Re: [Phys-L] breaking magnets

["John Clement" <clement@hal-pc.org>]

I left out of the experiment the fact that you can use a pencil to prevent
the magnets from slipping.  The effect is more pronounced when you try
testing opposing magnets.  The effect for magnets the same direction is not
2x because of the drop off of the dipole field.  The lower magnet does not
have as much effect on the floating test magnet as the the top magnet.  This
experiment can be done easily by students.

As to the bar magnets, if the original magnet is fairly long the change in
strength of breaking it will not be very noticeable.  But in the limit of
disc magnets breaking it into 2 pieces shouls be noticeable.  The very near
field of course is not affected as much.  The disc magnet experiment is
useful for getting students to think about how magnets work.  Side by side
bar magnets aligned the same way do have noticeably greater strength.  Once
students see their misconceptions refuted, the model of magnets having
"nanomagnets" inside them is very reasonable.  The big idea is nanomagnets,
and the domain structure is just a refinement.  The nanomagnet model can be
used to explain quite a lot of phenomena without going into the idea of
domains.

I did not invent this experiment, as it is a standard thinking activity from
Minds-on-Physics.  I just refined the testing to help the students see the
effect after they have come up with predictions.

John M. Clement
Houston, TX

> -----Original Message-----
> From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of 
> Ken Caviness
> Sent: Sunday, May 04, 2014 3:28 PM
> To: Phys-L@Phys-L.org
> Subject: Re: [Phys-L] breaking magnets
>
> Ok, that's a lower bound.  But is the force at any given 
> distance provided by the combination of 2 disc magnets on 
> some object = twice the force of one disc magnet at the same 
> distance on the same object?  Or is it greater/less than 
> twice the force of one disc magnet?  My quick and dirty model 
> says "=", but "slightly greater" wouldn't surprise me.
>
> KC suspects that the JC's answer wasn't actually what the 
> questioner was asking.
>
> -----Original Message-----
> From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of 
> John Clement
> Sent: Sunday, 4 May, 2014 2:02 PM
> To: Phys-L@Phys-L.org
> Subject: Re: [Phys-L] breaking magnets
>
> If you put together 2 disc magnets, the resulting magnet is 
> stronger than either individually.  This is easy to test if 
> they have a hole in the middle and you put a magnet opposing 
> them on the top.
>
> John M. Clement
> Houston, TX 
>
> > -----Original Message-----
> > From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of Larry 
> > Smith
> > Sent: Monday, April 28, 2014 12:28 PM
> > To: Phys-L@Phys-L.org
> > Subject: [Phys-L] breaking magnets
> >
> > If you break a simple dipole bar magnet in two unequal pieces, how 
> > does the strength of the pieces relate to the strength of the 
> > original?
> >
> > Larry
> >
> >
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