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Re: falling magnet, was Graphing by hand



REPOSTING BECAUSE THIS WAS PROBABLY LOST (DID NOT COME TO ME)

On Tue, 30 Sep 1997 Leigh Palmer, responded to this message:

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. Local current loops can exist in short
segments of copper, even when the input resistance of the oscilloscope
is very large. Just recall what happens when a magnet falls through a
copper pipe which has a slot from one end to another. The magnetic braking
is nearly as strong as it is in a pipe without a slot. I have a setup to
demonstrate this.
He wrote:

Those eddy currents must have an axial return path; that is not available
in a solenoid. Only circumferential paths are available. Eddy current
damping also requires joule heating dissipation. By hypothesis this
experiment is carried out with a high impedance measuring device; not
even an external current can be exerting significant braking force.

What do you mean? Eddy currents are created in any conductor near a moving
magnet. You certainly know this. I do not understand your objection. Take
two long aluminum (or copper) strips, place them vertically on the floor
so that the distance between them is only slightly larger than the diameter
of a strong bar magnet you may have. Drop the magnet and see how quickly
the terminal velocity is reached. The braking effect is obvious, it is not
very different from what happens when the circumferential (coaxial) paths
are avalable, as in the case of a copper pipe without a slot. I was very
surprised, several years ago, to discover that the terminal speed of a
magnet falling through an ordinary copper pipe did not change drastically
when a slotted pipe was used instead. The slotted pipe was prepared to
demonstrate something that did not happen.

I think 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
about equal (or smaller) than the diameter of the wire. The braking effect
(producing joule heating) is likely to be larger when a wire from which the
solenoid is made is thicker. Herb, can you confirm that short falling
magnets slow down in your long solenoid? By the way, when will the correct
answer to your initial question be posted?
Ludwik Kowalski