Date: Tue, 1 Apr 2003 13:05:02 -0800

Author: Paul Doherty

Subject: Re: cool new Lenz's law/eddy current demo

Post:

I just showed a demo at the NSTA convention in Philadelphia in which I fly
a neodymium magnet between two 4 cm thick slabs of copper by attracting it
to a longer neodymium magnet held above the upper copper plate.

to see a sketch of the demo go here.

http://www.exo.net/~pauld/activities/magnetism/floatingincopper.html

The eddy currents in the copper slow down the motion of the magnet to a
point where the human eye and brain can handle the negative feedback task.

The skin depth for slowly time varying eddy currents is quite large so we
use thick slabs of copper.

We use copper instead of aluminum to get better conductivity.

we have an exhibit at the Exploratorium in which visitors fly a neodymium
magnet between two "jaws of copper" made by slicing a meter-long, 10 cm
diameter copper cylinder in half. Expensive but nice.

http://www.exploratorium.edu/exhibit_services/exhibits/f/floating_copper.html

Now I'll have to cool the lower plate and see what happpens. thanks for the
idea.

Paul Doherty

>At the Physics Outreach conference in February, Doug Osheroff described
>a demo he takes on the road. We did the same demo today and floored
>students and impressed the prof too. I was amazed the first dozen or so
>times I tried it too.
>
>You need:
>
>NdFeB magnet
>Cu or Al block larger than the magnet
>LN2
>
>My stuff was a 2" OD x 1/2" NdFeB, a 6" x 6" x 1" Al block, and 3 l of
>LN2. Drop the magnet on the block - it goes ching! The magnet slightly
>slows as it approaches the block. Then cool the block in LN2, this
>takes a while and used about 3 l of LN2.
>
>When you drop the magnet, it stops abruptly before hitting the block and
>slowly falls to the block without making a sound. This is very very
>cool.
>
>Thanks for the great demo Doug!
>
>Sam


From gnper090@wviz.org Tue Apr 1 17:25:14 2003
From: "Dick Heckathorn"
To:
Subject: RE: vortex cannon
Date: Tue, 1 Apr 2003 17:29:40 -0500
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Greetings,

When introducing sound to elementary and middle school students, I focus
early on asking the question, "What is needed to hear a sound?

1st one needs a source such as a ringing bell, talking voice, rubbing
ones thumb over a piece of plastic with ridges that gives something
resembling some words, a vibrating object with frequency within hearing
range.

2nd the sound needs to get from the source to the listener. I set up
some dominoes, knock one down and the others are knocked down. What
traveled from beginning to end? I then use the vortex cannon to move
some papers on the opposite wall. What traveled from the cannon to the
paper on the wall? Nothing was seen. One can also aim it at students and
mess up their hair. Then one can put some smoke into the vortex cannon
and they can see the smoke ring travel from the cannon. Now we both know
that with the smoke ring, particles of smoke are traveling from point A
to B while with sound, nothing physical is traveling but it helps
students think of something traveling from one point to another without
being seen.

As a side, place a square opening over the circular one and ask the
students what the shape of the smoke ring will be. Those who answer
predict a square shape. I could let you try it to know what happens but
what one gets is a circular ring as before. Conclusion: Don't agree with
math teachers who say one cannot get a circle from a square.

Dick

"Science is nothing more than learning how to communicate with nature in
such a manner that it will talk back."

Helping teachers who facilitate, motivating students who learn.
Dick Heckathorn 14665 Pawnee Trail Middleburg Hts, OH 44130
440-826-0834
Physics Teacher CVCA 4687 Wyoga Rd Cuyahoga Falls, OH 44224
330-929-0575 VM 120




From thomason@colorado.edu Tue Apr 1 17:51:58 2003
Reply-To:
From: "Michael Thomason"
To:
Subject: RE: cool new Lenz's law/eddy current demo
Date: Tue, 1 Apr 2003 15:55:18 -0700
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Sam,

I too got excited when Douglas Osheroff described that Stanford
experiment. We did the same demonstration in lecture yesterday. It is
awesome.

