# Electromagnetic Field, H vs. B

I have a problem understanding the meaning of magnetic field strength, denotet H.

I have the following example from a textbook:

1. The path of magnetic field strength developed in a conductor loop is given by the formula H = (I*N*R^2) / (2*sqrt(R^2+x^2)^3)
I = current
N=number of turns
x=distance from the center.

Now, my textbook says "If a soft iron core is inserted into a coil-all other things remaining equal-then the force acting on a magnetic needle will increase. I*N remains constant and therefore so does the field strength. However, the flux density "B" -the total number of flux lines-which is decisive for the force generated has increased."

So if the magnetic force acting on a magnetic needle increases, why does not the field strength increase. And what exactly is the difference between H and B. Does anyone know any good web sites, tutorials etc on electromagnetism, inductive coupling.

t.

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Three good references are:
1. Intro to Electromagnetic Compatibility by Clayton Paul
2. Noise Reduction Techniques in Electronic Systems by Henry Ott
3. Electromagnetic Compatibility Handbook by Kenneth Kaiser

The first two are required reading for EMC/EMI engineers (or should be!). The third is a 2600 page book that instructs through examples. I would start with the first two. Additionally you could look at college level texts such as Electromagnetics by Kraus or Applied Electromagnetics by Shen and Kong (if it is still in print.

amt
Gosh it's been so long since I studie EM in college.
Difference between H and B is that, one is the Electric field and the other is the magnetic field. They always accompany each other perpendicular to each other.

I am sure someone here will be able to post something more detailed. Until try Google.

Doc Al
Mentor
amt said:
Difference between H and B is that, one is the Electric field and the other is the magnetic field. They always accompany each other perpendicular to each other.
Not so. Both B and H are vectors used in describing magnetism. B is the fundamental magnetic field, sometimes called "magnetic induction" to distinguish it from H; H, often called the "magnetic field strength", is useful in discussing magnetism in the presence of magnetic materials.

Doc Al
Mentor
temujin said:
So if the magnetic force acting on a magnetic needle increases, why does not the field strength increase. And what exactly is the difference between H and B.
The names of the magnetic vectors aren't particulary helpful. The magnetic field (B) increases, but the magnetic field "strength" (just the name attached to H) does not. H only depends on the "free" current (I in the coil).

You may find this helpful: http://hyperphysics.phy-astr.gsu.edu/hbase/solids/magpr.html

amt
Yes- I apologize for my mistake. I just referred to some text.
H=B/u, where 'u' is the permiability constant.

The Electric Field should have been 'E'.

Doc Al said:
The names of the magnetic vectors aren't particulary helpful. The magnetic field (B) increases, but the magnetic field "strength" (just the name attached to H) does not. H only depends on the "free" current (I in the coil).

You may find this helpful: http://hyperphysics.phy-astr.gsu.edu/hbase/solids/magpr.html

I agree with Doc Al. It took me a little while to get it, but there is a very important difference between B and H. Like Doc Al said, B is the magnetic field that is independent of source. There are two main sources of magnetism. The first comes from the material in the form of para/diamagnetism or ferromagnetism. The second comes from free current passing through the material. To find H, B is separated into the two parts, coming from free current and "bound" current. As Doc Al said, H is the name given to the free current part. H is used often by experimentalists because when they want to control the amount of magnetic field, they cannot easily change the magnetism inherent inside the material they're using. Instead, they change the amount of current passing through the material. So effectively, they are only changing H. Your example, however, is an example of them "changing the material" by filling the air with a piece of iron, changing the system's inherent magnetism.

Hope that helps Last edited: