# B and H

1. Nov 8, 2003

### Beer-monster

I'm currently writing a lab report on magnetic susceptibilty yet the notation most of theory behind the behaviour confuses me. What exactly (and simply) is the differences between the magnetic field B and the field H.

Thanks for any help as I can't really continue the report without being any clearer on this matter.

2. Nov 8, 2003

### futz

H=(1/&mu;0)B-M

B is magnetic field, M is magnetization.

M=&chi;m(H) (for linear media)

As well, B=&mu;0(H+M)=&mu;0(1+&chi;m)H=&mu;H

&mu; is the permeability of the material, and of course &chi;m is the magnetic susceptibility.

3. Nov 8, 2003

### Beer-monster

Um...what would that be in words. I have the equations but I have to define the symbols logically, but different texts have differents tersm for B and H, and both are often reffered to as the magnetic field strength. However I measured the strength B and need to use the equation H=B/[mu]0 to find H, but should say what the difference is between the values, and why I need to use one rather than the other. Unfortunately I have no idea what the difference is, and why i use one type of field in one part and the other in another. Its confusing.

4. Nov 8, 2003

### futz

You're right, it is a confusing topic. Some books use B as the magnetic field, others H. Basically, think of B as the magnetic field, like you are used to. H is introduced mostly as a convenient way to rearrange Maxwell's eqs. Specifically, it allows you to write the eqs in terms of something called the free current (as opposed to bound current). Free current is the current that you would actually put into the sample, and have complete control over. It is therefore often easier to calculate H for a particular material than B, which can depend on properties of the material that cannot be controlled.

The name for H itself varies, but many call it the auxiliary magnetic field.

BTW, H=B/&mu;, where &mu;=&mu;0(1+&chi;m)

5. Nov 8, 2003

Thanks