MadSci Network: Physics

Re: Will stacking magnets increase their overall strength?

Date: Mon Mar 13 17:32:51 2006
Posted By: John Link, Senior Staff Physicist
Area of science: Physics
ID: 1142202364.Ph

The question: "Will stacking magnets increase their overall strength? We are doing a lab report in science and I am pretty confused on how to explain that stacking magnets does increase their overall strength. Can you please email back as soon as possible? thanks"

It depends on what you mean by "increasing their overall strength", and how they are stacked. In fact, your question has been answered previously in our archives. I used our search engine to search on "refrigerator magnet" and found this previous answer by John Balbach, but I will add some information.

Imagine taking 5 magnets which can be stacked as follows, with each pair "[ ]" representing one magnet:
The magnets attract each other, and the magnetic field from each magnet is associated closely in space with an opposite pole, so the net effect is one long magnet of the same strength as the individual magnets. I have actually measured this effect. Because the magnetic fields are largely confined, on a refrigerator (or other ferromagnetic surface) the magnet assembly will "stick", but not terribly strongly, either by an end pole or along its long side.

However, stack the magnets this way:
(You will have to find a way to confine them because they will repel each other! I have a "Moo Magnet" that fits the needs, which you can find in many science supply places such as at allmagnetics.) This (more or less) is four magnets with double the strength of each individual magnet because each effective "pole" is comprised of two individual poles. This arrangement will "stick" to a refrigerator, along its long side, harder than the first configuration because the magnetic fields "leak" outside the physical confines of the assembly. However, the end poles are not any stronger than an individual magnet, and, in fact, are slightly less strong because the end pole is "buried" deeper inside the assembly. (The concept of "pole" is a convenience for our thinking. It is the "place" where the sum of all the little magnetic dipoles in the material seems to be, but in reality the magnetic "strength" is distributed throughout the magnet. Use our search engine to search on "magnet pole result" to find more information about this.)

There's another way to "stack" the magnets. Imagine attaching each one by one pole face to a wooden (or plastic, or anything that is not ferromagnetic) board, so that either
i) the magnets are widely spaced, or
ii) neighboring magnets are of opposite poles facing out.
You will have effectively "ganged" the magnets in a way that will "grip" a refrigerator (for instance) five times as hard (If there are 5 magnets!) as an individual magnet. In fact, so-called "refrigerator magnets" do this by making the magnetic material of alternating pole stripes, which you can read about more in this previous answer in our archives. The reason why refrigerator magnets are made this way is so that they are not effectively strong magnets at distances "far" (several inches) from their surfaces.

John Link, MadSci Physicist

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