Date: Tue, 15 Mar 2005 09:05:25 -0600

Author: Paul Nord

Subject: Re: Magnetic marble rolling down an aluminum channel


I'll venture a guess and say that the magnet would tend to roll with
the poles horizontal. If it got into any other rotation, the leading
pole would experience a greater force and tend to slow down. The
unbalanced force would even out if the poles were at the sides and
parallel to the surface. It could roll pole-over-pole but that seems
to be an unstable equilibrium. Off axis magnetization would complicate
that, of course.

Ok... If the above assumptions are true, the field doesn't change as
the magnet rolls. It simply moves down the slope. Before the magnet
passes by there is no field. Right under the magnet there is a high
field in one direction. After the magnet passes it goes back to zero

On the one side of the magnet the field goes up through the metal. On
the other side it goes down. Again, this changes from zero to a
maximum when the magnet is closest, and then back to zero.

The changing field produces a current. Check me on this... You'll
have two "swirls" of current following the magnet down the incline.
They rotate in opposite directions and meet in the middle; directly
under the path of the magnet. The duration of these, and the magnetic
field they produce depends on the conductivity of the metal. That
would be strictly true for a very thin metal incline. For a thicker
incline I'm picturing a continuous rotating "tube" of electric current
- like the vortex pairs left by a canoe paddle. The vortex pairs
connect under water.

In fact, the swirls should be centered just behind the magnet. That
way they exert a force pulling the magnet back. If they didn't die
out, the magnet would come to a stop. If the surface was
super-conducting, the marble would move a short distance before
generating a current that would (could) bring it to a stop. I just
went back and added "could" since I'm thinking that the ratio of the
mass of the magnet and its field might also have something to do with


On Monday, March 14, 2005, at 04:43 PM, Dick Heckathorn wrote:

> Greetings,
> A few months ago there was a discussion of rolling a magnetic marble
> down an aluminum channel.
> I mounted one inside a one inch plastic pipe used to enclose a
> fluorescent tube. I also put an aluminum ball with the magnetic marble.
> Rolling with the aluminum ball first shows it accelerating what I call
> normality with the magnetic slowed due to electromagnetic induction
> force.
> The question is: Where is the magnetic field? Where is the current? Is
> it constant?
> I took it to our section meeting but came away with no consensus?
> Close examination seem to show that it did not move with constant speed
> which I have not yet checked with a motion sensor.
> We also surmised that the induced force would be opposite the direction
> the ball is rolling. If this is the case, then the induced current and
> magnetic field would the right angle to this.
> But then we thought that the rolling marble could not have its magnetic
> field perpendicular to the direction of motion.
> Who was the one who introduced the demo? Can anyone give light to what
> is going on?
> Thanks, Dick
> 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 Lake Rd Cuyahoga Falls, OH 44224
> 330-929-0575 VM 120
> Physics is learning how to communicate with ones environment so that is
> will talk back.
> -----Original Message-----
> From:
> [] On Behalf Of Chuck
> Sent: Monday, March 14, 2005 2:00 PM
> To:
> Subject: RE: Activity for SLC
> well you might get to have it both ways in Utah...
> /healthy Utah or /heal thy Utah...
> cheers,
> chuck...
> -----Original Message-----
> From:
> [] On Behalf Of Zigmund J.
> Peacock
> Sent: Monday, March 14, 2005 6:17 AM
> To:
> Subject: Re: Activity for SLC
> Richard Richard Richard, such a goldmine of puns can derived from the
> URL alone.
> Zig
> Richard Berg wrote:
>> Zig,
>> Something you can arrange for everyone to do whilst in SLC:
>> It is there soon so you can probably get it back.
>> Dick
> ***********************************************************************
>> Dr. Richard E. Berg, Professor of the Practice
>> Director, Physics Lecture-Demonstration Facility
>> U.S. mail address:
>> Department of Physics
>> University of Maryland
>> College Park, MD 20742-4111
>> Phone: (301) 405-5994
>> FAX: (301) 314-9525
>> e-mail
> ***********************************************************************
> --
> Zigmund J. Peacock
> University of Utah/Physics
> 115 SOUTH 1400 EAST #201 Tel 801 581 6602
> SALT LAKE CITY UT 84112-0830 Fax 801 581 4801
> "We sleep safely in our beds because rough men stand ready in
> the night to do violence to those who would do us harm"
> -- George Orwell
> "All that is necessary for the triumph of evil is that good
> men do nothing!"
> -- Edmund Burke
From Tue Mar 15 11:19:31 2005