Naked Science Forum

On the Lighter Side => Science Experiments => Topic started by: ldx00 on 09/04/2016 23:29:07

Title: Induced magnetism
Post by: ldx00 on 09/04/2016 23:29:07
I saw a science program for kids while I was in Japan and despite being an engineer and knowing how to use Google, I genuinely am completely stumped by this one, being neither able to explain it, or to phrase the question in a way that Google might be able to give me an answer. Any explanation, greatly appreciated! Here is how you can replicate what I saw:

1. Tape a magnet down to the table, it appears that any permanent magnet will do
2. Get something non-magnetised, but with some iron in it or possibly anything ferromagnetic (they used ball bearings, although I can confirm it works with paper clips too, but only if you have a strong enough magnet)
3. Stack two of the objects in No.2 above, on top of the magnet (ie. magnet, ball bearing 1, ball bearing 2)
4. Now grab the upper ball bearing (2) and pull
5. Unbelievably, the ball bearing in contact with the magnet (1) comes away with the top ball bearing!?! Don't believe me, try it yourself!

Having thought it through, I would expect that if I pull the top ball bearing then I would pull just that one away, as I expect the ball bearing in contact with the magnet to be more strongly attracted to the magnet, than the ball bearing that has had magnetism induced in it only temporarily by the magnet itself. I have to come to the conclusion here that the magnetism induced in the ball bearings is stronger than that in the magnet that is inducing that magnetism. I'm not sure how this can be true but having done as much research as can, I think one of the below might be the explanation, not sure which, if any of the possible explanations is right though...

1. The shape of the objects I have tried, ie ball bearings and paper clips are all kind of circular. Maybe this is helping to concentrate the magnetic field. I don't unfortunately have a circular magnet, just ones that have flat surfaces. I tried it on the corner of one magnet though and I got the same result. Might mean that this theory is already proven incorrect...
2. The material that common permanent magnets are made from has a lower something (inductance? permeability?) than iron and therefore in the presence of the same magnetic field strength, won't be as strong a magnet. I think this one is probably my pick, but I don't know which magnetic property it is that may be the cause and therefore, I'm not sure this is truly an explanation...

There appear to be some caveats though. It doesn't seem to work very well for very strong magnets like neodymium and also for very small ball bearings. Otherwise, it's pretty consistent.

Sorry for the long post. Any help greatly appreciated!
Title: Re: Induced magnetism
Post by: hamdani yusuf on 29/06/2016 11:36:10
The parameter related to this experiment is magnetization.
https://en.wikipedia.org/wiki/Magnetization
Title: Re: Induced magnetism
Post by: wolfekeeper on 18/01/2018 01:35:29
Yes, I just tried this.

It seems to be because the iron in the ball bearings has a much higher magnetic saturation (about 1.6-2 Tesla) so in conjunction with the high permeability, the ball bearings become very highly magnetised, particularly in the small patch where they touch, and thus stick to each other more strongly than they stick to a ferrite magnet. Ferrites only have a saturation of about 0.35 Tesla, and very low permeability, so even though the magnet is a lot bigger than the ball bearings the attraction of the ball bearing to it has longer range, and is not so intense.