First off, hi, I'm new. Sorry if this is not the right board; I was unsure between Classical Physics, maybe Quantum since this involves fields, or Electrical Engineering. On to the question, say you have an electromagnet with an iron core. As I understand it, a magnetic field acts on a ferromagnetic material by aligning the magnetic spin of the latter's atoms. If you were to switch the electromagnets' polarity back and forth, can I assume this magnetic alignment propagates through the core instantly (or at the speed of light would more correct, I suppose), or are there any dynamic equations for this kind of thing? And if it is an "instantaneous" effect, is there anything limiting the frequency I can switch the polarity with, or can I go as fast I want and assume the iron can take it?