Date: Thu, 21 Jun 2001 07:38:50 -0700

Author: Roger Key

Subject: Re: unknown demo

Post:


This caught my attention because I have one too ......  Ours is a
rolling

magnet with a conductive "sheath" cover that runs on HO scale
rail tracks.


Looks to me like it's not in the bibliography .... but could be thought
of as

running 1K10.10 backwards .... sound about right?  I couldn't find
it as described.


- Rk


At 08:40 AM 6/21/01 -0500, you wrote:

The force on the bar is F=ilXB,
where i=current in the bar, l=length of

bar, B=magnetic field of other bar.  B supplied by other bar
is

B=mu*i/2*pi*r, where i= current in stationary bar, and r is the
separtion

between the bars at the instant in question.  Clearly, since r
changes,

this becomes an integral from r1 to r2, if you want to get the total
forces

moving the bar while the current is connected.


Symmetry allows you to exchange which bar is which, but you've got
one

fixed, and the other moveable, so that is the usual explanation. 
Unless,

of course, I'm still asleep...Karl


>Hi all,

>

>Could someone please enlighten me as to the name and details of a
demo

>that I have?

>

>A current is supposed to pass through parallel rods, upon which are
sitting a

>movable (rolling) bar.  The movable bar is sitting in a
permanent magnetic

>field.  When a small voltage is applied, a current flows, and
the bar

>rolls back

>toward the supply.

>

>I understand the "basic" concept of interacting fields
between the parallel

>conductors and conductors trying to force themselves apart because of
this

>interaction.  But, why does the interaction of the magnetic
field cause

>the rod

>to roll in one direction?  Is this demonstrating Lenz's
law?

>

>Thanks for any replies, I hope the question is not to dense!

>

>Regards

>Liane

>Physics Technician

>Faculty of Health & Sciences

>CPIT

>williamsl@cpit.ac.nz

>3649000 extn 8330