Re: falling magnet

[LUDWIK KOWALSKI <KOWALSKIL@alpha.montclair.edu>]

About a week ago Ludwik wrote:
> I suspect that the "magnetic braking" effect of eddy currents must play 
> a role in the case of the magnet falling near the copper winding. The 
> effect may be small but not negligible.

Leigh expressed a disagreement, a solenoid is not going to behave like a
tube which is often used to demonstrate terminal velocity. I finally 
conduct a simple experiment to clarify the issue. And was not able to 
observe any significant breaking.

Here are some details:

1) A neodymium magnet (mass=2.6 grams, diameter=9.5 mm, length=5 mm.)
2) A glass tube (length=64 cm, inner diameter=11.5 mm, outer diam=13.6 mm)
3) One layer of isolated copper wire (from a transformer, diam=0.6 mm)
   over the length of 26 cm (starting 11.5 cm from lower end of the tube).

The time of fall (estimated) was less than 0.5 s in all three cases:
         a) glass tube alone
         b) tube with the open coil
         c) tube with the shortened coil

Back to Herb's problem. I will be happy to hook the coil to a scope and
record the shape of volts-versus-time curve. But I will do this only if 
somebody responds to the challenge presented by Herb. Post the result
of your calculations (a table with two rows of numbers, for seconds and 
volts) and I will post the experimental data. 

The magnet was released from rest and traveled 26.5 cm before leveling 
with the upper end of the coil. Wires are closly wound; the insulation 
is negligibly thin.
                               Ludwik Kowalski
P.S.
I have no doubt that local eddy curls are induced (by a passing magnet) 
in each section of a long solenoid. The diameters of these local loops 
must be smaller than the diameter of the wire. But the experiment shows
that the effect of eddies is not significant in this situation.