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Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: neilep on 23/06/2025 14:37:41

Title: Do Magnets Have A zone of Non Magnetism At Their Half way point ?
Post by: neilep on 23/06/2025 14:37:41
Do Magnets Have A zone of Non Magnetism At Their Half way point ?

Considering a magnet with a length of exactly 4cm, what is the magnetic polarity at the exact midpoint, precisely 2cm from either end?


Zank ewe


Sheepy xxx




Title: Re: Do Magnets Have A zone of Non Magnetism At Their Half way point ?
Post by: paul cotter on 23/06/2025 17:07:57
Same as both ends. Imagine an arrow, 4cm long overlaid on the magnet. The only difference between the N and S ends of the magnet is the ORIENTATION of the arrow as seen from each end. The magnetic field is the same throughout, with minor differences because of anisotropy of the material. There is much confusion on the net about this with many so called experts saying there is a Bloch wall at the centre of the magnet- this is total nonsense as the field is the same throughout and a Bloch wall is a phenomenon at domain level.
Title: Re: Do Magnets Have A zone of Non Magnetism At Their Half way point ?
Post by: neilep on 23/06/2025 19:58:18
Quote from: paul cotter on 23/06/2025 17:07:57
Same as both ends. Imagine an arrow, 4cm long overlaid on the magnet. The only difference between the N and S ends of the magnet is the ORIENTATION of the arrow as seen from each end. The magnetic field is the same throughout, with minor differences because of anisotropy of the material. There is much confusion on the net about this with many so called experts saying there is a Bloch wall at the centre of the magnet- this is total nonsense as the field is the same throughout and a Bloch wall is a phenomenon at domain level.


As a sheep I of course understand all of your response, but (cough cough) on behalf of the younger viewers (cough cough)  does this mean that there is magnetism in the middle of the magnet but no polarity ? if so...does it nor attract or repel ?
Title: Re: Do Magnets Have A zone of Non Magnetism At Their Half way point ?
Post by: paul cotter on 24/06/2025 12:14:27
Ok, remember I suggested an arrow to represent the magnetic field. Now substitute that arrow with a multitude of mini arrows lined up head to tail and hopefully you can see that this is equivalent to the single arrow. The field at any point in a magnet of isotropic material is the same, apart from some fringing effects close to the ends. If one was to drill a small hole in the centre of a magnet and insert a molecule sized compass it would point in the same direction as our imaginary arrow. If I had access to graphics on this antiquated computer a simple diagram would explain this far better.
Title: Re: Do Magnets Have A zone of Non Magnetism At Their Half way point ?
Post by: Eternal Student on 24/06/2025 13:56:17
Some graphics....

(https://lh5.googleusercontent.com/proxy/onBOzIHwbTgey11ODPQK-a82IIEKCQnl9VzLvZ4RibEYWUK0W_L5FzXTCzsFpPfhVkEdRIK5EUijsLjtw3WNEpwHFajmxcyC-Epte7rz9Hofe3iG_Q)

[Image taken from  http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/elemag.html ]

   That has the magnetic field lines shown all the way through the bar magnet rather than just around the outside of the bar magnet like most diagrams will.


   Now, the only interesting thing is that if we had an isolated magnetic North monopole  (which we don't - there aren't any magnetic monopoles),  then where do we expect it to be pushed to (accelerated) if we dropped it directly on to the middle of the bar magnet?

   We might assume it will be pulled to the South pole of the bar magnet and pushed away from the North pole of the bar magnet, so overall it may seem reasonable to suggest it would be accelerated directly down the page in this diagram.    However, that isn't what this diagram is saying, the B field inside the bar magnet isn't directed down the page.   The magnetic field line in the bar magnet should be directed up the page (from the bar magnet south pole to the bar magnet north pole) and that should be the direction in which an isolated magnetic north pole would be pushed.

   It's kind of nice to have some basic ideas we might have held about magnetism challenged by a few simple questions that other people ask.   I do like that and it may take me a day to check this out.

Best Wishes.
Title: Re: Do Magnets Have A zone of Non Magnetism At Their Half way point ?
Post by: paul cotter on 24/06/2025 17:14:56
A diagram, thank you, Eternal Student.
Title: Re: Do Magnets Have A zone of Non Magnetism At Their Half way point ?
Post by: alancalverd on 24/06/2025 23:20:14
Sheep are more familiar with earthworms than magnets, but they are sort of similar. If you cut a bar magnet in half, you get two small magnets. Sadly, only the front half of the earthworm survives, but you get the picture.
Title: Re: Do Magnets Have A zone of Non Magnetism At Their Half way point ?
Post by: neilep on 24/06/2025 23:57:50
Quote from: paul cotter on 23/06/2025 17:07:57
Same as both ends. Imagine an arrow, 4cm long overlaid on the magnet. The only difference between the N and S ends of the magnet is the ORIENTATION of the arrow as seen from each end. The magnetic field is the same throughout, with minor differences because of anisotropy of the material. There is much confusion on the net about this with many so called experts saying there is a Bloch wall at the centre of the magnet- this is total nonsense as the field is the same throughout and a Bloch wall is a phenomenon at domain level.


cheers chum for taking the time to explain that concept to me! I really appreciate your patience and creativity in helping me understand. Your analogy of the mini arrows really helped to clarify things, and I really like the thought experiment to drive the point home.... And all that written on a Sinclair ZX81 !!

