Eddy Currents and Magnetic Braking

A spinning aluminum disc near a magnet has no wire, no circuit, nothing you can trace with your finger. But the metal itself is full of free electrons, and as the disc moves, different parts of it see a changing flux. Those electrons respond exactly like they would in a loop of wire: they swirl into closed current paths right inside the solid metal. These are eddy currents, named for the lazy circular eddies you see in a stream. Each swirling loop creates its own magnetic field, and Lenz's law says that field must oppose the change that made it. The result is a force pushing back against whatever motion or change caused the flux shift in the first place. That is magnetic braking: no contact, no friction, just induction fighting the motion and converting kinetic energy into heat.

To find the direction, treat each region of the sheet like its own tiny loop. Where the sheet is leaving the field, flux through that region is shrinking, so the induced current circulates to try to maintain it, flowing in the sense that makes its own magnetic field point the same way as the original field inside that region. Use the right-hand rule exactly as you would for any induced loop. The currents on different parts of the sheet can circulate in different senses, forming closed loops that close back on themselves within the metal.

This same effect is a nuisance in transformers, where you want changing flux to induce voltage in windings, not waste energy heating the iron core. A solid iron core gives eddy currents huge loops to follow and large losses. Slicing the core into thin, insulated laminations chops those loops down to narrow strips, shrinking the area each loop can enclose and cutting resistive heating dramatically, while the transformer's intended induction still works fine.