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Re: [Phys-l] drop a metal cylinder through a solenoid



Two interesting possibilities:
1. The falling magnet comes to a complete stop.
2. The magnet falls through without any friction.

When you cool an aluminum tube with LN2 you can demonstrate that the magnet slows down considerably. This is due to the greater persistence of the induced current in the tube caused by induction from the moving magnet. One could then argue that the current in a superconductor would persist forever and, once created would continue to match the force of the falling magnet. Therefore the magnet should come to a complete stop.

The second possibility is even more intriguing. The magnet will induce some current around it. But that current is free to move in any direction at all. Therefore it will not oppose the field of the magnet.

Both ideas are attractive. And I'd bet could allure that group of physicists with either.

Which is it Bernard? Or is it both?

Paul


On Mar 22, 2012, at 4:23 PM, John Denker wrote:

On 03/22/2012 12:52 PM, Bernard Cleyet wrote:

It has been nearly fifteen years since this question first was posed
to a small group of physicists and physics teachers - including, John
Rigden, David Goodstein, and Richard Feynman, and a number of others
- immediately following a meeting of the Southern California AAPT
(November, 1987). The animated discussion of the many speculations
that followed was sheer joy. Then a surprising answer was offered by
Caltech's Goodstein - which evoked an immediate and gleeful "Of
course!" from Feynman (and I might add, from the rest of us as
well!). It was like being on hallowed ground, as John Rigden was
later to write. The question is, what did Goodstein say that the rest
of us agreed with so quickly? And why do we think his answer was
right? This discussion will include how magnets and superconductors
interact, Faraday's law, the Meissner effect, and London's equation,
magnetic forces, induced currents and persistent currents - and which
of those principles and ideas are important to answering this
question. And the answer will be clear.

Huh? I've never heard that story before ... and I'm surprised
that there was any speculation involved, or that it took any of
those guys more than 15 femtoseconds to figure out the answer.

Unless there is something I'm overlooking, the answer is basically
the same as in Feynman volume II figure 10.9 ... and can be obtained
in the same way: Think about the energy in the field, then apply
the principle of virtual work.

=========

There is a hands-on demo that may be relevant: Get a short, fat,
powerful bar magnet, and decorate the field lines with iron filings.
This provides a literally hands-on feeling for the energy and force
required to distort the field-line pattern.

This makes a good demo at all levels, from preschool through grad
school. My father showed it to me before I was old enough to read
and write. I can remember being not able to read, but I can't
remember not having magnets to play with.

Hint: If the magnet is not to be dedicated to this purpose, put
it in a plastic bag before adding the iron filings. Otherwise
it is exceedingly laborious to get all the iron filings off.

On 03/22/2012 11:22 AM, Jeffrey Schnick wrote:
He dropped an aluminum meter stick, oriented so that it extended
vertically along its longest dimension, through the space between the
poles.

That's another good demo. I recommend it for department "open house"
day.

This requires a big, open magnet, perhaps one that looks like this:
http://www.nanomagnetics.org/instrumentation_and_characterization/images/electromagnet_photo.jpg

Such magnets cost a fortune, but they occasionally show up on surplus
equipment lists. Grab one if you can. If you have one, on open-house
day it's worth disengaging it from whatever it had been doing, and
setting up the demo as follows:

Make a thing that looks sorta like a hammer, with a light but stiff
plastic handle, and a symmetrical head made of thick-wall copper
tubing, of a size that will fit between the poles of the magnet.
The axis of the tube is perpendicular to the axis of the handle.

A hollow tube is preferable to a solid chunk of metal, because the
eddy current effects scale like the enclosed area, so metal near
the axis contributes to the weight without contributing much to
the desired effect.

Use OFHC copper, i.e. oxygen-free high-conductivity copper. Cool
it by soaking it in liquid nitrogen. This increases the conductivity
by a factor of 4, which is quite noticeable.

Observe the force required to move the copper from far away into the
gap of the magnet.

Observe the force (or lack thereof) required to wave the copper around
within the region of uniform field.

Observe the torque required to rapidly flip the copper end-over-end
by twisting the handle.

In particular, with the copper not in the field, demonstrate the desired
rapid flipping motion, by spinning the thing around the axis of the
handle. Demonstrate putting the thing into the gap and waving it
around without spinning. It's easy. Then hand it to a visitor. Let
them practice spinning it outside the magnet. Then tell them to put
it into the magnet and spin it like that. The effect is huge. It
takes a huge torque to produce even a slow flip.
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