If a bar magnet is dropped into a copper (conducting) tube, it reaches a terminal velocity. I'm trying to derive this terminal velocity.
We know from Lenz's law that $e=\frac{-d\phi}{dt}$.
I'm facing trouble in the first step itself, which is writing the expression for flux.
I did come up with the following expression for flux on a differential element dl, but it somehow seems incorrect because there's no differential element on the right hand side (like dl) $$d\phi=(\frac{\mu_0}{4\pi})(\frac{2M}{l^3})(\pi r^2)$$ where $l$ is distance from the magnet, $M$ is magnetic moment, and $r$ is radius of the tube.
I can then write the current in the tube and write the Force back on the magnet. How do i write the flux correctly?
