Magnetic field lines and iron filings

[swansont]

I thought so. That makes it clear that friction holds them in position on paper.

If you tap the paper, which temporarily reduces the friction, you can see filings move toward the magnet

 

If you stick two magnets together N-S end-to-end, and did the iron filings again, what would be the field configuration? Would it be like one magnetic field or two distinct fields, in some other pattern where they join?

You mean N-N (or S-S), or connecting N-S?

[StringJunky]

If you tap the paper, which temporarily reduces the friction, you can see filings move toward the magnet

 

 

You mean N-N (or S-S), or connecting N-S?

Connecting N-S with no gap between - magnetically attracted and in contact

[swansont]

Connecting N-S with no gap between - magnetically attracted and in contact

 

That will look just like a physically bigger magnet

[StringJunky]

 

That will look just like a physically bigger magnet

Yes. I thought so.

[CasualKilla]

 

 

 

Looks like my "blob" predictions was near the truth, but did not consider how much stronger the field is near the poles, so naturally the filing gather there first. we can still see some N-N, S-S attraction, but it is not as intense as expected.

 

For some reason none of these plebs leave the video on long enough to see the final steady state formation

Edited March 23, 2015 by CasualKilla

[Robittybob1]

 

 

 

Looks like my "blob" predictions was near the truth, but did not consider how much stronger the field is near the poles, so naturally the filing gather there first. we can still see some N-N, S-S attraction, but it is not as intense as expected.

 

For some reason none of these plebs leave the video on long enough to see the final steady state formation

It seemed that one end was forming more intensely than the other. (it ended up being tighter packed as well.)

[StringJunky]

It seemed that one end was forming more intensely than the other. (it ended up being tighter packed as well.)

The distribution of particles was probably unbalanced towards one end so it was attracting them at a higher rate.

[Robittybob1]

The distribution of particles was probably unbalanced towards one end so it was attracting them at a higher rate.

It would be worth looking at that and see what the reason was. I saw them shake the container to distribute the filings in the liquid in one of the videos.

[CasualKilla]

It seemed that one end was forming more intensely than the other. (it ended up being tighter packed as well.)

That is probably due to the initial uneven distribution of iron filings. Also imbalance will be compounded since the iron filing become magnetized adding their own magnetic dipole moments at the poles, thus creating a stronger field in that vicinity. Think of it as adding tiny little magnets to each side, the side with more magnets will naturally have stronger attractive ability and compete more strongly remaining iron filings. #monopoly#survivalofthefittest#darwinwasright

[Robittybob1]

That is probably due to the initial uneven distribution of iron filings. Also imbalance will be compounded since the iron filing become magnetized adding their own magnetic dipole moments at the poles, thus creating a stronger field in that vicinity. Think of it as adding tiny little magnets to each side, the side with more magnets will naturally have stronger attractive ability and compete more strongly remaining iron filings. #monopoly#survivalofthefittest#darwinwasright

That is a reasonable hypothesis but I don't think it is proven. All I see, and I think others agree with the observation, is that both ends of the bar magnet are surrounded in a different pattern and I'd like to know why.

[swansont]

That is a reasonable hypothesis but I don't think it is proven. All I see, and I think others agree with the observation, is that both ends of the bar magnet are surrounded in a different pattern and I'd like to know why.

The domain structure at either end need not be identical.

[Robittybob1]

The domain structure at either end need not be identical.

So it could be a property of the bar magnet. The iron filings are not little permanent magnets too are they. Are iron filings non-magnetized?

I have see iron filings stick to a file appearing as if they are slightly magnetized in filing the process. Have you experienced that? It could be the file or the filings.

[swansont]

So it could be a property of the bar magnet. The iron filings are not little permanent magnets too are they. Are iron filings non-magnetized?

I have see iron filings stick to a file appearing as if they are slightly magnetized in filing the process. Have you experienced that? It could be the file or the filings.

They're a ferromagnetic material, so they can become magnetized.

[Ynan]

About the explanation of why the layers form: If each iron feeling turns into N-S magnet, then we cannot expect to form a layer with more than one iron feeling perfectly parallel to each other within the same layer - can we check on this? 

Problems about the explanation: How can we form an entire 3D shell if each iron filling turns into a N-S magnet, there would be strong repulsion within each shell that would destroy it. The only way to for a complete shell to form is to align all iron fillings in such a way that the  N-end touches the S-end top and bottom., and side by side like how bricks are stocked on top each other. However, there is another problem with this, if the repulsions can be minimized this way, then how can we account for the huge distances between layers? 

Lastly, if a shell can form, using the arrangement shown in my picture, then notice that another shell that is completely attached to first shell can form on top and the bottom, thus we would expect a one big blob of iron around the magnet and not layers of shells like in onions. 

Can someone check my reasoning here?

[studiot]

14 minutes ago, Ynan said:

About the explanation of why the layers form: If each iron feeling turns into N-S magnet, then we cannot expect to form a layer with more than one iron feeling perfectly parallel to each other within the same layer - can we check on this? 

