# Figuring out North and South Magnetic Poles for Earth

I currently make my living as an electrophysicist, so am a bit embarassed at having not thought this one through, before.

Magnetically, the North pole is.... North. We get out our compass, and wait for the North-seeking needle to line up. I've always thought of this end of the needle as the north end.

In a current carrying loop, if we use the right-hand rule and point our thumb in the direction of conventional current, the fingers show the direction of the mag field lines around the wire. The direction the mag field lines point "north" when they pass through the interior of the loop. By stacking many such loops we can create a cylindrical electromagnet, for which it is easy to see which end is the "north" end. I resume permanent physical magnets follow the same naming convention: that is, mag field lines emanate from the north end and cycle around to the south.

We know that like poles of a magnet repel and opposites attract.

So, now, hanging a magnet from a string (or going back and using our trusty compass), if the north pole of the magnet points towards teh magnetic north pole of Earth, doesn't that mean that the pole near the Arctic circle is really magnetically-speaking the south pole? Or conversely, when we say "north-seeking" pole of a magnet, is that really the south pole of the magnet?

Which is it?

-

You are correct in saying that the pole near the Arctic circle is really the south pole of the Earth's magnetic field because the north pole of a bar-magnet points towards it. (Wiki)

That said, it is purely convention what you define on a bar magnet to be north and south - we've named the side that points towards the Arctic circle to be the north pole of the bar magnet, and since people don't immediately identify this seeking behavior as opposite poles attracting, we tend to think the earth's field at the Arctic circle is also a north pole when it is strictly speaking, a south magnetic pole.

Also, I would add that the magnetic poles don't coincide with the geographic poles - the latter define the points of intersection of the Earth's rotational axis with the surface of the Earth. We know for a fact that the magnetic poles have shifted (even reversed) with respect to the geographic poles during the history of the Earth. (Wiki)

-
Being a student pilot, this stuff is drilled in. Not only is the magnetic north pole over northern Canada (nowhere near the pole), but the lines of force descend at an angle, causing the magnetic compass to do funny things when the plane is turning, accelerating, or decelerating. –  Mike Dunlavey Dec 27 '11 at 18:35

In the context of magnets, "north" actually means "north-seeking," because the names were chosen before people knew much about magnetism. So yes, the magnetic pole of the Earth which is geographically north does in fact have "south" polarity (it's a sink of magnetic field lines). Wikipedia explains this in some detail.

-
This is the comment that I found most succintly answered my question, but found anna's and dbrane's answers also very helpful. The phrase, "it's a sink of magnetic lines," flipped the switch for me. I would have given all three of you a "vote up" but can't seem to get signed in. Maybe someone can add an extra vote to each of you for me. Thank you! –  Vintage May 19 '11 at 18:21
@Vintage: odd, the fact that you are able to post a comment seems to indicate that you are signed in. –  David Z May 19 '11 at 18:23
Agreed, David; but I can, indeed post , answer and comment, without being able to "vote up." When I try to do the last, I get a message saying, in essence, "sign in, please." –  Vintage May 23 '11 at 16:39
@Vintage: I'd suggest posting a bug report (a question tagged bug) on meta.stackoverflow.com. Make sure to include the exact text of the error message you're getting and describe the problem in as much detail as possible. –  David Z May 23 '11 at 17:56

Yes, north was defined as a geographic location long before magnets were studied in detail. Face the sun rise in the north hemisphere on the 21st of March, your left hand points north, your right south.

So it is the convention: a north pole is the pole of a magnet pointing to geographic north, and and ditto for south. It was the geography that interested the first compass users, not the physics. So magnetically earth's geographic north pole is a magnetic south pole :), as you pointed out.

-

The problem is that the terms "north pole" and "south pole" (and even "magnetic north" and "magnetic south") refer not to any magnetic polarities, per se, but to locations. I'm sure that you are aware that there is a good deal of evidence that the magnetic polarity of the Earth reverses itself every now and then. When it does that, the "north pole" doesn't change its name. North, as a cardinal direction, would still be toward the Arctic.

Magnetically speaking, though, you answered your own question. Indeed, the Earth's north pole is south-seeking. This question has been asked before.

-

just a foot note, most folks aren't aware that there is only one type of magnetic line that namely being a north line. there is no "south" magnetic line. magnetic lines are only emitted from the north pole of a magnet thus all magnetic lines are north lines.

-