MadSci Network: Physics

Re: Why don't refridgerator magnets have obvious poles?

Date: Wed Sep 23 10:58:22 1998
Posted By: Samuel Silverstein, faculty, physics, Stockholm University
Area of science: Physics
ID: 905538343.Ph

Hi there! This is a very good question, and it wasn't obvious for me how to answer it until I went home and played with a couple of my own refrigerator magnets, and then visited the websites of some of the companies which produce them :-)

The bar magnets you played with in school (and which get discussed in most textbooks) are a single piece of iron which have been given a preferred north and south pole. But the refrigerator magnets you are asking about are mainly molded rubber with small pieces of magnetized iron suspended inside. You can think of each of these small pieces of iron as a little bar magnet.

Now, if you wanted to, it would be possible to orient the poles of all of these small magnetic pieces in the same direction during manufacturing. Then you would have very powerful refrigerator magnets which would attract or repel each other depending on their orientation, as you would expect.

But in the real world, this is not actually what the customer wants! Powerful magnets like this would be hard to pack together for shipping, and would stick too strongly to your fridge.

So the manufacturers of refrigerator magnets make sure that the orientations of the different iron pieces are in different directions, so the surface of your magnet contains a mixture of north and south poles. When the magnet is very close to your refrigerator, it behaves like a whole bunch of small magnets which individually stick to the surface of the door. But when you take it even a small distance away, the average field coming from the magnet becomes close to zero, making it easy to remove.

This is also the reason why refrigerator magnets have very little effect on each other. If you bring two of them together, they will orient themselves so that there are a slightly larger number of opposite poles touching each other than like poles, since this is the lowest energy condition possible. But two refrigerator magnets repelling one another would be very unlikely.

I hope this helps. Let me know if any of this was unclear, or if I've raised new questions!


Sam Silverstein

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