Re: falling magnet

["Prof. John P. Ertel (wizard)" <jpe@nadn.navy.mil>]

This may seem like a stupid question, but I'll ask it anyway since I see
only one mention of a "connection."  What did we expect to see with the
windings "open" or not connected?  And when they were shorted, did we
really expect any observably effect?

When the ends are "open", there can be no damping due to "something
analogous to eddy currents" since the current in the windings will be at
best minuscule.  It will fall off almost exactly exponentially as the
capacitor formed by the two open ends of the wire windings "charges up"
through the resistance of the windings.  With this little current, the
impedance (or inertia) of the "inductor" formed by the windings of the
solenoid is certainly negligible.

I/we have already "done the experiment" of hooking up the ends of the
windings to an oscilloscope.  The results are immeasurable damping.  But
this should be no surprise!  The input impedance of the oscilloscope is so
high that little current will actually flow.  Yes, the oscilloscope can
certainly "see" the magnet go by, but the windings can do little to
"brake" the fall.  And why should they?  They cannot possibly come up with
the current that could/would be generated  in the standard Aluminum tube
(PASCO) commonly used for such demos.  Also, unless we wound a solenoid
that was of roughly the same length as the PASCO 2.0 m tube, the time of
exposure to whatever braking effects might be there would be so small that
it would essentially have no effect.


On Fri, 10 Oct 1997, LUDWIK KOWALSKI wrote:

> 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. 

ERTEL SENDS.
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