Often students are told that so-and-so is the case but not really why. For experienced physicists, not a problem, but for some newcomers just knowing that something is the case sometimes leaves us hanging. I realize that sometimes the explanation may be too complicated. But here is one re: electromagetism and mmf that I think I have puzzled out for myself. Maybe self-evident, but for a studnet I still think it needs be said.

In Chapter 14 we are told "the amount of magnetic field force generated by a coiled wire is proportional to the current through the wire multiplied by the number of "turns" or "wraps" of the wire in the coil." Great -- but why? How?

My take: the very small mmf generated by current through a straight wire over X distance (say 10 feet) is squeezed into a much shorter length when coiled, say to just 1 foot. The magnet field of the 10-ft wire is now piggy-backed on itself. The total mmf is the same, but it's now a flux stretching just over a 1-foot in length, so all that mmf of the ten feet (but coiled up) is having a bigger impact.

One thing I still can't understand--why is there no N and S pole in the single wire? When it moves the compass, does it move it randomly, first one way, then the other? It seems to me if there is a N/S in the coil, there has got to be a N/S in the wire. My guess would be it's too weak to be measured...but then, again, how just does it move the compass, and if it doesn't move it in a random pattern, it's got to have the N/S.

So, anyone out there who can comment on this?