# Thread: Ferromagnetic Plate B-Field Definition Question

1. Hello everyone. I am new to this forum and this is my first post. I am soon going to be starting a PhD at Exeter University later this year and my supervisor sent me a few questions relating to relevant work to make sure my learning is on the right tracks.

Basically I don't have much magnetism knowledge. I am currently learning but am having a few problems finding reliable sources on key topics, mainly "Demagnetizing Fields", "Exchange Interactions" and "Magnetic Dipolar Fields" (their interactions with one another and how they are inside objects of different shapes).

Anyway, here is the question I am struggling with:

Q: What is the B field inside of the infinitely thin and infinitely long ferromagnetic plate saturated perpendicular to the surface of the plane?

He said that it is more of a hypothetical definition than a direct answer if this helps.

So far I understand that there are no domains, all moments point in the same direction (i.e. saturated perpendicular to surface of the plane). Also, that the demagnetizing field affects only the magnetization perpendicular to the plane (not those parallel to the 2 planes the plate extends to, of which there is no magnetic field anyway).

Any help would be greatly appreciated. It doesn't even have to answer the question, reliable sources of information may be much more useful than this

Many thanks

2.

3. Ok it seems this one is too hard. My Professor did say it was a hard question. He then gave me this one to try instead but I'm also struggling with it. Any help please?

Q: A magnetic moment, M = (0, 0, Mz) is placed in the origin of the coordinate system (x, y, z). What is the B-field produced by this moment at the point R1 = (Rx, 0, 0)?

Apparently the equation I need is: H(r) =
1 x ( 3r (m . r) - m ) = B
4π r^5 r^3 μ0

My problem is I don't know what to input for values of m and r (magnetic moment and radius from origin, respectively). Is a matrix needed? If so how is this done as I as I have never studied matrices. An online guide would be very helpful if matrices are needed

Any help much appreciated!

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