1
$\begingroup$

While explaining magnets to some students, I just realized a very basic thing I never thought about: We often say that magnetic fields have a "direction", that magnetic field lines "exit" the north pole of the magnet and go back around to "enter" (oversimplified, I know) the south pole. But is there anything that makes magnetic north and south actually different, other than being opposites? In electric current, the electrons are actually moving in one, specific direction, but I have no idea why we keep taking the north's side in this? Is there a difference between how magnetic north and south work, or is the special status of north just completely arbitrary?

$\endgroup$
6
  • $\begingroup$ You seem to imply that the direction of electric current is not arbitrary since "the electrons are actually moving in one, specific direction" and the direction of the current is defined relative to that. In that case I would argue that the direction of the magnetic field is not arbitrary either since it is defined relative to the direction of electric current through Maxwell's equations. $\endgroup$
    – jkej
    Apr 15, 2016 at 23:32
  • $\begingroup$ Relative, yes (actually, defined by). But in a wire, something is moving in one direction only; there is nothing moving in the opposite direction of the electrons (the idea that the charge moves opposite the electrons is, AFAIK, an abstract/logical construct). If I understand correctly, nothing is really moving in the north to south direction in a magnet, nor south to north. The two poles are identical opposites, nothing more. But I felt unsure in that assumption, and other sources seemed to dance around the issue a bit. $\endgroup$ Apr 15, 2016 at 23:37
  • $\begingroup$ No, nothing is (necessarily) moving along the magnetic field lines. But there is a westward net electric current in Earth's outer core that causes the magnetic fields and if it was in the opposite direction we would draw the field lines in the opposite direction. Of course, which current direction is paired with which magnetic field direction is an arbitrary convention, but so is the convention of which electron movement direction is paired with which current direction in your wire (i.e. "an abstract/logical construct"). $\endgroup$
    – jkej
    Apr 16, 2016 at 0:09
  • $\begingroup$ I think we're actually agreeing. Electrons go in a concrete direction (kind of) and claiming they go in the other direction is inherently false, while magnetic field lines can be considered to go north-south or south-north without any real consequence for their theoretical basis. You're just a bit stronger on the details than I am :) $\endgroup$ Apr 16, 2016 at 0:29
  • $\begingroup$ "Electrons moving in one direction only" is only one part of how we define current direction. For a current flowing through an ionic fluid, electrons and positive ions are moving, in opposite directions; either one on its own causes a current. And if we used the same directional conventions but had originally defined electrons as positively charged and protons as negatively charged, rather than vice versa as we have, then our "directions" would be backwards from what they are now as well. $\endgroup$
    – Asher
    Apr 16, 2016 at 1:48

1 Answer 1

3
$\begingroup$

It is arbitrary. Just as the positive/negative convention for electrons is arbitrary.

The Earth's magnetic pole near the northern geographic pole is of course a south pole - because it attracts the north pole of a magnetic compass needle.

In the past the Earth's magnetic field has reversed, so if compasses had existed then, they would have pointed in the opposite direction.

Apart from this, electric currents are not always the result of electrons moving, the charge carriers can be positive ions.

$\endgroup$
3
  • $\begingroup$ I think you fully answered the question, but I wanted to be sure: You're not just talking about the naming convention, right? There is literally no difference between magnetic north and south, and no reason for bias when teahing it? They are just "identical opposites"? $\endgroup$ Apr 15, 2016 at 23:17
  • $\begingroup$ @Henry: See physics.stackexchange.com/q/10032/12613 $\endgroup$ Apr 15, 2016 at 23:18
  • $\begingroup$ Thank you. The link content was phrased a bit different from my understanding, thank you for clearing it up! $\endgroup$ Apr 15, 2016 at 23:29

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.