Ferromagnetism in an iron disc

1. Mar 26, 2015

samjohnny

1. The problem statement, all variables and given/known data

Attached.

2. Relevant equations

3. The attempt at a solution

For the first part I determined the net magnetic dipole moment of the disc by calculating the number of moles the iron the disc comprises of and hence the number of atoms. Then, by making the assumption that 30% of the magnetic domains, the ones that aren’t aligned, are instead distributed randomly such that the sum of their vector moments yields zero. Then, the net dipole moment is given by taking 70% of the maximum possible dipole moment which is the number of atoms multiplied by the dipole moment of an individual iron atom.
However, for the next part I’m not too sure on how to go about calculating the magnetic field due to the disc’s overall dipole moment. The hint says to consider the disc as being a loop of wire, although it’s not clear to me whether that is for the part that I’m having difficulty with or the subsequent parts on the topic of the wire's current.

Can anyone kindly provide some assistance?

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2. Mar 27, 2015

samjohnny

anyone?

3. Mar 27, 2015

rude man

OK. You have the dipole moment of the disc. Call it μ (it's a vector).
now, tale a 1-turn loop of wire of the same radius as your disc. If it carries a current I, what is μ for this loop? Obviously, make I such that the two μ's are the same.

Now you have a simple problem of computing the axial B field 10 cam away from the loop's center.

4. Mar 30, 2015

samjohnny

Thank you very much for the help, I've managed to get the answer.