Problem
Two vertical tubes of identical geometry, one copper and one plastic. A neodymium magnet is dropped into each. Explain in five lines why the fall is extremely slow in the copper tube and free in the plastic one, even though neither is ferromagnetic.
At the same instant the magnet is still high in the copper tube and already low in the plastic one.
Solution
In the copper tube. Copper is an excellent conductor. The falling magnet changes the flux through the cross-sections of the tube, inducing eddy currents in the walls. By Lenz’s law these currents oppose the change: they generate a braking force directed upwards, proportional to the velocity . The magnet accelerates until the braking force balances its weight, and then continues at nearly constant terminal velocity, very slowly. The potential energy lost is dissipated by the Joule effect in the induced currents.
In the plastic tube. Plastic is an insulator: no current can flow, so there is no magnetic braking force. The magnet falls essentially freely.
The conceptual point. The tube need not be ferromagnetic: it only needs to conduct. It is the conductivity of the material, not its magnetism, that makes the difference.
Collegamenti
Argomenti: Electromagnetic induction Concetti: Lenz’s law · Eddy currents · Joule effect