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I've read that when current flows in a solenoid it behaves like a bar magnet having both north and south poles. I was wondering how can we visualise north and south poles in a **Toroidal Solenoid".

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  • $\begingroup$ Take a bar magnet, curve it until both poles touch each other, and make it a circular ring. Does this "permanent toroid" have poles? $\endgroup$ Oct 25, 2017 at 16:43
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    $\begingroup$ @sammygerbil I'm sorry sir. I actually meant a toroidal solenoid (I.e. a coil wrapped around a torus). I'll make an edit! $\endgroup$
    – Serotonin
    Oct 25, 2017 at 16:55
  • $\begingroup$ Haha you're welcome, and please don't call me sir, I'm just 18! Mine wasn't a detailed explanation, not by a long shot. Hopefully, someone who is more experienced will pass by and answer your question. $\endgroup$ Oct 25, 2017 at 17:03

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Exquisite question. The answer is easy to get and somehow an illuminating insight.

enter image description here

Take the horseshoe permanent magnet and place a piece of metal between the poles. The magnetic dipole moments of the involved subatomic particles - mostly from the electrons - now are aligned in the piece of metal (like the metal shavings in the image) as well as this is so all the time inside the permanent magnet. You get a closed chain of aligned magnetic dipole moments. The meaning of poles gets lost.

But you have to take into account that any piece of magnetisable material will be magnetised if you hold it to this “closed“ magnet. And an permanent bar magnet gets aligned.

The same happens with your toroidal solenoid? Yes:

  1. Inside the toroid you have no poles
  2. Even if you have a metal core inside, the magnetic field is not fully inside. Any piece of metal approached to the device will be magnetised.
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