[davidk (OP)]

By plastic magnet, I am not talking about a magnet glued onto a plastic item.  I'm talking about the magnets made from plastic of sorts, flexible, roughly the same shape, size and thickness as a typical 'business card' and, just as thin.  Ads are usually printed on one surface; see photo below.

magnets3.jpg (54.67 kB . 656x492 - viewed 2002 times)

No matter how I orientated the two I could not sense any repelling and they always stuck together.
Note that the flat magnets that I used were not printed on either side.
Surely they must have polarity?

[yor_on]

Tried those?
https://www.wikihow.com/Determine-Polarity-of-Magnets

[chris]

@alancalverd and @evan_au will undoubtedly know the reason behind this observation.

[yor_on]

Ahh sorry, should have read you more carefully. Thought you were talking about this.
https://www.newscientist.com/article/dn6326-first-practical-plastic-magnets-created/

Heh, you were :)
Where did you get them?
=

They must have a polarity as their spins is aligned into one direction to make them magnetic. What a magnet does when it get stuck is to align the spins of whatever material containing magnetic properties, as a fridge. When you unstick it that fridge loses that alignment the magnet imposed on its material and its spins take each other out inside it. I know what you're suggesting btw :)  https://theconversation.com/explainer-the-mysterious-missing-magnetic-monopole-57661

[davidk (OP)]

As a newbie, I can't see how to response to answers.
Reply to yor_on:
Thanks.  Using a compass, as suggested, cleared up a lot; polarity is indeed present on edges and surfaces.
Because the fields are so weak,  the trick is to float one on water to to reduce friction and exclude gravity and approach with the other.  The other thing is to use only 1-2mm overlap.  Now I can feel weak repelling as loss of friction.  Attraction is far stronger.   

[Colin2B]

The magnets are rather clever in the way they are put together.
Imagine a series of horseshoe magnets side by side such that you get altenating NSNSNS etc. Now imagine that the horseshoes are the full width of the magnet so you get bands of NS etc.
This means one side (printed side) is not a magnet, but the other is and to get them to stick together you have to find the opposite bands. Try getting 2 exactly the same and sliding them across each other.

[yor_on]

Very cool, but I still wonder where one can buy them? The Internet was no help finding a supplier of those.

[davidk (OP)]

Colin2B - Great, that explains it.  Also answers why only small surfaces worked.

[They are distributed with adverts printed on one side.]

[Colin2B]

That’ Also why the small strips at the edge stick together.
That was a good idea to use a compass.

[alancalverd]

Google "flexible magnetic tape". There are plenty of suppliers.

As Colin said, the tapes consist of alternating micro-magnets, so the near-field is strong but the far field is very weak.

[evan_au]

Why do plastic fridge magnets not repel each other?

They do, but it is on a rather fine scale and you need to feel it rather than see it.
- As Colin2B says, they are magnetised in thin strips.

I find it best to start with two of the large flat sheets, of the same type
- This doesn't work if the magnetic strips have different widths...
- Place the "fridge" sides together, and rotate them until they "click" together. Now the magnetic strips are aligned in parallel, with N&S lining up and attracting.
- Now gently push the magnets parallel to each other
- In one direction they will slide easily, as they slide along the strips, with N&S lining up and attracting.
- At 90° to this, they will resist movement, as you are breaking the N-S attraction, and trying to make repulsive N-N and S-S alignments. But if you push harder, you will feel it "click over" into a new N-S alignment which attracts.
- If you now apply a steady force, you will feel it repeatedly jump from one strip to the next, creating a vibration as it jumps over the repulsive N-N alignments.
- If you push them very slowly and steadily, you will see the two magnets flapping together and apart as they attract and repel cyclically.

If you put the "advertising" sides together, there is very little attraction or repulsion. This is because they are often formed as a Halbach array - talk about blue-sky high-energy particle physics finding an everyday application in your kitchen!
https://en.wikipedia.org/wiki/Halbach_array

[RD]

...  Using a compass ...

Static magnetic-fields can be seen with this gadget ...


https://en.wikipedia.org/wiki/Magnetic_field_viewing_film

[chris]

...  Using a compass ...

Static magnetic-fields can be seen with this gadget ...


https://en.wikipedia.org/wiki/Magnetic_field_viewing_film

Wow - what is that and how does it work?

[RD]

Wow - what is that and how does it work?

Magnetic Viewing Film is a bit like the iron-filings demo,
 but it's flakes of nickel suspended in tiny cells of mineral oil, in a sheet of plastic ...

[yor_on]

That's cool, but what we're looking at is plastic material, made to magnetize. That's pretty weird as it is about the spins, not what we think of as iron etc, well, to me at least :) Don't think they exist in nature?

[Bored chemist]

That's cool, but what we're looking at is plastic material, made to magnetize. That's pretty weird as it is about the spins, not what we think of as iron etc, well, to me at least Don't think they exist in nature?

In the case of fridge magnets we are talking about plastic in which someone has mixed a lot of iron oxide, and then magnetised it.

[yor_on]

I gathered that this might be, still, what is weird to me isn't that but the way we can make a plastic magnetizing, without filling it with other materials. That makes me wonder.