Magnetism -- Clarification about North and South magnetic poles

[keith.gwilym]

When I look at images of the pattern of iron filings around a magnet I cannot see any difference between the Pattern at the 'North' and 'South' poles. The present model also says that the field 'flows' from North to South but if this were the case it would not be necessary to move a magnet near a conductor to produce electricity. Is there other evidence for the present model which still appears in texts?

 

[Drakkith]

It's not about evidence, but about convention. We could draw the arrows the other way, change the math slightly, and get the exact same results.

The present model also says that the field 'flows' from North to South

It does not. The arrows don't represent a 'flow', they are just a set of vectors in a vector field. Or, rather, they represent an infinite set of vectors along the path that the field lines take. Nothing is moving along the field lines.

 

[Ibix]

Summary:: What evidence is there for the current model of magnetism that says the magnetic field comes out of the North pole of a magnet and goes into the South pole?

As Drakkith says, this is just a sign convention. It's a decision we've made, and making the opposite decision would have no effect beyond the placing of some minus signs in our mathematical representation of reality. As this is a human choice, it isn't something that is or could be supported by evidence. The only thing that matters is that you use the convention consistently.

When I look at images of the pattern of iron filings around a magnet I cannot see any difference between the Pattern at the 'North' and 'South' poles.

There shouldn't be any such difference, so this does not surprise me.

The present model also says that the field 'flows' from North to South but if this were the case it would not be necessary to move a magnet near a conductor to produce electricity.

No - there's no flow. The "field lines" that you see drawn are what you get if you play join-the-dots with the magnetic field vectors at each point in space. They are just a way of representing a field - it works for any vector field, and is particularly popular for magnetism because of the way iron filings tend to align themselves due to their interaction with the field. But they aren't stream lines such as the ones you see in fluid dynamics, which do represent the flow of the fluid.

You might find this Insight article on what field lines are interesting.

 

[davenn]

Summary:: What evidence is there for the current model of magnetism that says the magnetic field comes out of the North pole of a magnet and goes into the South pole?

When I look at images of the pattern of iron filings around a magnet I cannot see any difference between the Pattern at the 'North' and 'South' poles. The present model also says that the field 'flows' from North to South but if this were the case it would not be necessary to move a magnet near a conductor to produce electricity. Is there other evidence for the present model which still appears in texts?

None, because you have it around the wrong way

from Wiki ( and it's found in many other places) ...

Magnetic poles[edit]​

Main article: Geomagnetic pole
The movement of Earth's North Magnetic Pole across the Canadian arctic.
Historically, the north and south poles of a magnet were first defined by the Earth's magnetic field, not vice versa, since one of the first uses for a magnet was as a compass needle. A magnet's North pole is defined as the pole that is attracted by the Earth's North Magnetic Pole when the magnet is suspended so it can turn freely. Since opposite poles attract, the North Magnetic Pole of the Earth is really the south pole of its magnetic field (the place where the field is directed downward into the Earth).[17][18][19][20]my bold

 

[Drakkith]

None, because you have it around the wrong way

Unless I'm mistaken, the OP is talking about the poles of a magnet, not the Earth, so he does have it the right way.

 

[davenn]

hahahaha to you too @nasu

go do some reading

 

[davenn]

Unless I'm mistaken, the OP is talking about the poles of a magnet, not the Earth, so he does have it the right way.

You may be right

But since he didnt state which, I gave the other explanation

 

[Drakkith]

You may be right

But since he didnt state which, I gave the other explanation

Huh?

What evidence is there for the current model of magnetism that says the magnetic field comes out of the North pole of a magnet and goes into the South pole?

 

[nasu]

hahahaha to you too @nasu

go do some reading

Maybe you read the question before answering? The convention for the direction of magnetic field lines may be a good reading too.

 

[vanhees71]

I always find it very confusing to talk about "north and south poles" of a magnet. It's an anachronism. Today we use the magnetic field $\vec{B}$ to describe magnetism, and as a vector field has a clear definition of its direction. The translation from the "anachronism" to the "modern view" simply is that by convention the north pole of a permanent magnet is that end where the magnetic field points away from (toward) the magnet is called north (south) pole.

The source of the magnetic field of such a permanent magnet macroscopically is the magnetization density (i.e., the magnetic dipole moment of the material contained in a little volume element per volume). If you look at the magnetic field not too close to the magnet it's to good approximation a dipole field with the dipole moment of the magnetization distribution as the source. In this sense you can approximate the magnet simply by its magnetic (dipole) moment. The dipole moment has a direction pointing from the "south to the north pole" of the magnet.

If you now consider a magnetic dipole moment in another magnetic field (e.g., the Earth's magnetic field) then you realize that the equilibrium position of the dipole is that it is directed parallel to this external magnetic field. Now be convention people call the end of the compass needle "north pole" that points towards north. But then it points in the direction of the magnetic field of the Earth, i.e., to that pole of the Earth, where the field points towards the Earth. Consequently the geographic north pole of the Earth is the south pole its magnetic field and vice versa, and that's why it's so confusing. I think a good compromise where to call the magnetic poles of the Earth the arctic and antarctic poles, but of course it's very difficult to change everyday language based on century (or even millenia) old convention :-)).

For more details on the Earth's magnetic field, see

https://en.wikipedia.org/wiki/Earth's_magnetic_field

 

[hutchphd]

Will we need to modify the weak interaction? Ms. Wu should somehow be informed.
I am astounded to see how far the pole has migrated during my sentient life. I have spent a lot of time in the state of Maine over my nearly 70 yrs and the artic pole's migration has significantly diminished the incidence of spectacular aurora events ( even including the 11 yr sunspot cycle) . Pity.

 

[vanhees71]

What has this to do with the weak interaction and Wu?

What's true is that the magnetic field of the Earth gets weaker over the last decades and also the (magnetic) pole drifts quite fast. Some scientists think that this may indicate that the Earth's magnetic field may flip in the not too far future as it has done several times before.

http://www.esa.int/Applications/Observing_the_Earth/Swarm/Magnetic_north_and_the_elongating_blob
https://www.esa.int/Applications/Ob...rm_probes_weakening_of_Earth_s_magnetic_field

 

[alan123hk]

I also heard that the north and south magnetic poles of the Earth will eventually flip in the distant future. I don't know if this is true. If flipping does occur, will it affect the climate or ecology, or even increase natural disasters during the long flipping process?

 

[Ibix]

I also heard that the north and south magnetic poles of the Earth will eventually flip in the distant future. I don't know if this is true.

The "distant" bit probably isn't true. Flipping has occurred many times in the past and I believe we're in an unusually long period without one, so it's overdue from what we can see.

If flipping does occur, will it affect the climate or ecology, or even increase natural disasters during the long flipping process?

No idea. Some creatures appear to navigate using a magnetic sense, though, so some disruption seems plausible.

 

[hutchphd]

What has this to do with the weak interaction and Wu?

Feeble attempt at "handedness" joke and unambiguous definition of North pole. Admittedly a little "loose".
But movement of the pole is impressively away from Maine:

 

[alan123hk]

No idea. Some creatures appear to navigate using a magnetic sense, though, so some disruption seems plausible

As another example, if the direction of the magnetic pole changes from a direction approximately parallel to the rotation axis of the Earth to a direction perpendicular to the rotation axis of the earth, this situation seems to have a more practical physical effect.