(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

An infinitely long cylinder, of radius R, carries a frozen-in magnetisation, parallel to the z-axis, M=ks k-hat, where k is a constant and s is the distance from the axis. There is no free current anywhere. Find the magnetic field B inside and outside the cylinder by two different methods:

i) Locate all bound currents, and calculate the field they produce; and

ii) Use Ampere's law, the loop integral of H.dl=I(subscript enc), to find H, then get Bfrom H=(1/mu0)B-M

3. The attempt at a solution

I found the answer on http://www.nhn.ou.edu/~shaferry/41832005_files/finalsol.pdf

but what I don't understand is why far away from the loop the B field must go to zero. I would be grateful if someone could explain please.

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# Infinitely long cylinder - locate bound currents and calculate field

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