Earth's north pole and bar magnets

OK, I searched the archives and Google - found lots of explanations with iron fillings and lines of force etc., but none that answered what must be a remedial physics question. I’m reasonably knowledgeable about physics but this question almost seems one of history or naming conventions rather than physics.

Here goes:

If the Earth’s north magnetic pole is a “north” pole, then why does the “north” end of a bar magnet (or compass etc.) point towards it?? Surely it is the “South” end of magnets that should be attracted to the Earth’s north pole?

My best guess is, we just conventionally label the end of the magnet that points north “N”, when in fact it is a South magnetic pole. Is the designation of what is the “North” and “South” magnetic pole of the Earth purely arbitrary?

I guess a related question is, is there any intrinsic asymmetry between “North” and “South” magnetic poles? Or would everything work just as well if you switched the two? (whatever that means) I think I’ve read somewhere about the concept of “charge symmetry” in particle physics? Now I’m thinking about antimatter, where protons and electrons (along with other charged particles) have the “wrong” charge.

Hmmm one last thing - for some reason in my mind, magnetic “north” is associated with positive electrostatic charge, and magnetic “south” with “negative”. Is there any logic behind in this? I believe that magnetism is caused by alignment of electrons/atoms at the atomic level anyway?

OK…flame away :wink: If anyone can answer any of these questions I’d be grateful (and enlightened) - even point me to a “physics for dummies” website that explains them. I’ve looked at various physics FAQs to no avail.

Actually, I believe that Earth’s ‘north’ magnetic pole is, in fact, the ‘south’ pole of the earthly magnet.

From here.

I think you may be confusing magnetic fields with electric fields. The confusion probably comes from the similarity of the diagrams for magnetic and electric fields. When drawing an electric field, the convention is for the field lines to radiate from the positive charge and to the negative charge. In a magnetic field diagram the magnetic field lines leave the north magnetic pole and enter the south magnetic pole.

Objects have measurable magnetic fields because the atoms within them have strong magnetic moments. In your everyday lump of iron, say, the moments are disorganized and tend to cancel each other out. Thus the lump is not ‘magnetic’. In a bar magnet, though, all of the atoms have been more or less “lined up” so that there is a net magnetic moment for the object. (It’s actuall a bit more complex, but this is essentially what happens.)

Moving charged particles, like the electrons in atom, cause magnetic fields. In most atoms the magnetic moment of one electron is cancelled by the magnetic moment of another, but in some elemets this doesn’t happen, leaving the atom with a net magnetic moment. (See also this thread about electron spin.)

I had this explained to me in a physics class just the other day- the north pole of a bar magnet is actually a shortened version of the term ‘north-seeking pole’. Shows what you get when you shorten terms.

Firx is correct, i.e. if you consider the Earth’s poles to be a big bar magnet, the Earth’s “north pole” is actually a south pole.

A few more tid bits:

1. I believe I’ve read somewhere that the Earth’s poles switch on a periodic basis, like every 50,000 years or so. I would assume, then, that this switching doesn’t have any adverse affect on nature. But I could be wrong.

2. As most of us already know, a bar magnet doesn’t point to the Earth’s geographical North Pole. But what many don’t realize is that it doesn’t necessarily point to magnetic north, either. A bar magnet simply aligns itself with the local magnetic field, and a compass assumes the local magnetic field points to magnetic north. This is usually a good assumption, but certainly not perfect. At some locations on the globe, the direction of the local magnetic fields are quite “strange,” and a significant correction factor is required if you’re relying on a compass for direction.

Other than that, the subject is above my head. Beatle is a geophysicists. Perhaps he can share some of his expertise…

Crafter_Man wrote:

As an example of this, here in the Dallas, Texas area, a compass needle points about six degrees to the East of true North, but the magnetic North pole actually is farther West than our longitude.

Did anyone ever see the movie where Anthony Hopkins and some others went down in a plane crash in Alaska, and they were trying to find their way back to the town? I think it was called The Edge. Hopkins’ character was suppsed to be some know-it-all, and he devised a compass by placing a sewing needle on a leaf, and floating this in water. The trouble with this is that you would have to know in detail how the magnetic field varies in that part of Alaska, because it’s really twisted around up there. But the biggest problem is that he was doing all this, and the Sun was shining brightly up in the sky!

