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Physics/The behavior of a magnetic files in an electromagnet.


closed magnetic loop
closed magnetic loop  

I am interested in using an electromagnet to repel a permanent magnet. The thing is, the electromagnet I want to use is a closed magnetic circuit design, where the core material is an O shape that provides a closed path for the magnetic field through the high permeability material of the core (image attached). I have read that an electromagnet like this is stronger because the magnetic field doesn't travel through free space, but I am wondering if  you could help me understand if the electromagnet could still repel a permanent magnet, given that the electromagnet's field is more contained. Would the electromagnet still repel the permanent magnet, or would the geometry of the core mean less of the magnetic field would act on the permanent magnet in a way that would make repelling the permanent magnet problematic?
(sorry if I have explained myself poorly)
Thanks very much, and have a lovely day.

ANSWER: None of the magnetic field will reach the permanent magnet in the geometry you describe/draw.  That's part of the reason that old telephone switching circuits used toroidal coils similar to this, there was no interference between them.  Since the field is entirely contained in the ferromagnetic core material, the permanent magnet will experience no force at all.

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Air gap in the magnetic circuit
Air gap in the magneti  
QUESTION: Thanks Steve,

Much appreciated. So, if I want the magnetic field to exert force on the permanent magnet, should I leave a gap in the core? Will the resulting leakage of the magnetic field reach the permanent magnet even if the gap is very small?

Thanks again!
Best regards,

If your goal is simply to repel the magnet, you will need more than just a gap.  A gap will result in a north and south pole.  Those will cancel.  You need the similar magnetic pole to be close to the permanent magnet's end, and the opposing pole to be far away.  Basically, I'd leave your magnet as a bar magnet for this purpose, or wind your coil directly up to and partly around the bottom end of your tube.


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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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