Permeability with static and dynamic H fields

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Discussion Overview

The discussion centers on the permeability of materials in the context of static and dynamic magnetic fields (H fields). Participants explore how permeability may vary with frequency and the implications for material selection in applications such as radio technology.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants propose that a material with higher permeability in a static H field may not necessarily have higher permeability in a changing H field, emphasizing the dependence on material properties.
  • Others argue that permeability is generally constant over frequency until eddy current effects become significant.
  • A participant mentions that the relationship between B and H is illustrated in magnetic hysteresis plots, suggesting that this relationship is important for understanding permeability.
  • Concerns are raised about the effects of saturation on permeability, with some participants stating that materials should be used in their linear region to avoid saturation where permeability drops.
  • It is noted that the B/H ratio may change with frequency, and after a certain frequency, this ratio starts to decline.
  • Some participants highlight that different ferrite materials exhibit varying effective frequency ranges and that certain materials are designed to become lossy at RF frequencies.
  • One participant suggests that absolute permeability may actually represent a range of values rather than a single constant.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the relationship between permeability and frequency, as well as the effects of saturation and hysteresis. The discussion remains unresolved with no consensus on the implications of these factors.

Contextual Notes

Participants reference specific material properties and behaviors, indicating that the discussion is influenced by the characteristics of different magnetic materials and their operational regimes.

SuccessTheory
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If a material has a higher permeability (electromagnetism) than another when in the presence of a fixed H field, would it generally also have a higher permeability when in the presence of a changing H field?
 
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no it doesn't : )

it depends on the material properties.
 
Heh, I'll consider this thread "solved". But our actual discussion in the other thread will hopefully be moved to a general forum since I pmed a moderator.
 
dude_ said:
no it doesn't : )

it depends on the material properties.

That is not correct. In general, the permeability is fairly constant over frequency, until eddy current effects start to become noticable.
 
SuccessTheory said:
If a material has a higher permeability (electromagnetism) than another when in the presence of a fixed H field, would it generally also have a higher permeability when in the presence of a changing H field?

Yes, the permeability is fairly constant over frequency. It is the relationship between B and H, as shown in the Magnetic Hysteresis plot on this page:

http://en.wikipedia.org/wiki/Hysteresis
 
berkeman said:
Yes, the permeability is fairly constant over frequency. It is the relationship between B and H, as shown in the Magnetic Hysteresis plot on this page:

http://en.wikipedia.org/wiki/Hysteresis

Thanks for your reply.
 
dude_ said:
https://www.physicsforums.com/showthread.php?t=244904

would you mind taking a look at this thread?

Lordy, that's an involved thread. But from a quick read, it seems off-target. The permeability for normal use of a ferrite or magnetic material has nothing to do with the magnetization M and saturation flux. You use the magnetic material in its linear region, so you stay away from saturation (where mu drops), and you use "soft" materials that do not have much Br. Especially if this is for a radio application, you are going to stay away from saturation and use a soft magnetic ferrite material.

Does that help?
 
Ah, thanks for that. But now I am at a loss as to how I can determine the permeability : ( because I was relying on the magnetization being an indicator of high permeability.
 
  • #10
yeah that is true you should stay away from the saturation, this was what i said in that thread.

but, what is more, i am saying that the B/H ratio would change as the frequency is swept. after a certain frequency B/H ratio starts to fall down.

berkeman said:
Lordy, that's an involved thread. But from a quick read, it seems off-target. The permeability for normal use of a ferrite or magnetic material has nothing to do with the magnetization M and saturation flux. You use the magnetic material in its linear region, so you stay away from saturation (where mu drops), and you use "soft" materials that do not have much Br. Especially if this is for a radio application, you are going to stay away from saturation and use a soft magnetic ferrite material.

Does that help?
 
  • #11
dude_ said:
yeah that is true you should stay away from the saturation, this was what i said in that thread.

but, what is more, i am saying that the B/H ratio would change as the frequency is swept. after a certain frequency B/H ratio starts to fall down.

It's certainly not constant over all frequencies. You can look at the datasheets for various ferrite materials to see their effective frequency ranges. There are even special ferrites (like material 43, IIRC) that become lossy on purpose at RF frequencies. They are used for EMI suppression, like in the form of ferrite beads on data lines. You definitely would not use one of them for signal processing!
 
  • #12
but, what is more, i am saying that the B/H ratio would change as the frequency is swept. after a certain frequency B/H ratio starts to fall down.

I see what you mean from those hysteresis plots, it seems as if there are "two types of slopes", the one going up and then a less steep one when you are fighting the remenance. So I guess this means absolute permeability is actually a range between these values.
 
  • #13
no this has nothing to do with hysterysis.

you would like to stay away from hysterysis as berkeman has suggested.

there is a linear region of operation of ferrite materials, where no hysterysis is observed, and this is the low field regime.

but what differs from ferrite to ferrite is that this linear region's slope changes with frequency and with material properties.

SuccessTheory said:
I see what you mean from those hysteresis plots, it seems as if there are "two types of slopes", the one going up and then a less steep one when you are fighting the remenance. So I guess this means absolute permeability is actually a range between these values.
 
  • #14
Ok I missed the last sentence of your last post, but I understand now. Thanks.
 

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