Is it possible to create a hollow magnetic sphere?

1. Jun 18, 2017

Rajan

Is it possible to create a hollow magnetic sphere?

2. Jun 18, 2017

eigenmax

Is the magnetic field spherical or is the magnet itself spherical?

3. Jun 18, 2017

rumborak

It depends on what part you want to be spherical. You can obviously just create a sphere out of magnets. But the resulting field will be haphazard.

If you are hoping for spherical symmetry of the magnetic field, that can't be done since the divergence of the magnetic field is always zero. What goes in must come out somewhere else.

The closest you can get is a Halbach sphere.

4. Jun 18, 2017

Rajan

Earth itself is a hollow sphere and it has a magnetic field which makes it a hollow magnetic sphere.

5. Jun 18, 2017

Rajan

Magnet itself is spherical don't know about field

6. Jun 18, 2017

JBA

@Rajan Where did you get the impression that earth is a hollow sphere? The earth is a solid sphere with its highest density is at its central core.

7. Jun 18, 2017

Staff: Mentor

There is nothing special about a hollow magnetic sphere. Consider a block of magnetic material. You could machine it into a sphere and then hollow it out.

It would still be magnetic with a north and south pole.

Last edited: Jun 18, 2017
8. Jun 18, 2017

eigenmax

Well I suppose you could just make a hollow, spherical permanent magnet. The field would be strange though.

9. Jun 18, 2017

This sounds good on paper, but I don't think it is the case. By the magnetic pole model of computing the magnetic field $\vec{B}$, if the magnetization vector $\vec{M}$ of the material remained intact after machining, the inner surface that has an outer "plus" pole (i.e. that particular hemisphere) would be a "minus" pole surface, largely cancelling the "plus" pole. (And visa versa for the other hemisphere). Thereby, I think the idea proposed by the OP @Rajan is impractical. For the same reason, they can not save on material and make a hollow bar magnet or hollow horseshoe magnet.

Last edited: Jun 18, 2017
10. Jun 18, 2017

Staff: Mentor

I thought about that considering stacked ring magnets and letting the ring get successively smaller. The magnetic properties should remain the same.

11. Jun 18, 2017

I think with the ring magnets, especially with a large radius, the magnetic flux lines will loop around down through the center and point in the opposite direction, thereby cancelling the effect of the magnets of smaller radius that are stacked on top of it. Alternatively, if you make a stack of ring magnets with small hollowed centers, you might as well just have a solid spherical magnet. I think it is impossible to make a large hollowing of the center without significantly affecting the external magnetic field strength. $\\$ Editing: To add to the above, the magnetic field from a ring magnet with a very large opening in the center would be relatively weak.

Last edited: Jun 18, 2017
12. Jun 18, 2017

CWatters

Rajan.. your original question is very poor. A hollow iron sphere would be magnetic.

However it sounds like you actually want to make a magnet that is in the shape of a hollow sphere. That might or might not be possible. It depends how to want the poles/field arranged and you haven't said.

13. Jun 18, 2017

Nik_2213

Assembling magnets to a hollow shell would be tricky, but welding two ferritic steel bowls rim-to-rim then magnetising that may be practicable...

14. Jun 19, 2017

CWatters

Reason we need to know the field pattern is that you can't make a monopole.

15. Jun 19, 2017

It might be worth mentioning, to the OP and anyone else who might have an interest in the subject, that (I believe this is correct), in general, thin surfaces do not make strong magnets because the minus pole will counteract the plus pole, but there are ways around this. One way consists of magnetic sheets where typically stripes of alternating plus and minus poles (e.g. 1/8" spacing) can be made to run along the length of the material (material may be 1/8 " thick), and at close range the magnetism can be relatively strong, but the field direction alternates regularly with position. In samples I have of this material, they made flat refrigerator magnets (1" diameter circle approximately of Disney characters). I could place two of these back to back, and there were detente positions, regularly spaced, where the plus rows of one were clearly lined up with the minus rows of another. Sliding them across each other with a little effort would get the position where plus was lined up with plus, and it would sort of snap into place to the next row where all of the pluses were again lined up with minuses. A google of "magnetic sheets" showed in fact, that my observations were correct. They are typically manufactured in this manner. $\\$ Note: I didn't want to digress too far from the original post, but I believe this is relevant to the discussion.