1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Magnetic reluctance

  1. Jul 26, 2010 #1

    htg

    User Avatar

    Is the concept of magnetic reluctance correct?
    If so, then the flux of magnetic induction in a toroidal solenoid, whose half length of core has relative permeability = 10, and the other half has relative permeability = 1000 or 10000 will be almost the same in both cases. Has anyone verified it experimentally?
     
  2. jcsd
  3. Jul 27, 2010 #2

    marcusl

    User Avatar
    Science Advisor
    Gold Member

    This is basically correct. The principal is used all the time in ferrite pot cores--see the picture here http://www.mag-inc.com/products/ferrite-cores/ferrite-pot-cores" [Broken]
    The coil bobbin fits over the post, and a second identical core fits over the first until the mating surfaces touch. Ferrites have permeabilities of thousands, but the overall reluctance can vary over an order of magnitude depending on the flatness and cleanliness of the mating surfaces, how much pressure is applied, etc. This is obviously untenable for an electronic circuit.

    To fix this, the posts are ground a little short so as to leave a precision air gap of a some thousandths of an inch. Since the gap has permeability one, the overall reluctance is set by the gap thickness independent of the core permeability or quality of mating surfaces. This gives highly accurate and reproducible performance.
     
    Last edited by a moderator: May 4, 2017
  4. Jul 27, 2010 #3

    htg

    User Avatar

    I have serious doubts if the concept of magnetic reluctance is correct. Since magnetization is caused by orientation of micro domains, I propose to talk about:
    1) H = Hfree, the intensity of magnetic field due to free currents (in conductors wound around a core)
    2) Hbound, the intensity of magnetic field due to bound currents (due to magnetization)
    3) Htotal = Hfree + Hbound
    4) B = MuZero * Htotal
    Such a conceptualization leads to a different picture which also enables one to talk about Hbound of a permanent magnet (something beyond the reach of the generally used conceptualization of description of magnetic fields).
    ALSO, consider a horseshoe electromagnet with a ferromagnetic core. It seems clear to me that magnetization acts like additional ampere-turns, so B in the air gap between the poles should be very significantly different in the case of permeability of the core = 1000 vs 10 000.
     
    Last edited: Jul 27, 2010
  5. Jul 27, 2010 #4
    See equation (7) and derivation in thumbnail. For a toroid of radius R, if half the toroid is an air gap, then G = πR. The denominator of Eq (7) is the reluctance.

    Bob S
     
  6. Jul 27, 2010 #5

    htg

    User Avatar

    I do not know what thumbnail you are referring to.
    What about B in the air gap of a horseshoe electromagnet mentioned above?
     
  7. Jul 27, 2010 #6

    marcusl

    User Avatar
    Science Advisor
    Gold Member

    The difficulty you are having in conceptualizing permanent magnets is not a failure of classical E&M theory, which treats magnetic phenomena quite successfully.
    Clear or not, magnetization does not "act like additional ampere-turns." Suggest you study a little further. We at PF can recommend some texts that you may find useful.

    B in the gap will be nearly identical in both cases.
     
  8. Jul 27, 2010 #7

    htg

    User Avatar

    If magnetization does not act like additional ampere-turns, then the widely known theory of magnetization by orientation of magnetic domains must be false.
     
  9. Jul 27, 2010 #8
    Here is the post again, with thumbnail.

    See equation (7) and derivation in thumbnail. For a toroid of radius R, if half the toroid is an air gap, then G = πR. The denominator of Eq (7) is the reluctance. Note that for very high permeability, the magnetic field in the air gap is independent of the permeability. This effect is very well known, and included in electromagnet design.

    Bob S
     

    Attached Files:

  10. Jul 28, 2010 #9

    htg

    User Avatar

    At least for a small gap, equations 4 and 5 contradict the Gauss' law.
     
  11. Jul 28, 2010 #10

    htg

    User Avatar

    I want to consider H significantly below the saturation field intensity. Will the B in the air gap of a horseshoe electromagnet, whose core permeability is 1000 or 10 000 be nearly the same in both cases?
     
  12. Jul 28, 2010 #11
    Div B = 0
    B longitudinal at gap is continuous
    H longitudinal is not continuous

    Bob S
     
  13. Jul 28, 2010 #12

    htg

    User Avatar

    When you look at the microscopic mechanisms of magnetization, it is clear that both H and B have to be continuous. What you say seems to be generally accepted, but it is pretty clear that it is not true.
     
  14. Jul 29, 2010 #13

    marcusl

    User Avatar
    Science Advisor
    Gold Member

    Ah, what a dilemma. You are certain of yourself, but what to do about all mainstream physicists and Nobel laureates of the past 150 years who must be wrong?
     
    Last edited: Jul 29, 2010
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Magnetic reluctance
Loading...