Error in text re magnetic fields going through iron

[littlechicoryParticipant]

Contrary to what is says in the text, one will find that a horseshoe magnet on one side of a tin can will hold a paperclip on the other. (When observations dispute the text it is the text that is wrong.) However, careful testing does reveal that the paperclip is held less strongly. Then there is the observation that a magnet will hold a chain a significant chain of paperclips. The last paperclip held in the chain will be at a distance where it would be unaffected by the magnetic field in the absence of the paperclips in between. The conclusion is that a magnetic field will follow along iron materials extending its reach along those materials. But as this occurs, its reach/strength is other directions is diminished (conservation of energy).

[NebelParticipant]

Are you using a horseshoe magnet or a button or bar magnet? When I wrote this, I had in mind a horseshoe magnet. The iron between the poles effectively “short circuits” the magnetic field so that the paperclip on the opposite side will not stick. On the other hand, if you are using a very strong horseshoe magnet, the field may not be totally short circuited and some sticking of the paperclip on the opposite side may be observed.

If you are using a button or bar magnet, however, there is only one pole touching the can and the field around that pole is conducted through. (It is like what happens in hanging a “chain” of paperclips from a magnet.)

Please let us know if this solves the problem. If not, please ask again.

Bernie Nebel

[littlechicoryParticipant]

Ah. We used button magnets.
I will go read the lesson again since I feel like I am not understanding how horseshoe magnets work.

[NebelParticipant]

A horseshoe magnet is simply a bar magnet bent into a “U” such that the two poles (north seeking and south seeking) are side by side.

Bernie Nebel

[knitgrlParticipant]

We had difficulty with this demonstration as well. The paperclip was attracted to the horseshoe magnet through the can. Our unlabeled horseshoe magnet appears to have the same pole and both legs and the opposite pole at the curved part. Does anyone know where to find a good horseshoe magnet?

[Bernard NebelKeymaster]

You have discovered and brought our attention to an error. Thanks “knitgrl”. On testing this further, you are quite right. I too find that the magnetic field extends through the can and attracts a paper clip on the other side. The attraction, I estimate, is only somewhat weaker than in the absence of the can’s “short-circuit”.

The science lesson is that what is observed will trump what it says in the text. Bernie Nebel

BTW, I think your horseshoe magnet is fine. I have never seen or heard of a magnet constructed as you describe.

[Bernard NebelKeymaster]

You have discovered and brought our attention to an error. Thanks “knitgrl”. On testing this further, you are quite right. I too find that the magnetic field extends through the can and attracts a paper clip on the other side. The attraction, I estimate, is only somewhat weaker than in the absence of the can’s “short-circuit”.

The science lesson is that what is observed will trump what it says in the text. Bernie Nebel

BTW, I think your horseshoe magnet is fine. I have never seen or heard of a magnet constructed as you describe.

[Tom MurzenskiParticipant]

It’s been a while, but I have something to add to this question.

I just received a new horseshoe magnet. It came with a small piece of steel that’s big enough to span across the two poles. The piece of metal is called a keeper. Wikipedia says the purpose of the keeper is to preserve the strength of the magnet by completing the circuit and keeping the magnetic domains aligned.

When I dip the horseshoe magnet into filings, it’s covered in filings stuck to the poles. When I dip the horseshoe magnet with the keeper in place into the filings, very few filings stick to the magnet. None stick to the (ferromagnetic) keeper. The only filings that stuck to the magnet are along the edges of the poles where they touch the keeper.

So apparently there is something to the idea of the magnetic field being blocked by short circuiting. Perhaps a tin can just doesn’t have enough oomph to produce the effect.

I never thought of magnetic fields as having a circuit, much less being able to short circuit a magnetic field, so this is all very interesting.

Yay, science!

[insightandlearningParticipant]

I was able to reproduce this experiment using the horseshoe magnet as described: it did not hold the paperclip. However, we naturally tried the bar magnet afterwards to see that it did work. And then, tried the horseshoe magnet again, finding that now it did hold the paperclip. I think that the bar magnet aligns the domains of the tin can and imparts some magnetism much like the chain of paperclips, which is why when the horseshoe magnet is used again it is able to hold the paperclip. I tried to hitting the tin can to “undo” its magnetism to no avail. But, I was able to replicate the same results with a fresh can.

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