# What is the difference between the B and H fields?

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?

Thanks.

## Answers and Replies

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Their ratio is certainly not dimensionless, as they're related by a permeability.

Their ratio is certainly not dimensionless, as they're related by a permeability.
"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.

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

Claude.

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

Claude.
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?

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.

Molu

Claude Bile
Science Advisor
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?
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.

Claude.

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.

Molu
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