The Earth’s magnetic poles can switch, and have numerous times in the past, most recently around 780,000 years ago. There has been some speculation about environmental effects during a transition (which can take several thousand years to complete), but since there’s no correlation between mass extinctions and polarity reversals I think it is safe to say that, generally speaking, there aren’t any harmful effects.

For animals that rely on a magnetic sense to navigate, there probably would be some problems. From the geophysical models constructed to simulate polarity reversals, it appears as though the non-dipole components of the Earth’s magnetic field (i.e., those components that don’t resemble the field of an imaginary bar magnet centered at the Earth’s core) become more prominent, and there is no clear “magnetic north” or “magnetic south.” We don’t really know whether migratory birds and the like have some other means of compensating for a screwed up magnetic field, though, so the total impact of a polarity reversal isn’t clear. (BTW, under ordinary conditions, about 90% of the Earth’s magnetic field can be modeled as the field of a dipole [bar magnet], and the remaining 10% has non-dipole behavior.)

The deviation from true magnetic north that Crafter_Man and CurtC mention is called magnetic declination. The deviation marks the influence of the non-dipole components of the Earth’s field, which can include things like the magnetic effects of large bodies of iron-rich rock (e.g., basaltic lava flows). There is some sort of correction that needs to be made for nearly every spot on the planet you can think of; to find out what the correction should be, you can check the bottom of a topographic map for the region you’re in. The topo map will tell you what the declination was in the year in which it was measured, and the rate of annual drift in declination (because non-dipole field components can shift with time).

I haven’t seen The Edge, but I’d say that apart from not knowing local declination, an ordinary sewing needle is not sufficiently magnetized to act as a compass needle - Tony would have had to stick a magnet to it. The sun would have no impact on this particular bit of business.


I think what CurtC meant was that Hopkins could have avoided this useless exercise if he’d used the sun to determine direction.

Of course he’d have needed to know what effect the time of year had on the path of the sun at that latitude, etc. etc.

Ah, I see what you mean, NutMagnet. But that far north, the sun isn’t going to help much more than a magnetic compass without declination correction, especially in the summertime. CurtC - can you tell us more about the movie? Then we can appropriately nitpick the science involved (or lack thereof). :smiley:

Fillet - I think CurtC’s referring to this movie.

I too have heard about the switch of the north and south poles. During the time it takes for the switch, what would people living on earth experience? Halfway through, would compasses point to the equator??

[hijack] You can use an analogue watch (set to the correct local time) and the sun to determine a north heading.

By pointing the hour hand at the sun and bisecting the hour hand and the 12 on the watch you get True North, you also get south but knowing whether it is morning or evening eliminates the 180 degree error.

I guess if you knew True North and the suns position you could set your analogue watch![/hijack]

I think the first reference to the sun up in the sky meant that if he was doing this at night he could have checked the makeshift compass against the stars to see how far off he was from true north.

But the sewing needle by itself still wouldn’t be enough… CurtC is going to have to come back & give more details so we can clear this up. :slight_smile:

No one is sure exactly what would happen. Some of the more sophisticated magnetohydrodynamic models of the Earth’s magnetic field suggest that the dipole component of the field would gradually weaken, letting the non-dipole components become more prominent (e.g. quadrupoles, octopoles, and so on in increasingly higher harmonics - think of poles of varying strengths peppering the Earth’s surface). Under these conditions, compasses would point to the strongest nearby pole of any kind, so you would not be able to navigate using a compass at all. Eventually, the dipole component of the magnetic field would begin to grow stronger again, this time with the polarity reversed, so now your compass would point to the south pole rather than the north pole.

Have a look here for a description & illustrations of one of the better models of Earth’s magnetic field to date.

There’s some speculation that a weakened dipole field would make the shielding properties of the Van Allen Belt less effective, thus letting more potentially harmful radiation down to the Earth’s surface and possibly causing an increase in genetic mutations. The fossil record doesn’t indicate any correlation between reversals and speciation events, though, so maybe the non-dipole components provide additional shielding during the transition.