Question

A student asserted that if a permanent magnet is dropped down a vertical copper pipe, it eventually reaches a terminal velocity even if there is no air resistance. Why should this be? Or should it?

Short answer

Yes, It eventually reaches a terminal velocity.

Step-by-step solution

Lens Law

The direction of any magnetic induction effect is such as to oppose the cause of the effect.

Faraday Law of EMF

Faraday Law of EMF states that the induced emf in a loop equals to the negative of the time rate of change of magnetic flux through the loop.

Mathematically it can be given by

$$\begin{gathered} \mathrm{e}=-\frac{\mathrm{d\varphi }}{\mathrm{dt}} \\ =\frac{\mathrm{Bd}\left(\mathrm{A}\right)}{\mathrm{dt}} \end{gathered}$$

Where, e is the EMF,$\mathrm{\varphi }$ is the magnetic flux, is applied magnetic field and is surface area.

Explanation

As, permanent magnet goes down in copper pipe it changes the magnetic flux through the different regions in the pipe which induces an EMF in such direction which the cause of effect (Lenz Law) and produces a magnetic field in opposite direction which causes magnet to reach a terminal velocity.