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Here is a picture from Wikipedia about how eddy current braking works

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(1) Conductor rotating in an external B field, in green, induces circulating currents in the conductor - the generator effect. (2) These circulating currents generate a magnetic field, in Blue, in accordance with Lenz's law to oppose the direction of the change in flux.

Now, I don't understand how an induced $B$ field, blue arrows, slows down the rotating disk. I can see that the direction of induced $B$ field on left side points up and the right side points down. This induced field interacts with the external field, in green, but I can't see how this interaction results in a braking force in the direction opposite to the motion of the disk.

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  • $\begingroup$ Author of the wikipedia explanation probably meant that both current whirls can be thought of as magnets with N and S pole. When you draw little magnets at both whirl centers, you can see that you get repulsion of the big magnet pole N with the left imaginary pole N, and attraction with the right imaginary pole S. If the whirl centers are far enough from the big magnet, these forces of repulsion and attraction have component in the plane of the disk, and act to stop its motion. $\endgroup$
    – Ján Lalinský
    Commented May 28 at 20:40
  • $\begingroup$ But this kind of explanation using imaginary poles and their position being far enough is not very good, as it replaces currents with imaginary magnetic poles and guesses their position. Farcher's explanation is much better, as it is based on the well-known action of external magnetic field on the current-carrying conductor just in between the two big pole of the magnet. It's like the magnetic force on a current-carrying wire. Only here the current flows in more complicated pattern, but the decisive part is the flow towards the center of the disk just below the big magnet N pole. $\endgroup$
    – Ján Lalinský
    Commented May 28 at 20:47

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The interaction of the induced currents (“away” from you) and the magnetic field (downwards) produces forces to the left, in the opposite direction to the rotation, and these forces generate the braking effect.
The currents to the left and right of the central region produce forces to the right but are weaker than the forces produced in the centre as the magnetic field is weaker than at the centre.

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