We used:

6" x 6" x 2" copper block (Guess how much it weighs! Ordering this is
one of the first things I did after getting back from that conference.)

.25" x 1" diameter NdFeB magnet (This size is probably not ideal.
Osheroff described a magnet 1" x 2" diameter. I will be looking for one
that size.)

This combination is impressive even without cooling. There is no
"ching". The magnet does not stop in mid-air when dropped onto the
copper from a height of 3 inches, but it makes a slow, silent soft
landing that would have made the Apollo astronauts proud. At liquid
nitrogen temperature it is gorgeous. (With the magnet above, .5" thick
copper works almost as well.)

As Sam says, thanks to Douglas Osheroff for attending the outreach
conference and for sharing that experiment,

Michael Thomason, Director, Physics Learning Laboratories
University of Colorado, Boulder, Department of Physics
http://physicslearning.colorado.edu
mailto:thomason@colorado.edu
303-492-7117

"As we, the people of the First World, the Romans of the twentieth
century, look out across our Earth, we see some signs for hope, many
more for despair. Technology proceeds apace, delivering the marvels
that knit our world together - ... the contemplated "information
highway" that will soon enable all of us to retrieve information and
communicate with one another in ways so instant and complete that they
would dazzle those who built the Roman roads, the first great
information system.

But that road system became impassable rubble, as the empire was
overwhelmed by population explosions beyond its borders. So will ours.
Rome's demise instructs us in what inevitably happens when impoverished
and rapidly expanding populations, whose ways and values are only dimly
understood, press up against a rich and ordered society. More than a
billion people in our world today survive on less than $370 a year,
while Americans, who constitute five percent of the world's population,
purchase fifty percent of its cocaine. If the world's population, which
has doubled in our lifetime, doubles again by the middle of the twenty
first century, how could anyone hope to escape the catastrophic
consequences - the wrath to come? But we turn our backs on such
unpleasantness and contemplate happier prospects of our technological
dreams.

What will be lost, and what saved, of our civilization probably lies
beyond our powers to decide. No human group has ever figured out how to
design its future. That future may be germinating today not in a
boardroom in London or an office in Washington or a bank in Tokyo, but
in some antic outpost or other ... in some unheralded corner where a
great-hearted human being is committed to loving outcasts in an
extraordinary way.

Perhaps history is always divided into Romans and Catholics - or,
better, catholics. The Romans are the rich and powerful who run things
their way and must always accrue more because they instinctively believe
that there will never be enough to go around; the catholics, as their
name implies, are universalists who instinctively believe that all
humanity makes one family. The twenty-first century, prophesied
Malraux, will be spiritual or it will not be. If our civilization,
which as Patrick would say, may pass "in a moment like a cloud or smoke
that is scattered by the wind" - if we are to be saved, it will not be
by Romans but by saints."

Thomas Cahill, "How the Irish Saved Civilization"


-----Original Message-----
From: owner-tap-l@listproc.appstate.edu
[mailto:owner-tap-l@listproc.appstate.edu] On Behalf Of Sam Sampere
Sent: Tuesday, April 01, 2003 12:52 PM
To: tap-l@appstate.edu
Subject: cool new Lenz's law/eddy current demo


At the Physics Outreach conference in February, Doug Osheroff described
a demo he takes on the road. We did the same demo today and floored
students and impressed the prof too. I was amazed the first dozen or so
times I tried it too.

You need:

NdFeB magnet
Cu or Al block larger than the magnet
LN2

My stuff was a 2" OD x 1/2" NdFeB, a 6" x 6" x 1" Al block, and 3 l of
LN2. Drop the magnet on the block - it goes ching! The magnet slightly
slows as it approaches the block. Then cool the block in LN2, this
takes a while and used about 3 l of LN2.

When you drop the magnet, it stops abruptly before hitting the block and
slowly falls to the block without making a sound. This is very very
cool.

Thanks for the great demo Doug!

Sam
From DWARN@boisestate.edu Tue Apr 1 18:19:04 2003

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