What is the difference between B and H?

[Heisenberg7]

A week ago, I started studying electromagnetism. I was introduced to a few new concepts and one of them was H. Now, in my book, they defined H as just magnetic field strength and B as magnetic induction. The thing is, I don't understand what those terms really are (in a physical way), let alone the difference between them. I know how they are connected and I've read at least a dozen definitions and watched a few videos but they always somehow manage to go past the explanation.

 

[berkeman]

Welcome to PF.

You will get other better answers, but for me the easiest way to think about it is in terms of the magnetic permeability $\mu$

https://en.wikipedia.org/wiki/Permeability_(electromagnetism)

 

[Gavran]

The magnetic field strength $\vec H$ (the amount of magnetizing force) is a fundamental magnetic field. Electric current $I$ generates the magnetic field strength $\vec H$ and the magnetic flux density $\vec B$ (the amount of magnetic force) is a response of the medium magnetized by $\vec H$.
The magnetic flux density is induced due to the magnetic field strength.

 

[martinbn]

Now, in my book, they defined H as just magnetic field strength and B as magnetic induction.

What are the definitions they gave? Can you quote or tell us which book and page?

 

[Paul Colby]

The same question may be asked about the $D$ and $E$ which are related by the permittivity, $\epsilon$, by $D=\epsilon E$. The E field is volts per meter, the local rate of change of potential while $D$ is the charge per square meter. The permittivity, like $\mu$, depends on the material and are often considered constant for many situations. Maxwells equations take a simple form in terms of E, B, D and H and make dealing with materials simpler.

 

[Heisenberg7]

What are the definitions they gave? Can you quote or tell us which book and page?

It's actually a local book. I'm from Bosnia and Herzegovina so it wouldn't be of much help if I told you which book it is. For that reason, I'm just going to quote the book: B : "The quantitative measure with which we describe a magnetic field is the magnetic induction B."; H : "Strength of a magnetic field, unlike the magnetic induction, does not depend on the characteristics of the medium in which we have the magnetic field. Magnetic induction B and strength of the magnetic field are connected in this equation: $B = \mu H$". Now this last definition is quite weird to me because it later on goes to say that for some materials $\mu = \mu_r\mu_o$ which means that it does depend on the medium in which we have the magnetic field (we use $\mu_r$ when we are not talking about vacuum so it doesn't make sense for us to say that it does not depend on the medium when it clearly does by this definition).

 

[DaveE]

It may be easiest to understand by duality (analogous) with electric circuits. Roughly speaking H is similar to an E field and the flux Φ is similar to current. See the links below for the real treatment. This is a great approach to modelling or designing complex magnetic circuits, like transformer cores.


https://en.wikipedia.org/wiki/Magnetic_circuit

https://ocw.mit.edu/courses/6-061-i...0996b1a78e25504a32819cb6f_MIT6_061S11_ch6.pdf

 

[DaveE]

for some materials μ=μrμo

I would prefer to say that $\mu = \mu_r \mu_o$ is always true. $\mu_o$ is the permeability of free (empty) space and is always present. Some materials add their own factor, relative permeability $\mu_r$, which is dimensionless, like gain. This is really just a convenience in design so we don't always have to write out $4 \pi \cdot 10^{-7} \frac{H}{m}$, which is ever present. Of course the "material" of free space has $\mu_r = 1$.

 

[Heisenberg7]

I would prefer to say that $\mu = \mu_r \mu_o$ is always true. $\mu_o$ is the permeability of free (empty) space and is always present. Some materials add their own factor, relative permeability $\mu_r$, which is dimensionless, like gain. This is really just a convenience in design so we don't always have to write out $4 \pi \cdot 10^{-7} \frac{H}{m}$, which is ever present. Of course the "material" of free space has $\mu_r = 1$.

I agree. I did kinda word it in a wrong way.

 

[Vanadium 50]

The flip answer is "the difference between B and H is 4πM".

The seed of truth in that is that the thing that appears in the Lorentz force, B, comes from the magnetic field H and in the presence of materials, a magnetization (4πM).

 

[WernerQH]

[...] to quote the book: B : "The quantitative measure with which we describe a magnetic field is the magnetic induction B."; H : "Strength of a magnetic field, unlike the magnetic induction, does not depend on the characteristics of the medium in which we have the magnetic field. Magnetic induction B and strength of the magnetic field are connected in this equation: $B = \mu H$".
Now this last definition is quite weird to me because it later on goes to say that for some materials $\mu = \mu_r\mu_o$ which means that it does depend on the medium in which we have the magnetic field (we use $\mu_r$ when we are not talking about vacuum so it doesn't make sense for us to say that it does not depend on the medium when it clearly does by this definition).

You are right, the definitions are contradictory. In general, B and H are not simply proportional to each other. Increasing the current in a solenoid will not proportionally increase the magnetic induction (B) in the iron core after it has reached saturation (when all elementary magnets are aligned).

In homogeneous media there can be an approximate proportionality, similar to the relation between stress and strain in a solid, or between pressure and density in a gas. But they are independent, "conjugate" quantities. The product has the dimension of energy density. In the case of a weakly magnetized medium you can express the energy density as $\frac 12 B \cdot H$. But for an iron core in a solenoid the relation between B and H is much more complicated. What is traditionally called magnetic field (H) is the field due to the external current only, not including the microscopic currents due to the magnetization M of the medium. As @Vanadium 50 has indicated, the magnetic induction B, i.e. what really exerts forces on the electrons, can be written as $$ {1 \over \mu_0} B = H + M \ , $$ (using SI units), and for a static magnetic field you have therefore $$ \nabla \times H = j \qquad \text{or} \qquad {1 \over \mu_0} \nabla \times B = j + \nabla \times M \ . $$ And remember that the magnetization M is not always strictly proportional to what is causing it. (B or H?)

 

[dMohler]

Simply put, consider an iron bar inserted into a coil of wire and a current in the coil. The magnetic flux induced in the iron will have a flux density, B=phi/A, phi being webers of flux and A being the area of the bar. Note that the source current times the number of coil turns gives ampere-turns (F), the electromagnetic potential. Then the flux level times the magnetic Reluctance of the iron bar gives its ampere-turns drop, F=phi x R. From this H=F x R, where H is expressed in ampere-turns/meter. The HARD PART is that the reluctance is determined by BH data for the iron in an iterative computer routine or by calculus if by hand calculations. These fundamentals allow the calculation of magnetic devices to determine flux levels because the summation of all the ampere-turns drops equals that of the coil ampere-turns. Ultimately the flux level of the device (a series circuit) determines the energy in say a motor air gap, to determine its force or torque.

 

[Meir Achuz]

The confusion is due to SI units where B and H do have a complicated connection. In Gaussian units, H is just B-4\pi M, where M is magnetic polarization of matter.

 

[weirdoguy]

The confusion is due to SI units where B and H do have a complicated connection

Is this a joke?

 

[Meir Achuz]

Is this a joke?

What a weird response