Re: [Phys-L] problems with the physics regents

["M. Horton" <scitch@verizon.net>]

Be careful with the magnetic field lines misconception.  There is no such 
thing as magnetic field lines.  Magnetic fields are continuously decreasing 
from the magnet.  The reason that we appear to see lines is that each of the 
pieces of iron filing becomes magnetized and creates a long, thin magnet 
when it stick to other pieces.  That long, thin magnet is repelled from the 
next long, thin magnet, so there is a gap in between.

Students may walk away with the misconception that there is no magnetic 
field in between field lines . . . and there is.  They might think that a 
compass only works when they are standing on one of the Earth's magnetic 
field lines . . .but compasses work anywhere on Earth.

Mike


----- Original Message ----- 
From: "Philip Keller" <PKeller@holmdelschools.org>
To: <Phys-L@Phys-L.org>
Sent: Thursday, June 14, 2012 9:24 AM
Subject: Re: [Phys-L] problems with the physics regents


> I do a silly little demonstration that always gets a more enthusiastic 
> response than I expect:
>
> 1.  First, cover a bar magnet with a piece of newsprint and sprinkle iron 
> filings.  Tap the paper to shake the filings so that they line up with the 
> field lines.
>
> (Already, my 11th graders are stunned.  Actually, so am I.  They are 
> stunned by the phenomenon which they swear they have never seen and I am 
> stunned that they did not see this in elementary school...)
>
> 2.  Now take one of those clear compasses used on overhead projectors and 
> move it around to different points over the filings.  Of course, the 
> compass lines up with the "local" magnetic field lines.
>
> 3.  Now pretend you are an explorer, following your compass to the "North" 
> pole.  You would just go the way it directed you.  So I slowly move my 
> compass over the filings, always moving in the direction that the compass 
> calls "North".  As expected, no matter where I start, I eventually 
> "arrive" at one end of the bar magnet.
>
> 4. Remove the newspaper to reveal that the end of the bar magnet we have 
> reached is actually the one with an S on it!
>
> Moral of the story:  while the Earth's magnetic field is NOT caused by a 
> giant buried bar magnet...if it WERE, the pole found in the northern 
> hemisphere would have a great big S on it!
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