Problems about the explanation: How can we form an entire 3D shell if each iron filling turns into a N-S magnet, there would be strong repulsion within each shell that would destroy it. The only way to for a complete shell to form is to align all iron fillings in such a way that the  N-end touches the S-end top and bottom., and side by side like how bricks are stocked on top each other. However, there is another problem with this, if the repulsions can be minimized this way, then how can we account for the huge distances between layers? 

Lastly, if a shell can form, using the arrangement shown in my picture, then notice that another shell that is completely attached to first shell can form on top and the bottom, thus we would expect a one big blob of iron around the magnet and not layers of shells like in onions. 

Can someone check my reasoning here?

 

First did you understand the explanation in the first page of this thread?

 

One observation was that the simple demonstration refers to 2D section like your picture.

But it is a 3D effect. So let us say that the white rectangles represent south seeking poles the what would you put into the space immediatedly adjacent to these south seeking poles above and below the plane of the paper?

[Ynan]

yes I did, and there is one thing that I can't seem to understand on the explanation. Magnetic fields are supposed to be continuous, (they are strongest from the source, becoming weaker through distance) hence, the first assumption in the explanation that "magnetic field lines" are uniformly distributed is already a divergence from the existing concept of what a magnetic field is due to the conception that there are magnetic field "lines". Are there really magnetic field lines? 

Then the explanation moves on to explain that gaps are partly due to the repulsions between the induced magnets (iron filings). My contention here is that, are those repulsions so strong that they could cause such large gaps? If they do, why aren't the iron filling within each line(shell) do not repel each other seeing that there are some of them that are perfectly aligned (on each side). 
 

My third contention is that if minimization of energy is the main reason why the iron filing tend to converge with each other to form lines and gaps, would it not more energy efficient for the magnet and the iron fillings to want to distribute themselves uniformly, tightly packed, attached to the magnet?

 

[studiot]

34 minutes ago, Ynan said:

yes I did, and there is one thing that I can't seem to understand on the explanation. Magnetic fields are supposed to be continuous, (they are strongest from the source, becoming weaker through distance) hence, the first assumption in the explanation that "magnetic field lines" are uniformly distributed is already a divergence from the existing concept of what a magnetic field is due to the conception that there are magnetic field "lines". Are there really magnetic field lines? 

Then the explanation moves on to explain that gaps are partly due to the repulsions between the induced magnets (iron filings). My contention here is that, are those repulsions so strong that they could cause such large gaps? If they do, why aren't the iron filling within each line(shell) do not repel each other seeing that there are some of them that are perfectly aligned (on each side). 
 

My third contention is that if minimization of energy is the main reason why the iron filing tend to converge with each other to form lines and gaps, would it not more energy efficient for the magnet and the iron fillings to want to distribute themselves uniformly, tightly packed, attached to the magnet?

 

 

We can best discuss your questions if you also consider mine.
They are designed to help you understand/point you in the right direction.
They are not designed to trip you up.

 

Think about a tapering funnel.

Now think about connecting it in line with a gas pipe so the gas flows through the funnel.
If the gas enters at the narrow part and leaves via the wide part it will reduce in density as is passes through the funnel.

But on average (to  a first approximation) we consider the density constant at any cross section to the flow.
So as the funnel widens the gas density reduces but is spread evenly at any cross section.
This is easy to derive from the continuity equation.

You can consider the field lines as a form of field density in the same way, so that it reduces as the field spreads out from the source, yet remain isopleths (the same strength) at any given distance (distance along the lines not a raidius).

The next thing to think about is the question of where and what is the source.
When the magnet is first introduced, the (say north pole end) end of the magnet is the beginning of the source.
Even before a sliver of iron attaches itself to the end, a north pole is induced in the end of the sliver remote from the magnet.

This north pole at the end of the sliver becomes the new 'source' and the sliver is then effectively part of the magnet.
The connection is not perfect but good enough.

Another sliver will attach at the end of the magnet, nearby but the two new areas of north pole will repel and so on.

As the stack of slivers build outwards they adopt a 3D dendritic pattern (the hedgehog look), as shown in the videos, if free to do so.
Swansont has mentioned that a paper surface provides some frictional restraint.

It should also be noted that some machining process partially magentise the slivers so they may already have a preferential direction.

 

Edited May 16, 2018 by studiot

[Ynan]

Based from your explanation (Thank you for it, by the way), the shape of the iron fillings has to do something with the gaps. What would happen if the filings are turned into spheres?  Can somebody do the experiment? 

I noticed in the video that the gaps aren't real gaps in the magnetic field (3D), they are simply brought about by the temporary friction of the structures (chains of iron filings) to the fluid. Once the resistance is overcome, the structures actually move straight towards that magnet, until the magnet becomes saturated with iron so that its magnetic field strength becomes shielded to further pull the remaining (chains) towards it. This absence (or minimal pull) of the downward pull of the magnet due to shielding is what keeps the remaining chains suspended in their places, thus giving the illusion that there are gaps in the magnetic field.