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Q & A: Poles on toroidal magnets?

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Most recent answer: 09/24/2015
Q:
Where are the poles on a torus-shaped magnet? Or is every point on the magnet a pole?
- Draken-Korin
A:
Great question!

You can picture a piece of a magnet as a small arrow, with the tail the S pole and the head the N pole. Now if you make a loop of these arrows, you've represented a toroidal (donut-shaped) magnet. At each head, there's also a tail, so no point is more N-like or S-like. Therefore a toroidal magnet has no poles. There's still a magnetic field inside, but it falls off more quickly outside than does the field from a magnet with poles. That can be very convenient if you don't want the magnetic field inside the torus to affect stuff outside, and vice-versa. Toroidal electromagnets are used to make transformers, and also used as inductors in electronic circuits.

Mike w (and Tom)

(published on 10/22/2007)

Follow-Up #1: donut-shaped magnets

Q:
Suppose we have a horseshoe magnet. Now we bend it in such a way that it becomes doughnut shaped and poles remain in contact with each other. In this situation, what will happen to the magnet? Will still behave as a magnet? Where would be its poles? What will happen to the domains inside the magnet?
- Razin Shaikh (age 16)
Navsari, Gujarat, India
A:

This is closely related to an old question: . This donut magnet has no poles.

We should slightly correct the old answer. In an ideal donut-shaped (toroidal) magnet, the field outside the donut is zero. In that sense, it doesn't behave like the horseshoe magnet, which can pick things up via the field outside the magnet.  The domains (again in this ideal case) join up to form one domain (as they would also do for an ideal horseshoe), but now with the field axis looping around the donut hole in circles.

 

Here's a site with some nice pictures of the field: 

Mike W.


(published on 09/24/2015)

Follow-up on this answer.