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suppose we have a bar magnet


|S_______________N| ... we can imagine it to be


|S-N-S-N-S-N-S-N|

So, why do we see the lines leave the north pole and enter the south pole externally? What happens internally?

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    $\begingroup$ possible duplicate of Magnetic field lines $\endgroup$ – user60063 Oct 26 '14 at 4:50
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Magnetic field lines do not go anywhere. Field lines are useful for visualizing vector fields. These are not physical that are actually present at certain locations. And the direction you are talking about is a convention. By convention, the field lines are taken to direct away from the N-pole and towards S-pole. Internally, these field lines complete a loop :-

enter image description here

Note: Reading this will help in understanding the nature of these field lines better link

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    $\begingroup$ This is a great answer! Like @user49111 said: 1) the names themselves don't matter--the takeaway is that the poles are opposites; 2) the field lines don't exist. They're tools to help visualize the strength & direction of magnetic fields. Re: N/S, sometimes people name stuff before they're really sure what it is or why it's causing it, but by the time we know more, the name has been around so long that it would be really hard to change it. These properties were first discovered through observation--read this, lots of great info: quora.com/Why-do-magnets-have-a-north-and-south-pole $\endgroup$ – Hee Jin Apr 11 '18 at 23:43
  • $\begingroup$ Why is the north picked that way, we could switch N and S consistently (and the direction of the arrows). $\endgroup$ – john Apr 22 '19 at 4:53
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First at all the magnetic lines 'connect' the two poles. There is no flow of energy or material nor preferred direction of these lines. In this sense the north and the south pole are equal.

Then, the magnet field lines are not connecting only the ends of a magnet bar (see this picture).

Internally exist small dipols, which are orientated more or less. This can be from natural materials or more powerfull this are pressed from powder magnets which get there orientation by the help of a magnetic fields during the production process.

Magnetic monopoles are not existing, hence there is always a "chain" of material dipols inside the magnet bar and outside there is a "chain" of electromagnetic dipols connecting the two poles together.

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