply to roughly 20-40 kHz) flows through the coil, producing a rapidly alternating magnetic field that induces strong eddy currents directly within the metal base of the cooking pan itself. These eddy currents dissipate a large amount of heat through ordinary Joule (resistive) heating inside the pan's own metal, which is exactly the heat used to cook the food -- the induction stove's cooking-zone surface itself stays comparatively cool …
What this figure shows. Two sub-figures show eddy-current braking, used in high-speed trains and roller coasters. Figure 4.14(a), the 'Linear Eddy current brake', shows a strong electromagnet (poles N and S) fixed just above a moving rail, with the rail moving with velocity v beneath it; when the electromagnet is switched on, its field induces eddy currents in the moving rail, and by Lenz's law these currents create a magnetic drag force that opposes and slows the rail's (and hence the train's) motion. Figure 4.14(b), the 'Circular Eddy current brake', shows a circular disc (connected through a common shaft to the train's wheel) rotating between the poles N and S of an electromagnet; the relative motion between the spinning disc and the fixed magnet again induces eddy …
What this figure shows. A coil of insulated wire, carrying an alternating current and hence producing its own alternating magnetic field, is held close to a 'Conductive material' specimen being tested for hidden defects. The figure labels the coil's own magnetic field ('Coil's magnetic field') and shows eddy currents induced within the specimen's surface, together with the separate magnetic field that these induced eddy currents themselves create ('Eddy current's magnetic field'). Where the specimen contains a hidden defect -- a surface crack or a trapped air bubble -- the local eddy-current pattern is disturbed, changing the phase and amplitude of the eddy current there in a way that can be detected electronically from outside the specimen, letting this …
What this figure shows. A moving-coil galvanometer's armature -- its rotating coil -- is shown wound on a soft iron cylinder, positioned between the N and S poles of a permanent magnet that produces a radial magnetic field. When the armature is deflected and begins to rotate, the relative motion between the soft iron cylinder and the surrounding radial field induces eddy currents within the cylinder itself, drawn in the figure as small closed loops. These eddy currents create a magnetic drag (damping) force that opposes the armature's motion and brings it to rest quickly at its correct, steady deflected position rather than letting it oscillate back and forth for a long time -- this deliberate use of eddy-current drag to quickly settle a moving pointer is called electromagnetic damping, and it is why a …