Re: magnet falling down tube

[Sam Sampere <sampere@suhep.phy.syr.edu>]

Here's an interesting demo I've been doing for years that relates to all
this stuff.  Get a strip of flexible magnet and curl the ends up.  Take a
chunk of YBCO and cool it.  Place the YBCO on the curled up end of the
magnet and let it go.  Not only is this a good mag-lev train demo, but
it's an excellent conservation of energy demo.

Later,

Sam


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Sam Sampere
Syracuse University
Department of Physics
Syracuse (Where all the snow is NOT..), NY 13244



On Fri, 1 May 1998, William Beaty wrote:

> On Fri, 1 May 1998, Chuck Britton wrote:
>
> > At 12:27 PM -0700 4/30/98, William Beaty wrote:
> > > Musings about LN2: as conductivity decreases, the magnet falls slower, but
> > > at some point a further decrease in conductivity will cause the magnet to
> > > fall faster.  The skin-depth effects and the Lenz-law resistive heating
> > > and braking effects don't vary with temperature in the same way.  At very
> > > low resistance, the skin depth becomes so small that significant fields
> > > won't "sink into" the metal, and so the braking force will be reduced and
> > > the magnet will fall.  Also, once the magnet starts to fall faster, the
> > > skin depth becomes even smaller and the braking becomes less.  As a
> > > result, the magnet would "break loose" from the braking effect, and
> > > suddenly drop down the tube with very little EM friction.  At least,
> > > visual/intuitive thinking tells me this is so.
> >
> > Sorry Bill, but I gotta disagree on this visual/intuitive thinking result.
> > MY visual/intuitive thinking sez you'll be approaching a superconductive
> > state and I use the copper pipe as a lead-in for the HTc levitation exp. If
> > the pipe were a PERFECT conductor the magnet wouldn't fall AT ALL!!!
>
> Right, it should hover over the mouth, but only until you push the magnet
> into the tube. Then the vertical forces vanish, and the magnet should fall
> frictionlessly (although repulsion forces might keep it from ever touching
> the metal.)
>
> Of course this "breaking loose" phenomenon might only occur at very low
> conductivity, and copper might not make it even at 0K.  But if the magnet
> was forced to move fast (shot out of a gun, say), then it would be in the
> "maglev flight" regime where the braking forces fall nearly to zero.
>
> This is similar to what I imagine happens with "maglev flight" trains, the
> kind with DC magnets and an aluminum slab as a track.  The train must be
> moving fast before the magnets are turned on, otherwise the fields would
> cause braking, not levitation.  If a magnet cannot slide down a
> superconductor tube, then wouldn't maglev trains have excessive energy
> loss from the same process?
>
>
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