See it can be viewed from derivations of the magnetic fields in materials u mentioned which can be found in standard text book or else it can be viewed practically in labs ( Not school labs :) )
As far as the theoretical explanation ,
I m not sure if there is one which we can understand !
So this time i m out of source !
Will tell u later sometime if i get it .... U also try searching for it in ur free time !
Good topic for research na ! :)
U can view this graph for an instance
The magnetic field distribution in and around a solid conductor of a nonmagnetic material carrying direct current.