H- & B-Fields

[kmortis]

>>>WARNING GEEK ALERT<<<

As some of you know, I'm an EMI Engineer. We recently hired a new technician and he ask me the following question:
"Why is an "H-field" called an "H-Field"?"

Not "How is an H-Field created?", not "What's the difference between an E-Field and an H-Field?" not a question that I can easily answer.

Now, I do know that current is representd by "I" because Ampere origionally named it "Intensity"; which became confusing once artificial light became common place. And E-Field is obvious....H-Field? What about B-Field? Where do these terms come from? Anyone?

[AK-Dave]

http://en.wikipedia.org/wiki/Henries ?

[Brown]

I have no answer to the question. I simply want to express my joy in finding out that this topic was actually about what I thought it was going to be about!

[Dilb]

http://en.wikipedia.org/wiki/Henries ?

A different H. Henries are a unit of inductance (used in circuit theory), the H-field is the component of the magnetic field due to the free charge (as opposed to electron spin in a material, like what happens in magnets). It's also called the magnetic field, magnetic flux, or similar, but it's generally best to just call it the H-field, because the same names are used for the B-field.

My best guess is that M was used for magnetization, and since the H-field is directly calculated from current I, or the current density J, H seemed like a convenient letter to use.

As for the B field, I seem to recall that some verion of the lorentz force law in my highschool textbook used Beta instead of B. Maybe it relates to the deflection of beta particles?

[Cyphermage]

That reminds me of one of my favorite bits of electromagnetic trivia.

The Lorentz Transformation was named after Hendrik Antoon Lorentz, a famous Dutch physicist.

The Lorenz Gauge Condition is named after Ludwig Lorenz, a less famous Danish physicist.

People have the habit of seeing "Lorenz" and saying "Oh look, someone misspelled Lorentz" and fixing it, thus dooming poor
Lorenz to a life of invisibility.

Quite a few popular physics texts now say "Lorentz Gauge," so the problem is unlikely to be corrected anytime soon, and Lorentz continues to get credit for the work of Lorenz.

Strange but true.

[davefoc]

kmortis, could you talk a bit about the difference between an H field and a B field.

I understand that it one takes into account permeability and the other one doesn't, but I don't quite understand what this is telling me.

It seems like the H field is the field that would be there if all you know about is the charge motion that is causing the field.

But the B field takes into account the permeability of the substance between the current source and the point in space where the field is being measured. Is this right?

[Cyphermage]

kmortis, could you talk a bit about the difference between an H field and a B field.

I understand that it one takes into account permeability and the other one doesn't, but I don't quite understand what this is telling me.

It seems like the H field is the field that would be there if all you know about is the charge motion that is causing the field.

But the B field takes into account the permeability of the substance between the current source and the point in space where the field is being measured. Is this right?

E and B are the electric and magnetic fields in free space, and are the "real" fields, defined by the force they exert at a point on test electric and magnetic charges. D and H are the fields in matter, where we have incorporated the effects of matter being like a continuous distribution of electric or magnetic dipoles, which align themselves with E and B fields passing through the matter.

Specifically, in the magnetic case, a magnetic medium will respond to a B field by developing a magnetic moment per unit volume, due to a magnetization current density. The total B inside the material incorporates this magnetic moment. H is just a fudge to B so that the notion of Curl H being J, the current density, works in materials as well as in free space.

A similar argument applies to D. D is a fudge to E so that the notion of Div D being the charge density also works in materials as well as in free space.

[kmortis]

kmortis, could you talk a bit about the difference between an H field and a B field.

I understand that it one takes into account permeability and the other one doesn't, but I don't quite understand what this is telling me.

It seems like the H field is the field that would be there if all you know about is the charge motion that is causing the field.

But the B field takes into account the permeability of the substance between the current source and the point in space where the field is being measured. Is this right?
E and B are the electric and magnetic fields in free space, and are the "real" fields, defined by the force they exert at a point on test electric and magnetic charges. D and H are the fields in matter, where we have incorporated the effects of matter being like a continuous distribution of electric or magnetic dipoles, which align themselves with E and B fields passing through the matter.

Specifically, in the magnetic case, a magnetic medium will respond to a B field by developing a magnetic moment per unit volume, due to a magnetization current density. The total B inside the material incorporates this magnetic moment. H is just a fudge to B so that the notion of Curl H being J, the current density, works in materials as well as in free space.

A similar argument applies to D. D is a fudge to E so that the notion of Div D being the charge density also works in materials as well as in free space.
Um...what he said. That'll teach me to ask a question on friday afternoon then not get back to it 'til monday morning.
A different H. Henries are a unit of inductance (used in circuit theory), the H-field is the component of the magnetic field due to the free charge (as opposed to electron spin in a material, like what happens in magnets). It's also called the magnetic field, magnetic flux, or similar, but it's generally best to just call it the H-field, because the same names are used for the B-field.

My best guess is that M was used for magnetization, and since the H-field is directly calculated from current I, or the current density J, H seemed like a convenient letter to use.

As for the B field, I seem to recall that some verion of the lorentz force law in my highschool textbook used Beta instead of B. Maybe it relates to the deflection of beta particles?

That could be. I was just curious if there was any more significance behind the choice. I know that in the olden days they would choose a letter to represent a descriptive term for the phenomenon. The only thing that came to my mind was "hysteresis", but I couldn't figure out why Gauss or Maxwell (or whomever gave the H-Field the name "H-Field") would call it that.

[Cyphermage]

Um...what he said. That'll teach me to ask a question on friday afternoon then not get back to it 'til monday morning.

That could be. I was just curious if there was any more significance behind the choice. I know that in the olden days they would choose a letter to represent a descriptive term for the phenomenon. The only thing that came to my mind was "hysteresis", but I couldn't figure out why Gauss or Maxwell (or whomever gave the H-Field the name "H-Field") would call it that.

In 1850, Sir William Thomson (Lord Kelvin), introduced the notion of magnetic permeability and susceptibility, and the letter H.

This pre-dates Maxwell's creation of the Maxwell Equations in 1864.

Thomson's papers aren't available for free on the Web, so I can't see if he gives a reason for picking H.

If not, it's just one of those things.