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I have the following conceptual missunderstanding (pressumably). If you have two bar magnets opposing each other separately, then the field lines go from the north- to the southpole of one bar to the other. However, since there are two south- and two-north poles, the field lines seem have opposite direction for the inner two poles. By interpreting these two bar magnets in the elementar magnet picture, combining them leads to a new larger bar magnet with one north- and one southpole. Now the field lines that originally were in opposite direction seem to be going in the same direction as for the outer two poles. Why did they flip? What's happening here?

two bar magnets becoming one

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    $\begingroup$ The first image should be more like this: qph.cf2.quoracdn.net/main-qimg-65c57bb379179f709e6d0ee6a00e7f2b $\endgroup$ Commented Feb 1, 2023 at 17:46
  • $\begingroup$ True, but that doesn't change my question. When I bring together these to magnets you still seem to have field lines from the left northpole to the left southpole. But that should be impossible, since the left magnet would become the southpole to which the lines from the right magnet go. $\endgroup$
    – Diger
    Commented Feb 1, 2023 at 20:54
  • $\begingroup$ Well, I the reason I posted that was because with a more accurate picture, you can begin to see how the field lines would bunch up together and bend, pinch-off and merge in some parts as you bring the two bars closer. $\endgroup$ Commented Feb 2, 2023 at 2:24
  • $\begingroup$ I've updated my paint skills. So does it work like this? When the magnets come closer to each other, the individual field lines from each magnets north- to southpole pile up vertically, so that when they come down to the inner right southpole and rise at the inner left northpole they extinct, because they run in opposite direction? $\endgroup$
    – Diger
    Commented Feb 2, 2023 at 19:04

3 Answers 3

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Every magnet is approximately made up of a zillion tiny dipole magnets. Each tiny dipole magnet has its own dipole magnetic field. The net magnetic field of the entire magnet is simply the linear sum of each tiny dipole magnet's magnetic field.

When you think of it in these terms, you will see that nothing magical or contradictory happens when you put together two magnets, because the same logic would apply to splitting apart one magnet. The picture linked by @Filip in his comment is very important, as it shows the qualitatively correct way of summing the magnetic fields of two dipole magnets.

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The basic answer to your question is that the field lines that, in the top diagram, ran from left to right between the two neighbouring poles of the two magnets are now (when the magnets have been allowed to come together) INSIDE the magnets.

Remember that a magnet's magnetic field lines don't start on its North pole and end on its South; rather they are continuous closed loops, emerging from the magnet at its North end, running through the air, re-entering the magnet at its South Pole, and running from South to North Pole inside the magnet.

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  • $\begingroup$ The field lines in between the two magnets will be inside, yes. But there are still field lines outside, that emerge from the northpole of (say) the left magnet and re-enter the southpole of the left magnet; for every finite distance of the two left and right magnets. Yet, when you bring them together (=zero distance), the resulting magnet should only have field lines emerging from the former right magnet (as this one becomes the northpole), that then re-enter the former left magnet (as this one becomes the new southpole). $\endgroup$
    – Diger
    Commented Feb 1, 2023 at 21:31
  • $\begingroup$ (a) "But there are still field lines outside, that emerge from the northpole of (say) the left magnet and re-enter the southpole of the left magnet" I note that you don't show these in your diagrams. (b) "the resulting magnet should only have field lines emerging from the former right magnet (as this one becomes the northpole)" I think the bit in brackets is an oversimplification. Its right hand end is a North Pole; its left hand end won't be a pole at all, or will still be a weak South owing to imperfect contact between the two magnets. (c) Decent diagrams of the field lines will help a lot. $\endgroup$ Commented Feb 1, 2023 at 21:48
  • $\begingroup$ (a) I'm refering to the graphic by Filip (b) I'm talking about the left hand end of the left magnet. It should be a south pole. $\endgroup$
    – Diger
    Commented Feb 1, 2023 at 22:02
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This picture shows the lines inside the magnets as well as outside. Perhaps it makes things a little more clear.

enter image description here

Image from https://www.reddit.com/r/explainlikeimfive/comments/97fcxr/eli5_why_does_breaking_a_magnet_in_half_result_in/

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