Reluctance of a bar magnet circuit?

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Main Question or Discussion Point

If you hold a bar magnet in the middle of an empty volume and put a gauss meter in front of a pole, there is evidently a measurable magnetic flux, so there is a magnetic circuit there.

What is the reluctance of the whole circuit, not counting the bar magnet itself? How do you work that out? Is it 'infinite' because the circuit is free to roam across empty space, or are there constrains on that, and how it is calculated?

Not a homework question, it's to address an engineering design question, so this is a question 'in practice'. Thanks.
 

Answers and Replies

  • #2
berkeman
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It's very similar to when you place a battery in a slighly conductive solution. The resistance varies with the path length for the electric field that transits the fluid outside the battery between the poles.

Can you show the Relevant Equations for the electric dipole case in a conductive fluid (like the battery example), and then extend them to the case of magnetic flux in free space from a bar magnet? :smile:

https://xaktly.com/Images/Physics/ElectricField/BatteryFigure.png

1586103466814.png
 
  • #3
cmb
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It's very similar to when you place a battery in a slighly conductive solution. The resistance varies with the path length for the electric field that transits the fluid outside the battery between the poles.

Can you show the Relevant Equations for the electric dipole case in a conductive fluid (like the battery example), and then extend them to the case of magnetic flux in free space from a bar magnet? :smile:

https://xaktly.com/Images/Physics/ElectricField/BatteryFigure.png

View attachment 260042
I don't need an equation, just want to know what the answer is?

The difference is that we are immersed in this particular magnetically conductive fluid, it's the same for everyone, so I am surprised no-one's ever measured it?

The significance is that this bulk reluctance value is what defines the magnetic field on the surface of a magnet in free space. I am not sure there is an equivalent for a battery (at least, nothing that is measurable) and the analogy falls down. So, yes, similar, but not the same.
 
  • #4
davenn
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I don't need an equation,
Yes you do, else you will never understand the process and therefore never learn anything


just want to know what the answer is?
Here at PF, we are not in the habit of just handing out answers,
we try and teach people to use some grey-matter and think things out with some guidance :smile:
 
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  • #5
cmb
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Yes you do, else you will never understand the process and therefore never learn anything




Here at PF, we are not in the habit of just handing out answers,
we try and teach people to use some grey-matter and think things out with some guidance :smile:
I don't believe this is a 'correct' response.

The first responder indicated this would be an empirical thing. You're now indicating it is something that can be deduced. These are contradictory things.

If someone wanted to know the speed of light, would you tell them to use their brain and deduce it?

OK, let me ask the question in a different way; imagine someone sold batteries and ALL of the buyers and users of their batteries used them under water in the sea. To serve the customer's needs, they'd have some means to design the batteries so they know how they'd behave under water.

This is now a direct analogy to making rod inductors. The only strict way to know what the energy content (thus inductance) is for a given current is to know what the reluctance of the the circuit outside the rod is, because all users use them under this particular permittivity sea.

If the reluctance of the space around a rod inductor was infinite then it could contain no magnetic flux. Clearly it is not infinite. Clearly it's actually quite low. How do rod inductor designers measure the reactance outside of the rod, and what do they measure/find/assume so as to tune their design?
 
  • #6
davenn
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I don't believe this is a 'correct' response.
sorry you feel that way. but you need to learn how to do work yourself
rather than having everything handed to you on a silver platter

If someone wanted to know the speed of light, would you tell them to use their brain and deduce it?
No, I would tell them not to be lazy and go and google it
 
  • #7
cmb
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I am googling 'reluctance of free space' and nothing's coming up.
 
  • #8
berkeman
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The first responder indicated this would be an empirical thing. You're now indicating it is something that can be deduced. These are contradictory things.
I didn't intend to say it has to be empirical. These kind of problems are usually solved using FEA sotware packages like COMSOL. You didn't seem to have any intuition for the situation, so that's why I used the battery in a conductive fluid analogy to try to get you started.

Have you used COMSOL Multiphysics or any other FEA packages yet?

1586185268897.png
 
  • #9
cmb
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Thanks, I have Maxwell SV, which comes up with 'a number' if I put in permanent magnet material.

I was hoping there would be some 'known' canonical solution, like reluctance outside a bar magnet being a function of its dimensions, length and cross-section/pole areas basically, not sure what else it'd be a function of?
 

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