I need a full explantation for this concept.

Magnetic field lines can be entirely confined within the core of a toroid, but not within a straight solenoid.

  • $\begingroup$ Why do you think so? A straight solenoid is clearly a limit of a very large thin torus, so if it were possible for any torus, it would be possible in the straight limit, too. If the currents are running strictly around the surface of the solenoid or torus, the field will be inside and strictly vanish outside. $\endgroup$ Sep 23 '12 at 7:11
  • $\begingroup$ This is not from my thoughts. I found this piece from a book. But lost it somewhere. $\endgroup$
    – BigSack
    Sep 23 '12 at 7:17

This is a solenoid and its magnetic field lines.


This is a toroid and its magnetic field lines


A solenoid by construction has two magnetic poles at the edges when current is flowing through its windings.

One can think of a toroid as a solenoid that has been curved and joined so no poles are open. A toroid can have magnetic fields outside its geometrical boundary according to the way the currents are flowing, if there is a circumferential current that has not been neutralized. Once neutralized there is magnetic field only inside.( as described in the link given above). A neutralizing design is shown below.

neutralizing circumferential current

In contrast a solenoid will always have two open poles.


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