 '
Title: Re: Do Magnets Have A zone of Non Magnetism At Their Half way point ?
Post by: neilep on 25/06/2025 00:05:54
Quote from: Eternal Student on 24/06/2025 13:56:17
Some graphics....

(https://lh5.googleusercontent.com/proxy/onBOzIHwbTgey11ODPQK-a82IIEKCQnl9VzLvZ4RibEYWUK0W_L5FzXTCzsFpPfhVkEdRIK5EUijsLjtw3WNEpwHFajmxcyC-Epte7rz9Hofe3iG_Q)

[Image taken from  http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/elemag.html ]

   That has the magnetic field lines shown all the way through the bar magnet rather than just around the outside of the bar magnet like most diagrams will.


   Now, the only interesting thing is that if we had an isolated magnetic North monopole  (which we don't - there aren't any magnetic monopoles),  then where do we expect it to be pushed to (accelerated) if we dropped it directly on to the middle of the bar magnet?

   We might assume it will be pulled to the South pole of the bar magnet and pushed away from the North pole of the bar magnet, so overall it may seem reasonable to suggest it would be accelerated directly down the page in this diagram.    However, that isn't what this diagram is saying, the B field inside the bar magnet isn't directed down the page.   The magnetic field line in the bar magnet should be directed up the page (from the bar magnet south pole to the bar magnet north pole) and that should be the direction in which an isolated magnetic north pole would be pushed.

   It's kind of nice to have some basic ideas we might have held about magnetism challenged by a few simple questions that other people ask.   I do like that and it may take me a day to check this out.

Best Wishes.

Wooo !!  Zank ewe ES. Your explanation has challenged my ovine understanding of magnetism. I appreciate how you've highlighted the importance of considering magnetic field lines. Thank ewe very much.
Title: Re: Do Magnets Have A zone of Non Magnetism At Their Half way point ?
Post by: neilep on 25/06/2025 00:12:13
Quote from: alancalverd on 24/06/2025 23:20:14
Sheep are more familiar with earthworms than magnets, but they are sort of similar. If you cut a bar magnet in half, you get two small magnets. Sadly, only the front half of the earthworm survives, but you get the picture.

Ha ha! I love the earthworm-magnet analogy! As a charming sheepy I of course am no stranger to attraction ? after all, I've got a flock of ewes drawn to my charisma !!But seriously, your comparison is a clever way to explain magnetism. When I grow up ( never).. I will market my own rod magnet..it will be a 'Baaaaa Magnet'..?.......* Extreme groan mode*

Title: Re: Do Magnets Have A zone of Non Magnetism At Their Half way point ?
Post by: Eternal Student on 25/06/2025 03:16:06
Hi.

1.   The idea of an isolated magnetic north pole being pushed up the page does seem to be correct.   There's some reasoning and explanation we could go into but it's not essential.

2.   You know one day we probably will find out who you are ( @neilep ) and won't have to assume you are a sheep.   None the less, there's nothing wrong with trying to maintain some anonymity online so I won't voice my suspicions here on the forum.   In any event I did quite like your baaa-magnet joke and it's been quite a pleasure trying to do something with some of the posts you create.

Take good care of yourself.
Title: Re: Do Magnets Have A zone of Non Magnetism At Their Half way point ?
Post by: neilep on 25/06/2025 12:13:45
Quote from: Eternal Student on 25/06/2025 03:16:06
Hi.

1.   The idea of an isolated magnetic north pole being pushed up the page does seem to be correct.   There's some reasoning and explanation we could go into but it's not essential.

2.   You know one day we probably will find out who you are ( @neilep ) and won't have to assume you are a sheep.   None the less, there's nothing wrong with trying to maintain some anonymity online so I won't voice my suspicions here on the forum.   In any event I did quite like your baaa-magnet joke and it's been quite a pleasure trying to do something with some of the posts you create.

Take good care of yourself.

Thank you for your thoughtful post! I'm glad we could have a productive discussion about magnetism.

And don't worry about my anonymity ? I'm not trying to be mysterious! I've always been open about who I am, and I typically sign my posts as "Neil". My sheep jokes are just a bit of fun.

Thanks again for your kind words, and I'm glad you enjoy our interactions.

Best regards,
Neil