A classic experiment shows that when we drop a magnet through a copper coil long tube, there is eddy current induced in the coil. Then, due to lenz law, the motion of the magnet will be opposed and it will move slower.

Now, let set up a similar experiment. Suppose we release a copper bar above a permanent magnet, what will happen to the subsequent motion of the bar?since copper is a good conductor, I think that eddy current once again exists in the copper bar, causing it to move slower. Is the phenomenon the same as the first experiment? I get confused as some materials say that the copper bar just falls with the gravity acceleration It sounds weird,isn't it?

  • $\begingroup$ How are eddy currents created? By the Lorentz force. What are the directions of field and velocity (of charges) in your second setup? $\endgroup$ – Bort Jan 8 '16 at 15:15

The magnet may decelerate "at the last minute" if it's sufficiently strong. The difference with the first setup (dropping in a tube) is that the change in flux (and therefore induced currents) in the copper plates is small until the magnet gets very close - the field of a dipole goes with $\frac{1}{r^3}$.

By contrast, as you are falling down a tube, there is a constant change of flux - first you increase the flux (as the magnet "appears"), then you decrease it (as the magnet "goes past"). That gives you double the kick, and double the force, at every point along the fall. When you drop onto a plate, you get the change just once, and only at the last minute. This makes the first (conventional) geometry a lot more favorable.


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