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Re: [Phys-L] maglev

-In the old demo one swings a disc of aluminum thru a U magnet and sees
considerable damping because of the eddy currents.
-But there would have been no damping with a superconducting disc, right?
-So, if the resistivity of the iron were less than that of the aluminum,
would this lead to less damping if an iron disc were used? There would
still be eddy currents in the iron, right?



On Tue, Apr 1, 2014 at 12:18 PM, John Denker <jsd@av8n.com> wrote:

On 04/01/2014 06:08 AM, Bill Norwood wrote:
That is a most illuminating explanation.

:-)

1. Where does the frequency come from? Are there significant differences
in energy dissipation relating to wheel rotational frequency?

Well, it depends on whether you think of "dissipation" as energy
per unit time or energy per unit distance. I am not an expert
and I don't have any good data, but I found a sketch
http://img7.imageshack.us/img7/1504/dragw.png
which alleges that the energy-per-unit-distance for rubber
tires is independent of speed (and quite nontrivial in magnitude,
especially at low speeds).

2. How could "plain old magnets" be chosen for train levitation when
modern
magnets are much stronger?

For the second time today I must apologize for yesterday's
sloppy terminology. What I meant by "plain old magnets"
was "ferromagnets" or perhaps "permanent magnets". This
stands in contrast to the temporary, induced magnetism
associated with Lenz's law.

We agree that modern ferromagnets are very much stronger
than "old" ferromagnets.

You can mention this to students as an example of modern
physics in the real world. There is a treeeemendous
amount of physics aka materials science that went into
the development of modern magnets. The result is motors
that are smaller, lighter, and more energy efficient.

================

BTW do not let small children play with small, strong
magnets. Eating /one/ magnet is usually harmless.
The problem arises from eating one magnet now and
another later. Getting them out requires surgery.

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