How do the magnetic strengths of compasses compare to those of magnets?

[feynman1]

How strongly magnetic is a compass compared to a magnet? If many compasses are put near a magnet, is it possible that the compasses are so strong that their orientations are affected by not only the magnet but also other compasses nearby?

 

[Charles Link]

The compass needle is a permanent magnet and has a north pole and a south pole. You might see a slight variation in where the needles point due to the Earth's magnetic field if you put two compasses in close proximity to each other at various relative positions, where the magnetic field from one needle will affect the other needle.

 

[feynman1]

The compass needle is a permanent magnet and has a north pole and a south pole. You might see a slight variation in where the needles point due to the Earth's magnetic field if you put two compasses in close proximity to each other at various relative positions, where the magnetic field from one needle will affect the other needle.

Let's ignore earth. Somewhere on the internet there's the following image. Some compasses' orientations differ from the field of the magnet. Is it due to the interactions between the compasses?

 

[Delta2]

I think it's because the horse-shoe magnet isn't a perfect one and not of interactions between the compasses. I consider the magnetic field of each compass to die very fast out of the plastic circle box in which each compass is encased so I think the interactions between compasses are really negligible.

 

[feynman1]

I think it's because the horse-shoe magnet isn't a perfect one and not of interactions between the compasses. I consider the magnetic field of each compass to die very fast out of the plastic circle box in which each compass is encased so I think the interactions between compasses are really negligible.

See the enclosed compasses as shown below. Opposite poles of neighboring compasses seem to attract each other a bit though they aren't so strong as to be attracted head on?

 

[Delta2]

There might be compass to compass interactions we can't be sure (though I doubt it as I said, it is most likely because the magnetic field of the magnet seems to have some irregularities). But this is predicted by theory.

 

[Ibix]

There might be compass to compass interactions we can't be sure (though I doubt it as I said, it is most likely because the magnetic field of the magnet seems to have some irregularities).

Cheap compasses with shoddy bearings is also a distinct possibility (particularly as putting compasses near magnets is one of the things you are recommended not to do), meaning that they may not even be pointing in the direction of the field. We also don't know what the magnet actually is - I've seen things that look like horseshoe magnets that, on closer inspection, turn out to be hollow plastic C shaped tubes with a pair of small button magnets glued to the end.

I agree that you could find that compass needles attract each other because they are magnets themselves. How strongly would depend how strong magnets they are - not very strong, I think. A quick google turns up a few forum posts about nearby compasses interfering with each other and being careful to store compasses separately, but I haven't seen anything quantitative (haven't really searched thoroughly, either).

 

[Meir Achuz]

A compass is affected by any nearby iron. That's why compasses on boat have to be 'boxed' by adjusted small iron balls (You can see them on ship's compasses.) to neutralize the effect of the ship on the compass.

 

[anorlunda]

When navigating at sea in a small boat, any external magnet or iron object can deflect the compass reading enough to spoil navigation.

My rule was simple. No magnets of any kind allowed on the boat. Of course there are hidden magnets in things like hard drives. But the rule helped ban things like refrigerator magnets that might get tossed aside and forgotten who knows where.

Thank goodness for GPS. With GPS, the compass becomes backup. With GPS, the compass heading can be checked against the GPS heading at any time, and the deviation recorded. Then, if the GPS dies, the compass backup can be used given the recorded table of deviations.

If the deviation suddenly changed significantly, that triggers and investigation of the source. And that source is more likely to be an ocean current than a magnet. A sailboat crossing the Gulf Stream might see a 60 degree deviation between compass heading and GPS track. It means that the boat is moving sideways.

 

[feynman1]

let's analyze possibilities in the pic of post #3

 

[berkeman]

let's analyze possibilities in the pic of post #3

https://upload.wikimedia.org/wikipe...gnet.svg/1200px-VFPt_horseshoe-magnet.svg.png

You can see from the diagram that the external magnetic field of a horseshoe magnet is concentrated around its poles. The compasses that are near the poles are obviously aligning with the magnet's external B-field.

Away from its poles and near the body of the magnet, the compass needles are just attracted to the ferrous metal of the horseshoe, and are not affected enough by the external magnetic field of the horseshoe magnet to align with that field.

Which N/S pole of the compass that points to the magnet body is probably random, just based on how the compass was pointing when it was set down next to the body of the magnet away from the poles. If you were doing the experiment yourself, you could probably verify that.

 

[anorlunda]

One test would be to leave the compasses where they are, take a picture, then lift out the horseshoe magnet vertically (avoid horizontal movement and rotations) and take a second picture. Now compare the needles in the two pictures.

Then put the magnet back where it was, but reversing the N-S poles.

 

[Charles Link]

One other test would be to use a single compass and move it to the various locations, and compare to the case of multiple compasses.

 

[hutchphd]

Heres a nice demo video with a table full of magnetic needles and a lab guy:

 

[feynman1]

View attachment 263746

https://upload.wikimedia.org/wikipe...gnet.svg/1200px-VFPt_horseshoe-magnet.svg.png

View attachment 263747

You can see from the diagram that the external magnetic field of a horseshoe magnet is concentrated around its poles. The compasses that are near the poles are obviously aligning with the magnet's external B-field.

Away from its poles and near the body of the magnet, the compass needles are just attracted to the ferrous metal of the horseshoe, and are not affected enough by the external magnetic field of the horseshoe magnet to align with that field.

Which N/S pole of the compass that points to the magnet body is probably random, just based on how the compass was pointing when it was set down next to the body of the magnet away from the poles. If you were doing the experiment yourself, you could probably verify that.

Thanks. How to describe 'the compass needles are just attracted to the ferrous metal of the horseshoe' by electric or magnetic forces?

 

[berkeman]

Thanks. How to describe 'the compass needles are just attracted to the ferrous metal of the horseshoe' by electric or magnetic forces?

Same as any bar magnet. Compass needles are just fine little bar magnets, no?

 

[vanhees71]

Well, the field of a bar magnet (be it a cylinder or a cuboid) is not too difficult to calculate (assuming homogeneous magnetization or some simple ansatz for it as found in some good textbooks on electromagnetism, e.g., Sommerfeld, vol 3), but for a horse-shoe magnet, I'm not so sure, how to do it. I've never seen it in any textbooks. I guess it can only be done numerically.

The compass needles can maybe treated as little rods and FAPP simply as magnetic dipoles.

 

[Charles Link]

The attraction of the compass needles to the pure (unmagnetized) iron should be similar in magnitude to the forces between two compass needles. It is difficult to sort this out completely without additional experiments, and I do think the Earth's magnetic field might also play a role here.
In response to the previous post by @vanhees71 , I think the assumption can be made of nearly uniform M along the horseshoe, so that the only magnetic poles (plus and minus) appear at the north and south pole endfaces.