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Suppose we take a solenoid:

enter image description here

I was wondering what the magnetic field pattern would look like if the black current-carrying wire in the depicted solenoid was replaced by a solenoid. In other words, what would the magnetic field pattern look like if we made a solenoid out of a current-carrying solenoid instead of out of a current conducting wire? This is not the same as a toroid:

enter image description here

I can imagine how the field pattern comes to be if we curl up a straight current-carrying wire to make a (multiple winded) solenoid, but I find it hard to imagine what pattern emerges if we curl up a (multiple winded) solenoid. For a toroid the situation is clear.

But the situation is different for the kind of solenoid I described. The field lines that enter (or leave) the solenoid at one side, leave (or enter) the solenoid on the other side. This means that the in- and outgoing magnetic field lines (the north and south pole) must be connected.

For simplicity, let's assume that the two ends of the solenoids are parallel. Will the field inside this solenoid be the same as in a toroid? Outside of the solenoid, at least as far as I think, doesn't there has to be a magnetic field pattern also? Going in (out) on one side of the solenoid and out (in) on the opposite side, Continuing the field inside and making a closed pattern of field lines?

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  • $\begingroup$ Curl up a solenoid. Are you talking about a toroid? You should definitely post a diagram depicting your shape. $\endgroup$ – Harshit Joshi Jan 7 '19 at 16:17
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If the ends of the coiled solenoid meet each other, there is no magnetic field outside the solenoid. See here. If the ends do not meet, then field lines will reach from one end to the other pretty much as if they were the ends of very long bar magnets.

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If a bundle of separate bars or helices are open-ended then there would be potential difference between these ends that are being kept apart. If endings are short-circuited no net mmf can develop.

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Try to think magnetic lines as electric lines . Magnetic field is sum of the electric fields created by moving electrons and nuclei vertical to the flow of current .

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