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B and H

  1. Sep 24, 2007 #1
    Hello all,

    I have recently taken up the study of electromagnetism because I found that I lacked severely in this area. This became apparent when I attempted to build a real motor from scratch. Sure I've built motors before, just the old high school coil suspended on two paper clips hooked to a battery etc.

    This time, I tried to build a real motor which could do real work. So I figure I know the basic function, as far as how it is supposed to work, attraction and repulsion. When it came to actually making it work... nothing. I got enough torque to set the poles of the armature parallel to the field poles, then no more. I did eventually, by trial and error, get it to function. And it now powers a small scroll saw (which I also built), but mostly for novelty purposes.

    With these failures I figure I must not know what I think I know and I start studying some old books (and I mean old, Hawkins Electrical Guide, 1914). Now these books really have alot of info.

    My question, right now is this:

    What is the difference between the B and H fields? Because their ratio is dimensionless, doesn't that mean they have the same units, so why are they described differently in my texts?

  2. jcsd
  3. Sep 24, 2007 #2
    Their ratio is certainly not dimensionless, as they're related by a permeability.
  4. Sep 24, 2007 #3


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  5. Sep 24, 2007 #4
    "The ratio of the number of lines B passing through a unit cross section of a solenoid core to the number of lines H passing through a unit cross section of the same solenoid with an air core"

    Is more or less how the Hawkins guide states it. And with the help from the "hyperphysics" link I think I see it clearly now>

    Thank you all.
  6. Sep 24, 2007 #5

    Claude Bile

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    You can think of H as the portion of the total B field that is solely due to free currents.

  7. Sep 24, 2007 #6
    Oh, I get it! Why can't they just say that? I think I lost myself between the definitions of "magnetization", "magnetizing force", "magne-whatever".

    So, then, the "magnetizing force" H is just the externally generated field? And the "magnetic induction" B is the total (H + internal) fields?
  8. Sep 25, 2007 #7
    Actually, they do say that, but not in the type of books you are reading. If you want to progress beyond building simple prototype motors, then I suggest you read Griffth's Introduction to Electrodynamics in order to have an in-depth understanding of the fundamentals. If that seems too mathy for you, try Purcell's Electromagnetism.

  9. Sep 25, 2007 #8

    Claude Bile

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    Yes, H is the total B-field with the internal response of the material removed, B is the total field with the response of the material included.

  10. Sep 26, 2007 #9
    I do have a modern text, I just wanted to start at the beginning and watch it evolve into what it is today. Seems like the guys who figured out the key characteristics of electromagnetism had a pretty firm grasp on how it worked, even if they had no idea of the quantum nature of matter. I have been studying these old texts by Steinmetz, Heaviside, the Hawkins Guide, and several dynamo and generator books from the turn of the 20th Century. I want to see it as they seen and calculated it, even in the old CGS electromagnetic units first, then move on to how we see it today.

    Just judging from a brief review of the two modern books I have, the classical view of the field seems unchanged from the old texts, except in the units used. Looking at it now from what I have learned just from the question I posed here, the descriptions of the B and H fields in the old texts make sense now, I guess it was just my lack of understanding how they worded it.

    I don't really see this as a bad approach to the subject. I think it will be quite enlightening
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