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Permeability with static and dynamic H fields

  1. Jul 16, 2008 #1
    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?
     
  2. jcsd
  3. Jul 16, 2008 #2
    no it doesnt : )

    it depends on the material properties.
     
  4. Jul 16, 2008 #3
    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.
     
  5. Jul 16, 2008 #4

    berkeman

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    That is not correct. In general, the permeability is fairly constant over frequency, until eddy current effects start to become noticable.
     
  6. Jul 16, 2008 #5

    berkeman

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    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
     
  7. Jul 16, 2008 #6
  8. Jul 16, 2008 #7
    Thanks for your reply.
     
  9. Jul 16, 2008 #8

    berkeman

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    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?
     
  10. Jul 16, 2008 #9
    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.
     
  11. Jul 16, 2008 #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.

     
  12. Jul 16, 2008 #11

    berkeman

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    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!
     
  13. Jul 16, 2008 #12
    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. Jul 16, 2008 #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.

     
  15. Jul 16, 2008 #14
    Ok I missed the last sentence of your last post, but I understand now. Thanks.
     
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